Guide to Aluminium Casting Alloys by Aleris


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Guide to Aluminium Casting Alloys by Aleris

  1. 1. Aluminium Casting Alloys
  2. 2. Aluminium Casting Alloys
  3. 3. Aluminium Casting Alloys Aluminium Casting Alloys
  4. 4. Aluminium Casting Alloys Content Introduction 5 Recycled aluminium 6 Technology and service for our customers • Quality Management 7 • Work safety and health 8 protection • Environmental protection Aluminium and aluminium 9 casting alloys • Aluminium – Material properties • Recycling of aluminium • Shaping by casting 10 Product range and 11 form of delivery • Technical consultancy 12 service Selecting aluminium 13 casting alloys • Criteria for the selection of 14 aluminium casting alloys • Influence of the 18 most important alloying elements on aluminium casting alloys Influencing the 19 microstructural formation of aluminium castings • Grain refinement 20 • Modification of AlSi eutectic 21 • Refinement of 23 primary silicon Melt quality and melt cleaning 24 • Avoiding impurities 25 • Melt testing and 28 inspection procedure • Thermal analysis 30 Selecting the casting process 31 • Pressure die casting 32 process • Gravity die casting process • Sand casting process 34 Casting-compliant design 35 Solidification simulation 37 and thermography Avoiding casting defects 38 Heat treatment of 40 aluminium castings • Metallurgy – fundamental principles • Solution annealing 41 • Quenching • Ageing 42 Mechanical machining of 44 aluminium castings Welding and joining 45 aluminium castings • Suitability and behaviour • Applications in the aluminium sector • Welding processes • Weld preparation 47 • Weld filler materials Surface treatment: corrosion 48 and corrosion protection Information on physical data, 50 strength properties and strength calculations Notes on the casting 51 alloy tables Overview: Aluminium casting 52 alloys by alloy group Eutectic aluminium-silicon 59 casting alloys Near-eutectic wheel 63 casting alloys The 10 per cent aluminium- 66 silicon casting alloys The 7 and 5 per cent 71 aluminium-silicon casting alloys Al SiCu casting alloys 76 AlMg casting alloys 81 Casting alloys for special 87 applications 4
  5. 5. Aluminium Casting Alloys In the second part, all technical aspects which have to be taken into account in the selection of an aluminium casting al- loy are explained in detail. All details are basedontheDINEN1676:2010standard. The third part begins with notes on the physical data, tensile strength charac- teristics and strength calculations of aluminium casting alloys. Subsequently, all standardised aluminium casting alloys in accordance with DIN EN 1676 as well as common, non-standardised casting alloys are depicted in a summary table together with their casting/technical and other typical similarities in “alloy families”. The aim of this new, revised and rede- signed Aluminium Casting Alloys Cata- logue is to give the user of aluminium Many of you have most certainly worked with the “old“ Aluminium Casting Alloys Catalogue – over the years in thousands of workplaces in the aluminium indus- try, it has become a standard reference book, a reliable source of advice about all matters relating to the selection and processing of aluminium casting alloys. Even if you are holding this Aluminium Casting Alloys Catalogue in your hands for the first time, you will quickly find your way around with the help of the following notes and the catalogue‘s detailed index. How is this Aluminium Casting Alloys Catalogue structured? The catalogue consists of three separate parts. In the first part, we provide details on our com- pany – a proven supplier of aluminium casting alloys. Introduction casting alloys a clear, well laid-out com- panion for practical application. Should you have any questions concerning the selection and use of aluminium casting alloys, please contact our foundry con- sultants or our sales staff. You can also refer to We would be pleased to advise you and wish you every success in your dealings with aluminium casting alloys! 5
  6. 6. Aluminium Casting Alloys Recycled aluminium Technology and service for our customers Employing approx. 600 people, Aleris Recycling produces high-quality cast- ing and wrought alloys from recycled aluminium. The company‘s headquar- ters are represented by the “Erftwerk” in Grevenbroich near Düsseldorf which is also the largest production facility in the group. Other production facilities in Germany (Deizisau, Töging), Norway (Eidsväg, Raudsand) and Great Britain (Swansea) are managed from here. With up to 550,000 mt, Aleris Recycling avails of the largest production capacities in Europe and is also one of the world‘s leading suppliers of technology and services relating to aluminium casting alloys. Aleris Recycling also offers a wide range of high-quality magnesium alloys. Aluminium recycled from scrap and dross has developed to become a highly-complex technical market of the future. This is attributable to the steady increase in demand for raw materials, the sustainability issue, increased envi- ronmental awareness among producers and consumers alike and, not least, the necessity to keep production costs as low as possible. This is where aluminium offers some es- sential advantages. Recycled aluminium can be generated at only a fraction of the energy costs (approx. 5%) compared to primary aluminium manufactured from bauxite with the result that it makes a significant contribution towards reduc- ing CO2 emissions. This light-alloy metal can be recycled any number of times and good segregation even guarantees no quality losses. Its properties are not impaired when used in products. The metallic value is retained which represents a huge eco- nomic incentive to collect, treat and melt the metal in order to reuse it at the end of its useful life. For this reason, casting alloys from Aleris Recycling can be used for manufacturing new high-quality cast products such as crankcases, cylinder heads or aluminium wheels while wrought materials can be used for manufacturing rolled and pressed products, for example. Key industries supplied include: • Rolling mills and extrusion plants • Automotive industry • Transport sector • Packaging industry • Engineering • Building and construction • Electronics industry • as well as other companies in the Aleris Group. State-of-the-art production facilities and an extensive range of products made of aluminium in the form of scrap, chips or dross are collected and treated by Aleris Recycling before melting in tilting rotary furnaces with melting salt, for example, whereby the salt prevents the aluminium from oxidising while binding contami- nants (salt slag). Modern processing and melting plants at Aleris Recycling enable efficient yet environmentally-friendly re- cycling of aluminium scrap and dross. The technology used is largely based on our own developments and – in terms of yield and melt quality – works significantly more efficiently than fixed axis rotary furnaces and hearth furnaces. The melt gleaned from these furnaces has a very low gas content thanks to the special gas purging technique we use as well as being homogeneous and largely free of oxide inclusions and/or contaminants. The resulting high quality of Aleris alloys enables our customers to open up an in- creasing number of possible applications. All management processes and the en- tire process chain from procurement through production to sale are subject to systematic Quality Management. Com- bined with Quality Management certified to ISO/TS 16949 and DIN EN ISO 9001, this guarantees that our clients‘ maximum requirements and increasing demands can be fulfilled. The product range offered by Aleris Re- cycling comprises more than 250 differ- ent casting and wrought alloys. They can be supplied as ingots with unit weights of approx. 6 kg (in stacks of up to 1,300 kg) as well as pigs of up to 1,400 kg or as liquid metal. Based on our sophisti- cated crucible technology and optimised transport logistics, Aleris Recycling sup- plies customers with liquid aluminium in a just-in-time process and at the appro- priate temperature. 6
  7. 7. Aluminium Casting Alloys Quality Management We believe that our most important cor- porate goal is to meet in full our custom- ers‘ requirements and expectations in terms of providing them with products and services of consistent quality. In or- der to meet this goal, our guidelines and integrated management system specifi- cations outline rules and regulations that are binding for all staff. As a manufacturer of aluminium casting alloys, we are certified according to ISO/ TS 16949. In addition, we operate ac- cording to DIN EN ISO 9001 standards. Due to its future-oriented corporate structure, Aleris Recycling supplies the market with an increasing number of applications involving high-quality sec- ondary aluminium. This service is not re- stricted to the area of casting alloys but also applies for 3000- and 5000-grade wrought alloys, for example. Aleris Re- cycling is also capable of offering some 6000-grade secondary aluminium alloys largely required by the automotive sector. For this so-called upgrade, Aleris applies special production technologies when it comes to manufacturing high-quality alloys from scrap. Recycled aluminium is increasingly be- coming a complex range at the interface between high-tech production, trade and service. In addition, customers demand intensive consulting as well as individual service. Aleris Recycling enjoys an ex- cellent position in this regard. At its various locations, the company units offer a high degree of recycling ex- pertise, manufacturing competence and delivery reliability for its customers. With the result that Aleris Recycling guarantees its customers a high level of efficiency and added value while supporting their success on the market. The principle of avoiding errors is para- mount in all our individual procedures and regulations. In other words, our priority is to strive to achieve a zero-error target. By effectively combating the sources of errors, we create the right conditions for reliability and high quality standards. We have also established a comprehen- sive process of continuous improvement (PMO, Best Practice, Six Sigma etc.) in our plants in response to the demands being placed on our company by the increasing trend towards business glo- balisation. This creates the right cli- mate for creative thinking and action. All members of staff, within their own area of responsibility, endeavour to en- sure that operational procedures are constantly improved, even if in small, gradual stages, with a clear focus on our customers‘ needs. 7
  8. 8. Aluminium Casting Alloys Work safety and health protection Our staff are our most valuable asset. Work safety and health protection, therefore, have top priority for us, and also make a valuable contribution to the success of our company. Our “Work safety and health protection” programme is geared towards achieving a zero accident rate, and towards avoiding occupational ill- nesses. Depending on the respective location, we are certified to OHSAS 18001 or OHRIS. All management members and staff are obliged to comply with legal regulations and company rules at all times, to pro- tect their own health and the health of other members of staff and, when en- gaged in any company operations, to do their utmost to ensure that accidents and work-related illnesses are avoided, as well as anything that might have a negative impact on the general company environment. Management provides the appropriate level of resources required to achieve these goals. There are regular internal and external training seminars on the topic of work safety, and detailed programmes to im- prove health protection. These help to maintain our comparatively low accident and illness rates. Environmental protection Following the validation of our environ- mental management system in conformity with EMAS II and certification to DIN EN ISO 14001, we have undertaken not only to meet all the required environmental standards, but also to work towards a fundamental, systematic and continual improvement in the level of environmental protection within the company. Our management system and environ- mental policy are documented in the company manual which describes all the elements of the system in easily understood terms, while serving as a reference for all regulations concerning the environment. The environmental impacts of our com- pany operations in terms of air purity, protection of water bodies, noise and waste are checked at regular intervals. By modifying procedures, reusing mate- rials and recycling residues, we optimise the use of raw materials and energy in order to conserve resources as efficiently as possible. We pursue a policy of open information and provide interested members of the public with comprehensive details of the company‘s activities in a particu- lar location, and an explanation of the environmental issues involved. For us, open dialogue with the general pub- lic, our suppliers, customers and other contractual partners is as much a part of routine operations as reliable co-op- eration with the relevant authorities and trade associations. Likewise, ecological standards are in- corporated in development and planning processes for new products and produc- tion processes, as are other standards required by the market or society at large. Our staff is fully conscious of all environ- mental protection issues and is keen to ensure that the environmental policy is reliably implemented in day-to-day op- erations within the company. 8
