Aluminium Alloy 6061 - Composition, Properties,Temper and Applications of 6061 AluminiumTopics Covered: Background Composition Key Properties Physical Properties Mechanical Properties Thermal Properties Typical Heat Treatment/Temper States Typically Available Forms ApplicationsBackground:Aluminium alloy 6061 is one of the most extensively used of the 6000 series aluminiumalloys. It is a versatile heat treatable extruded alloy with medium to high strengthcapabilities.Composition: Table 1. Typical composition of aluminium alloy 6061 Component Amount (wt.%) Aluminium Balance Magnesium 0.8-1.2 Silicon 0.4 – 0.8 Iron Max. 0.7 Copper 0.15-0.40 Zinc Max. 0.25 Titanium Max. 0.15 Manganese Max. 0.15 Chromium 0.04-0.35 Others 0.05Key Properties:Typical properties of aluminium alloy 6061 include: Medium to high strength Good toughness Good surface finish
Excellent corrosion resistance to atmospheric conditions Good corrosion resistance to sea water Can be anodized Good weldability and brazability Good workability Widely availablePhysical Properties: Density: 2.7 g/cm3 Melting Point: Approx 580°C Modulus of Elasticity: 70-80 GPa Poissons Ratio: 0.33Mechanical Properties:Temper Ultimate Tensile 0.2% Proof Stress Brinell Hardness Elongation Strength (MPa) (MPa) (500kg load, 10mm 50mm dia (%) ball)0 110-152 65-110 30-33 14-16T1 180 95-96 16T4 179 min 110 minT6 260-310 240-276 95-97 9-13Thermal Properties: Co-Efficient of Thermal Expansion (20-100°C): 23.5x10-6 m/m.°C Thermal Conductivity: 173 W/m.KElectrical Properties: Electrical Resistivity: 3.7 – 4.0 x10-6 Ω.cmTypical Heat Treatment/Temper States:Treatment DefinitionF As fabricated0 Annealed to obtain lower strength temperT1 Cooled from an elevated shaping process and naturally aged1T4, T4511 Solution heat treated and naturally aged2,3T51 Cooled from an elevated shaping process and artificially agedT6, T6511 Solution heat treated and artificially aged2,3
Note: This designation applies to products which are not cold worked after cooling from an elevated temperature shaping process, or in which the effect of cold work in flattening or straightening has no effect on mechanical properties This designation applies to products which are not cold worked after solution heat-treated, or in which the effect of cold work in flattening or straightening has no effect on mechanical properties 3. This designation applies to products which are not cold worked after solution heat-treatment, or in which the effect of cold work in flattening or straightening does not effect mechanical properties.Typically Available Forms:Being and extruded grade of aluminium, alloy 6061 is typically available as: Tube Bar Pipe RodApplications:Typical applications for aluminium alloy 6061 include: Aircraft and aerospace components Marine fittings Transport Bicycle frames Camera lenses Driveshafts Electrical fittings and connectors Brake components Valves CouplingsWhat is the definition of white layer?A thin layer of hardened material caused by a dull insert that gives the false impression ofa successful part until the surface fails. Hard turning often requires predicting thewearing of inserts so they can be changed before they cause the white layer.
