Study On Failure Behavior Of Hot Stamping High Strength Steel Spot Welds

With the immediately production and possession of the world automobiles,energy saving、environmental protection and safety have become the three topics ofautomobile industries in new century. However, the most effective way to solve theenergy and environmental problems is weight reduction. Application of hot stampingof high strength steel (HSHSS) is used to realize the weight reduction without thedecrease of strength and stiffness. Consequently, it gradually replaces the normalmaterial used in vehicle body such as low-carbon steels and so on. Resistance spotwelding is still the main joining technology for the body components of the vehicles,however, the high resistivity and low thermal conductivity of HSHSS and thetransient thermal cycle will not allow it to follow the continuous coolingtransformation of the ordinary steels, high peak temperature during welding thermalcycle and uneven local cooling rate will cause the difference of intensity matchbetween22MnB5and ordinary steels, all of the above resulted in the heterogeneity ofmicrostructure and mechanical performance of the welding joints, where fracture willoccur at different failure modes under different loading conditions. The traditionalcritical nugget formula4tcan not evaluate the weld quality of the high strengthsteel effectively, thus study of the failure mechanism for hot stamping of high strengthsteel and establishment of the quality evaluation model have become the mostimportant issues.Based on the background mentioned above,1.6mm thick22MnB5was selectedas the research material. Firstly, orthogonal experiment was carried out to study theinfluence of welding current, welding time and electrode pressure on microstructure,failure modes and mechanical properties of the weld joints; tensile-shear test wasdone to investigate the shear performance, cross tensile property and energyabsorption of the weld joints of HSHSS under hot stamping; fatigue test wasconducted to study the effect of spatter and welding heat input on fatigue life andfatigue performance of the weld joints. Secondly, fatigue failure modes of the weldjoints and crack propagation path were investigated under different stress levels;heterogeneity of mechanical performance and static failure mechanism were analyzedin terms of micro-hardness test, metallographic microscopy and scanning electronmicroscopy (SEM). It was shown that there are four main factors affecting the failuremodes of welding joint including sheet thickness, nugget diameter, weld defects andlocal toughness deterioration in the heat affected zone. Mechanical model of weld joints of hot stamped HSHSS was established toevaluate the critical weld diameter when the weld joints start to fail. Then,1.2mmand2.0mm thick22MnB5steel were used to validate the critical weld diameter.Results showed that the critical formula can accurately evaluate failure of the weldjoints without increasing the cost and wasting resources.