Kb alloys foundrymans guide to sr and ti bor


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Kb alloys foundrymans guide to sr and ti bor

  1. 1. .' KB Alloys, Inc. has been a leading producer of aluminum-based master alloys since its inception as Kawecki Chemical Company during 1950. KB Alloys manufactures a full line of master alloys, grain refiners, modifiers and hardeners to meet the metal treatment and alloying requirements of the aluminum cast metals industry. From strategically located manufac- turing and warehousing facilities in the USA, Europe and Asia, KB Alloys delivers consistently dependable prod- ucts anywhere in the world. To further serve the aluminum and non-ferrous foundry industry, KB Alloys' staff of technical specialists and experienced field sales engineers are available for technical assistance. They are supported by KB Alloys Metallurgical Services and Technology Departments. Aluminum..Base Master Alloys Today's foundryman realizes that close control of the as-cast structure and chemical composition of the alloy are major require- ments in the production of quality castings. Three types of master alloys are essential to the foundryman: grain refiners, modifiers, and hardeners. When grain refiners were first used in aluminum casting alloys, they were added as titanium and boron salts to the molten alloy in the furnace. Alloying elements were usually added in the form of pure metals. By present day standards, such practices are generally inefficient and enVironmentally unacceptable. Development of aluminum-base master alloys in 1955, by Kawecki Chemical Company, a predecessor of KB Alloys, made it possible to add the required elements faster, more economically and above all, more uniformly than was previously possible. KB Alloys produces a full line of aluminum- base master alloys which are convenient to use and provide the desired elemental addition. This assures uniformity and pre- dictability of the required alloy composition. These I master alloys are available in a variety of forms: waffle ingot, slab, button, bar, coiled and cut rod. A356 alloy as-cast structure. (2X) TIBOR® Grain Refiners In aluminum castings, a large dendritic grain structure is generally undesirable. The most effective way to provide a fine and uniform as-cast grain structure is to add nucleating agents to the melt to enhance crystal formation during solidification. KB Alloys TIBORilD family of aluminum master alloys containing titanium and boron provide a convenient means of introducing highly effective nucleating agents. Grain refinement of aluminum alloys provides a number of technical and economic advantages. Reduced Hot Tearing: Fine equiaxed grains provide a uniform network of grain bound- aries, and reduce the tendency for crack initiation and propagation. In foundry castings, this structure reduces the tendency for "hot tearing" and "hot cracking" during solidification. Improved Feeding: Fine grains promote an easier flow of the molten metal that feeds the shrinkage during the final stages of solidification, and result in smaller and more uniformly dispersed shrinkage porosity. Reduced Porosity: Voids from internal shrinkage or dissolved gas are intergranular; with fine grains these voids are smaller and more uniformly distributed at the grain boundaries, thus improving the soundness of the casting. Better Homogeneity: Secondary phases and impurities that accumulate along grain boundaries during solidification are also finer and more uniformly dispersed. Improved Mechanical Properties: Grain boundaries are high energy areas along which fracture cracks can initiate and propagate easily. Small closely knit grains minimize this tendency and provide higher mechanical properties. Improved Surface Finish: A fine grain structure improves the surface finish of a casting, especially when the piece is bright dipped or anodized. Reduced Cost: The improvements which result from grain refinement of the castings increase the product yield and reduce product costs. A356 alloy grain refined as-cast structure. (2X) Figure 1