(b)(a)Figure 6: A sample without white layer (a), and a sample with white layer (b).Whitelayers formed during machining have negative effects on surface finish and fatiguestrength of products. The whitelayer is generally a hard phase and leads to the surfacebecoming brittle causing crack permeation and product failure. This is a major concernwith respect to service performance especially in the aerospace and automotiveindustries. Numerous authors have investigated the formation of whitelayer underdifferent manufacturing processes. In turning, it was suggested that the whitelayerstructure is a martensitic phase whose formation is correlated to tool wear. Past studieshave tended to concentrate on the formation of whitelayers at conventional cuttingspeeds, but never examined the formation at high cutting speeds.Properties:White layer occurs on the surface of steel and may be up to 10 μm thick. The dark layerunderneath it may be two or three times thicker. (Ramesh, 2002) The transitionbetweenwhite and dark layer is usually abrupt, and occurs within a transition zone less than 1 μmin depth. (Akcan, Shah et al., 2002)Observation of white layers using a scanning electron microscope and an opticalmicroscope suggest that it has a nanocrystalline structure due to large strain deformationand dynamic recrystallization. It has been proposed that the white layer does not havevisible grain boundaries because the grains are small enough that they scatter light—notnecessarily because it is resistant to chemical attack. (Akcan, Shah et al., 2002)White layer hardness has been measured to be significantly greater than the martensite inthe bulk of the material. (Akcan, Shah et al., 2002)Using nano-indentation hardness measurements, the hardness was found to beapproximately 12.85 GPa, compared o about10.70 GPa for the bulk material. The grain size has been measured between 30-500nm.(Akcan, Shah et al., 2002 )Formation:The surface of a sample that has been electrodischarge
machined also has a white layer. White layer has also been observed onsurfaces that experience wear, such as on the surface of railroad tracks or a pin-on-disktest. (Griffiths, 1984)White layer is sometimes referred to as untempered martensite, and dark layer is referredto as over tempered martensite, because of their similar properties to heat treatmentby models that use heat effects as the primary cause of white layer formation.(Chou andEvans, 1999)In turning, when aggressive cutting parameters are used, even using a new tool, whitelayer accompanied by tensile stress is expected. This is an unacceptable condition that istypically undesirable. If less aggressive parameters are used with a new tool, one expectsno white layer and a compressive residual stress. As the tool wears however, thesedesirable characteristics diminish and a white layer develops. White layers may form ateither low or high cutting speeds. At low speeds it forms due to grain refinement, at highspeeds it forms due to rapid heating and quenching. (Akcan, Shah et al., 2002;Ramesh,2002) defects. . Counters EDM "White" Layer effect:The use of Electrical Discharge Machining (EDM)in the production of forming tools toproduce plasticsmouldings, die castings, forging dies etc., hasbeen firmly established inrecent years. Developmentof the process has produced significant refinementsinoperating technique, productivity andaccuracy, while widening the versatility of theprocess.Electrical Discharge Machining (EDM), while providing a rapid and relatively lessexpensive means for producing die casting die inserts, at the same time sets up some veryhigh and detrimental surface stresses. These stresses if not completely and properlyremoved, can accelerate thermal stress cracking. Metal is removed by a series of electrical spark discharges. The steel in the contact areamelts or vaporizes then solidifies on the surface of the cavity. Each spark erodes a tiny bit of metal, leaving a small crater in the surface of the tool. This leaves the immediatesurface in a high residual tensile stress condition. The topmost or recast "white" layer is a brittle, hard layer prone to cracking. This is the material that has melted and rapidly solidified. The white layer is densely infiltrated with carbon and has a distinct separate structure to that of the parent metal. Below this layer is the heat affected zone that has been structurally altered by the heat produced during EDM. This layer reaches the austenizing temperature of the steel. The zone may contain re-hardened or hard, brittle "untempered" martensite formed duringthe rapid cooling from the austenizing temperature. This can increase crack susceptibility since this microstructure stores considerable strain energy that decomposes with heat.
(Reference: www.metallife.com) After mandatory removal, by polishing, of the top "white cast" layer" it is important toprotect the next exposed layer. MetaLLife compressive stress removes the scratch stress risers created during polishing and closes the cracks that have propagated below the recast layer into the heat affected "untempered martensite" zone. This restores the desired residual compressive stress benefits to the tool. EDM layer zones*White layer - 5-15 microns - crack proneun tempered - 25-40 microns - crack pronetempered - 40-85 microns1 micron = 0.00003937"*Depth of zones is dependent on the spark density, volts, and amps of EDM equipment.EDM’s Effect on Surface Integrity:Article From: MoldMaking Technology, Jerry MercerPosted on: 2/1/2008Understanding the various layers of a cavity that are thermally altered by the EDMprocess will help you understand how EDM affects the integrity of the mold surface.Protecting the surface integrity of the cavity is one of the most critical facets of EDM. Theintegrity of the surface finish in the cavity is determined by the formation of thermallyaltered layers created by the EDM process, which involves the transference of a controlledelectrical discharge between an electrode and the workpiece. The current applied to theworkpiece during this discharge melts and vaporizes the metal, creating the thermallyaltered layers of the cavity. To understand how EDM affects the integrity of the mold
