Microstructural Characterization and Mechanical Properties of AZ31 Magnesium–Titanium Joints Produced by Friction Stir Welding

Magnesium alloys possess the lowest density among widely used structural metals and are extensively applied in aerospace and military industries. In this study, the welding behavior and mechanical properties of AZ31 magnesium alloy joints were investigated. After multiple trials, a rotational speed of 1200 rpm and a traverse speed of 8 mm/min were selected for friction stir welding (FSW) using an H13 tool steel pin. To fabricate a composite in the joint region, a thin titanium sheet was incorporated at the interface. Furthermore, to improve the distribution of reinforcing particles within the matrix, the number of welding passes was increased from one to three. Microstructural examinations revealed that friction stir welding not only homogenized the structure but also refined the grain size, leading to improved mechanical properties. The average hardness increased from approximately 70 HV for the as-received AZ31 alloy to 75 HV for the welded non-composite specimen, 78 HV for the one-pass composite specimen, and 82 HV for the three-pass composite specimen. In addition, the ultimate tensile strength increased from approximately 201 MPa for the base alloy to about 280 MPa for the three-pass composite specimen. X-ray diffraction analysis confirmed the presence of α-Mg and α-Ti phases, while no brittle intermetallic compounds were detected at the Mg–Ti interface. To evaluate the wear behavior, reciprocating wear tests were conducted under a load of 30 N over a sliding distance of 1000 m. The weight loss of the three-pass composite sample after 1000 m was approximately 6 mg. The steady-state coefficient of friction values were approximately 0.42, 0.40, 0.37, and 0.34 for the as-received, non-composite welded, one-pass composite, and three-pass composite specimens, respectively. Scanning electron microscopy (SEM) analysis of the worn surfaces showed that with increasing number of welding passes, the abrasive wear mechanism became more pronounced relative to delamination. In the non-composite sample, delamination was dominant, although abrasive wear was also observed. In the as-received sample, adhesive wear and delamination were identified as the dominant wear mechanisms.