Effect of rapid cooling on the microstructure and properties of fine-grained 7075 aluminium alloy under friction stir welding

Abstract

In this study, 7075 aluminium alloy plates with a fine-grained structure were produced by four-pass equal-channel angular pressing and subsequently joined at room temperature (297 K) using carbon dioxide (CO₂)-assisted cooling friction stir welding. Electron backscatter diffraction, x-ray diffraction, Thermocouple measuring instrument,optical microscopy and microhardness testing were used to investigate the microstructural and mechanical characteristics of friction stir welded joints. The results indicated that the maximum temperatures of welded joints at room temperature and CO₂-assisted were 673 K and 568 K, macroscopic surfaces of the welded joints under CO₂-assisted cooling were smoother than those created under room temperature; this was because the rapid cooling of liquid CO₂ inhibited the growth of grains and, following dynamic recrystallisation, the grain size was finer (∼2.9 μm). Compared with the fine-grained base material (BM), the proportion of large-angle grain boundaries in the stir zone region of the welded joints increased under both conditions, and the anisotropy was weakened. The precipitation hardening of the joints was obvious; the welded joints appeared to soften at room temperature, while the hardness of the joints was the same as that of the fine-grained BM under the CO₂-assisted cooling condition.