Optimizing Friction Stir Welding Process for Enhancing Strength and Hardness using Taguchi Multi-Objective Function Method

As a solid-state welding method, friction stir welding is widely employed for welding aluminium alloys. An important subject in this regard is the optimal adjustment of the parameters to maximize the ultimate tensile strength and the surface hardness. Four parameters have been selected for the multi-objective optimization of the 6061-T6 aluminium alloy, namely the rotational and the linear speed of the tool, the variation of the shoulder diameter with respect to the pin diameter (D⁄d ratio), and the shoulder base angle. The Taguchi's L9 Orthogonal Array has been employed for designing experiments. The experimental results have been examined using the Taguchi signal-to-noise (S/N) method, the analysis of variance, and regression. Optimization using the multi-objective Taguchi function revealed that a rotational speed of 800 rpm, a D⁄d ratio of 18/6, a shoulder base angle of 7°, and a linear speed of 80 mm/min yield both maximum strength and surface hardness. The results of the S/N analysis suggested the rotational speed of the tool and the linear tool speed have the most significant impact on the tensile strength with the average of 44.07 dB. On the other hand, the linear speed and the ratio of the diameters have the most significant impact on the surface hardness (around 36.91 dB). The results showed that using this optimization method, simultaneous improvement of tensile strength and surface hardness occurs. In fact, the tensile strength and hardness of the sheet surface were improved by 17.3% and 6.2%, respectively.