In this study, we investigated the solid-state weldability of Ti alloys with commercial purity (grade 2). First, as a solid-state welding approach, we conducted friction welding at a rotation speed of 1600 rpm and a friction pressure of 15 kgf on rodtype specimens with a size of 15 mm (dia.) × 50 mm (length). Subsequently, the grain boundary characteristic distributions such as the grain size, shape, orientation, and misorientation angle of the welds were clarified by means of the electron backscatter diffraction method. To study the mechanical properties of the welds, we conducted Vickers microhardness and tensile tests. We found that the application of friction welding to Ti alloy led to a significantly refined grain size in the weld zone (0.84 μm) relative to that in the base material (11.4 μm), accompanied by an orientation change from <0001> in the base material to <2-1-10> in the weld zone. In addition, the mechanical properties of the welds were more enhanced than those of the base material: the microhardness and yield strength of the weld were approximately 20% and 2% higher, respectively, than those of base material. These enhanced mechanical properties are mainly due to grain refinement and orientation development during welding.

We investigated the relationship between the microstructure and mechanical properties of Ti6Al4V alloy subjected to friction welding (FW). In this work, specimens with a diameter of 15 mm and length of 50 mm were prepared, and FW was performed at a constant rotation speed (1600 rpm) and different upset lengths (1 mm and 3 mm). The electron backscattered diffraction (EBSD) method was used to analyze the grain boundary characteristic distributions (GBCDs) such as the grain size, misorientation angle, and the phase distributions for the weld zone and base material. We found that the microstructures in the weld zone were developed by a combination of continuous dynamic recrystallization and phase transformation. In addition, a decrease in the upset length led to accelerated grain refinement, and the average grain sizes in weld zone were refined from that of the base material (5.75 µm) to 1.45 µm and 1.85 µm at upset lengths of 1 and 3 mm, respectively. Consequently, the maximum microhardness value increased by ³20% at an upset length of 1 mm relative to the base material, and the yield and tensile strengths of the welded materials were maintained at the same levels as those of the base material. Therefore, based on the developed microstructure and mechanical properties, the application of FW to Ti6Al4V alloy could be obtained joints with superior performance.

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.