silicon crystals in a eutectic matrix. Tis alloy does not require heat treatment, which may eliminate internal stresses that may cause fatigue failure. Te low coefficient of thermal expan-
sion, high hardness and excellent wear resistance of these alloys make them suitable for internal combustion engine pistons and blocks and cylinder bodies for compressors, pumps and brakes. Alloy 535—Alloy 535 is an alumi- num-magnesium alloy possessing a high, stable combination of strength, shock resistance and ductility. It is suited for parts in instruments and computing devices where dimensional stability is of major importance. In addition to the high ductility and tensile strength of 535, the Charpy impact is 10–12 lbs. (4.5-5.4 kg), which makes it suitable for shock-resistant applications. In addition, this alloy doesn’t require heat treatment. Brackets, C-clamps and machined parts that need strength, as well as impellers, optical equipment and similar applications re- quiring a high polish or anodized finish, are typical uses. In many cases, this alloy has replaced gray iron and malleable iron because it reduces weight without sacrificing strength.
Table 4. Common Aluminum Heat Treatment Tempers Temper Thermal Processing T4
T6 T61 T7 T71 T5
Solution treat and age naturally to a substantially stable condition. Natural aging may continue slowly, particularly at elevated service temperatures, so structural stability may not be satisfactory.
Solution treat and age artificially. In castings, T6 commonly describes optimum strength and ductility.
Solution treat, quench and age artificially for maximum hardness and strength. This variant of T6 yields additional strength and stability but at reduced ductility.
Solution treat, quench and artificially overage or stabilize. This temper improves ductility, thermal stability and resistance to stress corrosion cracking.
Solution treat, quench and artificially overage to a substantially stable condition. This temper further increases thermal stability and resistance to stress corrosion cracking and reduces strength.
Age only. Stress relief or stabilization treatment. Cool from casting temperature and artificially age or stabilize (without prior solution treatment). Frequently, the as-cast condition provides acceptable mechanical properties but is accompa- nied by microstructural instability or undesirable residual stresses. Perhaps the possibility of in-service growth is the only constraint against using a casting in the as-cast state. In each case, the T5 temper is appropriate.
Annealing Castings that have low strength requirements but require high dimensional stabil- ity are annealed. Annealing also substantially reduces residual stress, a need in die castings. Annealing is a severe stabilization treatment and an elevated temperature variant of the T5 temper. Softening occurs because annealing depletes the matrix of solutes, and the precipitates formed are too large to provide hardening.
Alloy 712—Alloy 712 is employed
when a combination of good mechani- cal properties without heat treatment is needed. It also shows good shock and corrosion resistance, machinability and dimensional stability. No distortion is exhibited when 712 is heated. After
brazing, the alloy will regain its original strength by natural aging. The alloy is used for marine cast-
ings, farm machinery, machine tool parts and other applications in which the part must have good strength or impact resistance.
2012 Casting sourCe DireCtory
Metal Casting Design & PurChasing
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