Study On The Weld Formation And Fracture Behavior Of Laser-Arc Hybrid Welding On BG890QL High Strength Steel With Mid-thickness

BG890QL low-alloy high-strength steel has been successfully used in the field of construction machinery due to its characteristics of high strength and good low temperature toughness.However,the welding efficiency of conventional arc welding method in intermediate thick plate is quite low,the laser-arc hybrid welding method is used in this paper to improve the welding efficiency of BG890QL low-alloy high-strength steel.Effect of the laser-arc hybrid welding parameters on the weld formation are studied,and conventional mechanical properties as well as fracture behavior of the welded joints are investigated.Influence mechanism of high-gradient microstructure on the driving force of crack growth is interpreted,internal relationship between the fracture resistance,microstructure and zone width is clarified.The results of this study can provide data support and theoretical basis for the service of the welded joint of BG890QL steel with intermediate thickness.Firstly,the influence of the laser-arc hybrid welding process on the weld formation of BG890QL steel plate was studied,and the effect mechanism of the groove shape on the full penetration behavior of the joint was discussed.The experimental results showed that based on the coupling effect of laser-arc double heat sources,when the laser was leading to the arc and the laser-arc distance was 4 mm during the hybrid welding process,the surface formation of the weld bead can be well controlled,no obvious defect occurred and the penetration depth was around 10 mm.By adopting the double-sided beveling process of hybrid welding,the laser-arc hybrid welding technique for 16mm thick plate was realized as the back penetration and front formation quality of the weld was improved,and defect such as hump on the back was eliminated.Based on the observation results with high-speed camera,coupling behavior between the laser and arc plasma in the groove was analyzed.It was found that when the laser-arc distance was 2 mm,strong coupling between laser and arc occurred and led to fierce fluctuationg of the molten pool,which caused more weld spatter and worse surface bead formation.When the laser-arc distance was4 mm,the coupling between laser and arc became weaker,area of the plasma was increased while the brightness was lower.With increasing groove filling height,arc-laser coupling effect as well as brightness of the plasma were further weakened,which promoted the best interaction of deeper penetration from laser welding and better weld surface formation from arc welding.During laser backing welding,the inverted V-type groove in the back surface of workpiece was adopted to obtain sound back formation.This was because the introduction of inverted V-type groove on the back allowed the high-temperature fluid to stretch out along the groove side wall and solidify directly,inhibiting the flow downward and avoiding the formation of overlap.Subsequently,the microstructure of hybrid welded joint was systematically characterized and analyzed,and the effect mechanism of microstructure on impact toughness was clarified.The weld metal was composed of equiaxed grains at the top and columnar crystals at both sides.The microstructural characterization indicated that the weld metal(WM)consisted of tempered martensite and granular bainite.Compared with BM,the WM showed higher hardness of 410 HV₁ due to formation of hardened microstructure during laser-arc welding process with high cooling rate.The heat affected zone(HAZ)was divided into coarse grain zone(CGZ),fine grain zone(FGZ),intercritical HAZ(IC-HAZ)and over-tempered zone(OTZ),and its microstructure was mainly tempered martensite.The lowest micro hardness in the HAZ was about 350 HV₁,which was located at the junction of OTZ and IC-HAZ.The CGZ has the highest hardness of 435HV₁ due to its coarse grain size and susaturated martensite.Fe₃C phases precipitated in both the WM and the HAZ,and size of the Fe₃C phase in the WM was about 350 nm,which is larger than that in the HAZ.The impact energy of the HAZ at room temperature and low temperature were147 J and 66.5 J respectively,which were both higher than that in the WM.This was due to the restraint effect of the low-hardness OTZ,IC-HAZ and BM on crack propagation.The presence of blocky martensite in the matrix also promoted crack to deflecte,making crack propagation more difficult.Meanwhile,the HAZ contained more stable small-angle grain boundaries and?3 grain boundaries(about 8%),which increased its resistance to crack propagation and improved the impact toughness.The impact toughness of the WM was lower than the HAZ,which was ascribed to the higher strain concentration at the phase boundary of martensite and granular bainite.In addition,the unstable random angle grain boundary(about 66%)and the large size of the Fe₃C phase can promote nucleation of micro-voids,leading to higher tendency for crack initiation in the WM.At the same time,the relatively complicated grain orientation and large degree of anisotropy in WM could easily produce severe strain concentration,which deteriorated the crack propagation resistance property.Finally,the fracture toughness and fracture driving force of different regions of the hiybrid welded joint were studied,and the fracture resistance property of the joint was also investigated.The Weibull stress was used to characterize the fracture driving force of brittle cracks.The calculation results showed that the driving force for crack growth of arc-WM was 2241MPa,the driving force for crack growth of laser-WM and hybrid-WM were144 MPa(6.4%)and 62 MPa(2.8%)higher than that of arc-WM respectively,indicating that the arc-WM had the highest fracture resistance,which was consistent with the experimental results of the fracture toughness test.The shielding effect improved the crack propagation resistance of laser-WM and hybrid-WM.The shielding effect obtained by laser-WM in welded joints was 126 MPa,which was the highest and followed by 77 MPa for hybrid-WM.Due to the shielding effect,the driving force for crack propagation of laser-WM and hybrid-WM were significantly reduced.In addition,the driving force for crack propagation in the HAZ of the laser welded joint was 2037 MPa,which was close to that in the hybrid-HAZ,and slightly lower than that of the arc-HAZ.This indicated that the ultra-narrow HAZ(<1 mm)of laser welding and hybrid welding both improved the crack propagation resistance and fracture toughness.