  9. 9. Aluminium Casting Alloys Aluminium and aluminium casting alloys Recycling of aluminium Long before the term “recycling” became popular, recycling circuits already exist- ed in the aluminium sector. Used parts made from aluminium or aluminium alloys as well as aluminium residue materials arising from production and fabrication are far too valuable to end up as land- fill. One of the great advantages of this metal, and an added plus for its use as a construction material, is that aluminium parts, no matter the type, are extremely well suited to remelting. • The energy savings made in recycling aluminium are considerable. Remelting requires only about 5 % of the energy initially required to produce primary aluminium. • As a rule, aluminium recycling retains the value added to the metal. Aluminium can be recycled to the same quality level as the original metal. • Aluminium recycling safeguards and supplements the supply of raw materials while saving resources, protecting the environment and conserving energy. Recycling is therefore also a dictate of economic reason. • Aluminium is light; its specific weight is substantially lower than other common metals and, at the same time, it is so strong that it can with stand high stress. • Aluminium is very corrosion- resistant and durable. A thin, natural oxide layer protects aluminium against decomposition from oxygen, water or chemicals. • Aluminium is an excellent conductor of electricity, heat and cold. • Aluminium is non-toxic, hygienic and physiologically harmless. • Aluminium is non-magnetic. • Aluminium is decorative and displays high reflectivity. • Aluminium has outstanding formability and can be processed in a variety of ways. • Aluminium alloys are easy to cast as well as being suitable for all known casting processes. • Aluminium alloys are distinguished by an excellent degree of homogeneity. • Aluminium and aluminium alloys are easy to machine. • Castings made from aluminium alloys can be given an artificial, wear-resistant oxide layer using the ELOXAL process. • Aluminium is an outstanding recycling material. Aluminium – Material properties Aluminium has become the most widely used non-ferrous metal. It is used in the transport sector, construction, the pack- aging industry, mechanical engineering, electrical engineering and design. New fields of application are constantly open- ing up as the advantages of this material speak for themselves: 9
  10. 10. Aluminium Casting Alloys Shaping by casting Casting represents the shortest route from raw materials to finished parts – a fact which has been known for five thou- sand years. Through continuous further development and, in part, by a selective return to classic methods such as the lost-form process, casting has remained at the forefront of technical progress. The most important advantage of the casting process is that the possibilities of shaping the part are practically limit- less. Castings are, therefore, easier and cheaper to produce than machined and/ or joined components. The general waiv- ing of subsequent machining not only results in a good density and path of force lines but also in high form strength. Furthermore, waste is also avoided. As a rule, the casting surface displays a tight, fine-grained structure and, consequently, is also resistant to wear and corrosion. The experience accumulated over ma- ny decades, the use of state-of-the-art technology in scrap preparation, remelt- ing and exhaust gas cleaning as well as our constant efforts to develop new, environmentally-sound manufactur- ing technology puts us in a position to achieve the best possible and efficient recycling rates. At the same time, they also help us to make the most efficient use of energy and auxiliary materials. The variety of modern casting process- es makes it possible to face up to the economic realities, i.e. the optimisation of investment expenditure and costs in relation to the number of units. With casting, the variable weighting of pro- duction costs and quality requirements are also possible. When designing the shape of the cast- ing, further possibilities arise from the use of inserts and/or from joining the part to other castings or workpieces. In the last decade, aluminium has at- tained a leading position among cast metals because, in addition to its other positive material properties, this light metal offers the greatest possible variety of casting and joining processes. 10
  11. 11. Aluminium Casting Alloys Product range and form of delivery Our casting alloys are delivered in the form of ingots with a unit weight of ap- prox. 6 kg or as liquid metal. We distinguish between ingots cast in open moulds and horizontal continu- ously cast ingots (so-called HGM). In- gots are dispatched in bundles of up to approx. 1,300 kg. The delivery of liquid or molten metal is useful and economic when large quanti- ties of one homogeneous casting alloy are required and the equipment for tapping and holding the molten metal containers is available. Supplying molten metal can lead to a substantial reduction in costs as a result of saving melting costs and a reduction in melting losses. The sup- ply of liquid metal also provides a viable alternative in cases where new melting capacities need to be built to comply with emission standards or where space is a problem. As ecological and economic trends sen- sibly move towards the development of closed material circuits, the clear dividing lines between the three classic quality grades of aluminium casting alloys are ever-decreasing. In future, people will simply talk about “casting alloys”. In practice, this is already the case. Metal from used parts is converted back into the same field of application. The DIN EN 1676 and 1706 standards with their rather fluid quality transitions take this trend into account. Aleris is one of only a few companies to produce a wide range of aluminium alloys; our product spectrum extends from classic secondary alloys to high- purity alloys for special applications. Production is in full compliance with the European DIN EN 1676 standard or international standards and in many cases, manufactured to specific cus- tomer requirements. We have also been offering several aluminium casting al- loys as protected brand-name alloys for many years, e.g. Silumin®® , Pantal®® and Autodur® . 11
  12. 12. Aluminium Casting Alloys Technical consultancy service The technical consultancy service is the address for questions relating to foundry technology. We provide assis- tance in clarifying aluminium casting alloy designations as stated in German and international standards or the temper conditions for castings. We also offer advice on the selection of alloys and can provide aluminium foundries or users of castings with information on: • Aluminium casting alloys • Chemical and physical properties • Casting and solidification behaviour • Casting processes and details regarding foundry technology • Melt treatment possibilities, such as cleaning, degassing, modification or grain refinement • Possibilities of influencing the strength of castings by means of alloying elements or heat treatment • Questions relating to surface finish and surface protection. Technical consultants also provide as- sistance in evaluating casting defects or surface flaws and offer suggestions with regard to eliminating defects. They sup- ply advice on the design of castings, the construction of dies, the casting system and the configuration of feeders. Technical consultants also provide tech- nical support to aluminium foundries in the preparation of chemical analyses, microsections and structural analyses. Customer feedback coupled with exten- sive experience in the foundry sector fa- cilitates the continuous optimisation and quality improvement of our aluminium casting alloys. In co-operation with our customers, we are working on gaining wider acceptance of our aluminium casting alloys in new fields of application. Where required and especially where fundamental problems arise, we arrange contracts with leading research institutes in Europe and North America. 12
  13. 13. Aluminium Casting Alloys As far as possible, the use of common aluminium casting alloys is recommended. These involve well-known and proven casting alloys and we stand fully behind the quality properties of these casting alloys which are often manufactured in large quantities, are more cost-effective than special alloys and, in most cases, can be delivered at short notice. In the European DIN EN 1676 and DIN EN 1706 standards, the most important aluminium casting alloys have been col- lated in a version which is valid Europe- wide. Consequently, there are already more than 41 standard aluminium casting alloys available. Aluminium foundries should – according to their respective structure – limit them- selves to as small a number of casting alloys as possible in order to use their melting equipment economically, to keep inventories as low as possible and to re- duce the risk of mixing alloys. With regard to the quality of a casting, it is more sensible to process a casting alloy which is operational in use than one which displays slightly better properties on paper but is actually more difficult to process. The quality potential of a cast- ing alloy is only exploited in a casting if the cast piece is as free as possible of casting defects and is suitable for subse- quent process steps (e.g. heat treatment). Our sales team and technicians are on hand to provide foundries and users of castings with assistance in select- ing the correct aluminium casting alloy. To supplement and provide greater depth to our technical explanations, we refer you to standard works on aluminium and aluminium casting alloys. Further details on other specialist literature are available and can be requested at any time. We would be delighted to advise you in such matters. Should you have any queries or com- ments, which are always welcome, please contact our technical service. Standard works on aluminium and alu- minium casting alloys: • “Aluminium-Taschenbuch”, Verlag Beuth, Düsseldorf • “Aluminium viewed from within - Profile of a modern metal”, Prof. Dr. D. G. Altenpohl, Verlag Beuth, Düsseldorf. Once the requirements of a casting have been determined, the selection of the correct casting alloy from the mul- titude of possibilities often represents a problem for the designer and also for the foundryman. In this case, the “Alu- minium-Taschenbuch” can be of great assistance. Selecting aluminium casting alloys 13
