Study On Microstrucrure And Properties Of Dp780/Qp980 Resistance Spot Welding Joints

With the development of the corporate average fuel consumption target in China,advancedhigh strength steel with good strength and plasticity is widely used in the main load-bearing structural parts of the body-in-white.At present,_the most widely used material in automobilesis still duplex steel,d_ue to its good welding performance and complete welding process.Du_eto t he higher carbon equivalent and emission during welding of the Quenching and Partitioningstee l,its welding process and microstructure of the joint have been challenged.In t_his work,the _cold-rolled DP780 and QP980 sheets with 1.6 mm _thick were used as the research material.In _order to ensure that the joint has high tensile shear strength and good toughness,the m_acroand mic ro characteristics of DP780/QP980 spot welding joints with different processparamet ers were studied by optical microscope(OM),elect_ron probe microanalysis(EPMA),micr_ohardness test and tensile test.The ma_in research and results are as follows:(1)Through the single pulse welding test,it was _found that the spot welding joint consistof four regions including fusion zone(FZ),parcial _melted zone(PMZ),heat aff_ected zone(HAZ)and base m etal(BM).There are liquation cracks(LC)in the heat affected zone of QP980 side,but no l_iquation cracks were observed on the DP780 side.Element sc_anning was performed onthe liquatio n crack area,and it was _found that C,Mn and P ele_ments were enriched in the interior.Under the c_onditions of electrode pressure of 4 kN,welding curre_nt of 7.8 kA and weldi_ng timeof 280 ms,the s hear tens_ile strength of the joint is 28.17 kN and the f_ailure energy is 81.72 J,the maxi_mum c_ross tensile strength is 10.35 kN and the f_ailure energy is 122.15 J.The ductil_ity_ratio is 0.37.(2)The sin_gle _pulse spot welding experiment found that there are four fracture modes inthe DP780/QP980 joints by the shear tensile test.The interface fa_ilure occurs when the currentvalue is 7.5 kA,th e partial _inter_face failure occurs when the current value is 7.8 kA,the partial_thick_ness-partial pu llout failure occurs when the current value is 8 kA and the pullout failureoccurs when the curr ent value is 9 kA,respectively.The mo_rphology of th_e notch on the jointshows that the crack a ppeared at the tongue root of the gap between the DP780 and QP980.(3)The double puls_{e s}pot welding process shows that the size of the secondary nuggetdepends on the currentvalue.When the current valu_e is 7.5-6.5 kA,the size of t_he s_econd_arynugget is smaller than t he initial nugget,and the local structure in the nugget area changes fromdendrite to block;When th e current value is 7.5-8.5 kA,the size of the _seco_ndary nugget is 1.60mm larger than that of the primary nugget.A line scan of the liquation crack region at thisp rocess was performed,_and the enrichment of C,_Mn,a_nd P elements was found to be lowerth an that of a single pulse.T_he best process parameters with double pulse are electrode pressure4 kN,we_lding current 7.5-8 kA,wel_ding time 280 ms.(4T)he mechanism of improving the mechanical properties of DP780/QP980 spot weldingjoints with double pulses is analyzed.It i_s found that the secondary pulse current value is higherthan 7kA,the n_ugget diameter is increased compared with the single pulse 7.5 kA,a_nd th_etensile strength of the joint is improved.At the _double-pulse 7.5-6.5 kA pro_cess,compared withthe single pulse,the nugge_t diameter of the joint does not increase,but the cr_oss tensile strengthand absorpti on energy are increased by 3.93 kN and 6₈.57 J,respec_tivel_y.Therefore,th_eimprovem_ent of cross tensile properties is not only related to the nugget size,but also affec_tedby the grain siz e.Through the str_ess analysis of the welding joint,it is found that the criticalnugget diameter ofthe DP780/QP980 is 6.40 mm,which is c_onsist_ent with the experimentalresults.