Study On Microstructure Characteristics And Aluminum Migration During Laser Welding Al-Si Coated 22MnB5

Aluminum-silicon coated hot stamping steel,one of advanced ultra-high-strength steels,is widely used in the manufacture of body-in-white,such as B-pillar or anti-collision beams.It can achieve energy saving and emission reduction,and improve body safety in automobiles.In a variety of advanced hot stamping forming technology,the laser tailor-welded blanks(TWB)can combine different materials or different sheet thickness flexiblely.Due to the existence of Al-Si coating,the strength of the TWB is greatly reduced.The coating removal before welding become the most effective mthod to solve the problem.However,in order to effectively remove aluminum-silicon coating,laser ablation must be carried out using picosecond and femtosecond laser source,which are very expensive.Therefore,it is of great theoretical significance and industry value to study the microstructure characteristics and aluminum migration mechanism in the Al-Si coated hot stamping steel.Based on Al-Si coated 22MnB5 in this paper,using experimental test,theoretical analysis,high-speed camera observation and simulation modeling,the microstructure characteristics and aluminum migration mechanism were investigated.The main research contents and conclusions of this thesis are as follows:(1)Factor analysis on weld quality of Al-Si coated 22MnB5 laser weldsIn view of the problem of Al-Si coating affecting the weld quality of 22MnB5welding,the laser welding experimental platform for thin-plate is set up,with the observation system of plume and weld pool flow during laser welding.The effect of welding process parameters and protective gas on the weld surface appearance and weld shape were investigatied.It is found that Al-Si coating is not fully dissolved in the melting pool during laser welding.The high Al causes the melting pool to oscillate,resulting in the coarse weld seam,weld root cut and edge caves.The joint is not suitable to paint on this surface.It is also found that the coating has less influence on the weld shape.The weld is"Y"shape when the laser power is low,and the shape transforms to"X"shape when the laser power is high.With the increase of welding speed,the"X"weld changes to the"I"shape."Y"and"I"weld respond to the blind keyhole,which did not go through the bottom melt pool but the plate was penetrated.At this time,the lower surface coating will float on the surface of the bottom melt pool.The flow of the bottom melt pool is slow,resulting in the slight influence on weld by the bottom coating.The study also revealed that the oxiditive gases such as CO₂ or air atmosphere can eliminate the weld cut,by reacting with the Al-Si coating slag on the surface of the melting pool.It also changes the weld shape.The transformation of"Y"shape welds to"X"-shaped welds requires higher laser power.The"Y"-shaped welds is more easily obtained in active gas.(2)Effect of Al-Si coating on microstructure characteristics and mechanical properties for tailor-welded blanksIn view of the reduction of the strength and plasticity of the Al-Si coated 22MnB5TWBs,a laser welding system was set up to study the effect of the coating on the welded joints,by comparing the microstructure transformation,chemical composition and mechanical properties in the coated and decoated conditions.The research shows that after heat treatment,the heat affected zone of the HPF welded joints disappears.Thus,there is no HAZ softening.The coating increases the aluminum content of the weld and high temperature?ferrite is generated.When the heating temperature increases,there is no change in the microstructure distribution,but the size of the two-phase increases and the corresponding tensile strength decreases.Compared to the decoated welded joints,?ferrite reduces the tensile strength from 1500 MPa to 1100 MPa and the elongation from 3%to 1%.By removing the coating,optimizing the process parameters of pure laser welding and laser welding with wire,the performance of the coated TWB is close to the joint with completely removal of coating.The research shows that increasing the decoating area can reduce the proportion of?ferrite.In addition,the effect of upper surface coating is greater than the lower surface.Studies have shown that changing the weld shape can also reduce ferrite and improve microstructure homogeneity.As the welding speed increases,the weld shape changes from"X"to"I"shape,and the tensile strength of the welded joint increases to 1500 MPa.However,too high