Effect of silicon nitride microstructure on characteristics of FSW tool for steel and tool life

Abstract

Abstract Friction stir welding (FSW) has been widely used in the welding of low melting point metal, such as Al alloy or Cu alloy. However, high melting point metal, such as Fe alloy, it is difficult to be welded by FSW because of short life or the high price of FSW tools. Silicon nitride tool can have both long life and low cost. In order to obtain design guideline for silicon nitride FSW tool, the relationship between the mechanical properties and tool life was examined. Two silicon nitride materials which had different microstructure were prepared. One of them had lower high-temperature mechanical properties and the other had higher properties. Tools made by them could join the SUS 304 plate under the following conditions; welding speed was lower than 1000 mm/min, and revolutional pitch was lower than 1.8 mm/r. Even the tool which had excellent mechanical properties at high temperature worn faster when the joining temperature was 950 °C or higher. The tool with excellent high-temperature mechanical properties had a tool life 6 times longer than the other tool. It is considered the life of the tool was improved when the crystallization of the grain boundary phase promoted the rise in the melting point of the grain boundary phase and the precipitation of silicon oxide during FSW.