Study On Microstructure And Properties Of Laser-Welded DP780/dp980 Dissimilar High-Strength Steel Joints

With the light weight of the automobile developing,it has become the developing trend of the big country and the enterprise which manufacture a lot of vehicles.DP780 and DP980 has been widely used in the automotive light-weighting.Compared with the welding joints of DP780/DP980,many scholars researched about DP780/DP780 and DP980/DP980,but the study of DP780/DP980 dissimilar welded joint was less.In view of this,we studied the microstructure and tensile properties of DP780/DP980 welding joint by laser welding,to providing important engineering practice reference and theoretical basis for its application in vehicle light-weighting technology.1.In order to optimize the laser welding parameters of DP780/DP980,we first studied the phenomenon of hardening and softening,and the transformation mechanism of martensite and austenite of DP780,DP980 samples by simulating laser welding thermal cycle.The results as follows:?1?In the study of simulate laser welding thermal cycle,when the peak temperature of DP780 was 735?and 788?,the softening degree of these DP780 samples was higher,and the two thermal cycle temperatures were close to Ac₁?757??of DP780.When the peak temperature of DP980 was 618?and 756?,the softening degree of these DP980 samples was also higher,and the two thermal cycle temperatures were close to Ac₁?740??of DP980.?2?In the study of simulating laser welding thermal cycle,when the peak temperature of DP780 was 735?,the hardness of the sample was 250.7?HV0.2?,which was 90.8%of the hardness of DP780 body material.However,when the peak temperature of DP980 was 618?,the hardness of the sample was 312.8?HV0.2?,which was 86.05%of the hardness of DP980body material.So the softening degree and the sensitivity of DP980 were greater than that of DP780 during the process of welding heat cycle.?3?When the peak temperature of welding simulated thermal cycle was higher than that of DP780 and DP980 Ac₃,the austenite was completely transformed into martensite during the cooling process due to the complete austenitizing of the samples,and obvious hardening phenomenon occurred.2.The effects of different heat input on the microstructure and tensile properties of DP780/DP980 dissimilar welded joints were studied.Our work studied the six kinds of welding heat input?85.7 J/mm,100 J/mm,120 J/mm,150 J/mm,200 J/mm,300 J/mm?.The results as follows:?1?Under the six kinds of welding heat input,Coarsa-Grain HAZ,Fine-Grain HAZ and fusion zone of DP780/DP980 dissimilar welded joints were mainly composed of martensite and a small amount of residual austenite,induced obvious hardening phenomenon in these zones.?2?Under the six kinds of welding heat input,the DP780 and DP980 heat affected zone had softened.When welding heat input was 120 J/mm,the softening degree of DP980 HAZ was higher than that of DP780 HAZ,which was consistent with the laser welding thermal cycle simulation results.?3?In the simple tension test of six kinds of welding heat input of welded joints,DP780/DP980 dissimilar welding joints broke in the tempering area of DP780 HAZ.And the lasticity of DP780/DP980 dissimilar welding joints was only 27.52%³9.77%of the DP780body material.The strength-plastic DP780/DP980 dissimilar welding joints declined65.68%⁷8.56%compared with the DP780 body material.The strength of DP780/DP980dissimilar welding joints was 1.11%¹1.31%of the DP780 body material.But the yield strength of DP780/DP980 dissimilar welding joints was 113.22%¹20.47%of the DP780body material.3)We studied microstrcture and tensile property of DP780/DP980,DP780/DP780 and DP980/DP980 welding joints under three heat inputs?100 J/mm,120 J/mm,150 J/mm?.The results as follows:?1?When the heat inputs were same,microstrcture and tensile property of three kinds of welding joints contrast as follows:i)Fnsion zone?FZ?:the grain size of DP780/DP980 dissimilar welded joints was bigger than that of DP780/DP780,DP980/DP980 welded joints.ii)Coarsa-Grain HAZ?C-G HAZ?,Fine-Grain HAZ?F-G HAZ?:the grain size of DP780/DP980 dissimilar welded joints?DP780 side HAZ?was smaller than that of DP780/D780 welding joint,and DP780/DP980 dissimilar welded joints?DP980 side HAZ?was greater than DP980/D980 welding jointsiii)Inter-Critical HAZ?I-CHAZ?:the martensite volume fraction of DP780/DP980dissimilar welded joints?DP780 side HAZ?was less than that of DP780/DP780 welded joints;the martensite volume fraction of DP780/DP980 dissimilar welded joints?DP980 side HAZ?was also less than that of DP980/DP980 welded joints.iv)Sub-Critial HAZ?S-CHAZ?:Microstrcture of DP780/DP980 dissimilar welded joints was similar with DP780/DP780,DP980/DP980 welded joints.?2?Under the same heat input condition,the yield strength of DP780/DP980 dissimilar welded joints was significantly higher than that of DP780/DP780 welded joints,and when the heat input was higher?150 J/mm and 120 J/mm?,the tensile strength and plastic of the DP780/DP980 dissimilar welded joints were better than that of DP780/DP780 welded joints.The plasticity of DP780/DP980 dissimilar welded joints was better than that of DP980/DP980welding joints.