  14. 14. Aluminium Casting Alloys different casting alloys are compared. These casting alloys are used for high- grade construction components, espe- cially for critical parts. “hard” The casting alloys of this group must display a certain tensile strength and hardness without particular requirements being placed on the metal‘s elongation. First of all, Al SiCu alloys belong to this group. Due to their Cu, Mg and Zn con- tent, these casting alloys experience a certain amount of self-hardening after casting (approx. 1 week). These alloys are particularly important for pressure die casting since it is in pressure die casting – except for special processes such as vacuum die casting – that pro- cess-induced structural defects occur, preventing high elongation values. Due to its particularly strong self-hardening characteristics, the Autodur casting al- Criteria for the selection of aluminium casting alloys In the following section, we provide an insight into the chemical and physical potentials of aluminium casting alloys by describing their various properties. The standardisation provided here helps to establish whether a casting alloy is suit- able for the specific demands placed on a casting. Degree of purity One important selection criteria is the de- gree of purity of a casting alloy. With the increasing purity of a casting alloy family, the corrosion resistance and ductility of the as-cast structure also increase; the selection of pure feedstock for making casting alloys, however, will necessarily cause costs to rise. The increasing importance of the closed- circuit economy means that, for the pro- ducer of aluminium casting alloys, the transition between the previous quality grades for aluminium casting alloys is becoming ever more fluid. Due to their high purity, casting alloys made from primary aluminium display the best corrosion resistance as well as high ductility. By way of example, Silumin-Beta with max. 0.15 % Fe, max. 0.03 % Cu and max. 0.07 % Zn can be mentioned. In many countries, the Silumin trademark has already become a synonym for alu- minium-silicon casting alloys. Casting alloys made from scrap are, with regard to ductility and corrosion resistance, inferior to other casting alloy groups due to their lower purity. They are, however, widely applicable and meet the set performance requirements. Strength properties Strength properties should be discussed as a further selection criterion (Table 1). A rough subdivision into four groups is practical: “strong and ductile” The most important age-hardenable casting alloys belong to this group. By means of different kinds of heat treat- ment, their properties can be adjusted either in favour of high tensile strength or high elongation. In Table 1, the typi- cal combinations of Rm and A values for Classification of casting alloys acc. to strength properties 1) Casting alloy Temper Tensile Elongation Brinell strength hardness Rm A5 [MPa] [%] HB Strong Al Cu4Ti T6 330 7 95 and ductile Silumin-Beta T6 290 4 90 Al Si10Mg(a) T6 260 1 90 Hard Al Si8Cu3 F 170 1 75 Al Si18CuNiMg F 180 1 90 Ductile Silumin F 170 7 45 Other Al Mg3 F 150 5 50 1) Typical values for permanent mould casting, established on separately-cast test bars. Table 1 14
  15. 15. Aluminium Casting Alloys Casting properties Further selection criteria comprise cast- ing properties such as the fluidity or solidification behaviour which sets the foundryman certain limits. Not every ideally-shaped casting can be cast in every casting alloy. A simplified summary of the casting prop- erties associated with the most impor- tant casting alloys is shown in Table 2. Co-operation between the technical de- signer and an experienced foundryman works to great advantage when looking for the optimum casting alloy for a par- ticular application. Given constant conditions, the fluidity of a metallic melt is established by de- termining the flow length of a test piece. Theoretically, low fluidity can be offset by a higher casting temperature; this is, however, linked with disadvantages such as oxidation and hydrogen absorption as well as increased mould wear. Eutectic AlSi casting alloys such as Silumin or Al Si12 display high fluidity. Hypoeutectic AlSi casting alloys such as Pantal 7 have medium values. AlCu and AlMg casting alloys display low fluidity. Hypereutectic AlSi casting alloys such as Al Si17Cu4Mg occupy a special posi- tion. In their case, very long flow paths are observed. This does not however necessarily lead to a drop in the melt temperature since primary silicon crys- tals already form in the melt. The melt still flows well because the latent heat of solidification of the primary silicon “ductile” Casting alloys which display particu- larly high ductility, e.g. Silumin-Kappa (Al Si11Mg), come under this general heading. This casting alloy is frequently used for the manufacture of automobile wheels. In this particular application, a high elon- gation value is required for safety reasons. “other” Casting alloys for more decorative pur- poses with lower strength properties, e.g. Al Mg3, belong to this category. loy represents a special case allowing hardness values of approx. 100 HB and a corresponding strength – albeit at very low ductility – in all casting processes. Hypereutectic AlSi casting alloys such as Al Si18CuNiMg and Al Si17Cu4Mg, for example, which display particularly high wear resistance due to their high silicon content, can also be classified in this group. Classification of casting alloys acc. to casting properties Fluidity Thermal Casting alloy Type of solidification crack susceptibility High Low Silumin Exogenous-shell forming Al Si12 Al S12(Cu) Exogenous-rough wall Al Si10Mg Endogenous-dendritic Silumin-Beta Al Si8Cu3 Pantal 7 Al Si5Mg Al Cu4Ti Al Mg3 Endogenous-globular Low High Al Mg5 Mushy Table 2 15
  16. 16. Aluminium Casting Alloys heats up the remainder of the melt. The already solidified silicon, however, causes increased mould wear and very uneven distribution in the castings. In these casting alloys, high melting and holding temperatures are necessary so that a casting temperature of at least 720 °C for pressure die casting and 740 °C for sand and gravity die casting has to be attained. The susceptibility to hot tearing is almost the opposite of fluidity (Tables 2 and 3). By hot tearing, we mean a separation of the already crystallised phases during solidification, e.g. under the influence of shrinkage or other tensions which can be transmitted via the casting moulds. The cracks or tears arising can be healed by, among other things, the feeding of residual melt. Eutectic and near-eutectic AlSi casting alloys also behave particularly well in this case, while AlCu and AlMg casting alloys behave particularly badly. In practice, there are mixed forms and transitional forms of these solidification modes. The solidification behaviour is responsible for the formation of shrink- age cavities and porosity, for example, or other defects in the cast structure as it determines the distribution of the volume deficit in the casting. To curb the aforementioned casting defects, casting/technical measures need to be taken: e.g. by making adjustments to the sprue system, the thermal balance of the mould or by controlling the gas content of the melt. A volume deficit occurs during transition from liquid to solid state. This is quite small in high silicon casting alloys since the silicon increases in volume during solidification. In any case, the volume deficit incurred Selection criteria for aluminium casting alloys Casting properties Strength characteristics Corrosion resistance* Shrinkage Fluidity Thermal crack High strength Strong Ductile Hard formation susceptibility and ductile (T6) and ductile Coarse High Low Silumin Silumin-Kappa Silumin-Delta Al Si12 Al Si12(Cu) Al Si12CuNiMg Al Si17Cu4Mg Al Si18CuNiMg Autodur Silumin-Beta Al Si10Mg Al Si10Mg(Cu) Al Si8Cu3 Pantal 7 Al Cu4Ti Al Mg3Si Al Mg3 Al Mg5 Fine Low High Al Mg9 * Analogue to DIN EN 1706 Table 3 16
  17. 17. Aluminium Casting Alloys needs to be offset as far as possible by casting/technical means (see also the section on “Avoiding casting defects”). Figure 1 indicates the main types of so- lidification; each type is shown at two successive points in time. With regard to aluminium, only high-purity aluminium belongs to Solidification Type A (“exog- enous-shell forming”). The only casting alloy which corresponds to this type is the eutectic silicon alloy or Al Si12 with approx. 13 % silicon. The hypoeutectic AlSi casting alloys solidify according to Type C (“spongy”), AlMg casting alloys according to a mix- ture of Types D and E (“mushy” or “shell- forming”). The remaining casting alloys also represent intermediate types. At high solidification speeds, the solidification types move upwards, i.e. in the direction of “exogenous-rough wall”. Shell-forming casting alloys with “smooth- wall” or “rough-wall” solidification are sus- ceptible to the formation of macroshrink- age which can only be prevented to a limited extent by feeding. Casting alloys of a spongy-mushy type are susceptible to shrinkage porosity which can only be avoided to a limited extent by feeding. In castings which demand feeding by material accumulation in particular and which should be extensively pore-free – as well as pressure-tight – the preferred casting alloys are to be found at the top of Table 3. For complex castings whose geometry does not allow each material accumu- lation to be achieved with a feeder, the casting alloys listed in Table 3 offer ad- vantages provided that a certain amount of microporosity is taken into account. Picture 1 A Smooth wall B Rough wall C Spongy Exogenous solidification types D Mushy E Shell forming Endogenous solidification types Mould Fluid Strong 17
  18. 18. SiSiFe Aluminium Casting Alloys Copper • increases the strength, also at high temperatures (high- temperature strength) • produces age-hardenability • impairs corrosion resistance • in binary AlCu casting alloys, the large solidification range needs to be taken into account from a casting/technical point of view. Manganese • partially offsets iron‘s negative effect on ductility when iron content is > 0.15 % • segregates in combination with iron and chromium • reduces the tendency to stickiness in pressure die casting. Magnesium • produces age-hardenability in combination with silicon, copper or zinc; with zinc also self-hardening • improves corrosion resistance • increases the tendency towards oxidation and hydrogen absorption • binary AlMg casting alloys are difficult to cast owing to their large solidification range. Zinc • increases strength • produces (self) age-hardenability in conjunction with magnesium. Influence of the most important alloying elements on aluminium casting alloys Silicon • improves the casting properties • produces age-hardenability in combination with magnesium but causes a grey colour during anodi- sation • in pure AlCu casting alloys (e.g. Al Cu4Ti), silicon is a harmful im- purity and leads to hot tearing susceptibility. Iron • at a content of approx. 0.2 % and above, has a decidedly negative influence on the ductility (elonga- tion at fracture); this results in a very brittle AlFe(Si) compound in the form of plates which appear in micrographs as “needles”; these plates act like large-scale micro- structural separations and lead to fracture when the slightest strain is applied • at a content of approx. 0.4 % and above, reduces the tendency to stickiness in pressure die casting. Nickel • increases high-temperature strength. Titanium • increases strength (solid-solution hardening) • produces grain refinement on its own and together with boron. 18
  19. 19. Aluminium Casting Alloys Influencing the microstructural formation of aluminium castings The marked areas in Figure 1 denote where it makes sense to carry out the respective types of treatment on AlSi casting alloys. Some of these measures are explained in more detail in the following section. Common treatment measures include: • grain refinement of the solid solution with Ti and/or B • transformation of the eutectic Si from lamellar into granular form • modification of the eutectic Si with Na or Sr • refinement of the eutectic Si with Sb • refinement of the Si primary phase with P or Sb. Measures influencing microstructural formation are aimed at improving the mechanical and casting properties. In practice, apart from varying the cool- ing speed by means of different mould materials, additions to the melt are usu- ally used. Types of treatment to influence grain structure Figure 1 Temperature [°C] 700 600 500 400 Primary Si refinement Grain refinement 0 2 4 6 8 10 12 14 16 18 20 22 24 Modification Eutectic temperature 577 C° Melt + Si Melt Melt + Al 660 °C Al Al + Si Al Si5 Al Si7 Al Si9 Al Si12 Al Si18 Silicon [wt. – %] 19
