Microstructural Characterisation and Mechanical Performance Evaluation of Friction Stir Welded AA7075-T651 Joints under Variable Tool Rotational Speed and Traverse Rate

Abstract

Friction stir welding (FSW) has emerged as a solid- state joining process of choice for aluminium alloys in aerospace, automotive, and marine applications owing to its ability to produce high- integrity, low- distortion joints without the deleterious effects of fusion welding. Aluminium Alloy 7075- T651, a high- strength aerospace- grade alloy, is notoriously difficult to fusion- weld due to its susceptibility to hot cracking and porosity. This investigation presents a comprehensive experimental study examining the effect of tool rotational speed (600- 1200 RPM) and traverse rate (40- 100 mm/min) on the microstructure, tensile strength, hardness distribution, and fracture morphology of FSW joints of 6- mm thick AA7075- T651 plates. Microstructural analysis via optical microscopy and scanning electron microscopy (SEM) revealed a well- defined nugget zone, thermomechanically affected zone (TMAZ), and heat- affected zone (HAZ) in all joints. Optimal conditions (900 RPM; 60 mm/min) yielded ultimate tensile strength of 462 MPa — a joint efficiency of 89.1% relative to the base material. Vickers microhardness traverses showed minimum hardness in the HAZ (134 HV), consistent with over- ageing effects. Fracture surfaces exhibited mixed ductile- brittle characteristics. A correlation matrix and regression analysis provide quantitative guidance for FSW parameter selection for AA7075- T651.