Microstructures and properties of CO₂ laser welded 600 MPa DP steel

CO₂ laser welding of cold rolled DP steel sheet with tensile strength of 600 MPa and thickness of 1.4 mm was carried out.The effects of welding speed on bead morphologies and cross-section shape were investigated.The microstructures of welded joint were observed.The hardness distribution,tensile strength and formability of the welded joint were tested.The study showed that for the same laser power porosities occurred on weld bead at low welding speed and spatters occurred at high welding speed.In addition,welding speed also had influence on penetration and bead width.The weld metal was mainly composed of martensite due to high cooling rate of laser welding.The amount of martensite decreased rapidly approaching base metal from weld bead center.Therefore,the weld metal or heat affected zone had maximum hardness for the whole welded joint.The tensile strength of welded joint perpendicular to the weld line was equal to that of the base metal.But the tensile strength of welded joint parallel with the weld line was higher than that of the base metal.The plasticity and formability of the welded joint were impaired because of the formation of martensite in the weld metal.