Investigation of mechanical and microstructural properties of friction stir welded dual phase (DP) steel

Abstract

The application of dual phase (DP) steels has been increasing significantly in the automotive industry because of their high strength as well as good ductility, thus, cold formability. These steels are generally joined using conventional welding methods such as resistance spot welding and laser welding in the production of automotive parts. In recent years, several studies have been conducted to investigate the possibility of joining the advanced high-strength steels such as DP steels using the solid-state friction stir welding (FSW) method due to its advantages over conventional fusion joining methods such as metallurgical benefits, energy efficiency, and environmental friendliness. The aim of this study is to investigate the microstructure, hardness and tensile properties of friction stir welded DP 600 steel plates. Thus, 1.5 mm thick DP 600 steel plates were friction stir butt-welded by a tungsten carbide stirring tool consisting of a concave shoulder having a diameter of 14 mm and a conical pin (angle=30°) with a diameter and length of 5 mm and 1.25 mm, respectively. In the weld trials conducted, the tool was tilted 2° and the down-force of the tool was kept constant at 6 kN. The tool rotation and traverse speeds used in FSW trials were 1600 rpm and 170 mm.min-1, respectively. The microstructure of friction stir welded zone comprised of main martensite, bainite, and refined ferrite. The average hardness of the stir zone has increased to about 400 HV. The tensile specimens failed in the base plate away from the weld zone and tensile strength as high as that of the base plate was obtained from the welded specimens, i.e., about 640 MPa. However, the elongation of the welded plates was significantly reduced, i.e. about 55% of that of the base.