Study on T5 treatment for improving dimensional stabilities of aluminum alloy castings

Studies on heat treatments of aluminum alloy castings (1st report)

Abstract

Heat treatment (T5 treatment) for improving dimensional stabilities was conducted on five sorts of typical aluminum casting alloys; AC2B, AC4B, AC4C, AC5A, and AC8B,
Casting stress, growth produced by heating to a high temperature, and mechanical properties were measured on the specimens, which had been heat treated under various T5 conditions, and optimum conditions of T5 treatment were determined
The principal results obtained were as follows:
1) Aluminum alloys containing appreciable amounts of slicon such as AC2B, AC4B, AC4C, and AC8B exhibited growths of 0.00100.0013mm/mm at maximum when heated to 250°C.
However, such a growth was not exhibited by an alloy containing less silicon such as AC5A.
For preventing such growths of aluminum alloys containing silicon, it was recommended to perform T5 treatment at 200°C for 10hrs., at 230°C for 3hrs., or at 250°C for 1hr.
(2) The casting stesss was increased with the increase in copper content in aluminum alloys.
AC5A exhibited the largest casting stress. It was diffcult to relieve its stress by conventional T5 treatments, but the stress was substantially relieved by further T2 treatment at 350°C for 3hrs.
Whereas, AC4C exhibited the smallest casting stress. Any T5 treatment was not needed to relieve its stress, or if necessary, the ageing at 250°C for 3hrs. would be enough.
3) Some of the methods of heat treatment specified in JIS, MIL, SAE, CSA, etc. are not effective in stress relieving or in preventing growth of castings.
You must be careful in application of these specifications to effective heat treatments of aluminum alloy castings.