HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding.The weld zone(WZ)was fully martensitic structure,and heat-affected zone(HAZ)contained newly-formed martensite and partially tempered martensite(TM)in both steels.The supercritical HAZ of the QP980 side had higher microhardness(~549.5 Hv)than that of the WZ due to the finer martensite.A softened zone was present in HAZ of QP980 and DP980,the dropped microhardness of softened zone of the QP980 and DP980 wasΔ21.8 Hv andΔ40.9 Hv,respectively.Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain,leading to the formation of low angle grain boundaries(LAGBs).Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs,which led to significant dislocation interaction and formation of cracks.The electron back-scattered diffraction(EBSD)results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing,resulting in the failure of joints located at the sub-critical HAZ of DP980 side.The QP980-DP980 dissimilar steel joints presented higher elongation(~11.21%)and ultimate tensile strength(~1011.53 MPa)than that of DP980-DP980 similar steel joints,because during the tensile process of the QP980-DP980 dissimilar steel joint(~8.2%and 991.38 MPa),the strain concentration firstly occurred on the excellent QP980 BM.Moreover,Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value(~5.92 mm)and the peak punch force(~28.4 kN)due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.

In-situ observation of microcrack initiation and damage nucleation modes on the HAZ of laser-welded DP1180 joint

In-situ SEM can intuitively observe the changes in microstructure and microcrack initiation during the deformation process of the specimen.In this paper,the microstructural evolution,mechanical properties,and damage nucleation modes of DP1180 laser welded joints were systematically discussed by the trans-mission electron microscope(TEM),focused ion beam(FIB),and in-situ tensile test.The precipitation of carbides and disappearance of dislocations tangles deteriorated the mechanical properties of tempered martensite in the sub-critical heat affected zone(SCHAZ),which caused its microhardness slightly de-creased(∼27 HV)and also called the softened zone.The joints fractured in the SCHAZ during the tensile and Erichsen cupping tests and microcracks initiated from the tempered martensite,while the microc-rack initiation of base material(BM)occurred at the ferrite/martensite interface.The dominant damage nucleation mode of the joint was tempered martensite cracking,and that of BM was ferrite/martensite interface decohesion during tests.