Study On Microstructures And Mechanical Properties Of Al-Si Coating 22MnB5 Laser Welded Joints

In order to solve the problems of energy and environment,the focus of the experts’ research is to increase the service life of automotive body steel and automotive lightweight.Al-Si coating UHSS not only has extremely high strength,but also has the oxidation resistance and corrosion resistance during high-temperature processing.Laser welding has high energy density,which has gradually become the main processing technology for welding UHSS.Therefore,this paper studied the microstructures and mechanical properties of the Al-Si coating 22MnB5 laser welded joints.In addition,the influence of laser power,welding speed and defocusing value on the joints has also been discussed.The optimal parameters were obtained using orthogonal experimenting,and the performance of the joint after heat treatment was also analyzed.At room temperature,the Al-Si coating 22MnB5 laser welded joint was composed of weld zone,heat-affected zone and base metal,the fusion zone was in fusion line form.The microstructure of the weld zone was a mixed structure of martensite and ferrite.The heat-affected zone could be subdivided into coarse-grain HAZ,fine-grain HAZ and inter-critical HAZ.The microstructure of the coarse-grain HAZ and the fine-grain HAZ was martensite.And the microstructure of the inter-critical HAZ was a mixed structure of martensite,ferrite and pearlite,while the grain size and microstructure were unevenly distributed.The microhardness of the weld zone fluctuated in the range of 489~520 HV,with an average value of 509.8 HV.The microhardness of the coarse-grain HAZ and the fine-grain HAZ was 510 HV.The microhardness of the inter-critical HAZ was 350 HV with a significant decrease.At room temperature,the tensile strength of the joint was 581 MPa,which was 91.7 % of the BM.The elongation of the joint was 18.8 %,which was 86.1 % of the BM.The fracture position of the joint was on the side of the BM with obvious necking.The influence of welding parameters(laser power,welding speed and defocusing value)on the microstructures and mechanical properties of joints was studied.The results show that increasing the laser power has increased the fusion width and grain size of the joints,the tensile strength has peaked and all were fractured in the BM.Increasing the welding speed has reduced the fusion width and grain size of the joints.Tensile strength has shown a tendency to increase first and then decrease.The defocusing value didn’t have an obvious influence on the joints within a reasonable range(-2.0 mm~0),and it wasn’t easily to penetrate through when using positive defocusing value(+1.0 mm).By orthogonal experimenting,the optimal parameters of the laser welding joints were P=2.0 kW,V=50 mm/s and F=-1.0 mm.After heat treatment,the microstructure of the weld zone was a mixed structure of martensite and ferrite,the boundary between the HAZ and the BM was not obvious,and the microstructure of both areas were martensite.The average microhardness of the WZ,HAZ,and BM were 484.6 HV,537.2 HV,and 538.9 HV.The tensile strength of the joint was increased to 1196 MPa,which was 80.4 % of UHSS.The joint has become brittle,and the location of the fracture was the WZ.