surface, you must first understand the various layers of the cavity that are thermallyaltered. EDM changes not only the surface of the work metal, but also the subsurfacelayers.Thermally Altered Layers:The various layers affected by the EDM process will be referred to as the altered metalzone. Figure 1 shows that the altered metal zone is comprised of two thermally affectedsub-layers of material: the recast layer or white layer and the heat affected zone.The white layer is the layer that has been heated to the point of a molten state, but notquite hot enough to be ejected into the gap and be flushed away. The EDM process hasactually altered the metallurgical structure and characteristics in this layer as it is formedby the unexpelled molten metal being rapidly cooled by the dielectric fluid during theflushing process and resolidifying in the cavity.This layer does include some expelled particles that have solidified and been re-depositedon the surface prior to being flushed out of the gap. The white layer is densely infiltratedwith carbon to the point that its structure is distinctly different than that of the basematerial. This carbon enrichment occurs when the hydrocarbons of the electrode anddielectric fluid break down during the EDM process and impenetrate into the white layerwhile the material is essentially in its molten state. The first layer of the heat-affected zone isthe re-cast or “white layer”. This layer has been heated above the melting point of the tool
steel and quickly cooled, subsequently producing an extremely brittle surface subject tomicro-cracking. If this condition is left untreated, then propagation of the cracks canultimately lead to failure of the tool. Immediately below the re-cast layer is the re-hardened layer. This layer has been heated to the austenizing, or hardening temperatureand rapidly cooled, leaving an un-tempered brittle surface condition. Below the re-hardened layer is the re-tempered layer. The re-tempered layer has been heated above thenormal tempering temperature of the tool steel, leaving the area with a lower hardnessthan typically useful for the tool. The unaffected base layer is the last layer of the HAZand this layer is in the same condition as it was prior to the EDM process. Multiple passes using a lowercurrent, on the final passes, can minimize the amount of heat-affected zone, andtherefore reduce the chance of the tool cracking while in service. The use of newer EDMequipment, with better control of the amperage and frequency, produce less “whitelayer”. Removal of the white layer is vital to the longevity of the tool. Grinding or stoningand polishing of the HAZ should be performed on the tool to remove the white layer.Stress relieving of the tool may also reduce the chance of cracking by tempering the re-hardened layer. Stress relieving should be performed about 25-50 degrees F below the lasttempering temperature. See the tool steel data sheets for specific temperingtemperatures.Influence of machining parameters on the surfaceintegrity in electrical discharge machining:Purpose: The aim of this research is to make a study of the influence of machiningparameters on the surface integrity in electrical discharge machining. The material used
for this study is the X200Cr15 and 50CrV4 steel for dies and moulds, dies castings, forgingdies etc. Design/methodology/approach: The methodology consists of the analysis anddetermination of the white layer thickness WLT, the material removal rate MRR, theelectrode wear ratio EWR and the micro hardness of each pulse discharge energy andparameters of electrical discharge machining.Findings: The Results of the tests undertaken in this study show that increasing energydischarge increase instability and therefore, the quality of the workpiece surface becomesrougher and the white layer thickness increases. This is due to more melting andrecasting of material.With the increase of the discharge energy, the amount of particles inthe gap becomes too large and can form electrically conducting paths between the toolelectrode and the workpiece, causing unwanted discharges, which become electric arcs(arcing). these electric arcs damage the electrodes surfaces (tool and workpiece surfaces)and can occur microcracks.Keywords: EDM; Energy discharge; White layer thickness WLT; Metallographicaspect; Cracks; HAZReference to this paper should be given in the following way: M. Boujelbene, E.Bayraktar, W. Tebni, S. Ben Salem, Influence of machining parameters on the surfaceintegrity in electrical discharge machining, Archives of Materials Science and Engineering37/2 (2009) 110-116. Fig. 1.(a) The composition of the heat affected zone HAZ, (b) Influence of the white layerthickness WLT on the discharge energy W in EDM
Fig. 2. Analyse of the White Layer Thickness WLTas a function of the machined energy W observed under an optical microscope; (a) W =38.4 J, WLT = 13.7μm; (b) W = 99.84 J, WLT = 18.7μm; (c) W = 384 J, WLT = 31.95μm Fig. 3. The influence of thematerial removal rate MRR and the electrode wear ratio EWR on the thickness of thewhite layer.
