Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

Abstract

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (∆T max) between various locations within samples during quenching. When ∆T max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ∆T max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.