Microstructure and properties of fiber laser welded high strength steel butt joints

A YLS-6000 fiber laser was used to weld high strength steel E36 with a thickness of 6 mm. The main composition (wt.-%) of high strength steel E36 is 0.15C, 1.40Mn, 0.31Si, 0.04Al, and 0.08Ni. The morphology, tensile strength and microhardness of butt joints were studied by using scanning electron microscopy (SEM), a tensile tester and a microhardness tester. The results showed that butt joints of high strength steel E36 consist of a weld seam (WS), a heat affected zone (HAZ) and a base metal (BM). The microstructure of the WS is mainly composed of lath martensite, and the microstructure of the HAZ is mainly composed of martensite, bainite and ferrite. The hardness of the WS increases with an increase in laser power, and the maximum hardness of the butt joint appears in the transition zone between WS and HAZ. When laser power reaches 3500 W, the maximum microhardness is 450 HV. The laser power has little effect on the tensile strength of the butt joint. All tensile specimens were broken at the base material. There is many dimples on the fracture, and the butt joint displays a ductile fracture.