Thermal Treatment

Special mechanical properties are obtained in many alloys through accurate control of chemistry, manufacturing and heat treating. Reliable ingot producers can supply foundries with ingot and heat treating specifications designed to obtain specific mechanical properties. However, while certain mechanical properties are improved, it is at the expense of others. For example, tensile and yield strengths can be increased, but this results in lower elongation and higher hardness. Higher elongation and lower hardness result in losses in tensile and yield strengths.

Sand and permanent mold castings may be heat-treated to improve mechanical and physical properties. Die castings only can be stress relieved (and not solution heat-treated) because of their porous internal structure with gas inclusions, which may blister at high temperatures. Some alloys, such as 443.0 (which is cast in sand or permanent molds but contains little or no copper, zinc or magnesium), do not respond to heat treatment to increase mechanical properties.
Following are thermal treatment designations (tempers) and what they specify. For aluminum castings, “–T” designates thermal treatment and is always followed by one or more digits that indicate specific sequences of basic treatments. A second digit indicates a modification of the heat treatment to obtain specific properties.
F—As-cast.
O—Stress relieve or anneal.
–T4—Solution heat treat and quench. This is an unstable treatment. While it improves mechanical properties (such properties increase through aging at room temperature over a period of weeks), it is a usual practice to artificially age to attain maximum mechanical values.
–T5, –T51—Artificially age. This type of heat treatment is done at a comparatively low temperature and serves to eliminate growth of thermal cycle. It also is used to stabilize castings dimensionally (improving mechanical properties somewhat) to improve machinability and to relieve stress.
–T6, –T61—Solution heat treat, quench and artificially age. Such heat treatment usually results in maximum tensile and yield strengths with adequate elongation. Aging stabilizes the properties.
–T7, –T71—Solution heat treat, quench and artificially overage. This heat treatment improves mechanical properties to a large degree, stabilizes the castings, and usually results in a slightly lower tensile and yield strength but an increased elongation value compared to the –T6 series of heat treatments.

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