| Magnesium(Mg)alloys,which are recognized as the lightest commercially structural alloys,have great develop potential in automobile industry,aerospace field,electronic instruments and chemical engineering due to the extraordinary combination of excellent properties such as high specific strength,high specific stiffness,excellent electromagnetic shielding property and machinability,etc.Besides,Titanium(Ti)alloys are widely used in shipbuilding industry and aerospace series owing to its outstanding combination of low density,high specific strength,excellent corrosion resistance,good plasticity and toughness.In order to exploit the individual properties of both variants,reduce the cost,weight reduction,energy conservation and emission reduction,it is necessary to obtain the welding of Ti-Mg dissimilar metals.However,it is quite challenging to achieve robust joining of Ti-Mg dissimilar alloys owing to the notable mismatch in physical and mechanical properties.Such as,the tremendous difference in the melting point and boiling point between Ti-Mg will cause catastrophic vaporization of Mg when they contact with each other.Besides,overlarge residual stress tends to occur because of the difference in thermal conductivity and thermal expansion coefficient.Tungsten inert gas(TIG)welding-brazing technology is characterized by high efficiency,simple process and well welding quality.During the TIG welding-brazing process,the low melting of base material and filler wire firstly melt and subsequently wetting and spreading on the solid high melting of base plate,resulted in the formation of reliable joint.In the present study,the TIG welding-brazing technology is developed to the welding of Ti-Mg and the effects of welding parameters on the joint appearances,microstructure,mechanical properties and corrosion resistance are investigated.Furthermore,ultrasonic vibration assisted process and high energy shot peening treatment are developed to improve the microstructure and mechanical properties of Ti-Mg joint.The main conclusions are shown as follows:The Ti-Mg dissimilar metals were welded by TIG welding-brazing process and the effects of welding current and welding speed on the microstructure and mechanical properties were studied.Results reveal that,with welding current of 40 A,Mg alloy base metal and filler wire are not completely melting owing to insufficient heat input,which resulted in the occurred of welding pores.With welding current of 80 A,Mg alloy evaporated with overlarge heat input,leading to the formation of pores at the top surface of joint.The maximum joining strength of 193.5MPa was obtained with welding current of 60 A and welding speed of 0.2m/min.The top surface of Ti-Mg joint is characterized by crow's feet morphology and Ti-Mg solution layer with thickness of 5?m is observed at the brazing zone of joint,which is the key to realize the reliable joining of Ti-Mg.However,coarse ?-Mg grains with the maximum size of 200?m form at the fusion zone of joint and fracture occurs at the fusion zone.The Na Cl solution immersion testing shows that the corrosion resistance of fusion zone is poor and pitting corrosion easy to occur and expand to general corrosion.Hydrogen evolution testing and weight loss testing results are consistent with the above immersion testing results.Ultrasonic vibration assisted process is developed to improve the microstructure and mechanical properties of Ti-Mg joint.The results show that,with the aid of ultrasonic vibration,the width of welding seam increases slightly and the coarse grains of fusion zone are refined effectively.With ultrasonic power of 1.2k W,the average grain size of fusion zone is decreased from 200?m to 45?m and a large amount of equiaxed grains appear in the fusion zone of Ti-Mg joint.The maximum joining strength of 228 MPa is obtained and fracture occurs at the heat affected zone.With the increase of ultrasonic power,the further grain refinement is not apparent and pore form at the fusion zone,which tends to decrease the mechanical properties of joint.Ultrasonic vibration has no obvious effect on the brazing zone of Ti-Mg joint.The Na Cl solution immersion testing,hydrogen evolution testing and weight loss testing results indicate that the refinement of fusion zone is beneficial to improve the corrosion resistance.Furthermore,the vertical direction and horizontal direction of ultrasonic vibration have similar influence on the microstructure and mechanical properties of Ti-Mg joint.High energy shot peening treatment is applied in order to further improve the microstructure and mechanical properties of Ti-Mg joint.During the high energy shot peening process,projectiles strike the top surface of joint and resulted in plastic deformation,which could improve the micro hardness,increase the dislocation density and generate strain strengthening.The plastic deformation of surface layer could repair the micro crack of joint surface.Besides,the dislocation movement occur owing to the high strain of surface layer,which resulted in the formation of high density dislocation.Under the plastic deformation,the dislocations accumulate into the subgrain boundary and lead to grain refinement.With shot peening pressure of 0.15 MPa,the maximum joining strength of 241 MPa is obtained and fracture location is at Mg alloy base plate.Similar with the aid of ultrasonic vibration,high energy shot peening treatment has no effect on the microstructure of brazing zone. |
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