Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters

This novel study presents an unconventional approach to find the best governing process parameters of high frequency mechanical impact technique based on multi-objective optimization method. In this investigation, the post-weld mechanical treatment is aimed to enhance fatigue resistance of structural friction-stirred weld subjected to fluctuating loads by obtaining nominal sub-surface hardness. The experimental study was conducted on aluminium alloy AA 6061 with thickness of 6 mm under varied parameters centred on indenter diameter, air pressure and impact frequency. The investigation began with obtaining optimum parameters for single response by using conventional Taguchi method with L9 orthogonal array. Next, advanced optimization approach by means of multi-objective Taguchi method attempts to consider the multiple responses simultaneously which are sub-surface hardness and structural life. As the final results, the optimum value was acquired by calculating the total normalized quality loss and multiple signal-to-noise ratios based on unequal desirability. The significant level of the parameters was evaluated by using analysis of variance. Furthermore, the second-order model for predicting the objectives was derived by applying response surface methodology. It can be summarized that, first, the affecting parameters to obtain superior structural life can be ordered at significant level of ca. 65, 25 and 10% for air pressure, impact frequency and indenter diameter, respectively. Secondly, using subsequent post-weld mechanical treatment, the life cycle number can be extended up to 12 times on friction-stirred weld. Finally, based on the experimental confirmation test, the proposed method can effectively estimate the structural life and surface hardness within the acceptable range of relative error.