  20. 20. Aluminium Casting Alloys Grain refinement The solidification of many aluminium casting alloys begins with the formation of aluminium-rich dendritic or equiaxed crystals. In the beginning, these solidified crystallites are surrounded by the remain- ing melt and, starting from nucleation sites, grow on all sides until they touch the neighbouring grain or the mould wall. The characterisation of a grain is the equiaxed spatial arrangement on the lattice level. For casting/technical or optical/decorative reasons as well as for reasons of chemical resistance, it is often desirable to set the size of these grains as uniformly as possible or as finely as technically possible. To achieve this, so-called grain refinement is frequently carried out. The idea is to offer the so- lidifying aluminium as many nucleating agents as possible. Since grain refinement only affects the α-solid solution, it is more effective when the casting alloy contains little silicon, i.e. a lower fraction of eutectic (Figure 2). Grain refinement is particularly important in AlMg and AlCu casting alloys in order to reduce their tendency to hot tearing. From a technical and smelting perspec- tive, grain refinement mostly takes place by adding special Al TiB master alloys. We pre-treat the appropriate casting al- loys when producing the alloys so that grain refinement in the foundry is either unnecessary or only needs a freshen- up. The latter can be done in the form of salts, pellets or preferably with titanium master alloy wire, following the manu- facturer’s instructions. Since every alloying operation means more contaminants in the melt, grain refinement should only be carried out for the reasons referred to above. To make a qualitative assessment of a particular grain refinement treatment, thermal analysis can be carried out (see section on “Melt testing and inspection procedure”). Effect of silicon content on grain refinement with Al Ti5B1 master alloy Mean grain diameter Casting temperature 720 °C [µm] holding time 5 min 1400 1200 1000 800 600 400 200 0 Silicon [%] Columnar and equiaxed crystals Without grain refinement With grain refinement Al Ti5B1: 2,0 kg/mt 0 2 4 6 8 10 12 Figure 2 20
  21. 21. Aluminium Casting Alloys Figures 3 and 4 depict the formation of microstructural conditions or the degree of modification as a result of interaction between sodium and strontium and the phosphorous element. It can be ascer- tained that the disruption of modification due to small amounts of phosphorous is relatively slight. In Sr modification, a high phosphorous content can be offset by an increased amount of modifying agent. In aluminium casting alloys with a silicon content exceeding 7 %, eutectic, silicon takes up a larger part of the area of a metallographic specimen. From a silicon content of approx. 7 to 13 %, the type of eutectic formation, e.g. grained or modified, thus plays a key role in determining the performance characteristics, especially the ductility or elongation. When higher elongation is required in a workpiece, aluminium cast- ing alloys containing approx. 7 to 13 % silicon will thus be modified by adding approx. 0.0040 to 0.0100 % sodium (40 to 100 ppm). In casting alloys with approx. 11 % silicon, particularly for use in low-pressure die casting, strontium can also be used as a long-term modifier since the melting loss behaviour of this element is substantially better than that of sodium. In this case, the recommended addition is approx. 0.014 to 0.04 % Sr (140 to 400 ppm). With suitable casting alloys, the required amount of strontium can be added during alloy manufacture so that, as a rule, the modification process step Modification of AlSi eutectic (refinement) By “modification”, we mean the use of a specific melt treatment to set a fine-grained eutectic silicon in the cast structure which improves the mechanical properties (and elongation in particular) as well as the casting properties in many cases. As a general rule, modification is carried out by adding small amounts of sodium or strontium. To facilitate an understanding of the possible forms of eutectic silicon, these are depicted in Figure 2 (a-e) for Al Si11 with a varying Na content: a) The lamellar condition only appears in casting alloys which are virtually free of phosphorous or modification agents, e.g. Na or Sr. b) In granular condition which appears in the presence of phosphorous without Na or Sr, the silicon crystals exist in the form of coarse grains or plates. c) In undermodified and d) to a great extent in fully-modified microstructural condition, e.g. by adding Na or Sr, they are significantly reduced in size, rounded and evenly distributed which has a particularly positive effect on elongation. e) In the case of overmodification with sodium, vein-like bands with coarse Si crystals appear. Overmodification can therefore mean deterioration as regards mechanical properties. a) Lamellar b) Granular e) Overmodified c) Undermodified d) Modified Picture 2Types of grain structure 21
  22. 22. Aluminium Casting Alloys can be omitted in the foundry. At low cooling rates, strontium modification is less effective so that it is not advisable to use this in sand casting processes. To avoid the burn-off of strontium, any cleaning and degassing of Sr-modified melts should be carried out with chlorine- free preparations only, preferably using argon or nitrogen. Strontium modifica- tion is not greatly impaired even when remelting revert material. Larger losses can be offset by adding Sr master alloy wire in accordance with the respective manufacturer‘s instructions. At the right temperature, the addition of sodium to the melt is best done by charging stand- ard portions. For easy handling, storage and proportioning, the manufacturer‘s recommendations and safety instruc- tions should be followed. Since sodium burns off from the melt relatively quickly, subsequent modifi- cation must take place in the foundry at regular intervals. In melts modified with sodium, any requested cleaning and degassing should be carried out with chlorine-free compounds only (argon or nitrogen). A certain amount of sodium burn-off is to be reckoned with, however, and needs to be taken into account in the subsequent addition of sodium. When absolutely necessary, the melt can be treated with chlorine- releasing compounds long before the Phosphorous [ppm] Overmodified Granular Modified Lamellar Undermodified Microstructural formation in relation to the content of phosphorous and sodium Al Si7Mg Sodium Sand casting [ppm] cooling rate 0.1 K/s 140 120 100 80 60 40 20 0 0 5 10 15 20 25 30 35 40 45 50 55 60 Figure 3 Phosphorous [ppm] Modified Undermodified Granular Lamellar Microstructural formation in relation to the content of phosphorous and strontium Al Si7Mg Strontium Gravity die casting [ppm] gravity die cast test bar cooling rate 2.5 K/s 450 400 350 300 250 200 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 Figure 4 22
  23. 23. Aluminium Casting Alloys first addition of sodium. If such treat- ment is carried out after adding sodium or strontium, chlorine may react with these elements and remove them from the melt, thereby preventing any further modification. Modification with sodium or strontium increases the tendency to gas absorp- tion in the melt. As a result of the reac- tion of the precipitating hydrogen with the rapidly-forming oxides, defects can occur in the casting, especially cumulant microporosity. In many practical cases, this potential for micropore formation is even desirable. Then, the purpose of modification is also to offset the expected macroshrinkage by forming many micropores. An accurate assessment of the effects of modification can only be made by means of metallographic examination. As a quick test, thermal analysis can be carried out if it is possible to establish by means of a preliminary metallographic examination which depression value is necessary to attain a sufficiently-modi- fied grain structure (for more information on thermal analysis, please refer to the section on “Methods for monitoring the melt”). Under the same conditions, rapid determination of the modified condition is also possible by measuring the elec- trical conductance of a sample. In aluminium casting alloys of the type Al Si7Mg, a refinement of the eutectic silicon with antimony (Sb) is possible. A Sb content of at least 0.1 % is required. This treatment, however, only produces a finer formation of the lamellar eutec- tic silicon and is not really modification in the traditional sense. The danger of contamination of other melts by closed- circuit material containing Sb exists as even a Sb content of approx. 100 ppm can disturb normal sodium or strontium modification. What‘s more, refinement with antimony can be easily disturbed by only a low level of phosphorous (a few ppm) (Figure 5). In contrast to modi- fication, refinement with antimony can not be checked by means of thermal analysis of a melt sample. Refinement of primary silicon In hypereutectic AlSi casting alloys (e.g. Al Si18CuNiMg), the silicon-rich, polygonal primary crystals solidify first. To produce as many fine crystals as pos- sible in the as-cast structure, nucleating agents need to be provided. This is done with the aid of prepara- tions or master alloys which contain phosphorous-aluminium compounds. This treatment can also be carried out when the alloy is being manufactured and, in most cases, the foundryman does not need to repeat the process. If required, the quality of such primary refinement can be checked by means of thermal analysis. Phosphorous [ppm] Influence of antimony and phosphorous content on the form of the eutectic silicon of Al Si7Mg Antimony [%] 0.30 0.20 0.10 0.00 0 2 4 6 8 10 Coarse-lamellar Acceptable Coarse-lamellar to granular High-purity base Figure 5 23
  24. 24. Aluminium Casting Alloys Melt quality and melt cleaning To achieve good melt quality, the for- mation of oxides and the absorption of hydrogen have to be suppressed as much as possible on the one hand, while other hydrogen and oxides have to be removed from the melt as far as pos- sible on the other, although this is only possible to a certain extent. All factors which come under the gen- eral term of “melt quality” have a direct effect on the quality of the casting to be produced. Inversely, according to DIN EN 1706, the cast samples play a valuable role in checking the quality of the melt. Most problems in casting are caused by two natural properties of liquid melts, i.e. their marked tendency to form oxides and their tendency towards hydrogen absorption. Furthermore, other insolu- ble impurities, such as Al-carbides or refractory particles as well as impurities with iron, play an important role. As mentioned in other sections, the larger oxide film can lead to a material separation in the microstructure and, consequently, to a reduction in the load- bearing cross-section of the casting. The solubility of hydrogen in aluminium decreases discontinuously during the transition from liquid to solid so that as solidification takes place, precipitating gaseous hydrogen reacting with exist- ing oxides can cause voids which can in turn take various forms ranging from large pipe-like blisters to finely-distrib- uted micro-porosity. Segregation factor [(Fe)+2(Mn)+3(Cr)] Al Si8Cu3 Al Si6Cu4 Al Si12(Cu) Critical melting temperatures in relation to the segregation factor Temperature [°C] 650 640 630 620 610 600 590 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Figure 6 24
