| 초록 |
Friction stir welding (FSW) invented by The Welding Institute (TWI) is considered a promising process for non-metallic materials such as aluminum alloys and magnesium alloys. Previously, the FSW technique was mostly applied to lightweight and low-melting-point materials, as a solid-state welding technique. However, in recent years, FSW with high-melting-point and high-strength materials have been tried. Some FSW studies on microstructure and mechanical properties of pure Ti and Ti alloys have been reported. However, the studies about the effect of welding process parameters on microstructure and mechanical properties are significantly insufficient. In addition, Ti-6Al-4V alloy is extremely hard, and it is a material that undergoes allotropic transformation and shows considerable microstructural evolution during friction stir welding. For this reason, it is hard to study on FSW of Ti-6Al-4V alloy sheets. Therefore, there are few studies on relationship between microstructures and mechanical properties of the FSW joints with Ti-6Al-4V alloy sheets. In the present study, Ti-6Al-4V alloy sheets were friction-stir-welded at different rotational speeds with fixed feed rate. The main aim of this paper is to investigate the effect of tool rotational speed on microstructural evolution and mechanical properties. The relationship between microstructure formation and mechanical properties of the FSW joints was also discussed. Texture study has been done by EBSD for better understanding the phase transformation in the stir zone. This study also investigated the microstructure and mechanical properties of friction stir welded beryllium-copper alloy joints. Friction stir welding (FSW) was successfully conducted using 700 rpm and 60 mm/min combination of parameter. Post-weld heat treatment (PWHT) of the specimens was performed at 315˚C, with range of 30 minutes to 8 hours to analyze the behavior of the microstructural evolution. While the microstructure of the specimens was characterized in detail using optical microscopy, scanning electron microscopy, and transmission electron microscopy, the mechanical properties were evaluated by Vickers hardness test, Charpy impact test and tensile test. The softening of the microstructure was observed during FSW due to the dissolution of the precipitates. Whereas, the specimens were strengthened after PWHT due to the formation of gamma prime (CuBe) precipitates. |