  2. 2. - . . TIBOR@ 1 Figure 3. Grain refining response of 5%Ti-1.0%B when added to A356 alloy containing residual titanium and strontium. Case A ~ 0.005% Ti and no Sr. Case B =0.005% residual TI and 0.015% Sr. Case C ~ 0.15% residual Ti and no Sr. Case D ~ 0.15% residual Ti and 0.015% Sr. Grain refiner addition is 2 Ibs per 1,000 ibs A356 alloy in all cases. 0.15 o o 1.711.4 C c Figure 2(a) Figure 2(b) B 5.011.0 A A In today's practice, Aluminum-Strontium Master Alloys provide a reliable method for adding strontium to molten aluminum. Recovery is high, and loss during hold- ing is reduced significantly compared to sodium, even to the extent that aluminum ingot preViously modified with strontium can be remelted with good retention of strontium. This led to the development of "permanently modified" aluminum silicon alloys. Strontium Modification The search for alternative-elements for modifying aluminum silicon alloys revealed that strontium master alloy could be used in place of sodium. Fortunately, none of the special precau- tions required in the use and handling of sodium apply to the strontium master alloy, and superior recovery and perfor- mance are achieved with strontium as a modifying agent. 4 5 0 . - - - - - - - - - - - - - - - - - - - - . 4 5 0 , - - - - - - - - - - - - - - - - - - - . 4 5 0 . , - - - - - - - - - - - - - - - - - - - . 2001....l...-...J.........- 2OO..J...._....L.._ _ 0.005 Residual To (%) Fogure 1. Grain refining response of 5% Tl-l.0%B and 1.7% Ti-1.4%B When added 10 A356 alloy having low and high residual To levels. Grain refiner addilion is 2 100 per 1,000 100 A356 alloy In all cases. 1 Ftgure 2. GraIn refining response of 1.7% TI-1.4%B when added 10 A356 alloy con1aining residual titanium and strontium. Case A ~ 0.005% Ti and no Sr. Case B ~ 0.005% residual nand 0.015% Sr. Case C ~ 0.15% residual To and no Sr. Case 0 = 0.15% residual To and 0.015% Sr. Grain refiner addition is 2 Ibs per 1,000 Ibs A356 alloy in all cases. .,§ 400 (; I 350 ~ iii 300c; 'E CJ 250 .,c; 400 ~ :[350 ~ iii 300c; 'E CJ250 ., e400 ":[350 ..N ;;)300 c; 'E Cl250 The improvements in properties that resulted were greatly responsible for the increase in use of these alloys. However, sodium is a very reactive metal. It can react when ex- posed to air and can burn violently during addition to the molten aluminum silicon alloy, therefore, close control and the level of additions is difficult. Modification produces a silicon phase that is fibrous and finely dispersed. Ductility of the castings markedly improves, and the tendency for cracking or brittle fracture is less. For many years, sodium was the only means available for the modification of aluminum silicon alloys. Modifiers A major portion of the aluminum alloys used to produce castings in the foundry industry contain silicon in the range of 5% to 12%. When unmodified melts of these alloys are used, coarse platelet crystals of the aluminum silicon eutectic phase form in the casting during solidification. These par- ticles are brittle and reduce the strength and ductility of the casting by inhibiting flow of molten metal ("feeding") into areas of the casting as it solidifies. but does contain 0.15% residual Ti. Finally, Case D which represents the smallest grain size obtained contained 0.15% residual Ti and 0.015% Sr. Clearly, when grain refiner performance is evaluated on a pound per pound basis, TIBORfI 5% Ti-1.0%B is the most powerful product for use with Sr modified alloys such as A356. After addition to molten aluminum, sodium tends to volatilize during holding of the melt, leading to further losses. Excessive additions to compensate for loss can lead to "over modification" with the formation of coarse AI-Si-Na compounds and SUbsequent deterioration in structure and mechanical properties. The need for a non-sodium modifier was clear. 2001....l..._...J..._ _ Grain Refiner Interactions In the production of foundry alloy ingot, it is common practice for the ingot producer to add titanium to his alloy to enhance the alloy's response to later additions of TIBOR(!l grain refiners. We commonly refer to this as "residual titanium" and it is typically present at levels ranging from -0.15 - 0:30%. In addition, aluminum-silicon foundry alloys are typically modified via additions of strontium, either by the ingot producer or by the foundryman. KB Alloys Technology Group has studied these factors and interactions relative·to the performance of 5% Ti-1.0%B and 1.7% T-1.4%B TIBORfI. It was confirmed that both products producer a smalle grain size when added to an alloy containing residual titanium as illustrated in Figure 2(a). In both cases, the grain size was reduced from -415 microns to -355 microns. This work also investigated possible interactions between strontium and grain refiners. In the presence