  25. 25. Aluminium Casting Alloys Avoiding impurities Ingot quality An essential prerequisite for a good casting is good ingot quality. The metal should be cleaned effectively and the in- gots should display neither metallic nor non-metallic inclusions. The ingots must be dry (there is a risk of explosion when damp) and no oil or paint residue should be present on their surface. When using revert material, this should be in lumps, if possible, and well cleaned. Melting When melting ingots or revert material, it must be ensured that the metal is not exposed unnecessarily to the flame or furnace atmosphere. The pieces of metal should be melted down swiftly, i.e. follow- ing short preheating, immersed directly in the liquid melt. Large-volume hearth or crucible furnaces are best suited to melting. Furnaces with melting bridges are oxide producers and they lead to expensive, unnecessary and irretrievable metal losses. The type and state of the melt in contact with refractory materials are of particular importance in the melting and holding of aluminium. Aluminium and aluminium casting alloys in a molten state are very aggressive, es- pecially when AlSi melts contain sodium or strontium as modifying agents. With an eye to quality, reactions, adherences, infiltrations, abrasive wear and decompo- sition have to be kept within limits when using melting crucibles and refractory materials as well as during subsequent processing. The care and maintenance as well as cleanliness of equipment are equally important. Adhering materials can very easily lead to the undesired redissolving of oxides in the melt and cause casting defects. Melting temperature The temperature of the melt must be set individually for each alloy. Too low melting temperatures lead to longer residence times and, as a result, to greater oxidation of the pieces jut- ting out of the melt. The melt becomes homogeneous too slowly, i.e. local un- dercooling allows segregation to take place, even as far as tenacious gravity segregation of the FeMnCrSi type phases. The mathematical interrelationship for the segregation of heavy intermetallic phases is depicted in Figure 6. Furthermore, at too low temperatures, autopurification of the melt (oxides ris- ing) can not take place. When the temperature of the melt is too high, increased oxide formation and gassing can occur. Lighter alloying ele- ments, e.g. magnesium, are subject to burn-off in any case; this must be off- set by appropriate additions. Too high melting temperatures aggravate this loss by burning. 25
  26. 26. Aluminium Casting Alloys Conducting the melting operation As long as the melt is in a liquid condi- tion, it has a tendency to oxidise and absorb hydrogen. Critical points during subsequent processing include decanta- tion, the condition or maintenance of the transfer vessel, possible reactions with refractory materials as well as transport or metal tapping. The addition of grain refiners and modifying agents above the required amount can lead to an increase in non-metallic impurities and greater hydrogen absorption. To minimise an enrichment of iron in the melt, direct contact between ferrous materials and the melt is to be avoided. For this reason, steel tools and contain- ers (casting ladles) must be carefully dressed. Similarly, but also on economic grounds, the feed tubes for low-pressure die casting – made from cast iron up to now – should be replaced by ceramic feed tubes. Even during the casting process itself and especially due to turbulence in the flow channel, oxide skins can once again form which in turn can lead to casting defects. Casting technology is thus re- quired to find ways of preventing the excessive oxidation of the melt, e.g. by means of intelligent runners and gating systems (please refer to the section on “Selecting the casting process”). Type of melt treatment Al Si8Cu3 Pantal 7 Al Mg5 Hydrogen content of various casting alloy melts after different types of treatment Hydrogen [ml/100g] 0.50 0.40 0.30 0.20 0.10 0.00 10 20 30 0.5 2 4 24 10 20 Aftermelting Rotary degassing [min] Rotary degassing [min] Gassing 24h Holding in[h] Figure 7 26
  27. 27. Aluminium Casting Alloys Cleaning and degassing the melt Our casting alloys consist of effectively cleaned metal. Since reoxidation always takes place during smelting, and in practice revert material is always used, a thorough cleaning of the melt is nec- essary prior to casting. Holding the aluminium melt at the cor- rect temperature for a long time is an ef- fective cleaning method. It is, however, very time-intensive and not carried out that often as a result. Foundrymen are thus left with only intensive methods, i.e. using technical equipment or the usual commercially available mixture of salts. In principle, melt cleaning is a physical process: the gas bubbles rising through the liquid metal attach oxide films to their outer surfaces and allow hydrogen to dif- fuse into the bubbles from the melt. Both are transported to the bath surface by the bubbles. It is therefore clear that in order for cleaning of the melt to be effective, it is desirable to have as many small gas bubbles as possible distributed across the entire cross-section of the bath. Dross can be removed from the surface of the bath, possibly with the aid of ox- ide-binding salts. Inert-gas flushing by means of an im- peller is a widely-used, economical and environmentally-sound cleaning process. The gas stream is dispersed in the form of very small bubbles by the rapid turn- ing of a rotor and, in conjunction with the good intermixing of the melt, this leads to very efficient degassing. To achieve an optimum degassing effect, the vari- ous parameters such as rotor diameter and revolutions per minute, gas flow rate, treatment time, geometry and size of the crucible used as well as the alloy, have to be co-ordinated. The course of degassing and reabsorption of hydrogen is depicted for various casting alloys in Figure 7. When using commercially available salt preparations, the manufacturer‘s instruc- tions concerning use, proportioning, storage and safety should be followed. Apart from this, attention should also be paid to the quality and care of tools and auxiliary materials used for cleaning so that the cleaning effect is not impaired. If practically feasible, it is also possible to filter the melt using a ceramic foam filter. In the precision casting of high- grade castings, especially in the sand casting process, the use of ceramic filters in the runner to the sand mould has proved to be a success. Above all, such a filter leads to an even flow and can retain coarse impurities and oxides. In the gravity die casting of sensitive hydraulic parts, or when casting sub- sequently anodised decorative fittings in Al Mg3, ladling out of a device which is fitted with in-line filter elements and separated from the remaining melt bath is very common. 27
  28. 28. Aluminium Casting Alloys Melt testing and inspection procedure To assess the effectiveness of the clean- ing process or the quality of the melt, the following test and inspection methods can be used to monitor the melt: Reduced pressure test This method serves to determine the tendency to pore formation in the melt during solidification. A sample, which can contain a varying number of gas bubbles depending on the gas content, is allowed to solidify at an underpressure of 80 mbar. The apparent density is then compared with that of a sample which is solidified at atmospheric pressure. The so-called “Density Index” is then calculated using the following equation: DI = (dA - d80)/dA x 100 % DI = Density Index dA = density of the sample solidified at atmospheric pressure d80 = density of the sample solidified at under 80 mbar The Density Index allows a certain infer- ence to be drawn about the hydrogen content of the melt. It is, however, strongly influenced by the alloying elements and, above all, by varying content of impurities so that the hydrogen content must not on any account be stated as a Density Index value (Figure 8). The assessment of melt quality by means of an underpressure density sample there- fore demands the specific determination of a critical Density Index value for each casting alloy and for each application. The underpressure density method is, however, a swift and inexpensive meth- od with the result that it is already used in many foundries for quality control. To keep results comparable, sampling should always be carried out according to set parameters. Determination of the hydrogen content in the melt Reliable instruments have been in opera- tion for years for measuring the hydrogen content in aluminium melts. They work according to the principle of establish- ing equilibration between the melt and a measuring probe so that the actual gas content in the melt is determined and not in the solid sample. In this way, the effec- tiveness of the degassing treatment can be assessed quickly. The procurement of such an instrument for continuous quality monitoring is only worthwhile when it is used frequently; in small foundries, the hiring of an instrument to solve problems is sufficient. 28
  29. 29. Aluminium Casting Alloys Determination of insoluble non-metallic impurities For determining the number and type of insoluble non-metallic impurities in aluminium melts, the Porous Disc Filtra- tion Apparatus (PoDFA) method, among others, can be used. In this particular method, a precise amount of the melt is squeezed through a fine filter and the trapped impurities are investigated metallographically with respect to their type and number. The PoDFA method is one of the determination procedures which facilitates the acquisition, both qualitatively and quantitatively, of the impurity content. It is used primarily for evaluating the filtration and other clean- ing treatments employed and, in casting alloys production, is utilised at regular intervals for the purpose of quality control. This method is not suitable for making constant routine checks since it is very time-consuming and entails high costs. Hydrogen content [ml/100g] Correlation between the hydrogen content and density index in unmodified Al Si9Mg alloy Density index Measurement acc. to Chapel [%] at vacum 30 mbar 35 30 25 20 15 10 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Figure 8 29
  30. 30. Aluminium Casting Alloys Thermal analysis To evaluate the effectiveness of melt treatment measures, e.g. modification, grain refinement and primary silicon re- fining, thermal analysis has proved itself to be a fast and relatively inexpensive method in many foundries. The test method is based on the comparison of two cooling curves of the investigated melts (Figures 9 and 10). The undercooling effect (recalescence) occurring during primary solidification allows conclusions to be made about the effectiveness of a grain refinement treatment, whereby the recalescence values do not however allow conclusions to be drawn as regards the later grain size in the microstructure. Modification is shown in thermal analysis by a decrease in the eutectic temperature (depression) in comparison to the unmodified state. Here too, the level of the depression values depend strongly on the content of accompanying and alloying elements (e.g. Mg) and, consequently, the de- pression values required for sufficient modification must be established case by case, by means of parallel microstruc- tural investigations. Time [t] Thermal analysis for monitoring the grain refinement of Al casting alloys Temperature [T] With grain refinement Without grain refinement Liquidus temperature [TL ] TL TL Figure 9 Time [sec] Thermal analysis for monitoring the modification of Al casting alloys Temperature [°C] 585 580 577 575 570 565 560 0 10 20 30 40 50 Modified Undermodified Eutectic temperature Figure 10 30
