A review of applications of friction stir welding in joining difficult to weld metals

Friction stir welding (FSW) represents a promising solid-state welding method that is effective in welding challenging-to-weld structural materials, including steel, titanium and nickel-based magnesium alloys. These materials are rather difficult to weld together through traditional fusion welding because of high melting points, hard oxide layers as well as forming brittle phases and residual stress. The current review includes an in-depth and dedicated evaluation of the recent developments in friction stir welding of high-strength and high-temperature alloys. It is a critical study of the advancements in friction stir welded joint tool materials, optimization of the process, microstructure development and mechanical performance. The review also points out the main issues linked to the tool wear condition, heat generation, and joint integrity and elaborates on the measures implemented to enhance weld quality and process efficiency. The results of this review offer a coherent technical view of friction stir welding of hard to weld materials and can be used to suggest future studies and industrial practice.