Fig. 4. Influence of the tool material on the white layer thickness WLT in roughing EDM; (a) Copper electrode WLT = 53.65 μm, (b) graphite electrode WLT = 51.88 μm Detecting White Layer in Hard Turned Components Using Non-Destructive Methods:Title: Detecting White Layer in Hard Turned Components Using Non-Destructive MethodsAuthor: Harrison, Ian SpencerAbstract: Hard turning is a machining process where a single point cutting tool removes material harder than 45 HRC from a rotating workpiece. Due to the advent of polycrystalline cubic boron nitride (PCBN) cutting tools and improved machine tool designs, hard turning is an attractive alternative to grinding for steel parts within the range of 58-68 HRC, such as bearings. There is reluctance in industry to adopt hard turning because of a defect called white layer. White layer is a hard, 1-5 쭠deep layer on the surface of the specimen that resists etching and therefore appears white on a micrograph. When aggressive cutting parameters are used, even using a new tool, white layer is expected. If more conservative parameters are selected, one does not expect white layer. There is some debate if white layer actually decreases the strength or fatigue life of a part, but nevertheless it is not well understood and therefore is avoided. This research examines the use of two different non-destructive evaluation (NDE) sensors to detect white layer in hard turned components. The first, called a Barkhausen sensor, is an NDE instrument that works by applying a magnetic field to a ferromagnetic metal and observing the induced electrical field. The amplitude of the signal produced by the induced electrical field is affected by the hardness of the material and surface residual stresses. This work also examines the electrochemical properties of white
layer defects using electrochemical impedance spectroscopy. This idea is verified by measuring the electrochemical potential of surfaces with white layer and comparing to surfaces without any. Further corrosion tests using the electrochemical impedance spectroscopy method indicate that parts with white layer have a higher corrosion rate. The goal of this study is to determine if it is possible to infer white layer thickness reliably using either the Barkhausen sensor or electrochemical impedance spectroscopy (EIS). Measurements from both sensors are compared with direct observation of the microstructure in order to determine if either sensor can reliably detect the presence of white layer.Type: ThesisURI: http://hdl.handle.net/1853/6982Date: 2005-01-20Publisher Georgia Institute of TechnologySubject: Residual stress White layer Hard turning Barkhausen effect Electrochemical impedance spectroscopyDepartm Mechanical Engineeringent:Advisor: Committee Chair: Kurfess, Thomas; Committee Member: Liang, Steven; Committee Member: Melkote, ShreyesDegree: M.S. Title: White layer formation and tool wear in high speed milling of 57HRc tool steel using coated and uncoated tools:
Author: Paul T. Mativenga, Aamir MubasharAddress: Manufacturing and Laser Processing Research Group, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, M60 1QD Manchester, UK. Manufacturing and Laser Processing Research Group, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, M60 1QD Manchester, UKJournal: International Journal of Agile Systems and Management 2007 - Vol. 2, No.2 pp. 172 - 185Abstract: Advances in process technology have opened new possibilities for rapid manufacturing. High Speed Machining (HSM) is one of these innovative areas. One demanding application is the HSM of hardened steels for die and mould tooling applications. A significant impediment in the wide- spread use of HSM in hard machining is a lack of understanding and subsequent control of possible micro-structural changes to the surface of machined components. These changes can occur in the form of surface and sub-surface layers induced by grain refinement, rapid heating and quenching and or reactions with the environment. Some surface layers are known for decreasing the material fatigue life due to their brittleness. Generally, these affected surface layers are referred to as the white layer. This paper focuses on formation of white layers during high speed milling of hardened tool steels. The machining was carried out using uncoated and TiAlCrN coated micro-grain carbide end mills. The cutting tools were also analysed for tool wear. The paper explores the correlation of white layer formation to tool wear progression and how this is affected by the PVD coating. Surface hardening, sub-surface tempering, surface finish and compositional changes are also presented. The results show that in milling, tool wear is a significant driver for white layer thickness progression. Moreover, increased oxygen content suggests that oxidation could also play a role in white layer formation.Keywords: tool coatings; high speed machining; HSM; microhardness; white layer formation; tool wear; high speed milling; tool steels; rapid manufacturing; agile systems; hardened steels; microstructure; carbide end mills; PVD coating; oxidation.DOI: 10.1504/IJASM.2007.015787