  31. 31. Aluminium Casting Alloys Selecting the casting process Squeeze-casting is another casting pro- cess to be mentioned; here, solidification takes place at high pressure. In this way, an almost defect-free microstructure can be produced even where there are large transitions in the cross-section and insufficient feeding. Other special casting processes include: • Precision casting • Evaporative pattern casting • Plaster mould casting • Vacuum sand casting • Centrifugal casting. The considerations above concern cast- ing as an overall process. In the following notes on casting prac- tice, the actual pouring of the molten metal into prepared moulds and the subsequent solidification control are looked at in more detail. From the numerous casting processes, which differ from one another in the type of mould material (sand casting, per- manent dies etc.) or by pressurisation (pressure die casting, low-pressure die casting etc.), a few notes are provided here on the most important processes. nesses can be favourably influenced with the help of risers. Cylinder heads for water-cooled engines represent a typical application. In the low-pressure gravity die process with its upward and controllable cavity filling, the formation of air pockets is re- duced to a minimum and, consequently, high casting quality can be achieved. In addition to uphill filling, the overpressure of approx. 0.5 bar has a positive effect on balancing out defects caused by shrinkage. The low-pressure die casting process is particularly advantageous in the casting of rotationally symmetrical parts, e.g. in the manufacture of pas- senger vehicle wheels. Pressure die casting is the most widely used casting process for aluminium casting alloys. Pressure die casting is of particular advantage in the volume production of parts where the require- ment is on high surface quality and the least possible machining. Special ap- plications (e.g. vacuum) during casting enable castings to be welded followed by heat treatment which fully exploits the property potential displayed by the casting alloy. In addition to conventional pressure die casting, thixocasting is worthy of men- tion since heat-treatable parts can also be manufactured using this process. The special properties are achieved by shaping the metal during the solid- liquid phase. As mentioned in the introduction, the entire “casting” process is the shortest route from molten metal to a part which is almost ready for use. All sections of this catalogue contain advice on how the entire experience should be carried out. The casting process is selected ac- cording to various criteria such as batch size, degree of complexity or requisite mechanical properties of the casting. Some examples: The sand casting process is used predominantly in two fields of appli- cation: for prototypes and small-scale production on the one hand and for the volume production of castings with a very complex geometry on the other. For the casting of prototypes, the main arguments in favour of the sand casting process are its high degree of flexibility in the case of design changes and the comparably low cost of the model. In vol- ume production, the level of complexity and precision achieved in the castings are its main advantages. When higher mechanical properties are required in the cast piece, such as higher elongation or strength, gravity die cast- ing, and to a limited extent pressure die casting, are used. In gravity die casting, there is the possibility of using sand cores. Large differences in wall thick- 31
  32. 32. Aluminium Casting Alloys Gravity die casting process The gravity die casting which includes the well-known low-pressure die casting process is applied. The main fields of application are medium- or high-volume production using high-grade alloys, and also low to medium component weight using heat-treatable alloys. Compared with sand casting, the aluminium cast- ings display very good microstructural properties as well as good to very good mechanical properties which result from the rapid cooling times and the other easily-controlled operating parameters. The castings have high dimensional ac- curacy and stability as well as a good surface finish, are heat-treatable and can also be anodised. The basis for good quality castings is, not least, the right melt treatment and the appropriate casting temperature (see section on “Melt quality and melt clean- ing”). For castings with high surface or microstructural quality requirements, such as in decorative or subsequently anodised components or in pressure- tight hydraulic parts, it is useful to filter the melt before casting. Parts generated using the horizontal pressure die casting process are light- weight as low wall thicknesses can be achieved. They have a good surface finish, high dimensional accuracy and only require a low machining allowance in their design. Many bore holes can be pre-cast. The melting and casting temperatures should not be too low and should be checked constantly. Pre-melting alu- minium casting alloys is useful. The melt can thus be given a good clean in order to keep the melt homogeneous and to avoid undesirable gravity segregation (see Figure 6). From a statistical point of view, more casting defects arise from cold metal than from hot. It is particu- larly important to keep a sufficiently high melting temperature, even with hypere- utectic alloys. These comments are also valid for other casting processes. Pressure die casting process This process takes up the largest share. The hydraulically-controlled pressure die casting machine and the in-built die make up the central element of the process. The performance, the precise control of the hydraulic machine, the quality of the relatively expensive tools made from hot work steel are the deci- sive factors in this process. In contrast, the flow properties and solidification of the aluminium casting alloys play a rather subordinate role in this “forced” casting process. The pouring operation in horizontal pres- sure die casting begins with the casting chamber being filled with metal. The first movement, i.e. the slow advance of the plunger and the consequent pile-up of metal until the sleeve is completely filled, is the most important operation. In doing this, no flashover of the metal or other turbulence may occur until all of the air in the sleeve has been squeezed out. Immediately afterwards, the actual casting operation begins with the rapid casting phase. High injection pressure is essential to achieve high flow velocities in the metal. In this way, the die can be filled in a few hundredths of a second. Throughout the casting operation, the liquid metal streams are subject to the laws of hydrodynamics. Sharp turns and collisions with the die walls lead to a clear division of the metal stream. 32
  33. 33. Aluminium Casting Alloys Demands on the casting system To keep disadvantages and defects – which constantly arise from an oxide skin forming on the melt – within limits, the gating system must guarantee low turbulence in the metal stream and also a smooth, controlled filling of the die cavity. With the transition from a liquid to a solid condition, volume contraction occurs; this can amount to up to 7 % of the volume. This shrinkage is con- trollable when the solid-liquid interface runs – controlled or directed – through the casting, mostly from the bottom to the top. This task, namely to effect a directed solidification, can be achieved with a good pouring system. The castings are usually arranged “up- right” in the die. The greatest mass can thus be placed in the bottom of the die. Quality requirements can be, for example, high strength, high-pressure tightness or decorative anodising quality. One example of an “ideal” gating system which meets the highest casting require- ments is the so-called “slit gate system”. Here, the metal is conducted upwards continuously or discontinuously to the casting via a main runner. During mould filling, the melt is thus superimposed layer upon layer with the hotter metal always flowing over the already solidifying metal. The standpipe ends in the top riser and supplies it with hot metal. This way, the solidification can be directed from below, possibly supported by cooling, towards the top running through the casting and safeguarding the continuous supply of hot metal. When there is a wide flare in the casting, the gating system has to be laid out on both sides. This symmetry en- sures a division of the metal and also an even distribution of the heat in the die. In low-pressure die casting, directing the solidification by means of the gat- ing system is not possible. Nor is there any great possibility of classic feeding. Directional solidification is only possible by controlling the thermal balance of the die during casting. This mostly requires the installation of an expensive cooling- heating system. Simulation calculations for die filling and solidification can be useful when laying out and designing the die and possibly the cooling. In actual production, the cooling and cycle time can be optimised by means of thermography (see section on “Solidification simulation and ther- mography”). 33
  34. 34. Aluminium Casting Alloys Sand casting process This process is used especially for in- dividual castings, prototypes and small batch production. It is, however, also used for the volume production of cast- ings with a very complex geometry (e.g. inlet manifolds, cylinder heads or crank- cases for passenger vehicle engines). During shaping and casting, most large sand castings display in-plane expan- sion. With this flat casting method, gating systems like those which are normal in gravity die casting for directing solidifica- tion are often not applicable. If possible, a superimposed filling of the die cavity should be attempted here. Another generally valid casting rule for correct solidification is to arrange risers above the thick-walled parts, cooling (e.g. by means of chills) at opposite ends. This way, the risers can perform their main task longer, namely to conduct the sup- ply of molten metal into the contracted end. Insulated dies are often helpful. The cross-section ratio in the sprue system should be something like the following: Sprue : Sum of the runner cross-section : Sum of the gates: like 1 : 4 : 4. This facilitates keeping the run-in laun- der full and leads to a smoother flow of the metal. This way, the formation of oxides due to turbulence can be kept within limits. The main runner must lie in the drag, the gates in the cope. In the production of high-grade castings, it is normal to install ceramic filters or sieves made from glass fibre. The selection of the casting process and the layout of the casting system should be carried out in close co-operation between the customer, designer and foundryman (see section on “Casting-compliant design”). 34
  35. 35. Aluminium Casting Alloys Casting-compliant design Only through good cast quality can the technical requirements be met and the full potential of the casting alloy be ex- ploited. Every effort and consideration must be made therefore to design a light, functionally efficient part whose manu- facture and machining can be carried out as efficiently as possible. For this and subsequent considerations, the use of solidification simulation is available (see section on “Solidification simulation and thermography”). Casting alloys shrink during solidifica- tion, i.e. their volume is reduced. This increases the risk of defects in the cast structure, such as cavities, pores or shrinkage holes, tears or similar. The most important requirement is thus to avoid material accumulations by hav- ing as even a wall thickness as possible. In specialist literature, the following lower limits for wall thickness are given: • Sand castings: 3-4 mm • Gravity die castings: 2-3 mm • Pressure die castings: 1-1.5 mm. In the valid European standard, DIN EN 1706 for aluminium castings, there are strength values only for separately-cast bars using sand and gravity die casting. For samples cut from the cast piece, a reduction in the 0.2 % proof stress and ultimate tensile strength values of up to 70 % and a decrease in elonga- tion of up to 50 % from the test bar can be anticipated. When the alloy and the casting process are specified, so too is the next point within the framework of the design, i.e. determination of the die parting line. Die parting on one level is not only the cheapest for patterns and dies but also for subsequent working and machining. Likewise, every effort should be made to produce a casting without undercuts. This is followed by designing and determining the actual dimensions of the part. The constant guideline must be to achieve a defect-free cast structure wherever possible. The following notes on the design of aluminium castings are provided to help exploit in full the advantages and design possibilities of near net shape casting. They also align practical requirements with material suitability. Aluminium casting alloys can be pro- cessed in practically all conventional casting processes, whereby pressure die casting accounts for the largest volume, followed by gravity die casting and sand casting. The most useful casting process is not only dependent on the number and weight of pieces but also on other tech- nical and economic conditions (see sec- tion on “Selecting the casting process”). To find the optimum solution and produce a light part as cheaply and rationally as possible, co-operation between the de- signer, caster and materials engineer is always necessary. Knowledge concern- ing the loads applied, the distribution of stress, the range of chemical loading and operation temperatures is important. 35
