Effect of tool tilt angle on weld joint strength and microstructural characterization of double-sided friction stir welding of AZ31B magnesium alloy

Abstract

Mechanical and metallurgical behavior of double-sided friction stir welded (DS-FSW) joints of 6 mm thick magnesium alloy (AZ31B) plates, under 0° (Joint W1) and 2° (Joint W2) tool tilt angle, using scrolled shoulder and screwed pin tool were investigated for the first time. Joint fabrication using tool rotational speed of 800 rpm and welding speed of 50 mm/min resulted in tunnel defect in W1 joint and defect free W2 joint. The tensile test results show the joint efficiency of 74% for W1 joint and 87% for W2 joint. The defect free micro tensile samples extracted from first pass (FP), second pass (SP) and overlap region (OZ) of both the welds shows higher degree of grain refinement. Also, microhardness in stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) of W2 joint is high compared to W1 joint. These metallurgical enhancements in each zone of 2° tilt angle joint led to the remarkable improvement in joint efficiency of FP-2 (80%), OZ-2 (71%) and SP-2 (91%) tensile specimens compared to FP-1 (76%), OZ-1 (63%) and SP-1 (67%) for W1 welded joint. Significant improvement by 26%, 19% and 32% and in percentage elongation was observed in the first pass, overlap region and second pass respectively for W2 joint over W1 joint. Scanning electron microscopy (SEM) characterization revealed ductile and shear fracture modes, wherein, larger area fraction of dimpled morphology and tearing ridges was observed for the macro and micro tensile specimens of W2 joint.