Study On CO₂ Laser Welding Of Zinc Coated High Strength Steel

High strength galvanized steel with 800MPa possesses characteristic of less spring-back, good formability, corrosion resistance. So it was extensive applied to high strength power-absorbing of initial body of car, such as A-B-C reinforced plate, door sill, seat skeleton, reinforced plate of bumper, carline, beam and so on. The welding of high strength steels is one of the critical problems to apply these steels. Therefore, research on the weld ability of high strength steels is of great signification in theoretical study and engineering practical value.laser welding is one key method to carry out the welding of high strength steels, in this paper, firstly on the base of the simulation of heat conduction temperature field, welding experiments are done to high strength galvanized steel and tailor-weld the high strength galvanized steel plate with cold-rolled plate; then based on the research of thermal cycling of laser welding, the grain growth and microstructure transformation of welding joint in laser welding of 800MPa grade high strength galvanized steel were studied, especially the combined influence of impurity element, grain size and microstructure on mechanical properties of the laser-welded joint was analyzed. The major conclusions are summarized as the followings:1. Mathematic model of thermal conduction of laser welding was established in this paper, and it was calculated by using finite difference method (FDM). When laser power is lower, laser welding is belong to thermal conduction welding model; with the increasing of laser power, the surface happen to appear evaporation, then it transforms from thermal conduction welding model into deep penetration welding model. By studying the transition, the relation of critical power and welding speed in laser penetration welding of high strength galvanized steel is acquired.2. On the basis of experiments of laser welding high strength galvanized steel, by analyzing the influence of technology parameter to laser penetration welding, optimizing technology parameter was acquired. For guaranteeing good mechanical property of tailor-welded board; adopting side blown protective gas can effectively control the influence of welding joint quality caused by zinc coated, comparing with maximal penetration, rigidity and toughness of weld joint, Erichsen number of welds respectively adopting N2 and Ar as side-blown protective gas. After inspecting mechanical properties, corrosive properties and microstructure, CO 2 laser welding can effectively avoid softening HAZ of welding of zinc coated high strength steel, it also can control the gas hole and crack, meanwhile fine corrosion-proof properties of welding seam can be gained.3. Based on tailor-welded tests of high strength galvanized steel sheet and cold-rolled plate, connection between laser power, welding speed and depth, width of welding seam are studied, optimized technology parameter was acquired; it appears fine mechanical property of welding joint, no softening of HAZ. Forming property of tailor-welded board depends on strength ratio and thickness ratio of two materials, welding line is prone to high strength galvanized steel sheet, the offset of welding line tends toward bigness with thin cold-rolled plate. Corrosion test indicates that the corrosion resistance of tailor-welded board of high strength galvanized steel sheet with cold-rolled plate is low because of bimetallic corrosion, the corrosion rate of it is faster than that of the joint in the high-strength galvanized steels welded joint, it should take measures to protect it.4. Considering elimination of heat of sheet, on the basis of theoretical calculating a simplified point-line thermal model, theoretical thermal cycling curve of welded joint is obtained. Aim at thermo junction can't trace heating-cooling procedure, in this paper concept of time constant was put forward and compensatory formula of dynamic error was deduced. For transforming compensating voltage to temperature, adopting least squares techniques to fit thermoelectric curves of thermo junction, the thermoelectric curve is acquired. Actual thermal cycling curve of welded seam of laser welding is obtained to zinc coated high strength steel. Theoretical thermal cycling curve matches well with actual thermal cycling curve, it demonstrates that point-line thermal mathematical model can explain actual thermal cycling character of laser welding high strength galvanized steel with thickness of 1.5mm. Utilizing mathematic model forecasts galvanized layer width of burning loss while laser welds high strength galvanized steel.5. The microstructure of laser welding joint of high strength galvanized steel was observed, it consists of martensite, lower bainite, upper bainite, similar pearlite, pearlite and ferrite. Test shows that austenite deposits based on phase interface in transiting process, alloy carbide organization homogeneously distributes on ferrite basal body, and meanwhile the structural pattern and generative mechanism was analyzed. By adopting model of laser deep penetration welding optimum growth zone of crystal grain in HAZ is forecasted, mathematical model of primary austenite crystal is established, and its computing result matches well with actual result. The theoretical and experiment results show the grain growth of the welded joint of high-strength galvanized steels was inevitable and more severe than low carbon steel; crystal grain growth leans to easy with primary thin crystal grain of base metal; crystal grain growth leans to obvious with lower impurity content of base metal; the speed of grain growth could be faster with bigger heat input.As a summary, with the study on laser welding 800Mpa grade high strength galvanized steel it provides experimental and theoretical basis for the development and application of laser welding of car body. Based on the experimental condition, the samples acquired have been successfully employed in door safety of PoLo by Shanghai Dazhong Automotive CO.LTD.