Research on Microstructure and Mechanical Properties of Rapid Cooling Friction Stir Welded T2 Copper Joint

doi:10.3901/JME.2022.24.094

Abstract

Abstract: Conventional friction stir welded (FSW) T2 copper joint usually exhibits a low yield strength due to the decreased dislocation density and coarsened grain size caused by post-annealing effect. To eliminate the post-annealing effect and improve mechanical properties, 2-mm-thick T2 copper plates was subjected to liquid CO₂ assisted rapid cooling FSW technology. Microstructural evolution, mechanical properties, and strain hardening behavior of the welds are investigated by optical microscopy, electron backscatter diffraction, transmission electron microscopy, microhardness measurement, and tensile tests. The results show that the grain refinement mechanism of the FSW copper weld is attributed to discontinuous dynamic recrystallization, continuous dynamic recrystallization, and twinning-induced geometric dynamic recrystallization. The rapid cooling FSW weld exhibits a fine grain structure with nano-twins and massive dislocations, and thus it had a high strain hardening rate at stage Ⅲ of strain hardening behavior. During the stage Ⅳ, the nano-twins provide enough storage space for dislocation movement, resulted in reduced strain hardening rate and improved ductility. Compared with the conventional FSW, the yield strength and elongation of rapid cooling FSW weld increase by 31.1% and 25.7%, respectively. The liquid CO₂ assisted rapid cooling FSW technology proposed can prepare heterogeneous fine grain structure in the welded joint by improving the thermal cycle, which is helpful to improve the yield strength. A welded joint with good matching of strength and ductility is achieved by using this advanced FSW technology.

Key words: friction stir welding, copper, microstructure, mechanical properties, strain hardening behavior

CLC Number:

TG453

XU Nan, CHEN Lei, LIU Kun, LIU Lutao, SONG Qining, BAO Yefeng. Research on Microstructure and Mechanical Properties of Rapid Cooling Friction Stir Welded T2 Copper Joint[J]. Journal of Mechanical Engineering, 2022, 58(24): 94-101.

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