of strontium, TIBOR® 5% Ti-1.0%B produces a smaller grain size than does TIBORfi 1.7%T-1.4%B. In fact, no interaction was observed between strontium and TIBOR@ 1.7% Ti-1.4%B as demonstrated in Figure 2(b). The results however are different with 5%Ti-1 %B as illustrated in Figure 2(c). The largest grain size is represented by Case A where there was no Sr addition and residual Ti was at the low level of 0.005%. Case B is the same as A, but includes a Sr addition of 0.015%. Case C received no Sr addition, KB Alloys TIBOR(!l family of master alloy grain refiners are available in a range of chemical compositions and titanium to boron ratios. Howeverthe most effective and most commonly used grain refiners for aluminum casting alloys contain either 5% Ti-1.0% B or 1.7% Ti-1.4%B. Both products are available in button, waffle ingot, bar, coiled and cut rod form. Choice of TIBOR® Alloy for Grain Refinement KB Alloys produces TIBORfi in button, waffle ingot, bar, coiled and cut rod form. Product form and performance attributes can be tailored to fit customers production practices. Beyond the differences in chemical com- position, the intermetallic boron phases differ significantly between the two products. In the 5%Ti-1%B composition, the boron intermetallic phase is present as TiB2 particles. The 1.7%Ti-1.4%B composition has a "mixed boride" intermetallic phase. Figure 2(c)
  3. 3. 0 ••1 I~.l Aluminum-Strontium Master Alloys KB Alloys produces and markets a variety of aluminum master alloys containing strontium. A residual concentration of 0.01 % to 0.02% strontium is usually adequate for full modification of hypo-eutectic and eutectic alloys. However, excess additions do not cause over modification, although concentrations greater than about 0.1 % should be avoided because detrimental AISrSi intermetallics may start to form. Furnace practice, alloy composition, and solidification rate of the casting will influence optimum level to be used in production. Proper strontium additions to aluminum silicon alloys improves as-cast mechanical properties. Improvements in elongation from 50% up to 200% can be achieved. Increases in ultimate tensile strength of 20%, have been reported as well as improved surface texture and machinability perfor- mance of the castings. There is evidence that strontium promotes the formation of finer particles of iron-rich intermetallic compounds instead of the relatively large particles of more brittle iron- aluminum-silicon phase. These fine particles increase the ductility of aluminum silicon casting alloys with high iron content. As shown in Figure 2(c) strontium additions promote a positive interaction with titanium and boron. Together strontium and TIBOR~ interact to further refine grain structure than TIBO alone. KB Alloys TIBOR- products are an ideal family of grain refiners for use in both modified and unmodified alloys. Summary 1. KB Alloys strontium aluminum master alloys provide reliable, effective means of adding strontium to modify aluminum alloys. . 2. Castings made from melts properly modified with strontium are more sound and have significantly improved mechanical properties, particularly ductility, than castings made with unmodified melts. 3. Strontium is a more cost effective modifier than sodium for aluminum silicon hypoeutectic casting alloys. Under controlled conditions, strontium modified ingots can be remelted and retain the modified structure. 4. The use of aluminum strontium master alloys avoids the need for the special precautions associated with use of metallic sodium. 5. Strontium tends to reduce the size of the iron-rich compounds, if present, resulting in improved ductility of iron-containing aluminum silicon casting alloys. 6. Strontium modified ingot and sodium modified ingot may be melted and mixed together without loss of modification. If the modification melt mixture requires al;lditional modification, more strontium may be added to obtain the desired structure. 7. Additions of sodium as a metal to a melt of strontium modified ingot are not recommended because of the Strontium modified, as-cast structure of A356 alloy. Note finely dispersed fibrous structure of silicon phase. (400X) possibility of "over modification", i.e., formation of undesirable AL-Si-Na compounds. 8. Degassing of a strontium modified melt should be performed_ with dry nitrogen or argon gas. 9. The use of salts for grain refining or fluxing should be avoided because chlorine and flourine will remove strontium from the melt. 10. Phosphorous, even in small amounts should be avoided because it will ' poison the ability of strontium to modify the silicon phase. 11. When grain refiner performance is evaluated on a pound for pound basis, TIBOR- 5%Ti-1 %B is the most powerful and effective grain refining product for use with strontium modified alloys such as A356. Unmodified, as-cast structure of A356 alloy. Note the coarse platelet crystals of silicon eutectic phase. (400X) Figure 3
