Study On Microstructure And Properties Of Laser Welding Ultra-high Strength Steel DP980 Joints

In this paper,the laser butt welding of DP980 ultra high strength dual phase steel with Nd:YAG laser was carried out.The mechanical properties of DP980 laser welded joints were studied by means of hardness test and tensile test,evolution of microstructures were observed through optical microscope(OM)and scanning electron microscope(SEM),fatigue properties of base metal and welded joints were got by high frequency fatigue test.The welding process,microstructure and mechanical properties of laser welded DP980 butt joints were paid much attention,especially the fatigue properties of welded joints.During the research of welding process on DP980 dual phase steel of 1.6mm,the effects of defocusing amount,laser power and weld rate on weld formation were studied respectively.It was found that with-1mm defocusing amount,it was easier to obtain the large depth width ratio.At the welding speed of 10mm/s,good weld performing was obtained with 1250w~1350w laser power.When the laser power was 1350 W,the welding speed was between 15mm/s and 25mm/s,which could obtain good weld.Observation and analysis of microstructure of welded joints showed that laser-welded DP980 joints could be divided into weld metal zone(WM),heat affected zone(HAZ)and base metal zone(BM).The weld metal zone and complete phase-transformation zone consisted of martensite,exhibiting higher hardness.The organization of HAZ changed greatly including complete phase-transformation zone,incomplete phase-transformation zone,tempered zone.The softening of welded joints occurred in incomplete phase-transformation zone and tempered zone: In incomplete phase-transformation zone,the ferrite content increased because of transformation of martensite while in tempered zone carbide precipitated in martensite due to tempering.In the tensile test of the base metal and welded joints,the welded joints witnessed slight decrease in tensile strength and greater decrease in elongation rate due to the inhomogeneous deformation originating from softening of welded joint.The tensile tests revealed that fracture took place in HAZ,presenting plastic fracture,the fracture was observed with different sizes of dimples.The fatigue limit of DP980 laser welding joint which was obtained under 1300 w laser power and 10mm/s weld rate was 280 MPa which decreased significantly comparing with base metal of 695 MPa fatigue limit.The existence of the softening zone causes a big decrease in the fatigue limit.Fatigue crack initiation was initiated at the surface of the samples,and the expansion area was in the presence of fatigue crack and second-cracks,and the morphology of the transient zone was the same as that of the tensile fracture surface.