welding speed will lead to inadequate transfer of the upper surface coating,resulting the weld area in the macrosegregation:the lower half of the fusion zone is the lath martensite,the upper half fusion zone is a?ferrite-rich area.The study also shows that adding filler wire can dilute the aluminum element in welds,and also enhance the melting pool stirring.The wide strip partially distributed?ferrite will fragment into the disperse distribution of fine ferrite.By analyzing the aluminum-rich microstruture of partial penetrated welds,the influence of aluminum content on the solidification process and solid phase change is summarized.The results show that with the increase of aluminum content,the volume fraction of ferrite increases.There are six microstructure characteristics.One is composed of fully lath martensite(completely decoated welds with 0.5 wt.%Al).The second is the lath maritone matrix,with the slender long strip of ferrite uniformly dispersive distributed among the lath martensites(the welds of 1.5 mm thichkness sheet has 1.5 wt.Al).The third one(vermicular structured ferrite)is evenly distributed among the lath martensites(the welds of 1 mm thickness has 2.5 wt.%Al).The fourth one is?ferrite matrix with island-like lath martensite evenly distributed at the grain boundary(with 3.7 or 4.7 wt.%Al in the conduction mode welds).The fifth is the striped martensite is distributed at the ferrite grain boundary.There are many crossed needle-like phases(with 5.3 wt.%Al in the conduction mode welds)inside the ferrite grain.The needle-like phase proves to be a long-period ordered structure phase,with fault stracked structure.The fracture mechanism of Al-Si coated 22MnB5 TWBs is as follows:the crack is priamarily generated in delta ferrite of this dual phase,and the micro-crack then expands along the ferrite until it is hindered by the lath bundle.When the ferrite is vermicullar or wide strip,the micro-cracks are easier to expand rapidly along its length direction,accelerating the formation of macro cracks and fracture.It results in a sharp decline in the plasticity of welding joints.Therefore,the reduction of ferrite can reduce its size and change its shape.Finally,the finer microstructure improves the mechanical properties of the welded joints of Al-Si coated 22MnB5.(3)Numerical analysis of weld pool flow and Al migration during laser welding Al-Si coated 22MnB5In view of the inhomegeneous microstructure of Al-Si coated 22MnB5 weld,we establish a novel 3D model to describe the weld pool flow and Al migration during laser welding.The Al element is set as a mark.Based on the mixing laws,the physical properties of fluid are modified.The melting pool flow behavior and aluminum element migration in conduction mode and keyhole mode laser welds are analyzed in visually.The formation of inhomogeneity distribution of elements is revealed.The decrease of welding speed is proposed to improve the homogeneity.The research also shows that the aluminum content of the conduction mode laser weld is simulated about 5 wt.%,which is approximate to the measurement.Increasing melting time helps aluminum to migrate.Due to the short melting time in fusion line,there is possiblely an aluminum rich area.The research also shows that the aluminum content of deep-melt welded seamis is reduced to 1.2 wt,and the aluminum content distribution is more chaos.The rich aluminum area of the"X"-shaped weld is located in the upper fusion line and the center area of the lower part.The"Y"weld appears only in the aluminum-rich area in the upper part of the weld.In the lower part of the weld,it is less aluminum,which is consistent with the distribution of the"Y"-shaped weld microstructure.Increasing the welding speed will lead to a wider gap of aluminum content between the upper and lower part of"Y"welds.Studies have also shown that keyholes will hinder the migration of aluminum,increasing the migration distance.The aluminum rich area is initially appearred from the front wall of the keyhole,and come across the keyhole to enter the tail part of the melting pool.When the keyhole penetrates the melting pool bottom,the lower surface coating also enters the tail,which increases the aluminum content of the"X"-shaped weld.When the front wall of the keyhole is collapsed and attachs the back wall of keyhole,the aluminum rich zone will migrate through the liquid bridge.The aluminum-rich area migrates to the tail of the melting pool in advance,which disturbs the pattern flow,resulting in randomly distributed aluminum-rich area in the weld.