Microstructure And Mechanical Behavior Of Hot Stamped Laser-Welded Blanks Of Ultra-high Strength Boron Steel With Al-Si Coating

Energy shortage and environmental pollution are the most serious problems facing the society,on the premise of meeting the requirements for automotive property and cost,realizing automobile lightweight is an important way to meet the demand for energy saving and emission reduction.Ultra-high-strength hot-formed boron steel solves the contradiction between the strength of automotive steel plates and hot-forming.The Al-Si coating on the surface of ultrahigh-strength boron steel can effectively prevent oxidation and decarburization in the hot forming process,and make it have good corrosion resistance.Laser welding has the advantages of high energy density,fast welding speed,small heat-affected zone,and high welding seam quality.What’s more,laser tailor welding can weld various steel plate with different thicknesses and coatings,improving automobile structure strength and safety.Laser welded joints with filler wire can improve the weld formation,decrease the welding defects and reduce the assembly precision.The Al-Si coating on the surface of boron steel will seriously reduce the mechanical properties of the tailor-welded joint,and it is necessary to remove the coating by laser ablation technology before laser tailor welding.therefore,the load-bearing structural components of the car body must withstand the effects of high-speed dynamic loads and different cyclic stress loads during the preparation and service process.The service behavior and deformation mechanism are particularly important to ensure the service safety and reliability of the vehicle.Based on the above research background,this paper selects automotive aluminum-silicon coating ultra-high-strength hot-formed boron steels B1500HS,Usibor1500,and automotive advanced high-strength steels MBW500,MBW600 as research objects.The laser ablation technology was used to pretreat the Al-Si coating on the surface of the B1500HS ultra-highstrength boron steel to different variables,followed by laser tailor welding and hot-forming of the similar plate;the Al-Si coating on the surface of the plate was retained,and the non-equal thicknessMB W500/B1500HS,MBW600/B1500HS,MBW500/Usibor1500 and MBW600/Usibor1500 dissimilar joints were prepared by laser wire-fill welding technology.The micro structure、microhardness distribution、tensile properties and fatigue properties at different strain rates with different ablation treatments and different dissimilar joints were studied,analyzing the effect and mechanism of the surface coating state on the micro structure and properties of welded joints.The effects of strain rate on mechanical properties and deformation behavior of Al-Si coated boron steel base metal and welded joints were investigated.The main conclusions obtained in this paper are as follows:after hot forming,the Al-Si coating has obvious delamination phenomenon.From the substrate to the surface,the Al element content gradually increases,and the Fe element content gradually decreases.The main constituent phases are Fe-Al intermetallic compounds and Fe-Al-Si intermetallic compounds.,the Ⅰ layer is the FeAl3 phase from the surface,the Ⅱ layer is the Al5FeSi and Al8Fe2Si phases,the Ⅲ layer is the FeAl3 phase,and the Ⅳ layer is the Fe3Al phase.High-power fiber lasers can be used to weld different ablation treatment plates to be welded to produce joints without welding defects.After the hot forming treatment,the microstructure of the fusion zone of the partially ablated Al-Si coating joints is mainly lath martensite and aluminum-rich lower bainite,and there is a small amount of ferrite at the boundary of the fusion zone,its the average microhardness(524 HV)is lower than the fusion zone(full lath martensite structure,562 HV)of full ablated Al-Si coating joints.At five different strain rates(0.05 s-1,25 s-1,150 s-1,500 s1,and 1000 s-1),the strength and elongation of full ablated welded joints were lower than the partial ablated welded joints,the tensile fracture micromorphology is mainly quasi-cleavage features,with a large number of dimples and ridge.The fatigue limit of full ablation joints(5166.03 N)was lower than that of partial ablation joints(5272.22 N).Laser filler wire welding technology can be used to weided non-equal thickness MBW500/B1500HS,MBW600/B1500HS and equal thickness MBW500/Usibor1500,MBW600/Usibor1500 dissimilar tailor welded joints with good welding quality.After the hot forming,the microstructure of the fusion zones of the MBW500/B1500HS and MBW600/B1500HS joints are lath martensite and polygonal ferrite with different contents and sizes,and their average microhardness is 413 HV and 424 HV.Respectively,the microstructure of MBW500 and MBW600 are composed of martensite and ferrite,and their average microhardness is 321 HV and 343 HV,which are much lower than the microhardness of the fusion zones and the B1500HS lath martensite(608 HV);at five different strain rates(0.05 s-1,25 s-1,150 s-1,500 s-1,and 1000 s-1),the tensile strengths of MBW500/B1500HS dissimilar joints are lower than that of MBW600/B1500HS dissimilar joints during tensile test,but the plasticity is higher,The tensile fractures occurred on the side of the base metal MBW500 and MBW600 with relatively weak microhardness and strength in the parallel section.The microstructure of the tensile fracture was dominated by dimples and the fracture mode was mainly ductile fracture.The fatigue limit(5857.26 N)of the MBW500/B1500HS dissimilar joint is lower than the fatigue limit(5963.33 N)of the MBW600/B1500HS dissimilar joint.After the hot forming treatment,the microstructure of the fusion zones of the MBW500/Usibor1500 and MBW600/Usibor1500 joints are lath martensite and polygonal ferrite with different contents and sizes,and their average microhardness is 380 HV and 430 HV.Respectively.the microstructure of MB W500 and MB W600 are composed of martensite ferrite and pearlite,and their average microhardness is 232 HV and 243 HV,which are much lower than the microhardness of the fusion zones and the Usibor1500 lath martensite(612 HV);at five different strain rates(0.05 s-1,25 s-1,150 s-1,500 s-1,and 1000 s-1)The tensile strengths of MBW500/Usibor1500 dissimilar joints are lower than that of MBW600/Usibor1500 dissimilar joints during tensile test,but the plasticity is higher.The tensile fractures occurred on the side of the base metal MBW500 and MBW600 with relatively weak microhardness and strength in the parallel section.The microstructure of the tensile fracture was dominated by dimples and the fracture mode was mainly ductile fracture.The fatigue limit(6052.22 N)of MBW500/Usibor1500 dissimilar joint is higher than the fatigue limit(5477.16 N)of the MBW600/Usibor1500 dissimilar joint.