Research On Laser Welded Joint Weldability Of Ultra-high Strength Steel

The reduction of body-in-white（BIW） weight is intensively desired in order to reduce fuelconsumption and comply with CO₂emission regulations. Meanwhile, consumers are payingmore attention to the improvement of crash safety. The application of ultra-high strengthsteel（UHSS） can solve the conflict requirement. UHSS is becoming a kind of lightweightmaterials for use in BIW due to its high tensile strength（more than1500MPa）、high energyabsorption rate and good crash concave performance. However, for the special physical andchemical properties, it is very difficult to meet the demand of welding performance by theconventional welding process. Laser welding has prominent strongpoints such as concentratedlaser power, quick speed, narrow HAZ and fine welded joint quality, and it is graduallydeveloping into the main process of welding UHSS. In this paper, the performance oflap-welding and butt welding is investigated, and the influence of welding speed on themicrostructure and mechanical properties is analyzed.Laser lap-welding studies indicate that good-quality joint can be obtained, and the defectssuch as gas hole and crack don’t appear in the fusion zone if the optimized parameters areemployed. Cross-sectional dimensions of the fusion zone decrease with the increasing of thewelding speed, partial penetration occurs if the welding speed exceeds35mm/s. Microstructuresof the fusion zone and its approaching HAZ are mainly lath martensite, the widths of martensitelath aren’t sensitive to the welding speed. However, the welding speed affects the grain width ofprior autensite of the fusion zone. When changing the welding speed from20mm/s to35mm/s,the mean widths of prior austenite grains decrease from10.67μm to7.68μm. Hardness of thefusion zone is close to the base metal, hardness of the HAZ firstly increases, and when it comesto maximum, then decreases suddenly in the tempered zone resulting from the soft ferrite,obvious softening phenomenon occurs. Shear resistance of the joint increases with the increasingof the welding speed, and it comes to the maximum at the welding speed of20mm/s. As a result that there exists strong stress concentration near the fusion line, the specimens all fracture at thelap interface of the fusion zone. Dimples are the primary fracture morphologies, it indicates thatthe fractures carry out in the form of microporous gathering, and it’s not ductile fracture.Laser butt-welding studies indicate that the size of the fusion zone decreases with theincreasing of the welding speed and partial penetration occurs if the welding speed exceeds100mm/s. Microstructures of the fusion zone and its approaching HAZ are lath martensiteleading to the high strength and hardness. The hardness distribution is close to the lap-welding.Hardness of the HAZ firstly increases, when it comes to maximum, then decreases suddenly inthe tempered zone resulting from the appearance of soft ferrite, obvious softening phenomenonoccurs. There exists a tempered zone, where phase transformation happens, the supersaturatedcarbon dissolved in the martensite precipitates in the form of carbides and then the martensite isdecomposed into ferrite. When changing the welding speed from70to130mm/s, the width of thetempered zone decreases from100.4μm to48.2μm and the degree of softening slightly decreaseswith the minimum hardness ranging from310HV to336HV. The result of the tensile test showsthat the specimens fracture in the HAZ. The strength and elongation increase with the increasingof the welding speed. The highest tensile strength and the maximum elongation are both obtainedat the welding speed of90mm/s. The fracture morphologies is dimples and cleavage planes, aswell as some tearing ridges, the fracture surface shows quasi-cleavage fracture characteristics.