Laser Welding Performance Of Advanced Automotive High Strength Steels

The laser welding performance of 1000 MPa grade Fe-0.28C-2Mn-0.93Al-0.97Si(wt.%)Transformation Induced Plasticity(TRIP)steels and 1200 MPa grade Fe-18.8Mn-0.63C(wt.%)Twinning Induced Plasticity(TWIP)steels were investigated in this paper.The effect of welding speed and heat input per unit length on the microstructures and mechanical properties of laser welded joints of the two kinds of advanced high strength steels(AHSS)were analyzed respectively.The microstructures of welded joints were characterized by means of optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron backscatter diffraction(EBSD),and the microhardness measurements and tensile tests were utilized to evaluate the mechanical properties of the welded joints.Weld thermal simulations were designed to systematically study the microstructural evolution of the heat-affected zone(HAZ)of TRIP steels.In addition,in-situ SEM and in-situ EBSD technique were performed to investigate the deformation and strengthening mechanisms of the fusion zone(FZ)of TWIP steels under tensile stress with the tensile direction perpendicular to the weld seam.The conclusions are shown as follows:(1)During laser welding process of TRIP steels with a 0.28 wt.%carbon content and 0.66%carbon equivalent,lath martensite and lower bainite formed in the weld seams.The microstructural morphologies and phase constituents of FZs varied little with the welding speed or heat input per unit length.The spatial distribution of HAZ microstructures could be influenced by the welding parameters due to the significant changes of the temperature gradient over the narrow HAZ.(2)The FZ had the highest hardness values followed by HAZ and finally the base metal(BM),and the average microhardness of FZs varied little with welding parameters.No HAZ softening occurred in all laser welded joints of TRIP steels.The welded samples possessed good tensile properties with the tensile direction perpendicular to the weld seam,and their tensile strength were almost equal to that of BM,while their slight ductility loss were related to the widths of both FZs and HAZs.The tensile properties of welded samples with the tensile direction parallel to the weld seam decreased significantly,and the brittle intergranular fracture of FZs was attributed to the microsegregation of alloy elements which occurred in the interdendritic regions during solidification process.(3)The weld thermal simulations of HAZ were conducted using a quenching dilatometer which was applied to simulate the rapid thermal cycles of the laser welding process.The results showed that the microstructural evolution of HAZ mainly involved in the decomposition of metastable phases(i.e.the retained austenite(RA)and the caride-free bainite)in the thermal cycle with a peak temperature below Acl temperature,while involved in the reaustenitization and matensitic transformation in the thermal cycle with a peak temperature above Ac1 temperature.(4)When experiencing the thermal cycles with a peak temperature ranged from 400 ℃ C to Ac1 temperature,no HAZ softening occurred because both the precipitation hardening by cementite particles and the hard-phase martensite that transformed from the carbon-depleted RA effectively compensated the hardness loss induced by the decomposition of RA and the caride-free bainite.Reaustenitization of HAZ microstructure took place when experiencing the thermal cycles with a peak temperature above Ac1 temperature,and the decrease of carbon content in the reformed austenite led to the formation of lower bainite in HAZ when the peak temperature was much higher than Ac1 temperature.(5)The microstructural characterization of the laser welded joints of Fe-18.8Mn-0.63C(wt.%)TWIP steels showed that microsegregation of both Mn and C occurred in the interdendritic regions during the solidification of FZ.The secondary phase particles in FZ are mainly divorced eutectic phases(Fe,Mn)3C and inclusions,which distributed in the interdendritic regions.(6)The investigation of the microstructural evolution and strengthening mechanism of FZ under tensile stress showed that dislocation slip was the main deformation mechanism at low strains,while twinning became the main deformation mechanism at relatively high strains,and the formation of mechanical twins played an important role in improving the strength and ductility as well as the work-hardening effect of FZ.In addition,the activation of slip systems in dendrite grains complied with Schmid’s law and the lattice rotation path in each dendrite at the early deformation stage depended on the slip system possessing the maximum Schmid factor.The initial dendrite grain orientations were essential for the deformation and strengthening mechanisms of FZ.(7)Fully austenitic welded joints of TWIP steels were obtained by using varriedwelding parameters.The shrinkage porosities surrounding the weld centerline are the main welding defects in FZ.The decrease of average secondary dendritic arm spacing,the dendrites refinement and the increase of<0 0 1>//RD fiber texture fraction and density in FZ could be achieved by increasing welding speed or decreasing heat input per unit length.(8)The FZs exhibited the lowest microhardness values in the hardness distribution of laser welded TWIP steel joints,and the average microhardness of FZs varied little with welding parameters.The FZ strength was mainly affected by the fraction and density of<0 0 1>//RD fiber texture in FZ.The tensile properties of welded sample with the tensile direction perpendicular to the weld seam depended on both the fraction and density of<0 0 1>//RD fiber texture and the average dendrite grain size in FZ.The tensile properties of welded samples with the tensile direction parallel to the weld seam were almost equal to that of BM.