Friction Stir Welding of Unequal-Thickness Magnesium Sheets with a Cover Sheet

In the friction stir welding (FSW) process, it becomes challenging to utilize conventional FSW for welding unequal-thickness sheets due to the shoulder's difficulty in simultaneously acting on two plates with different thicknesses. This study proposed an enhanced FSW technique for achieving unequal-thickness welding by employing a 1 + 2 butt joint structure. By doing so, the direct force exertion of the shoulder on the thinner plate is prevented, thereby eliminating plate thinning. Orthogonal experiments were conducted to determine the optimized parameter: 1800 rpm-30 mm/min-0.3 mm, with tensile strength reaching 86.56% of the base material (BM). The microhardness profile exhibits an asymmetric “w” shape in the cross-section of the weld joint. The weld nugget zone (WNZ) shows the highest hardness value at 73 HV, whereas the heat-affected zone (HAZ) demonstrates the lowest hardness value at 39.7 HV. The average grain size of the BM measures 12 µm. Recrystallization occurred in the WNZ, forming a fine equiaxed grain structure with an average grain size of 4.7 µm. The average grain sizes in the two HAZ are 23 µm (thicker sheet) and (thinner sheet) 20 µm, respectively. The peak temperature of the thicker plate, located at an equal distance from the weld's centerline, exceeds the temperature of the thinner plate by 20-35 °C. Consequently, the softening degree of the HAZ in thinner sheets decreased.