  36. 36. Aluminium Casting Alloys The minimum values are also dependent on the casting alloy and the elongation of the casting. In pressure die casting, the minimum wall thickness also depends on the position of and distance to the gate system. Generally speaking, the wall thickness should be as thin as possible and only as thick as necessary. With increasing wall thickness, the specific strength of the cast structure deteriorates. Determining casting-compliant wall thicknesses also means, especially with sand and gravity die casting, that the die must first of all be filled perfectly. During subsequent solidification, a dense cast structure can only occur if the shrinkage is offset by feeding from liquid melt. Here, a wall thickness extending upwards as a connection to the riser may be necessary. Another possible way of avoiding material accumulations is to loosen the nodes. At points where fins cross, a mass ac- cumulation can be prevented by stag- gering the wall layout. The corners where walls or fins meet should be provided with as large transi- tions as possible. Where walls of different thickness meet, the transitions should be casting-compliant. Where the casting size and process permit, bores should be pre-cast. This improves the cross-section ratio and structural quality. Apart from the points referred to above, a good design also takes account of practical points and decorative appear- ance as well as the work procedures and machining which follow the actual casting operation. Fettling the casting, i.e. removing the riser and feeders, must be carried out as efficiently as possible. Grinding should be avoided where possible. Reworking and machining should also be easy to carry out. Machining allowances are to be kept as small as possible. Essential inspections or quality tests should be facilitated by constructive measures. 36
  37. 37. Aluminium Casting Alloys Solidification simulation and thermography Thermography Even after a casting goes into volume production, it is often desirable and nec- essary to optimise the casting process and increase process stability. Besides the aforementioned solidification simula- tion, periodic thermal monitoring of the dies by means of thermography is used in particular. In this process, a thermogramme of the die or casting to be investigated is made with the aid of an infrared camera. This way, the effectiveness of cooling, e.g. in pressure or gravity die casting, can be checked or optimised and the optimum time for lifting determined. Possible positive effects of simulation calculations include: • Optimisation of the casting before casting actually takes place • Avoiding casting defects • Optimisation of the feeding system (reducing material in the recycling circuit) • Optimisation of the casting process (reducing cycle times) • Increasing process stability • Visualisation of the die-filling and solidification process. A simulation programme does not opti- mise on its own and can not, and should not, replace the experienced foundry- man. To exploit the potential of die-filling and solidification simulation to the full, it should be applied as early as possi- ble, i.e. already at the design stage of the casting. Solidification simulation A basic aim in the manufacture of cast- ings is to avoid casting defects while minimising the amount of material in the recycling circuit. Optimisation of the manufacture of cast- ings with regard to casting geometry, gating and feeding system and cast- ing parameters can be achieved via numerical simulation of die filling and the mechanisms of solidification on the computer. Casting defects can thus be detected in good time and the casting design and casting system optimised before the first casting operation takes place. In principle, flow and thermal con- duction phenomena which occur during casting can be calculated numerically using simulation programmes. In calculation models, the casting and die geometry – which first of all must be available in a CAD volume model – is thus divided into small volume elements (Finite Difference Method). The flow velocities and temperatures in the individual vol- ume elements are then calculated using a numerical method. 37
  38. 38. Aluminium Casting Alloys Avoiding casting defects The type of solidification is also impor- tant when considering suitable casting/ technical measures. In AlSi casting al- loys with approx. 13 % Si, a frozen shell forms during solidification while, in hy- poeutectic AlSi casting alloys as well as in AlMg and AlCu casting alloys, a predominantly dendritic or globular so- lidification occurs. In gravity die casting processes, the feeders are laid out in particularly critical or thick areas of the casting. The feed- ers require hot metal in appropriately large volumes to execute their task. The combination of feeding and cooling is useful. Heat removal to accelerate and control solidification at the lower end of the casting or in solid areas can be effected by means of metal plates or surface chills (cooling elements). quality and melt cleaning” as well as “Methods for melt monitoring” and “Se- lecting the casting process”. Here are a few key points: • Use good quality ingots • Quality-oriented melting technology and equipment • Correct charging of the ingots (dry, rapid melting) • Temperature control during melting and casting • Melt cleaning and melt control • Safety measures during treatment, transport and casting Volume contraction during the transition from liquid to solid state can - depend- ing on the casting alloy - be up to 7 % volume. Under unfavourable conditions, part of this volume difference can be the cause of defects in castings, e.g. shrink marks, shrink holes, pores or tears. To produce a good casting, the possibility of feeding additional molten metal into the contracting microstructure during solidification must exist. In pressure casting processes, this occurs by means of pressurisation; in gravity die casting, this is done primarily by feeding. As shown in Table 4, there are two phenomena which – individually or in combination – can lead to defects in emergent castings: 1. The continuous (new) formation of oxides in the liquid state and 2. volume contraction during the trans- ition from liquid to solid state. During transition from liquid to solid state, the dissolved hydrogen in the melt precipitates and, on interacting with ox- ides, causes the well-known problem of microporosity or gas porosity. The task of melt management and treatment is to keep oxide formation and, consequently, the dangers to cast quality within limits. Information about this is provided in the sections on “Melt 38
  39. 39. Aluminium Casting Alloys As already shown in the section on cast- ing processes, an uncontrolled or tur- bulent filling of the die cavity can have a negative influence on the quality of the casting. A gating system which allows the solidification front to be controlled upwards through the casting from the bottom up to the feeder is helpful. A good casting system, e.g. side stand pipe-slit gate, begins the filling in the lower part of the die and always layers the new hot metal on the lower, already solidified part and also supplies the feeder with hot metal. A casting system of this type can par- tially cushion the negative effect caused by volume contraction while conducting the molten metal in such a way that fresh oxidation of the melt due to turbulence is avoided. Two methods can be used to reduce the number of defective parts due to porosity: In hot isostatic pressing (HIP), porous castings are subjected to high pressure at elevated temperatures so that shrinkage and pores inside the cast- ings are reduced; they do not, however, completely disappear. A second and less costly possibility is the sealing of castings by immersing them in plastic solutions. The shrinkage and pores, which extend to the surface, are filled with plastic and therefore sealed. Classification of casting defects Source of defect Consequences Optimisation for the casting possibilities • Oxidation and • Pores • Melt treatment hydrogen- • Aeration and degassing absoption • Inclusions • Melting and • Leakiness casting temperature • Surface defects • Filter • Machining • Loss of strength and elongation • Volume contraction • Cavity • Gating system • Shrinkage • Solidification control • Aeration • Feeding • Leakiness • Grain refinement • Loss of strength • Modification and elongation Table 4 39
  40. 40. Aluminium Casting Alloys Heat treatment of aluminium castings In ageing, mostly artificial ageing, pre- cipitation of the forcibly dissolved com- ponents takes place in the form of small sub-microscopically phases which cause an increase in hardness and strength. These tiny phases, which are techni- cally referred to as “coherent or semi- coherent phases”, represent obstacles to the movement of dislocations in the metal, thereby strengthening the previ- ously easily-formable metal. The following casting alloy types are age-hardenable: • Al Cu • Al CuMg • Al SiMg • Al MgSi • Al ZnMg. Metallurgy – fundamental principles For age-hardening to take place, there must be a decreasing solubility of a par- ticular alloy constituent in the α-solid so- lution with falling temperature. As a rule, age-hardening comprises three steps: In solution annealing, sufficient amounts of the important constituents for age- hardening are dissolved in the α-solid solution. With rapid quenching, these constituents remain in solution. Afterwards, the parts are relatively soft. Heat treatment gives users of castings the possibility of specifically improv- ing the mechanical properties or even chemical resistance. Depending on the casting type, the following common and applied methods for aluminium castings can be used: • Stress relieving • Stabilising • Homogenising • Soft annealing • Age-hardening. The most important form of heat treat- ment for aluminium castings is artificial ageing. Further information is provided below. Ageing time [h] Yield strength of gravity die cast test bars (Diez die) in Al Si10Mg alloy Yield strength Rp0,2 [MPa] 280 240 200 160 120 0 0 2 4 6 8 10 12 14 16 As-cast state160 °C 180 °C 200 °C Figure 11.1 Ageing time [h] Elongation of gravity die cast test bars (Diez die) in Al Si10Mg alloy Elongation A5 [%] 5 4 3 2 1 0 0 2 4 6 8 10 12 14 16 As-cast state160 °C 180 °C 200 °C Figure 11.2 40
  41. 41. Aluminium Casting Alloys Ageing time [h] Tensile strength of gravity die cast test bars (Diez die) in Al Si10Mg alloy Tensile strength Rm [MPa] 360 320 280 240 200 160 0 2 4 6 8 10 12 14 16 As-cast state160 °C 180 °C 200 °C Figure 11.3 Solution annealing To bring the hardened constituents into solution as quickly as possible and in a sufficient amount, the solution anneal- ing temperature should be as high as possible with, however, a safety margin of approx. 15 K to the softening point of the casting alloy in order to avoid in- cipient fusion. For this reason, it is often suggested that casting alloys containing Cu should undergo step-by-step solution annealing (at first 480 °C, then 520 °C). The annealing time depends on the wall thickness and the casting process. Com- pared with sand castings, gravity die cast- ings require a shorter annealing time to dissolve the constituents sufficiently due to their finer microstructure. In principle, an annealing time of around one hour suffices. The normally longer solution annealing times of up to 12 hours, as for example in Al SiMg alloys, produce a good spheroidising or rounding of the eutectic silicon and, therefore, a marked improvement in elongation. The respective values for age-hardening temperatures and times for the individual casting alloys can be indicated on the respective data sheets. During the annealing phase, the strength of the castings is still very low. They must also be protected against bending and distortion. With large and sensitive cast- ings, it may be necessary to place them in special jigs. Quenching Hot castings must be cooled in water as rapidly as possible (5-20 seconds de- pending on wall thickness) to suppress any unwanted, premature precipitation of the dissolved constituents. After quench- ing, the castings display high ductility. This abrupt quenching and the ensuing increase in internal stresses can lead to distortion of the casting. Parts are often distorted by vapour bubble pres- sure shocks incurred during the rapid immersion of hollow castings. If this is a problem, techniques such as spraying under a water shower or quenching in hot water or oil have proved their value as a first cooling phase. Nevertheless, any straightening work necessary at this stage should be carried out after quenching and before ageing. 41
