Study on failure mode and mechanism of tool in friction stir welded steel

Abstract

The failure modes and mechanism of the tool in friction stir welding process are stud ied by welding of Q235 steel of 4mm thick, with the tool made of YG8 Cemented carbide. The results show that the main failure modes of the tool in FSW of Q235 steel are the mechanical wear, the oxidation, the brittle fracture of the pin, and the concave and convex ring area on the shoulder. The microstructure, fracture morphology and micro-hardness of the tool are analyzed, it can be shown that the main reason for the failure are the high temperature friction of the tool for a long time, resulting in uneven or partial diffusion of the Co phase, and the tungsten carbide particles are peeled off from the matrix under the action of force, which seriously destroys the matrix model of the cemented carbide. The free carbon and the hard-brittle phase Co6W6C in the crystal boundary greatly weakens the solid solution strengthening of the Co phase to tungsten carbide, which decreases the hardness, strength and wear resistance of the tool.