Study On Technique And Mechanism Of Laser/DP-MIG Hybrid Welded Aluminum To Steel

In the automotive industry,the aluminum/steel joining is a significant technique for the application of aluminum/steel hybrid body.It is of great importance to accomplish lightweight manufacturing and improve quality and reliability.However,the aluminum/steel dissimilar metals have great differences in physical properties including low mutual solubility,thermal conductivity and melting points as well as thermal expansion,which is easy to generate brittle intermetallic compounds（IMC）and residual stresses.The poor mechanical properties greatly limit the application level of aluminum/steel dissimilar joints in automobile and ship fields.In this work,the Al-Si coated boron steel and 5052 aluminum was selected as base metals,the laser-double pulse MIG（laser/DP-MIG）hybrid welding-brazing was developed to achieve the excellent Al/steel dissimilar joint.The applicability and reliability of this process was verified.The following studies including process parameters,optimization,microstructure,interface structure,fracture behavior,IMC growth and regulation theory by means of the methods of experiments and theories.（1）The weld formation,interface structure and mechanical properties of dissimilar Al/steel joint by laser/DP-MIG hybrid welding-brazing,laser-MIG hybrid welding-brazing and traditional MIG welding-brazing were comparatively studied at the same heat input.The results show that the laser/DP-MIG hybrid welding-brazing can suppress the formation and growth of IMC,improve the wettability and formation quality,and reduce welding defects such as cracks,pores and weld overburnt.A better Al/steel joint with satisfactory strength could be obtained by laser/DP-MIG hybrid welding-brazing.（2）The effects of welding parameters on weld formation,IMC thickness,mechanical properties and porosity of laser/DP-MIG hybrid joint were studied,and the process window was optimized.The spread length and the IMC thickness can be adjusted by varying the process parameters,mainly determining the tensile-shear strength of the joint.The three factors,i.e.,IMC layer thickness,wetting length and porosity,play a significant role to determining the joint tensile-shear strength.The mathematical relationship between factors and tensile-shear strength is established at the different fracture modes.In interface fracture（IF）mode,there is a positive correlation between joint tensile-shear strength and wetting length.The maximum critical wetting length D_CR to occur IF mode is 4.8 mm.In the melted zone fracture（MF）mode,there is a negative correlation between the tensile-shear strength and the porosity percentage.When the porosity percentage of joint was below 3%,an excellent tensile-shear strength of 170 MPa could be achieved.（3）Based on the response surface method（RSM）,the mathematical models of tensile-shear strength and porosity percentage were established and verified,respectively.The multi-objective optimization was carried out by genetic algorithm（GA）.The optimized process are 464 W≤P≤543W、61.1 A≤I≤62.5 A、3.49 Hz≤f≤3.73 Hz、9.74 mm/s≤v≤10.32mm/s.An excellent tensile-shear strength of 170 MPa and porosity percentage below 2%could be achieved.The latter could reach more than80%of the tensile-shear strength of 5052 aluminum alloy.（4）The fracture failure of the joint mainly occurred at two weak positions:the pore concentration area and the brazing interface.There are three fracture modes:interface fracture（IF）,melted zone fracture（MF）and partial interface-partial melted zone fracture（PIF-MF）.The IF mode is mainly brittle fracture.Only a few dimples are found at the root of the fracture.The initial crack originates from the notch at the root of the weld and then propagates to the head interface along the IMC/steel interface.The MF mode exhibited a mixture of plastic fracture and brittle fracture.The major area of fracture occurs plastic fracture due to the presence of dimples,while the only minor area in the middle of the fracture occurs cleavage surface due to the presence of smooth cleavage surface.The initial crack originates from the root notch,extends to the fusion zone,and accelerates propagation along the pore during the pore concentration area.In addition,the critical load conditions of different fracture modes are given.When the effective wetting length is less than 4.8 mm,the joint fracture is mainly IF mode;When the effective spreading length is more than 4.8 mm,no matter how large the porosity is,the joint mainly fracture in the melted zone;When the porosity is less than 2%and the effective wetting length is between 4.5 mm and 5.1 mm,the mode competition may lead to the occurrence of PIF-MF mode.（5）The thickness and shape of IMC layer are non-uniform distribution characteristics.The IMC at the head interface is composed ofτ₅-Fe₂Al₈Si andθ-Fe（Al,Si）₃;The IMC at the center interface is mainly composed ofτ₅-Fe₂Al₈Si、θ-Fe（Al,Si）₃、η-Fe₂（Al,Si）₅.Theξ-Fe（Al,Si）₂ precipitates are dispersed inη-Fe₂（Al,Si）₅ phase;The phase at the root of the interface isτ₅-Fe₂Al₈Si.The formation and growth of IMC is simultaneously determined by Gibbs free energy and formation temperature.The growth sequence of IMC:η-Fe₂（Al,Si）₅ phase is to form firstly,and thenξ-Fe（Al,Si）₂ andθ-Fe（Al,Si）₃,and finallyτ₅-Fe₂Al₈Si.The formation temperature from low to high:τ₅-Fe₂Al₈Si、θ-Fe（Al,Si）₃、η-Fe₂（Al,Si）₅.The growth models of IMC at different interfaces were established according to the growth sequence and formation temperature.The theoretical basis of regulating the IMC during laser/DP-MIG hybrid welding-brazing is revealed,and the IMC dendrite fragmentation model by droplet impact force is established.The periodic droplet impact force is mainly determined by four factors:peak current,base current,moving distance and droplet transition frequency.The molten pool oscillates and vibrates,breaking or fragmentizing the extendingτ₅-Fe₂Al₈Si dendrite to suppress the growth rate of IMC.