Microstructure and Mechanical Properties of Dissimilar Friction-Welded Commercially Pure Ti and Ti–6Al–4V Alloy

Abstract

Here, we investigated the microstructure and mechanical properties of dissimilar joints between grade 2 commercially pure titanium (CP-Ti-2) and Ti6Al4V alloy welded via friction welding. Accordingly, specimens with 15 mm diameter and 50 mm length were prepared, and friction welding was performed at a constant rotation speed (1600 rpm) and different upset lengths: 1, 2, and 3 mm. Electron backscattered diffraction (EBSD) method was introduced to systematically analyze grain boundary characteristic distributions (GBCDs), such as grain size, grain orientation, phase distribution, and misorientation angle distribution. In addition, mechanical properties of dissimilar joints were evaluated using the micro-Vickers hardness and tensile test machine. The microstructures at both sides of the CP-Ti-2 and Ti6Al4V alloy were refined materials, compared with each base material. These grain refinements were determined via dynamic recrystallization accompanied by frictional heat and severe plastic deformation. In addition, the increase in the upset length distinctively affected the change trend of the grain size of the CP-Ti-2 and Ti6Al4V alloy. The hardness values in regions around the weld interface were higher than those of base materials. Furthermore, yield and tensile strengths were maintained at a similar level to those of CP-Ti-2, and the tensile specimens were fractured at the base material zone of CP-Ti-2, not at the weld interface and its periphery. Consequently, friction welding to dissimilar joints between the CP-Ti-2 and Ti6Al 4V alloy was successfully conducted and sound welded materials without welding defects were obtained.