Study On The Characteristics Of Laser Screw Welding Galvanized Steel

Galvanized steel sheets are widely used in the automotive industry.At present,resistance spot welding is the most common method in the manufacture of body-in-white lap joints with galvanized steel sheets as the main material.But resistance spot welding requires repeated movements due to its weld process need welding torch to touch the sheets,it greatly limits the production efficiency,so more and more car manufacturers are beginning to use lasers as a heat source for white body lap joint welding.Although laser welding has the advantages of low heat input,small welding deformation and high precision,laser seam welding requires high assembly precision,and it is suitable for the change of different gaps when the two-layer or three-layer lap joint of the body-in-white is used.When the gap is large,it is easy to depress on the upper surface of the weld joint and defects may be created owing to incomplete fusion of the upper and lower sheets.When the gap is too small,due to the poor weldability of the galvanized steel,the zinc vapor which has been vaporized sharply when the steel has not melted will be nowhere to escape.Then it will lash the melt pool and cause huge splashs and a severe depression or huge blowholes in the joint.With the rapid improvement of laser beam quality,the oscillating laser welding technology has attracted more and more attention.The PFO laser weld torch produced by Trumpf can make the laser spot move quickly in a certain area at a speed of up to 1000mm/s.Based on the programming of computer,different scanning is realized now and the path is designed to expand the welding area.The method of the laser screw welding is to control the laser spot moving in a circular area by a spiral trajectory to realize a large fusion surface width weld seam without defects and to improve the laser welding adaptability of the lap joint of galvanized steel sheets.This paper mainly studies the effects of the parameters?microstructure and mechanical properties of laser screw welding galvanized steel sheets.The effects of various process parameters on the formation of laser screw welding(LSW)were analyzed,and the influence of zinc vapor on the formation of LSW was observed and studied to avoid the occurrence of defects.Through the comparative analysis of the macro profile and mechanical properties of the LSW obtained under different gap conditions,the process window of laser screw welding of galvanized steel sheets was obtained.The fracture behavior of laser screw welding was further analyzed.Provide experimental and theoretical data support for the application of this new technology?In this paper,the laser screw welding process of two-layer galvanized steel sheet with single layer of 1.8mm ST12 galvanized steel sheet is studied.The process can adapt to the stable welding of 0.2-0.6mm lap joint,0.2-0.6mm and the laser screw welding is smooth and uniform.The optimized welding process parameter is determined as follows: the beam is spirally scanned from inside to outside,with an outer diameter of 2.4-2.7 mm,a laser power of 5 kW,and a scanning speed of 65-85 mm/s.The microstructure of the edge of the laser screw welding is columnar crystal,and the center is equiaxed crystal.Because of the small heat input,the U-shaped solder joints have obvious columnar crystal orientation,and the columnar crystal regions of the H-type solder joints are less than the U-shaped solder joints,and the equiaxed crystal regions are significantly increased.Unlike conventional laser spot welding,this method has a larger cooling rate and a larger columnar crystal size.The mechanical performance analysis results show that when the inter-sheet gaps of 0.2-0.3mm,the welding spot can obtain better tensile shear performance.The maximum load of the shearing shear is 15.3kN at 0.2mm,and the maximum load of the cross tensile test is 12.7 kN.Compared with resistance spot welding of the same fusion surface size,the laser screw welding has better shear tensile properties,and the cross tensile performance is close to the resistance spot welding.There are two failure modes of laser screw welding: button-out failure,pullout failure.Pullout failure is an ideal fracture type because it absorbs more energy.In this paper,the button out failure mode mainly occurs in the inter-sheet gaps of 0.2mm-0.5mm,and the tensile shear strength is generally 15 kN.When the inter-sheet gaps is 0.1mm and 0.6mm,then button-out failure mode occurrs.The former has a hole in the edge of the solder joint because the overlap gap is too small,and the latter has a button gap,and the bearing area of the solder joint is seriously reduced,resulting in a button break.The tensile shear strength is generally 14kN.