Process,microstructure And Properties Of Aluminum Alloy/Stainless Steel Butt Joint Using Laser-metal Inert-gas Welding-brazing

Al/steel dissimilar metal joint is one of the most important lightweight structures,and can be prepared with excellent properties using welding-brazing process.For the Al/steel butt joint using the single-sided arc process,the uneven heated on Al/steel interface and the poor rear heated of steel side limited the properties of Al/steel butt joint.The laser-metal inert-gas（MIG）hybrid welding-brazing was adopted to optimize the heat source distribution of 6061-T6 aluminum alloy and 304 stainless steel butt joint,and improve the poor rear heated of stainless steel with a high welding efficiency.As a result,the uniform and continuous intermetallic compound（IMC）layer located at 6061-T6/304interface was obtained,and a sound 6061-T6/304 butt joint with excellent forming and properties was achieved.The effect of Nocolok flux and Al-Si-Mg foil on the forming,microstructure and properties of 6061-T6/304 butt joint achieved by laser-MIG hybrid welding-brazing process was investigated.The results indicated that the application of Nocolok flux significantly improved the wettability of molten metals on stainless steel surface,and enhanced the stability of MIG arc and reduced welding spatters during the welding process.Furthermore,the tensile strength of the joint using Nocolok flux was better than that of the joint without Nocolok flux.The application of Al-Si-Mg foil had not a significant effect on the forming,microstructure and properties of 6061-T6/304 butt joint.The 6061-T6 aluminum alloy and 304 stainless steel with 2 mm thickness were used as base metals,and the effect of the main welding parameters in laser-MIG hybrid welding-brazing process on the forming and wettability of 6061-T6/304 butt joint was studied.The results indicated that the forming and the wettability of the molten metals on steel surface depended on welding heat input.The spreading widths of molten metals on both sides of steel surface were increased with the enhancement of welding heat input,and the joint-section shape was improved.The precise control of the laser energy acted on steel side was the key of the laser-MIG hybrid welding-brazing technology applying in6061-T6/304 butt joint,and the surface condition of stainless steel and the joint properties were sensitive to the change of laser energy.The laser energy should enable the steel surface to be enough heated while preventing steel surface from melting.As a result,a sound4-mm-thick 6061-T6/304 butt joint with excellent forming and properties was achieved using laser-MIG hybrid welding-brazing process.The IMC layer located at 6061-T6/304 interface was analyzed by scanning electron microscope（SEM）and energy dispersive spectrometer（EDS）.The results indicated that the microstructure and component of the IMC layer depended on welding heat input.When using a low welding heat input,the IMC layer was monolayer structure and consisted of Fe₄Al₁₃.With the enhancement of welding heat input,the IMC layer had transformed gradually into the bilayer structure which consisted of Fe₄Al₁₃3 and Al₈Fe₂Si.The formation of Al₈Fe₂Si layer restricted the growth of Fe₄Al₁₃ layer,and the morphology of Fe₄Al₁₃ layer had transformed into needle-like.The tensile strength of 6061-T6/304 butt joint was tested,and the fracture location and the fracture mechanism of joint were analyzed.The results indicated that the tensile strength of joint depended on the thickness and microstructure of Fe₄Al₁₃ layer.The fracture of the joint without reinforcement occurred and propagated along Fe₄Al₁₃ layer,and the average tensile strength of the joint without reinforcement was improved to 178±7 MPa by optimizing welding parameters.The fracture of the joint with reinforcement initiated at Fe₄Al₁₃ layer,and was divided into three fracture modes according to the spreading width on both sides of steel surface and the pores in both sides of reinforcement.With optimizing welding parameters,the average tensile strength of the joint with reinforcement was improved to 200±4 MPa（76.9%of 6061-T6 tensile strength）,and the fracture occurred at the heat-affected zone（HAZ）of 6061-T6.The H₂O remained in Nocolok flux led to the massive pores occurred in the weld seam of 6061-T6/304 butt joint achieved using laser-MIG welding-brazing process.Due to the application of Nocolok flux was necessary,and the pores can be restricted by enhancing molten pool flow and slowing the cooling rate of molten pool.The porosity of weld-section was applied as the response value,and the significance analysis was adopted to study the effect of main welding parameters on the porosity of weld seam.The results indicated that the effect of welding speed on the response value was the most significant among three welding parameters.Furthermore,considering the interaction among three parameters,the interaction between welding speed and wire feed rate had a significant effect on the response value.