The influence of aluminium alloy quench sensitivity on the magnitude of heat treatment induced residual stress

To produce useful strengthening, precipitation hardenable aluminium alloys rely on rapid quenching from the solution heat treatment temperature to suppress the formation of coarse equilibrium second phases. An unavoidable consequence of the rapid quenching of thick sections is the severe thermal gradients that quickly develop in the material. The attendant inhomogeneous plastic flow can then result in the establishment of residual stresses. The surface and through thickness residual stress magnitudes present in heat treated high strength aluminium alloy components are frequently reported to exceed the uniaxial yield stress of small specimens of the same alloy measured immediately after quenching. In thick section plate and forgings it is proposed that these high residual stress magnitudes are a consequence of hardening precipitation that occurs during quenching which allows for a greater elastic stress to be supported. To investigate this theory, thick sections of the quench sensitive alloy 7175 and the less quench sensitive alloy 7010 were heat treated in such a way as to allow the internal hardness to be measured immediately, after quenching. The rate of cooling was also monitored during quenching and these data were used in conjunction with time temperature property data to predict the degree of precipitation and subsequent loss of hardening potential in the fully heat treated condition. The magnitudes of the residual stresses induced during quenching were determined using standard x-ray diffraction techniques.