| Summary: | 碩士 === 國立中山大學 === 機械與機電工程學系研究所 === 103 === In this study, friction stir lap welding (FSLW) of thin steel sheets was conducted with high rotating speed and constant load. Using a pin-less tool made from tungsten carbide (WC) with 6 mm diameter, the welding characteristics and welding mechanism of thin steel sheets were investigated under different operation conditions, such as the load (150~500 N), the rotating speed (6000~24000 rpm), the welding time (10~60 sec)and the feeding speed (0~2 mm/sec). Effects of load, rotating speed, welding time and feeding speed on the appearance, surface profile, shear fracture load, and interface temperature between the tool and steel sheet were investigated.
Based on the results of spot welding, the operation conditions were selected for the feeding welding. Experimental results showed that the optimum lap condition for thin steel sheets at the load of 300~500 N, rotating speed of 18000 rpm with the feeding speed 1~2 mm/s and the load of 400 N, rotating speed of 12000~24000 rpm with the feeding speed 1~2 mm/s, where the appearance was smooth with the protrusion no more than 0.06 mm. The shear strength is about 1.26 times as strong as that for the base metal at the load of 500 N, rotating speed of 18000 rpm, and feeding speed 2 mm/s.
Based experimental results, the empirical equation of maximum shear fracture load (Ff) suitable for thin steel sheets of the FSLW process is derived in terms of load (Fd), rotating speed (Ns), feeding speed (ƒ) as Ff=0.0408∙Fd^0.2391∙Ns^0.2325∙ƒ^0.1041. This formula indicated that Ff is proportional to the Fd, Ns and ƒ. Moreover, the influence of load on shear fracture load is greater than rotating speed, and the influence of rotating speed is greater than feeding speed.
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