Statistical analysis and optimization of parameters affecting the hardness of butt joint cross-section of Al7075 produced by FSW and SFSW using RSM and desirability approach

The thermal cycles involved in friction stir welding (FSW) cause softening in the joint. This phenomenon generally occurs in heat-treatable aluminum alloys leading to decrease in mechanical properties. Submerged friction stir welding (SFSW) has been developed to overcome this limitation. In this study, firstly, the butt joints were produced from Al7075-T6 alloy using FSW process. To this end, the RSM was selected as the experiment design technique. So, the factors such as: tool rotational speed, tool feed rate, tool shoulder diameter, and tool tilt angle were identified as the input variables. Then, statistical analysis and optimization of variables affecting the hardness of cross-sections was performed. In attention to the high value of desirability function (0.976), it could be found that the procedure of optimization has well fulfilled the pre-determined target. In addition, the optimal condition has been confirmed by implementing the verification test. Afterward, the butt joints were produced using SFSW based on the optimal values of tool feed rate and tool tilt angle. The obtained results from analysis of experimental data, confirmed the accuracy of regression equation. Furthermore, it is shown that the final model can predict the MHD parameter with an error less than 5%. Also, the linear terms of the tool shoulder diameter and tool rotational speed are effective on the hardness of cross-sections of the underwater welded joints.