The Effect Of Microstructure Heterogeneity On Mechanical Properties Of Laser-Welded DP800 Steel Butt Joints

In recent years,DP(Dual-Phase)steel has been widely used in the construction of automotive structural parts to meet the requirements of automobile lightweight due to the excellent performance.However,due to the microstructure inhomogeneity of the welded joints of DP steel,the mechanical properties of the joints vary widely in different zones.This kind of inhomogeneity is bound to have a great impact on the safety and reliability of service.Therefore,it is necessary to research the inhomogeneity of welded joints and explore the real reason for the difference of mechanical properties.The mechanical properties of welded joints were studied by hardness test and conventional tensile test.The effects of microstructure inhomogeneity on the mechanical properties of welded joints were studied by means of optical microscope(OM),scanning electron microscope(SEM)and digital image correlation method(DIC).The results showed that the hardness distribution of welded joint is inhomogeneous.The welded joints contain the softened zone and the hardened zone.The softened zone is about 2 mm-3.6 mm away from the weld center,and the hardness value ranges from 221 HV-243 HV.The hardened zone is mainly distributed in the weld metal zone and the heat-affected zone on the side of the weld metal,and the hardness value ranges from 263 HV to 395 HV.The yield strength and tensile strength of the welded joint are not different from that of the base metal,while the average elongation after fracture of the joint(11.77%)is significantly lower than that of the base metal(24.92%).The strain localization first appears in the softened zone during the tensile process.As the load increased,the strain localization in the softened zone increases,and the value of the strain localization reaches 0.58 before fracture.According to the analysis,martensite is tempered in the SCHAZ and the content of ferrite is increased in the ICHAZ,forming the softened zone of the joint.The microstructure of martensite in the FGHAZ,CGHAZ and WM constitute the hardened zone of the joint.The inhomogeneous microstructure of welded joint causes the difference of mechanical properties.In order to solve the problem of two-phase microstructure inhomogeneity in the softened zone including the SCHAZ and the ICHAZ,the tensile behavior of the softened zone was studied by means of nano-indentation test,finite element method and in-situ tensile test.The effect of microstructure inhomogeneity on the mechanical properties of the softened zone was investigated.The results showed that the mechanical property curve of the SCHAZ is lower than that of the ICHAZ after the welded joint specimen enters the plastic stage.The phenomenon of strain segmentation is found in the microstructure of the SCHAZ and the ICHAZ during the tensile process.The strain localization is mainly localized in the softer ferrite,and the average plastic strain value of the SCHAZ is higher than that of the ICHAZ.The in-situ tensile test validates the prediction of finite element simulation based on microstructure.After the specimen necking,the strain localization of the SCHAZ is more serious.The accumulation of plastic strain localization in the SCHAZ causes the final fracture of welded joint.