Research On Oscillating Laser Butt Welding Process And Joints Micristructure And Properties Of Zinc-coated Thin Sheets

Due to its excellent corrosion resistance,zinc-coated sheets are widely used in many fields such as automobile manufacturing,packaging,bridge construction and industrial facilities.As a joining and forming process of zinc-coated sheets,welding plays an important role in the wide application of zinc-coated sheets.Laser welding with less heat input and narrower heat-affected zone has gradually become the main welding method for zinc-coated sheets.However,the existence of Zn-coated layer brings challenges to laser welding of zinc-coated sheets as the violent evaporation of the Zn-coated layer seriously damaged the stability of the welding process,thus the weld is very prone to burn through and pore defects.What's more,when the thickness of the plate to be welded is reduced,controlling of the weld formation becomes even more difficult.In order to reduce the influence of zinc evaporation on the weld formation,lap joint is usually used to discharge the zinc vapor through the lap gap to control the weld formation.Therefore,the current welding method of zinc-coated sheets is still dominated by lap welding,but the main application scenarios of lap welding are limited to the manufacturing of body in white in automobile industry.For butt welding,its application scenarios are more extensive and butt welding can optimize welding structure and reduce the weight of welded joints.Therefore,butt welding has more advantages than lap welding in the engineering application of zinc-coated sheets,but the influence of zinc vapor on the welding process is more serious during the butt welding,the perforation defect of the weld is obviously aggravated and the formation of the weld of zinc-coated sheets faces more severe challenges.Therefore,the research on the butt welding process of zinc-coated sheets has important engineering application value and significance.In this study,a series of butt welding tests with oscillating laser were performed on zinc-coated thin sheets of 0.8 and 1.2 mm thicknesses.Firstly,the weld formation of the zinc-coated sheets is compared and observed under several different oscillating modes(circular oscillation,transverse oscillation,“8” oscillation,“?” oscillation,non-oscillation),and the welding process test was carried out in the optimized oscillating mode.The effects of laser power,welding speed,oscillating frequency and amplitude on weld formation are studied,and the optimized oscillating welding parameters are obtained.At the same time,in order to further study the weldability of the zinc-coated sheets,the influence of the thickness of plates,the state of butt surface and the thickness of the Zn-coated layer on the weld formation of the zinc-coated sheets was discussed,and the microstructure and mechanical properties of the welded joints are compared.Finally,the mechanism of oscillating laser welding and the corrosion resistance of oscillating welded joints were analyzed in detail.The results of tests showed that:(1)Under non-oscillating laser welding conditions,the weld of zinc-coated sheets with a thickness of 0.8 mm showed more serious perforations than that with a thickness of 1.2 mm.A thinner zinc-coated sheet made the control of weld formation more difficult.Using oscillating laser welding,the welded joint with the favorable formation and no perforations was successfully obtained even for the welding of the Zn-coated sheets with a thickness of 0.8 mm.(2)The laser power,welding speed,oscillating frequency and amplitude have a certain degree of influence on the weld formation.When the laser power increases or the welding speed decreases,the weld is more prone to perforation,while the laser power decreases or when the welding speed increases,the weld is prone to incomplete penetration.When the laser oscillating frequency is reduced,the trajectory of the welding seam is obvious,and the undercut defect becomes more serious.The influence of the laser oscillating amplitude on the weld is mainly manifested in the melting width.The width of the weld increases with the increase of the amplitude.(3)The state of the butt surface can affect the weld appearance and mechanical performance of oscillating welded joints to a certain degree.When the butt surface was processed via laser cutting,the welded joint showed a good appearance with a low undercutting degree.However,the welded joint processed by laser cutting had a slightly worse mechanical performance than that processed by shear cutting.Overall,the oscillating welded joints have higher mechanical characteristics than the base metal.(4)The microstructure of the WM of the laser oscillating welded joint comprised acicular and polygonal ferrites,with a small amount of Widmanstatten ferrites.The HAZ had nearly the same microstructure as the BM,containing mostly ferrites and a few pearlites.In the welded joint with the butt surfaces processed via laser cutting,on account of greater thermal input,Widmanstatten ferrites' content in the WM was higher than that with butt surfaces processed by shear cutting.This can also account for the lower mechanical performance of the welded joint processed via laser cutting.(5)In the test of salt spray corrosion and electrochemical corrosion,the oscillating laser welding joint was slightly corroded,and its corrosion resistance shows not significantly deteriorated.The results of this research have been applied in engineering applications in the liquid metal packaging industry.