Enhancing surface integrity in friction stir welding through deep rolling and post-weld heat treatment

Abstract

This study aimed to investigate mechanical surface treatment through deep rolling (DR) and post-weld heat treatment (PWHT) to enhance the surface integrity of an AA7075-T651 aluminum alloy welded through friction stir welding (FSW). The surface integrity of the welded joint was evaluated by analyzing residual stress, microhardness, surface roughness, microstructure, and fatigue life. The experimental design comprised three sets: one set exclusively applied FSW, another set applied FSW followed by DR (FSW-DR), and the last set applied FSW followed by PWHT (FSW-PWHT). Fatigue testing (screening) was performed through a four-point bending test with a bending stress of approximately 300 MPa, test frequency of 2.5 Hz at room temperature, and stress ratio (R) of 0. The FSW parameters included a tool rotational speed of 1600 rpm, welding speed of 30 mm/min, immersion depth of 0.1 mm, dwell time of 15 s, and tool tilt angle of 0°. The DR parameters included a rolling pressure of 150 bar and rolling speed of 1400 mm/min. The results revealed that the residual stress significantly influenced fatigue life. The fatigue test demonstrated that FSW workpieces treated with the DR process (FSW-DR) exhibited the highest fatigue life, showing a remarkable increase of 33.5 % compared with that of untreated FSW workpieces. Moreover, the residual stress changed from tensile to compressive, accompanied by noticeable enhancements in microhardness and surface roughness. These results highlight that FSW followed by the DR process (FSW-DR) positively affects weld surface integrity, resulting in an extended service life for welded components.