| In view of the safety and reliability of the automotive industry and the demand for energy saving and emission reduction,advanced high-strength steel(AHSS)plays an irreplaceable role in the development trend of automobile lightweight.Laser tailor welding is an efficient and easy to automatic production method in automobile body manufacturing.With the advent of the third generation of AHSS,such as press-hardened steel and QP steel,automotive components have the potential for higher performance,while higher demands and new problems are brought to the body welding process.In this paper,two types of the third generation of advanced high-strength steel,the Al-Si coated 22 Mn B5 steel and QP980 steel,were selected as test materials.Laser welding of 1.5mmthick 22 Mn B5 steel welded joint and 1.2mm-thick 22 Mn B5/QP980 dissimilar steel welded joint were carried out.The process optimization research of laser tailor welding process was carried out.The influence of Al-Si coating on microstructure evolution and mechanical properties of 22 Mn B5 steel on welding and hot stamping process was clarified.The strength and formability of 22 Mn B5/22 Mn B5 tailor welded blank and 22 Mn B5/QP980 tailor welded blank were compared.Firstly,the response surface method was used to design the laser welding process optimization experiment of the original Al-Si coated 22 Mn B5 steel.The results showed that the influence of process parameters on the tensile strength of joints was ranked as: welding speed>laser power>defocus.Through the regression model optimization,combined with the macroscopic forming and section morphology of the weld under different parameters,the optimal process parameters are as follows: 2.2k W for laser power,2m/min for welding speed,+2mm for defocusing amount.Subsequently,the effects of Al-Si coating on microstructure evolution and comprehensive mechanical properties of the joints were studied,which caused by the laser welding and hot stamping process.The results showed that the Al-Si coating contributes to the increase of the high-temperature ferrite region in the phase diagram.The microstructure of fusion zone was composed of 15% ferrite + 85% martensite structure.The hardness drops to 350 HV.The maximum load is reduced to 65% of the de-coated condition,but the joint fatigue performance is improved and the fatigue limit reaches 125 MPa.The Al-Si coating in the fusion zone further affects the evolution of the structure during hot stamping.Due to the slow cooling rate in the hot stamping process,the effect of Al element is more obvious.The microstructure of fusion zone consisted of 60% ferrite + 40%martensite.The hardness value of the fusion zone is reduced to 300 HV.The tensile strength is about 1150 MPa,which is 85.2% of the de-coated condition,and the elongation is about 2.1.%;the forming property of the tailor welded blank is seriously degraded,and the cupping value fell from 6.9 mm under de-coated condition to 2.9 mm.Finally,the laser welding of hot stamped Al-Si coated 22 Mn B5/QP980 dissimilar steel was carried out.The stable welded joint was obtained when the laser power was 1.8k W-2.2k W.The microstructure of fusion zone is lath martensite.The size and degree of softening zone increase with the increase of laser power.At 2.0kw,the dissimilar steel joint has the best mechanical properties,the tensile strength reaches the strength of QP base metal,which is about 1050 MPa,and the elongation is 13%.Although the thickness of welded blanks is reduced,good forming ability is achieved with a cupping value of 4.0mm,which beyond the value of 22 Mn B5/22 Mn B5 welded joints in coated conditions.Compared to the welded joint of 22 Mn B5/22 Mn B5 laser tailor welding,the tensile strength of the dissimilar steel is reduced while the fracture absorption energy is significantly increased due to the large increase in the elongation,so that the tailor welded blank has better forming properties. |
PDF Full Text Request