Improving tensile properties by varying the welding conditions of the passes of the double-sided friction stir welding of AZ31B magnesium alloy

The present study aims to improve the tensile properties of the double-sided friction stir welded joints of AZ31B magnesium alloy by simultaneously varying the process parameters of the 1st and 2nd passes based on the design matrix developed using response surface methodology. The twenty numbers of welded joints fabricated as per the design matrix were evaluated for tensile strength, followed by the microstructural characterization, micro -hardness evaluation, and fractographic analysis of the optimized welded joint. A strong interaction among the rotational speed of the 1st and 2nd passes was noticed on the ultimate tensile strength. The EBSD analysis of the optimized welded joint revealed that the mutual variation of the process parameters for 1st and 2nd passes led to the inhomogeneous distribution of fine and ultra-fine grains within the overlap region (OR). The extensive grain refinement in the OR compared to the stir zones (SZ) of 1st and 2nd passes significantly improved the micro -hardness of the joint, followed by the attainment of approximate to 84 % joint efficiency and similar to 12 % percentage elongation. The fractographic analysis of the fractured tensile specimen revealed the mixed brittle and ductile fracture morphology for the DS-FSW joint as compared to the dominating ductile fracture behavior for the base material. The abnormal texture variation introduced within the interface of the OR and SZ of the 1st pass formed the weakest bonded region of the optimized DS-FSW joint.