  4. 4. AluQ'linum Hardener Alloys Alloying elements are added to aluminum to improve the mechanical and physical properties of the final product. In order to overcome the disadvantages of add- ing pure elemental metals to the melting furnace, aluminum-base master alloys were developed that are rich in one or more of the desired addition elements. This family of master alloys, frequently referred to as hardeners, is used to add alloying elements to aluminum to produce alloys with improved strength, hardness, fracture toughness and corrosion resistance. Master alloys of copper, magnesium, manganese, bismuth and chrome are examples. 68% Mg KB Alloys introduced a new formulation to its magnesium aluminum hardeners line of alloys. By raising the concentration of magnesium from the traditional 25% and 50% levels to that of 68%, the new formulation takes advantage of the traditional benefits associated with the use of a master alloy, while the higher composition rivals the economics of alloying with pure magnesium through the benefits of better recoveries, improved through-put with lower melting points and cleaner melts. The new product is exclusiv~ to KB Alloys and comes in a variety of sizes and forms to suit the needs of a wide range of customer applications. The 3.5 ounce button is designed for small furnace alloying or "touch-up" for larger additions. The waffle and slab ingots provide a convenie'nt alloying method for medium to large furnace additions. All shapes are produced under an ISO registered process to insure consistent chemical composition, ingot weight control and metallurgical cleanliness. The 68% Mg product is design~d to benefit the experienced alloyer. Because the density is greater than that of pure magnesium (2.0g/cc vs. 1.7g/cc) the 68% Mg-AI alloy has less tendency to float and burn-off. Less burn-off means fewer oxides, better recoveries and cleaner melts. With its low melting temperature (43rC vs 650°C) the 68% Mg alloy melts ultra-fast to keep production lines moving. Modification Rating System for Hypoeutectic Aluminum Silicon Alloys The structure of an aluminum casting varies with different casting parameters and must be controlled in order to provide consistent castings. To achieve optimum properties, it is necessary to modify the morphology of the eutectic phase in hypoeutectic aluminum silicon casting alloys. The cast structure of hundreds of aluminum silicon alloy samples have been examined to establish the degree of modification. (1) Extensive experience with Aluminum Association Alloy A356, which contains 6.5-7.5% silicon and 0.20-0.45% magnesium, has led to the formulation of the silicon phase modification rating system shown in Figure 4. 1. D. Apefian. G. K. Sigworth and K. R. Whaler: "Assessment of Grain Refining and Modification of AISi Foundry Alloys by Thermal Analysis", AFS 7{ansactions. pp. 297-307 (1984). Sample Preparation A small section is cut from the casting to be examined, then polished on successively finer grits of SiC sandpaper until a smooth and flat surface is obtained. The sample can then be polished on cloth wheels using "A" and "B" grade aluminum oxide powders until a mirror-like shine is observed. The silicon phase can clearly be seen at 200x on the as-polished surface; a brief etch in a 5-10% HF solution will darken the silicon phase to make viewing easier. Master Alloys Sample Evaluation The polished sample is examined at (200X) magnification. The microstructures observed can be placed in one of six overall classes. These are listed on Figure 4 with a numerical scale of Type 1 through Type 6 along with a description of the structure. Photomicrographs of each type of structure at 200X are presented for use as standards representative of each class. It is now possible to assign any casting of hypoeutectic AI-Si alloy, a numerical value, which reflects its internal structure to that of the rating system shown in figure 4.
