Tilted-tool Friction Stir Welding of Steel Material by using Hemispherical Silicon Nitride Tool

Silicon nitride is one of the inexpensive and attractive tool materials for friction stir welding (FSW) of steel material. The improvement of silicon nitride tool life has been studied by controlling welding conditions and the tool geometry. In this study, the tool life of a hemispherical silicon nitride tool tilted toward the retreating side during FSW of steel plate was investigated. The defect-free welded SPHC plate was obtained using the hemispherical silicon nitride tool under the optimum welding condition. The tool life was evaluated at tool rotation speeds of 400, 500 and 700 rpm. At 400 rpm, the tool broke at a welding distance of 100 mm. At 700 rpm, the tool wear was observed at the welding distance of 3.0 m, and the tool broke before reaching the welding distance of 6.0 m. In contrast, at 500 rpm, tool wear or tool breakage was not observed up to the welding distance of 10.0 m. The tool life of the hemispherical silicon nitride tool strongly relates to the welding condition. The temperature of the hemispheric silicon nitride tool was 1078 K at the tool rotation speed of 500 rpm. Tool wear depends on the high temperature strength of the silicon nitride. The use of a simple shape such as the hemispherical tool allows FSW to perform at relatively low temperatures, which prevents tool wear and tool breakage.