This aluminum piece (below) serves as the chassis supension support for a snowmo- bile. The one-piece casting replaced multiple stamped and riveted pieces.


for strength and ductility. Recent research includes the use of fl uidized beds to reach solution temperature rapidly and provide for quicker heat treatment cycles. The benefi ts of heat treatment include:


• homogenization of alloying elements— this is desirable to distribute elements evenly throughout the matrix, so prop- erties in the casting will be uniform;


of these three heat treatments are called tempers (Table 4). The principal purpose for heat treating aluminum castings is to develop the best combination of mechanical proper- ties that will meet the critical needs of the component application.


The three basic thermal operations


often are combined into heat treatment cycles that provide various properties. Al- though aluminum casting-related books


offer “typical” or “recom- mended” solutions, quench and age times and tempera- tures for each alloy and tem-


per, these heat treatment cycles often are varied and manipulated to change the mechanical properties of the casting to meet specifi c component requirements


Table 4. Common Aluminum Heat Treatment Tempers Temper Thermal Processing T4


T6 T61 T7 T71 T5


• stress relief—residual stresses are created during cooling from elevated casting and solution temperatures; heating the casting to an intermediate temperature can relieve these residual stresses;


• improved dimensional stability and ma- chinability—changes in the microstruc- ture can cause castings to grow over time; to maintain tight dimensional tolerances during and after machining, castings should be heat treated to form stable precipitate phases;


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 artifi cially. In castings, T6 commonly describes optimum strength and ductility.


Solution treat, quench and age artifi cially for maximum hardness and strength. This variant of T6 yields additional strength and stability but at reduced ductility.


Solution treat, quench and artifi cially overage or stabilize. This temper improves ductility, thermal stability and resistance to stress corrosion cracking.


Solution treat, quench and artifi cially 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 artifi cially age or stabilize (without prior solution treatment). Frequently, the as-cast condition provides acceptable mechanical properties but is accompanied 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 stability 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.


32 Metal Casting Design & PurChasing


• mechanical property improvement—the greatest use of heat treatment is to enhance mechanical and corrosion properties through spheroidizing constituent phase particles and by precipitation hardening. Rarely are all of the desired proper-


ties optimized in a single casting. More often, heat treatment is a compromise, maximizing some properties at the ex- pense of others. For example, tensile and yield strengths can be increased, but this results in lower elongation. Contrarily, higher elongations result in lower tensile and yield strengths.


METAL


For more information, consult: “Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot,” (Pink Sheets), Aluminum Assn., Washington, D.C., www. aluminum.org; “Aluminum Casting Technology,” American Foundry Society, Schaumburg, Ill.; and “Design and Procurement of High-Strength Struc- tural Aluminum Castings,” American Foundry Society, Schaumburg, Ill.


2010 Casting sourCe DireCtory


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