Microstructural characterization and in-process traverse force during friction stir welding of austenitic stainless steel

Welding AISI 304 stainless steel is challenging, especially as fusion-based welding processes (such as arc welding) severely undermine the material's corrosion resistance due to sensitization. Solid-state friction stir welding is one of the most suitable alternatives. Friction stir welding of high-strength high-softening materials such as AISI 304 is difficult mainly because of the non-availability of affordable tools and tool life. In this study, AISI 304 stainless steel was successfully butt-welded by friction stir welding. The experiments were performed using Taguchi's L27 orthogonal array. Shoulder diameter, tool r/min, and traverse speed were selected as the most influential welding parameters. A Tungsten Carbide (WC) tool was employed with a tapered pin profile. Defect-free joints were fabricated successfully for all the welding conditions. Microstructural examinations using optical microscopy and scanning electron microscopy revealed significant grain refinement in the stir zone and the presence of distinct structural features such as stepped, dual, and ditch in various characteristics zones. The presence of precipitates was also observed in samples and was confirmed via energy-dispersive X-ray spectroscopy analysis. The in-process traverse force was measured by a special arrangement of force measuring units attached to the work fixture. The traverse force data were analyzed and optimized. The results of an analysis of variance reveal that the traverse speed was the most important parameter, followed by tool r/min, interaction between the tool shoulder diameter and traverse speed, interaction between the tool shoulder diameter and tool r/min, and, finally, the tool r/min.