Study On Heat Transfer,Microstructure And Properties Control Of Dissimilar Metal Joints Of Aluminum/Steel By Dual-Beam Laser Welding

With the rapid development of the automotive industry,energy conservation and emission reduction of automobiles are highly desirable.Lightweighting of the car body is an important method to save energy and reduce emissions for automobiles.The use of dissimilar metal components made of aluminum and steel is a crucial strategy for achieving vehicle lightweighting.However,the huge differences in melting point,density,and thermal properties between aluminum and steel make it susceptible to the formation of rigid and fragile intermetallic compounds in the welded seam,thereby impacting the mechanical properties of the joint.The composition,amount,and dispersion of intermetallic compounds are influenced by the combined effects of the molten pool material and heat.Due to its high energy density,rapid heating,and rapid cooling,a laser beam is an ideal heat source for dissimilar metal welding of aluminum and steel.However,single-beam laser welding may present certain challenges,including elevated peak temperatures in the weld pool,inadequate weld formation,non-uniform microstructure,and insufficient mechanical properties of the joint are prone to occur during laser welding.The utilization of a dual-beam laser can efficiently disperse energy density,reduce the peak temperature of the molten pool,and enhance the formation of welds.However,the heat transfer mechanism of dual-beam laser welding of aluminum/steel is not clear.The welding parameters for dual beam laser welding of aluminum/steel dissimilar metals need to be systematically optimized.The microscopic mechanism needs to be further studied.Therefore,this thesis employs a combination of numerical simulation,laser welding experiments,and thermodynamic calculations to investigate the fundamental principles of dual-beam laser welding of dissimilar aluminum/steel metals.The aim is to establish a theoretical basis for the reliable dissimilar metals joining of aluminum/steel.The impact of beam shaping mode and dual beam laser characteristics on heat transfer during dissimilar metal lap welding of aluminum/steel was investigated through numerical simulation.The findings indicate that,（1）dual-beam serial laser welding offers several advantages,including preheating before welding with a low peak temperature,which can avoid issues associated with high heating speed and peak temperature of the single beam,and can overcome the shortcomings such as small welding penetration.（2）Since the main beam is fronted.The energy ratio between the main beam and the auxiliary beam is 7:3.The center point distance is 0.6 mm.The temperature field has characteristics of low peak temperature,significant preheating effect,and weak interference between the two laser beams.It is important for the stability of the welding pool.（3）The placement method of"aluminum above and steel below"is adopted,and the light spot covering 1/3 of the aluminum alloy,which has the characteristics of sufficient joint fusion,low peak temperature,and a slow cooling rate during the overlap welding of aluminum/steel.（4）The arrangement of"steel above and aluminum below"can enhance the formation of welds during the deep penetration lap welding technique.The filler material can effectively decrease the maximum temperature and rate of cooling in the weld zone.The dissimilar metal end lap joints of aluminum/steel were taken as the research object.The effects of multiple factors on the stability of laser wire filling,weld pool stability,weld formation,and microstructure evolution were explored.The results showed that:（1）A good match between laser power,wire filling speed,and welding speed is an important issue for achieving promising weld formation.When the wire filling speed is too small and the welding speed is too large,it is prone to form discontinuous"humps"of the weld zone.（2）During the welding process,the welding wire undergoes five stages:"large curvature curling→end roughening→stable melting→end roughening→small curvature curling".The molten drop gradually changes from"droplet transition"to"liquid bridge transition"as the laser power increases.The dissimilar metal deep penetration welding of aluminum/steel was taken as the research object.The effects of no filler material,Al-Si powders,and Fe Co Cr Ni Mn powders,as well as the laser powers on the weld formation,microstructure evolution,and mechanical properties of the welded joints,were investigated.It was found that:（1）There is a welding parameters window within the range of high and low line energies as the deep penetration laser welding without filler material is performed.The joints obtained with low line energy have the best shear resistance of 130.70 N/mm.（2）Similar to the Al-Si filler powders,multi-principal powders of Fe Co Cr Ni Mn cannot inhibit the generation of Fe-Al intermetallic compounds in the weld zone.There are a large number of intermetallic compounds such as Fe₂Al₅ and Fe₄Al₁₃ in the mixed zone near the aluminum layer.（3）Compared to the Al-Si powders,Fe Co Cr Ni Mn powders are beneficial for improving weld formation,reducing microhardness values,and improving joint shear properties.Moreover,the weld formation and joint shear resistance are better by using a low heat input in welding.Based on the thermodynamic calculations,the influence mechanism of Fe-Al intermetallic compounds on the fracture of welded joints was investigated.It was found that the Gibbs free energy of Fe-Al intermetallic compounds decreased first and then increased as the temperature is increasing.Fe₄Al₁₃ and Fe₂Al₅ were mainly formed in the weld zone of dissimilar metal joints of aluminum/steel.The content of the former was much higher than that of the latter.Moreover,Al-rich intermetallic compounds such as Fe₂Al₅ and Fe₄Al₁₃ have relatively high hardness and brittleness,resulting in fracture in the mixed zone near the aluminum layer for the dissimilar metal joints of aluminum/steel.