상용순수 타이타늄과 Ti-6Al-4V 합금의 마찰접합 특성에 관한 연구

Abstract

In this study, a systematic investigation on the microstructural evolution and mechanical properties in the weld zone of similar and dissimilar joint of commercial pure (CP) titanium and Ti-6Al-4V alloy welded by friction welding (FW) was carried out. Specimens with a diameter of 15 mm and length of 50 mm were prepared, FW was performed at constant rotation speed changing different process parameters such as friction pressure, upset pressure, and upset length. The electron backscattered diffraction (EBSD) method was used to analyze the grain boundary characteristic distributions (GBCDs) such as grain size, misorientation angle, orientation, and phase distributions. Mechanical properties of each joint were evaluated using the microhardness and tensile tester. It was found that the application of the similar FW to CP titanium led to significantly refined grains, of which the average size is 0.71 μm, in the weld zone while that of the base material was 11.4 μm. The grain orientation was changed from <0001> in base material to <21 ̅1 ̅0> in the weld zone. These microstructural evolutions resulted from the dynamic recrystallization caused by frictional heat and material flow during FW. On the contrary, microstructures in the weld zone of Ti-6Al-4V alloy during similar FW were developed by combination effect of dynamic recrystallization and phase transformation. Consequently, the microstructures in the weld zone consisted of equiaxed and lamellar structures, and its grains (1.4 μm) were markedly refined compared to that of base material (5.74 μm). Dissimilar FW between CP titanium and Ti-6Al-4V alloy was carried out. It was found that the microstructures in the weld zone at the CP titanium and Ti-6Al-4V alloy were differently developed due to the different thermal conductivity of each material. Microstructures in the weld zone at CP titanium side were changed in the same way as those during the similar FW while those at Ti-6Al-4V alloy side were differently developed due to the fragmentation resulted from insufficient heat-input. The average grain sizes in the weld zone at both sides of CP titanium and Ti-6Al-4V alloy were 1.03 μm and 1.01 μm, respectively. Because of the refined grains, the micro-hardness in the weld zone of similar and dissimilar titanium joints were higher than those of initial materials. Besides, the yield and tensile strengths were maintained at the same levels as those of the base materials. The tensile specimens were fractured at base materials not at the weld zone or heat-affected zone. The microstructures and mechanical properties of welds for similar and dissimilar joint using FW were investigated. Through this study, it was found that the continuous dynamic recrystallization, which determined the characterization of the final microstructure in weld zone, was dominant recrystallization phenomenon during FW. The strengthening mechanism by applying FW is expected to give insight into securing the titanium joints with superior performance. Furthermore, the results of the microstructures and mechanical properties obtained through this study are expected to be used for basic research data for the reliability evaluation of dissimilar titanium joint and for the study on dissimilar solid-state welding between α and α+βphase titanium alloys.