Abstract: An aluminum casting of composition that is hardenable due to the presence of suitable amounts of silicon, magnesium and, optionally, other hardening constituents such as copper, nickel and the like is heat treated for improved tensile strength at 300.degree. C. The casting, which as formed has a microstructure of aluminum rich dendrites, silicon particles and hardening particles, is reheated above 500.degree. C. to redissolve the hardening particles and redistribute hardening constituents through the aluminum dendrites, cooling the casting to 350.degree. C. to 450.degree. C. and holding there to reform an abundance of large stable hardening precipitates in the aluminum dendrites and then air cooling the casting. The thus treated casting may then be artificially age hardened such as by a T5 temper practice.