Regulation On Microstructure And Properties Of Al-Si Coated 22MnB5 Steel Laser Welded Joint

Automobile lightweight is one of the main ways to achieve the strategic goal of "carbon peak and carbon neutralization".Using Al-Si coated 22MnB5 steel to manufacture structural parts such as automobile body A/B pillar and front and rear bumper can reduce the weight of the automobile while ensuring the safety performance.Among them,scholars at home and abroad have conducted a lot of research and exploration on the degradation of laser welded joint performance of 22MnB5 steel with Al Si coating,which is mainly divided into four aspects:removing coating,optimizing welding process,weld alloying and developing new coating.In this paper,the existing research is analyzed and summarized.In this paper,the existing research was analyzed and summarized.The laser tailor welding of 22MnB5 steel with Al-Si coating was carried out by using fiber laser,and the effect of Al-Si coating on the microstructure and properties of 22MnB5 steel laser welded joint was studied;The effects of nickel content and distribution and CO2 content in shielding gas on the microstructure and properties of laser welded joints of Al Si coated 22MnB5 steel were studied through weld nickel alloying and changing the type of shielding gas(Ar,Ar+20%CO2,Ar+60%CO2,CO2),so as to provide theoretical basis and basic data for the regulation of microstructure and properties of laser welded joints of Al Si coated 22MnB5 steel.The main research contents and conclusions were as follows:(1)Effect of Al Si coating on microstructure and properties of 22MnB5 steel laser welded joint:The uneven distribution of Al led to the disorder of flow on the surface of the molten pool.The violent non periodic oscillation of liquid metal impacts on both sides of the molten pool,making the weld boundary non-linear and the periodic ripple on the weld surface disappear.The numerical simulation results show that Al segregation will occur near the fusion line driven by different orders of speed vectors,which will change the solidification process of the molten pool and promote the coarse Al enrichment δ ferrite formation.The microstructure did not undergo phase transformation during subsequent heat treatment(simulating the hot stamping process in actual production);After heat treatment,the tensile fracture position of laser welded joint appeared in the weld area,which the phase interface between ferrite and martensite is the main channel of crack propagation.Reducing the content of solid solution aluminum in weld or regulating the existing form of aluminum can effectively inhibit coarseness δ ferrite formation.(2)Effect of nickel content on Microstructure and properties of laser welded joints:the addition of nickel can inhibit the effect of Al on solidification processes and promote the formation of austenite.The increase of nickel content can reduce the solidification primary phaseδ ferrite.Further increase of nickel content will change the type of primary solidification phase δferrite(δ ferrite→γ austenite).When the thickness of nickel foil was 60 μm,the structure of the welded joint was full martensite,the strength reached 1670MPa and broke on the base metal.The analysis of experimental results and three-dimensional lattice Boltzmann numerical simulation results showed that the oscillation laser promoted the uniform distribution of Ni and Al in the weld due to the asymmetry of velocity vector and the increase of convection intensity.(3)Effect of CO2 content in shielding gas on the microstructure and properties of laser welded joints:CO2 in shielding gas can reduce the average Al content of weld(1.81wt.%→1.39wt.%),which was due to the ionization and dissociation of CO2 during laser welding,and the Al in the coating reacted with CO2 and its ionized oxygen to form oxide Al2O3,which existed on the weld surface in the form of black particles.However,the segregation of Al and the existence of flocculent floating matter inhibited the further reaction of Al,so δ ferrite content can only be reduced(5.5%),but can’t be completely eliminated.After adding CO2 into Ar,the strength of welded joint increased significantly,and the maximum strength can reach 95%of the base metal.(4)In contrast,nickel alloying can inhibit the effect of Al in weld on the solidification processes of molten pool.When the thickness of nickel foil was 60 μm and the Ni content at 7.5wt.%-10wt.%,the full martensite structure was obtained,and the high-quality laser welding of 22MnB5 steel with Al-Si coating was realized.When pure CO2 was used as shielding gas,the Al in the coating can be transformed into Al2O3 and existed on the weld surface,so as to reduce the content of solid solution Al in the weld,and finally significantly improve the strength of laser welded joint of 22MnB5 steel with Al-Si coating(up to 95%of the base metal).