| Summary: | 博士 === 國立中山大學 === 機械與機電工程學系研究所 === 101 === In recent year, a trend in FSW toward the use of pinless tool to replace the pin tool in sheet welding is to achieve increasing feeding speed and to avoid key hole. Hence, a novel dynamometer is proposed to measure the time histories of welding forces including the downward, clamping, and feeding forces during friction stir welding (FSW). The FSW experiments involve the butt joining of aluminum alloy 6061-T6 plates using the tool with pin of 3 mm diameter to investigate the effects of rotating speeds and feeding speeds on the downward force and feed force. To compare with the result of using pinless tool, the formation mechanism of the feeding force was investigated. Empirical equations of the mean downward force and the mean feeding forces in terms of rotating speed and the feeding speed are derived. The mean downward force and the mean feeding force are physically influenced by the interface temperature between the tool and the workpiece material. The feeding force is the difference between the friction forces, respectively acting on the advancing side and the retreating side of workpiece.
A pinless tool with an embedded rod is proposed in the present study. A rod made of the same material as the workpiece was selected to enhance heat generation by the higher friction coefficient. Further, the higher compatibility with higher temperature can also enhance the adhesion behavior between the tool and the workpiece. The effects of rod diameter (d) on the welding properties and mechanism of weld formation were investigated. For friction stir spot welding (FSSW), the maximum rate of temperature rise was about 12.8 oC/s, and the ultimate load was about 2 kN for this pinless tool (d = 10 mm), while these values are only about 7.4 oC/s and 1.35 kN for the plain one. For FSW, the ultimate load was about 1.46 kN for this tool, and about 0.72 kN for the plain one. The abutting edges of sheets with a thickness of 3 mm could be welded using this pinless tool.
Cu-Ni alloy and low-carbon steel were joined by friction stir lap welding (FSLW) with the help of a nickel coating. The appropriate coating thickness and appropriate welding parameters were investigated. A thickness of 5 μm on the Cu-Ni side and 20 µm on the steel side are the most appropriate settings for the Ni coating. Using these parameters, the micro-voids were reduced in size due to the self-diffusion in the Ni/Ni interface at high interface temperature and high contact pressure. The shear strength was about 2.9 times as high as that of the joints without Ni coating. The strong bonding mechanism using a nickel coating could be explained by the iron and copper atoms diffusing into the coating through the interface at high interface temperature and holding time. The maximum shear strength was achieved for the most appropriate thicknesses of Ni coating at the rotating speeds of 800-1400 rpm, and the feeding rate of 10 mm/min. With these welding parameters, the maximum interface temperature was higher than 900◦C and the holding time was longer than 83 s. The feeding rates had significant effects on the holding time and the shear strength of the joint.
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