The aluminum base alloys may in general be characterized as eutectic systems, containing intermetallic compounds or elements as the excess phase. Because of the relatively low solubilities of most of the alloying elements in aluminum and because of the complexity of the alloys that are produced, any one aluminum base alloy may contain several metallic phases which sometimes are quite complex in composition. These phases usually are appreciably more soluble near the eutectic temperature than at room temperature, making it possible to heat treat some of the alloys by solution and aging heat treatments.

The effects of the various elements, with which aluminum is alloyed of course, influence all the properties of interest. The principal alloying elements in aluminum base casting alloys are copper, silicon, magnesium, zinc, chromium, manganese, tin and titanium. Iron is an element normally present and usually considered as an impurity.

Although aluminum base alloys are considered as commercial alloys today, they lack the centuries tradition of other casting materials. Their selection and proper application in many cases is dependent upon understanding solution heat treating and aging heat treatments. Solution heat treatment of aluminum casting alloys consists of a thermal cycle of heating, a suitable period of holding the metal at some elevated temperature, and then rapid cooling of the castings, usually by quenching in water. The temperature and time of holding are exceedingly important factors in the treatment. The temperature must be high enough to cause a substantially large amount of the alloying elements to dissolve in the aluminum rich solid solution phase. The end microstructure after solution heat treating then is a supersaturated aluminum rich solid solution phase. Since solution heat treating results in a more uniform distribution of soluble alloying elements, it also assists in minimizing the harmful effects of segregation developed during solidification. Accompanying the microstructural effects of solution heat treatment are improvements in mechanical properties. A marked increase in tensile and yield strengths and an improvement in ductility. Most important is the fact that solution heat treatment is the necessary step in preparing the alloys for age or precipitation hardening from which further benefits may be obtained.

Natural age hardening is a gradual increase in hardness (and strength) which occurs with the elapse of time at atmospheric temperatures. The increased hardness may reach a maximum value in a few days but may require several years in some alloys. More rapid aging can be caused to occur at elevated temperature, 300 to 400 F. Heat treating to cause aging is called artificial age hardening, or precipitation hardening. Aging effects by either method are obtained only from alloys which have been previously solution heat treated. The metallurgical changes associated with aging are exceedingly complex. Aging or precipitation temperatures are such as to promote precipitation from the super-saturated solid solution remaining from solution heat treatment. The most beneficial aging effects are obtained before microstructural evidence of precipitation is revealed. In fact, when the precipitating phase is metallographically visible, overaging has occurred. Overaging results in a substantial decrease in hardness, strength and other properties.

The most important requirement of aluminum casting alloys is adequate mechanical strength for the intended use. In addition, factors such as machinability; ability to be polished, welded brazed or surface-treated; appearance; corrosion resistance; conductivity; leak tightness; and cost are extremely important. Selection of a particular alloy from the wide spectrum of choices depends on the most favorable compromise of all the factors.