  42. 42. Aluminium Casting Alloys Ageing The procedure of ageing brings about the decisive increase in hardness and strength of the cast structure through the precipitation of the very small hard- ening phases. Only after this does the part have its definitive service properties and its external shape and dimensions. Common alloys mostly undergo artificial ageing. The ageing temperatures and times can be varied as required. In this way, for example, the mechanical prop- erties can be adjusted specifically to at- tain high hardness or strength although, in doing this, relatively lower elongation must be reckoned with. Conversely, high elongation can be also achieved while lower strength and hardness values will be the result. When selecting the age- ing temperatures and times, it is best to refer to the ageing curves which have been worked out for many casting al- loys (Figures 11.1-11.4). In Al SiMg casting alloys, a further pos- sibility of specifically adjusting strength and elongation arises from varying the Mg content in combination with different heat treatment parameters (Figure 12). Ageing time [h] Brinell hardness of gravity die cast test bars (Diez die) in Al Si10Mg alloy Brinell hardness [HB] 160 140 120 100 80 60 0 2 4 6 8 10 12 14 16 As-cast state160 °C 180 °C 200 °C Figure 11.4 Magnesium [%] Influence of Magnesium on the tensile strength (Diez bars) Tensile strength Rm Alloy Al Si7 auf 99.9 base [MPa] + 200 ppm Sr + 1 kg/mt Al Ti3B1 n=5 300 250 200 150 100 50 0 0 0.1 0.2 0.3 0.4 0.5 0.6 8 h to 525 °C, H2 O As-cast state 8 h to 525 °C, H2 O +6 h to 160 °C Figure 12 42
  43. 43. Aluminium Casting Alloys If the heat treatment does not work first time, it can be repeated beginning with solution annealing. By doubling the so- lution time, a coarsening of the eutectic silicon can arise in the grain structure. Since the solution treatment is performed close to the alloy‘s melting temperature and the precipitation rate is highly sen- sitive to variations in ageing tempera- ture, it is essential that a high degree of consistency and control is assured. Regular maintenance, especially of the measuring and control equipment, is therefore absolutely essential. For slightly higher hardness or strength requirements, there is the non-standard possibility of “simplified age-hardening”. This can be used in gravity die casting and pressure die casting when age- hardenable alloys are being poured. Decisive here is a further rapid cooling after ejection from the die, e.g. by im- mediately immersing the part in a bath of water. Artificial ageing in a furnace at approx. 170 °C brings about the desired increase in hardness and strength. The procedure used in artificial ageing as well as typical temperatures and times are shown in Table 5. Procedures used in artificial ageing 1) Casting type Example Solution heat treatment Age-hardening Temperature Time Temperature Time [°C] [h] [°C] [h] Al SiMg Al Si10Mg 530 4 - 10 160 - 170 6 - 8 Al SiCu Al Si9Cu3 480 6 - 10 155 - 165 6 - 2 Al MgSi Al Mg3Si 550 4 - 10 155 - 175 8 - 0 Al CuMg 530* 8 - 18 140 - 170 6 - 8 1) Typical temperature and time values * Poss. gradual annealing at approx. 480 °C / approx. 6 h Table 5 43
  44. 44. Aluminium Casting Alloys Mechanical machining of aluminium castings High-speed steel and hard metal or ceramic plates are used as cutting tool materials; for microfinishing, diamonds are often utilised. The following machining allowances are given for the main casting processes: • sand castings: 1.5-3 mm • gravity die castings: 0.7-1.5 mm • pressure die castings: 0.3-0.5 mm. In order to minimise value losses, turnings and chips should be sorted out according to casting alloy type and stored possi- bly in briquettes. In addition, dampness, grease and free iron reduce the value of chips and turnings. Aluminium chips and turnings are not hazardous materials and there is no risk of fire during storage. Whengrindingaluminiumparts,explosion- proof separation of the dust is stipulated. With softer materials and also with most hypoeutectic AlSi casting alloys, narrow tools, i.e. with a large rake angle, cause the least possible surface roughness. These casting alloys produce narrow- spiral or short-breaking turnings. When machining aluminium, suitable emul- sions with water are used as cooling agents and lubricants. Friable and chips and fine to powdery Si dust arise when machining hypereutectic casting alloys. In combination with the lubricant, this powder produces an abradant which is often processed when dry. In some re- spects, the machining of these casting alloy types is similar to grey cast iron. With workpieces made from Al Si12 casting alloys with their very soft matrix, a large volume of long curly spirals are produced. In addition, the plastic mate- rial tends to build up edges on the tool. This leads to lubrication and, as a result, a poor surface appearance. When this occurs, it often gives the machinist the subjective impression of bad machina- bility although tool wear is not the cause in this case. In general, parts made from aluminium casting alloys are easy-machinable. This also applies for all metal-cutting processes. Low cutting force allows a high volume of metal to be removed. The surface finish of the cast piece depends on the machining conditions, such as cutting speed, cutting geometry, lubri- cation and cooling. The high cutting speeds required in alu- minium to achieve minimum roughness necessitate, with regard to processing machines and tools, stable, vibration- free construction and good cutting tools. Besides the microstructure – including defects, pores or inclusions – the silicon content of the casting has a strong ef- fect on tool wear. Modified, hypoeutectic AlSi casting alloys have, e.g. the highest tool time, while hypereutectic aluminium- silicon piston casting alloys can cause very considerable tool wear. 44
  45. 45. Aluminium Casting Alloys Welding and joining aluminium castings using argon. The process is suitable for both manual welding and for fully- mechanised and automatic welding. In fully-mechanised and automatic welding, both the power source and burner are water-cooled. With the wire electrode acting as the positive pole, the energy density is so high that it is able to break open the tenacious and high-melting oxide layer by means of local, explosive metal vaporisation underneath the ox- ide. With appropriate heat conduction, it is possible to achieve a relatively nar- row heat-affected zone with satisfactory strength and elongation values. A further development of MIG welding is represented by MIG pulse welding. Here, the welding current alternates between a so-called pulsed current and background current. Using this process, it is possible to carry out difficult tasks, i.e. thin wall thicknesses (1 mm) and out-of-position work (overhead). Today, MIG welding is the most frequently used aluminium welding process be- cause, in addition to its easy manipula- tion, the investment and running costs are favourable. The production welding sector should not be underestimated, e.g. for repair- ing defects in castings. Besides casting defects, there is also the possibility of correcting dimensional discrepancies, removing wear by build-up welding and repairing broken components. Welding processes The most frequently used fusion weld- ing processes for joining castings are metallic-insert-gas welding (MIG weld- ing) and Tungsten-inert-gas welding (TIG welding). Metal inert-gas welding (MIG welding) In MIG welding, an inert-gas arc weld- ing process, a continuous arc burns between a melting wire electrode and the workpiece. The process works with direct current, the wire electrode acting as the positive pole. The process is car- ried out under an inert gas in order to protect the melt area from the hazard- ous influences of the oxygen contained in air and moisture. Argon and/or helium, both inert gases, are used as shielding gases. Normally, it is cheaper to weld Suitability and behaviour Similar to most wrought aluminium alloys, castings made from aluminium casting alloys can, in principle, also be joined by means of fusion welding. Near-eutectic and hypoeutectic aluminium-silicon cast- ing alloys are the best to weld. Poor to unweldable are parts made from Al Cu4Ti alloys types since the Cu-content can cause the casting alloy to crack during welding. In AlMg casting alloys, the ten- dency to tearing must be counteracted by selecting a suitable weld filler. Applications in the aluminium sector Although near net shape casting gives the designer the greatest possible free- dom in the design of castings, welding is becoming increasingly important for the joining of aluminium cast components, either for welding two or more easy-to- cast parts (e.g. half shells) – whereas they would be difficult to cast as one – to form hollow bodies on the one hand or for joining extruded sections or sheet to castings to give a subassembly on the other, such as the case in vehicle construction, lamp posts, lamp fittings and heat exchangers. 45
  46. 46. Aluminium Casting Alloys Tungsten-insert-gas welding (TIG welding) In TIG welding, an inert-gas shielded arc welding process, an arc burns con- tinuously between a non-consumable electrode made of a tungsten alloy and the casting. Alternating current is nor- mally used when welding aluminium. The welding filler is fed in separately from outside either by hand or mechanically. The process is carried out under an in- ert gas in order to protect the melt area from the hazardous influences of the oxygen contained in air and moisture. Argon and/or helium, both inert gases, are used as shielding gases. Welding is usually carried out with alternating cur- rent and argon which is cheaper. This is primarily a manual welding process but there is a possibility to work with a full degree of mechanisation. In TIG weld- ing, the power source and the burner are both water-cooled. By using alternating current, the tenacious and high-melting oxide layer is broken open during weld- ing, similar to the MIG process. Weld- ing normal diameter material with direct current and a reverse-polarity tungsten electrode would lead to destruction due to electric overload. The electrode diam- eter, however, can not be increased since the current density required for welding is no longer sufficient. In one process variant, which has an electrode with negative polarity as in the welding of steel, welding is carried out using direct current under a helium shield. Compared with argon, helium displays better thermal conductivity so that less current is required to break open the ox- ide layer. Consequently, the electrode is not overloaded. In TIG welding, there are also process variants which work with the pulsed-current technique. With regard to freedom from porosity, the cleanest seams can be achieved using TIG welding. One disadvantage of the TIG welding process, however, is the high local energy input. This leads to considerable softening of the zone adjacent to the weld which is also the case with MIG welding. TIG welding, for example, is an excellent process for the repair of small casting defects. Com- pared with the MIG process, however, TIG welding operates at lower speeds. Other thermal joining processes The group of so-called “pressure welding processes” also includes friction stear welding (FSW) which is frequently used for welding aluminium castings. Since this welding process works without any filler material, it is possible to join materials together which are not fusion-weldable since they would form brittle inter-metallic phases. By means of friction welding, aluminium and steel, for example, can be joined together. The principle behind the process is to heat the workpieces up to a pasty condition followed by subjecting them to strong compression. A weld upset is thus de- veloped and, if necessary, subsequently machined. The heating is done by rotat- ing one or both parts and finally press- ing them against each other until they stop moving. It even allows workpieces of circular and square cross-sections to be joined together. As a result of the rotary movement and in order to keep the compression load from increasing too much, a certain cross- sectional area may not be exceeded. Another welding process is represented by electron beam welding. Particular in- terest is being shown in this process at the moment for the welding of aluminium pressure die castings. 46