This paper describes the examination of the insert piece shape for the joint of a low carbon steel (LCS) fixed specimens using a brass insert piece by an autocompleting friction welding method, that method was developed by authors. The weld faying surface of the LCS specimen had a 10mm diameter, and the joint was made with a friction speed of 27.5rps (1650rpm) and a friction pressure of 90MPa. Then, the effect of the thickness and that at the bottom of the grooves (groove bottom thickness) for the brass insert piece on the joining phenomena and joint properties were investigated. The joint, which was made with an insert piece thickness of 4mm and a groove diameter of 11mm, had the circumferential shear fracture during the friction process at the groove bottom thickness of 0.95mm or below. However, some joints did not have the circumferential shear fracture although other joints had its fracture at a groove bottom thickness of 1.15mm or above. The joint efficiency of those joint had scatter, although that increased with increasing the groove bottom thickness and one of the joint had 100% joint efficiency at 1.45mm. That is, the joint, which had reliably the circumferential shear fracture with the 100% joint efficiency, was not obtained at an insert piece thickness of 4mm. To reliably obtain the joint which had the circumferential shear fracture with the 100% joint efficiency, the effect of the insert piece thickness was investigated. The joint efficiency increased with increasing insert piece thickness, and the scatter of that was decreased. When the joint was made with an insert piece thickness of 8mm and a groove diameter of 11mm, those had reliably the circumferential shear fracture with the 100% joint efficiency at the groove bottom thickness of 0.70 and 0.75mm. Also, when the joint was made with an insert piece thickness of 6mm and a groove diameter of 12mm, the joint had also reliably the circumferential shear fracture at the groove bottom thickness of 0.70mm. Therefore, a joint with 100% joint efficiency, which was made with the LCS with the brass insert piece, should be made with opportune brass insert piece shape of that thickness, the groove diameter, and the groove bottom thickness. In conclusion, a dissimilar joint was able to make by an autocompleting friction welding method.

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