Elucidation of tool wear phenomenon in FSW using silicon nitride tool

Friction stir welding (FSW) has been widely used in welding of metal. However, it is difficult to weld high melting point metal by FSW because of short life or high price of FSW tools. Silicon nitride tool can have both long life and low cost. Although the silicon nitride tool, which has high temperature mechanical properties, could have a long tool life for FSW of steel, it is not clear how FSW conditions affected the tool life. In this study, changes in the shape of the FSW tool was focused. A comparative verification of the tool shape changes due to the FSW conditions was carried out, and specific shape changes in silicon nitride tools were verified. The silicon nitride tool was worn only at the shoulder and not at the probe. Furthermore, the wear shape was different depending on rotation speed and welding speed of the tool. The factors that cause them were analyzed using temperature and stress simulations for silicon nitride tools. Increasing the rotation speed broadened temperature distribution, increased the peak temperature and narrowed the stress distribution. When the welding speed was increased, the temperature distribution became narrower and peak temperature was lower, and the thermal asymmetry increased, while the stress distribution expanded and the stress asymmetry increased. When the welding speed was high, the agitation in the plastic flow zone was insufficient, and there were places where the stress and temperature were low around the shoulder. It is considered that these decrements of the stress and temperature lead to the concave part remaining on the outer circumference. Therefore, it is important to control both the welding speed and the rotation speed of the silicon nitride tool in order to suppress the protrusion which is main reason of the tool breakage.