  5. 5. Modification Rating System (200x) Type 1. Fully Unmodified Structure Type 2. Lamellar Structure Type 3. Partial Modification Structure Figure 4 . ~~'-..,c ...... _ ",.' -~ . Type 4. Non-Lamellar Structure Type 5. Modified Structure Type 6. Super Modified Structure
  6. 6. ~~~~~===================================~~--------- FOUNDRYMEN'S GUIDE TO GRAIN REFINER ADDITIONS* SELECT CHARGE SIZE, CHOICE OF TIBOR·, PRODUCT FORM. READ QUANTITY REQUIRED. TIBOR· 5%Ti /1 %B TIBOR·l.7%Ti /l.4%B Nominal addition level: Nominal adClition level: 2 Ibstl,OOO Ibs (0.01 %Ti) 2 Ibst1 ,000 Ibs (0.01 %TO .907 kg/454 kg (0.01 %Ti) .907 kg/454 kg (0.01 %Ti) 5%TI/1%8 'CUT 'WAFFLE Kg WAFFLE 1.7%Ti/1.4%B 'CUT 'WAFFLE CHARGE TIBO~ ROD "BUDON SECTION INGOT INGOT TIBO~ ROD "SUDON SECTIONGRAIN GRAINSIZE REFINER (1 oz) (50z) (1 Ib) (2.2Ibs) (16Ibs) REFINER (1 oz) (50z) (1 Ib) (Ibs or kg) REQUIRED (.03 kg) (.14 kg) (.454 kg) (1 kg) (7.25 kg) REQUIRED (.03 kg) (.14 kg) (.454 kg) 100lbs 0.21b 3 1 0.21b 3 1 45 kg .1 kg Rods Button .1 kg Rods Button 250lbs 0.51b 8 2 0.51b 8 2 113 kg .23 kg Rods Buttons .23 kg Rods Buttons 500lbs 1 Ib 16 3 1 1 Ib 16 3 1 26 kg .454 kg Rods Buttons Section .454 kg Rods Buttons Section 1,0001bs 2 Ibs 7 2 1 21bs 7 2 454 kg .907 kg Buttons Sections Ingol .907 kg Buttons Sections 1,5001bs 31bs 10 3 1 31bs 10 3 680 kg 1.36 kg Buttons Sections Ingol+ 1.36 kg Buttons Sections 2,0001bs 4 Ibs 13 4 2 41bs 13 4 908 k9 1.81 kg Buttons Sections Ingots 1.81 kg Buttons Sections 5,0001bs 10lbs 10 5 10lbs 10 2268 kg 4.54 kg Sections Ingots 4.54 kg Sections 10,0001bs 20lbs 20 9 1 20lbs 20 4540 kg 9.07 kg Sections Ingots Waffle+ 9.07 kg Sections 4 Sections , *Note: The addition levels shown are typical. Depending upon the casting method employed and the difficulty of the alloy, it may be necessary to increase the addition level by a factor of 2 to 3x. FOUNDRYMEN'S GUIDE TO STRONTIUM ADDITIONS* SELECT CHARGE SIZE& CHOICE OF STRONTIUM MASTER ALLOY. READ QUANTITY REQUIRED. r I STRONTIUM MASTER ALLOY 1O%Sr / AI Nominal addition level: , 1.5 Ibst1 ,000 Ibs (0.15% Sr) .68 kg/454 kg (0.15%Sr) CHARGE STRONTIUM CUT WAFFLE WAFFLE MASTER ROD SUDON SECTION SECTION SIZE ALLO~ (1 oz) (80z) (1 Ib) (16Ibs) '(Ibs or kg) REOUIR (.03 kg) (.23 kg) (.454 kg) (7.25 kg) 100lbs 0.151b 3 45 kg .07 kg Rods 250lbs 0.381b 6 1 113 kg .17 kg Rods Button 500lbs 0.751b 12 2 1 26 kg .34 kg Rods Buttons Section 1,0001bs 1.51b 24 3 2 454 kg .68 kg Rods Buttons Sections 1,5001bs 2.251bs 5 680 kg 1.02 kg Buttons 2,0001bs 3 Ibs 6 3 908 kg 1.36 kg Buttons Sections 5,0001bs 7.51bs 15 8 2268 kg 3.4 kg Suttons Sections 10,0001bs 151bs 15 1 4540 kg 6.8 kg Sections Waffle *Note: The addition levels shown are typical. Depending upon the casting method employed and the difficulty of the alloy, it may be necessary to increase the addition level by a factor of 2 to 3x.