Study Of TIG Welding Brazing Joining Of 6061 Aluminum Alloy To HSLA 350 Steel And Process Optimization

With rapid economic development,global warming,smog and other issues are increasing.The efficient use of resources,energy conservation and emission reduction are the key methods for alleviating these problems.Aluminum alloys are widely used in automotive,aerospace and other fields due to their light weight,high specific strength and excellent corrosion resistance.However,most of the materials used in practice have to meet both structural properties and high energy efficiency,welding of dissimilar materials of aluminum/steel becomes a hot spot.In this paper,the TIG welding-brazing joining of aluminum alloy/steel was studied,the effect of high-energy shot peening on the microstructure and mechanical properties of the weld was studied.Furthermore,ultrasonic vibration technology was used to assist the aluminum/steel TIG welding brazing joining,and to study its effect on the weld.In this paper,Al-Si12 welding wire was used as the filling material,and the effects of welding current?40-80 A?and welding speed?0.15-0.25 m/min?on microstructure and properties of aluminum/steel TIG welding brazing joint were studied.Under the condition of welding speed of 0.2 m/min:no metallurgical reaction occurred at the interface of aluminum/steel welded joint with welding current of 40 A,the joint tensile shear strength of joint was poor?120 MPa?;When the welding current increased to 60 A,the optimum strength of the joint is 170 MPa,and the intermetallic compound?Al₃FeSi?with the thickness of 4.4?m is formed at the interface of the joint.When the welding current is 80 A,the weld joint with good bonding properties was also obtained,but larger grains were formed in the fusion zone and the intermetallic compound with thickness about 6.1?m was formed.Under the condition of welding current of 60 A:when the welding speed is 0.15 m/min,the joint strength is 165 MPa,and the intermetallic compound?Al₃FeSi?with the thickness about 5?m is formed at the interface of the joint;When the speed is 0.25 m/min,partial metallurgical bonding occurs at the joint interface due to insufficient heat input,and the joint strength is only118 MPa.The best welding parameters of 6061 aluminum alloy/HSLA350 steel are as follows:the welding current is 60 A,and the welding speed is 0.2 m/min.High energy shot peening is an effective and economical treatment to improve the materials strength.The thickness of the deformed layer on the surface of the welded joint increases with the increase of the shot peening pressure,when the shot peening pressure was 0.15 MPa,the maximum tensile shear strength of the joint reached 199.6MPa,which is about 30 MPa higher than the raw joint strength.The deformation layer having a thickness of²40?m was generated in the surface layer of the joint,meanwhile,a grain refining zone is formed on the surface layer near the joint,and the grain size is about 11.5?m.The hardness of the joint surface is 116 HV,which was approximately 1.48 times higher than the joint without HESP treatment.At the same time,residual compressive stress is introduced into the joint,which is greatest at the joint surface.The joint strength and surface hardness were remarkably improved owing to grain refinement strengthening and work hardening.Ultrasonic vibration has a unique advantage in improving the microstructure and performance of molten metal pools.When the ultrasonic vibration power is 0.8 kW,the tensile strength of the joint reached the maximum value of 191 MPa,which is about 20MPa higher than the raw joint strength.Grain refinement and homogenization occurred in the fusion zone,and the grain size?52?m?was significantly smaller than that of the original joint.Meanwhile,the precipitates are broken and dispersed.The coarse column dendrites located in the FL are broken.Finally,the strength of the joint was improved due to grain refinement and dispersing strengthening induced by ultrasonic vibration.An additional longitudinal magnetic field is used to improve the aluminum/steel TIG welding brazing joint.Compared with the original joint,the grain size of the magnetic field assisted joint is 71?m,the grain size is significantly reduced,and the Al₃FeSi phase distributed in this fusion zone is broken and dispersed,and the average size of the Al-Si eutectic is decreasing,which distribution is more uniform;the columnar grains in the fusion line zone are broken into equiaxed grains,which are similar to the microstructure of the fusion zone,achieving a smooth transition from the fusion line zone to the fusion zone;The thickness of the compound layer was reduced from 5?m to 3.5?m.Due to the refinement of the microstructure and the dispersion of the second phase,the microhardness of the fusion zone is slightly higher than raw joint;the tensile shear strength of the aluminum/steel welded joint is 194.5 MPa,which is¹4%higher than the raw joint.