Investigation on the Microstructure and Mechanical Properties of AZ91D Magnesium Alloy Plates Joined by Friction Stir Welding

Abstract

Friction stir welding (FSW) is an effective technique to join magnesium-based alloys which are difficult to fusion weld. In this work, similar AZ91D Mg alloy sheet of 3 mm thick butt joint was produced via friction stir welding at welding parameters such as rotational speed, welding speed, and tilt angle. The rotational speed was kept constant of 720 rpm, the welding speed varied from 25 to 75 mm/min, and tilt angle from 1.5° to 2.5°. Defect-free weld was obtained under 75 mm/min welding speed and tilt angle of 1.5°. The microstructure of the parent alloy consists of phases, namely primary α and eutectic β (Mg17Al12) in the as-received condition (gravity die-cast) which was confirmed by X-ray diffraction (XRD) analysis. Microscopic studies, tensile tests, hardness test, and fractographic studies were conducted. Metallographic studies revealed different features in each zone depending on their thermomechanical condition. A significant increase in hardness was observed in the stir zone of weldment compared to parent alloy due to the recrystallized grain structure. The dendrite grain structure present in weldment was completely disappeared and was transformed to fine grains in stir zone (SZ). The transverse tensile test result of the weld specimen indicated that weldment was about 44.9% higher than the parent alloy. Fractographic analysis of the friction stir welded specimen indicated that the weld specimen failed through the brittle failure.