Study on T7 treatment for improving dimensional stabilities of aluminum alloy castings

Studies on heat treatments of aluminum alloy castings (2nd report)

Abstract

A study on T5 treatment for improving dimensional stabilities of aluminum alloy castings as cast was described in the 1st report.
This paper describes T7 and other heat treatments for improving dimensional stabilities for five sorts of typical aluminum alloy castings as quenched; AC2B, AC4B, AC4C, AC5A and AC8B.
The optimum conditions of heat treatments were determined by test results of quenching stress, growth produced by heating to a high temperature, and mechanical properties.
The principal results obtained were as follows:
1) Aluminum alloys as quenched, containing appreciable amounts of silicon such as AC2B, AC4B, AC4C, and AC8B exhibited growths of 0.00090.0012mm/mm at maximum when heated to 250°C.
However, such a growth was not exhibited by an alloy containing less silicon such as AC5A.
These results of the alloys as quenched were almost the same as those of as cast in the 1st report.
2) It was effective to perform T7 treatment at 200°C for 15hrs., 230°C for 8hrs., or 250°C for 3hrs. for preventing such growths of aluminum alloys containing silicon.
3) AC5A and AC8B exhibited the largest quenching stress and AC4C exhibited the smallest.
When the alloys liable to exhibit large quenching stresses were subjected to T7 treatment or a short time annealing after T6 treatment, the quenching stresses were not perfectly relieved and mechanical properties were markedly reduced.
T6 treatment involving oil or air quenching was effective in obtaining the optimum conditions of the alloys with high strength and small residual stress.