Mechanical strength and corrosion resistance of Al-additive friction stir welded AZ31B joints

Abstract

Compared to other structural alloys, magnesium alloys have a relatively poor corrosion resistance and low mechanical strength, which can be further deteriorated when these alloys are subjected to joining processes using the existing joining methods. Herein, we propose for the first time an additive friction stir-welding (AFSW) using fine Al powder as an additive to improve the mechanical strength as well as corrosion resistance of AZ31B weld joints. AFSW is a solid-state welding method of forming a high-Al AZ31B joint via an in-situ reaction between pure Al powders filled in a machined groove and the AZ31B matrix. To optimize the process parameters, AFSW was performed under different rotational and transverse speeds, and number of passes, using tools with a square or screw pin. In particular, to fabricate a weld zone, where the Al was homogenously dispersed, the effects of the groove shape were investigated using three types of grooves: surface one-line groove, surface-symmetric grooves, and inserted symmetric grooves. The homogenous and defect-less AFS-welded AZ31B joint was successfully fabricated with the following optimal parameters: 1400 rpm, 25 mm/min, four passes, inserted symmetric grooves, and the tool with a square pin. The AFSW fully dissolved the additive Al into α-Mg and in-situ precipitated Mg17Al12 particles, which was confirmed via scanning electron microscopy, transmission electron microscope, and X-ray diffraction analyses. The microhardness, joint efficiency, and elongation at the fracture point of the AFS-welded AZ31B joint were 80 HV, 101%, and 8.9%, respectively. These values are higher than those obtained for the FS-welded AZ31 joint in previous studies. The corrosion resistance of the AFS-welded AZ31B joint, evaluated via hydrogen evolution measurements and potentiodynamic polarization tests, was enhanced to 55% relative to the FS-welded AZ31B joint.

Keywords

• AZ31B magnesium alloy
• Additive friction stir-welding
• In-situ reaction
• Mg17Al12 particles
• Joint efficiency
• Corrosion resistance