Recent advances in welding of high-strength bainitic steel for automotive applications

Abstract

Bead-on-Plate Friction Stir Welding (FSW) joints were produced on 3-mm thick Ultra-fine Bainitic Steel using polycrystalline cubic boron nitride (PcBN) tool. The rotational speeds employed include 80, 100, 150, and 200 rpm. All the weld joints were prepared at a traverse speed of 35 mm/min. The noncontact online thermography has been used to measure interface temperature between the top surface of plate and bottom of the tool shoulder bottom. This measurement enabled to determine the peak temperature reached in the stir zone (SZ) during FSW. The interface temperatures at all rotational speeds are ~below 673 K. The microstructure in the base metal (BM) is composed of carbide-free bainite and retained austenite. After FSW, the weld joint revealed BM, stir zone (SZ), and thermomechanically affected zone (TMAZ). The retained austenite decreased with increasing rotational speed in SZ and there has been a gradual transition of microstructure in the TMAZ. Stir zone exhibited higher hardness compared to BM. Maximum hardness in SZ occurred at 150 rpm.