Mechanistic insights into abnormal grain growth suppression in friction stir welded 7075-T651 aluminum alloys through variation in tool rotational speed

Abstract

Friction stir weld joints of precipitation-hardenable aluminum alloys invariably show abnormal grain growth (AGG) during the post-weld T6 heat treatment. This study investigates the influence of tool rotational speed on AGG in FSW joints and elucidates the underlying mechanisms. Friction stir welding (FSW) of heat-treatable 7075-T651 aluminum alloy was conducted at various tool rotational speeds such as 386 rpm, 664 rpm, 931 rpm, and 1216 rpm with a constant traverse speed of 20 mm/min, followed by T6 heat treatment. Further, metallurgical characterization of FSW joints, such as optical stereotype microscopy and electron backscattered diffraction (EBSD) analysis, was employed. In addition, micro-hardness distribution and transverse tensile test of the FSW joints were studied to understand the effect of AGG on the mechanical performance of the weld joint. Results indicate that higher tool rotational speeds lead to more uniform deformation and a higher strain rate, resulting in the formation of a strong B/B̅ texture and higher geometrical necessary dislocation density (GND) within the nugget zone. The presence of this texture, coupled with a higher fraction of high-angle grain boundaries, proves less susceptible to AGG. Conversely, welds produced at lower tool rotational speed exhibit a strong C texture with relatively lower high-angle grain boundaries and GND formation, rendering them more susceptible to AGG. Microstructural analysis reveals that continuous grain growth dominates in welds developed at higher tool rotational speeds, while discontinuous grain growth prevails in those developed at lower tool rotational speeds. A weld joint with AGG leads to a lower tensile strength of 502 MPa and higher ductility of 14.1 %, and the fracture mechanism was found to change from ductile to quasi-cleavage fracture. However, no significant effect of AGG was found on micro-hardness. This study establishes the critical role of optimized tool rotational speed in controlling AGG during FSW and provides a comprehensive understanding through EBSD analysis. The finding of this study bridges the gap between controlled AGG and a detailed mechanistic comprehension, offering valuable insights for enhancing the reliability and performance of FSW joints in precipitation-hardenable aluminum alloys.