Influence of Short-Time Post-Weld heat treatment on the performance of friction stir welded AA7075 aluminum sheets

Abstract

Zinc-containing aluminum alloys of the 7xxx series represent materials with highest potential for lightweight applications due to their superior strength combined with low density. Unfortunately, robust processing of these alloys remains a challenge, as these alloys are still considered not being weldable by conventional methods, where hot cracking is a critical issue. However, it has been numerously shown that joints being characterized by excellent properties can be obtained by friction stir welding (FSW). Here, one possibility for further improvement is to perform a post-weld heat treatment (PWHT) to stimulate precipitation hardening after welding. Two different PWHT with 300 s and 10 s soaking time at 480 °C followed by water quenching and artificial aging for 24 h at 120 °C were performed. The heat treatment with a short soaking time led to a higher quasi-static strength as compared to the condition upon PWHT with 300 s soaking time and the FSW condition. Fatigue tests in the low-cycle and high-cycle fatigue regimes revealed a reduction in fatigue strength due to the onset of grain growth near the surface for both PWHT conditions. In consequence, removal of the surface layer led to an improvement in fatigue strength.