Study On Processing Improvement Of TIG Welding-brazing,Micro Structure And Mechanical Properties Of Ti/Al Dissimilar Alloy Joints

With the development of science and technology,manufacturing industry has been rapidly advanced.At the same time,the diversified production has challenged the environment protection.Nowadays,lightweight design becomes the major method to energy saving and emission reduction,especially titanium and aluminum alloy have attracted wide attention with their sound physic and chemical properties.However,because of the poor metallurgical compatibility between Ti/Al,prospective Ti/Al joint is difficult to obtain.Thus,the TIG welding-brazing process of Ti6Al4V/Al5052dissimilar alloys was studied in this research,and the sound welded joints in overlap configuration were achieved with assistance of zinc foil interlayer.Comparing the seam spreading behaviors in different experimental parameters,the optimized welding parameters were obtained.The mechanical properties and microstructures of joints were examined to understand the effect of welding heat input.In order to improve the weld quality,the as-welds were treated by high energy shot-peening（HESP）,and the effects of peening pressure on microstructure evolution of seam surface,mechanical property and fracture pattern were studied.Moreover,the strengthening mechanism induced by HESP was analyzed.The wettability and spreadability of liquid metal on the Ti6Al4V substrate improved by using zinc foil in welding process.Welding current,welding speed and filler metal component determined the joint forming,and comparing their effects on joint quality we could come to the conclusion:when the welding current of 80 A,welding speed of 0.2 m/min,AlSi12 filler wire and zinc interlayer were used,the better spreadability and morphology of welded seam were obtained.The fusion zone containedα-Al matrix,Si,Al-Si eutectic and TiAl₃,the brazing zone formed by metallurgical joining of intermetallic compounds（IMCs）.The number and morphology of TiAl₃ were affected by welding current mainly.Especially for brazing,the double-layer TiZn₁₆+TiAl₃ were found in a low current,and the TiAl₃ was the only layer in a high current.When the heat input was insufficient,zinc gathered in Ti/Al interface to form an invalid physic joining.With heat input increased,zinc diffused evenly in the seam in the solid solution and there was no zinc compound observed.But a excessive heat put could lead to the overmuch IMCs to deteriorate the joint properties.After HESP treatment,the surface strengthening of Ti6Al4V/Al5052joint was achieved,the phase components were same as ones before HESP but grain refinement in seam surface could be observed.Following the severe surface plastic deformation,the Si and Al-Si eutectic in near-surface region were broken into small particles and dispersed in matrix metal.Meanwhile,the tensile stress state in the near-surface region of welded seam was shifted to a compressive stress state.The average microhardness in seam metal was 90 HV,and the minimum value was tested in heat affected zone.Affected by IMCs,the microhardness in brazing zone reached 140 HV.The tensile shear strength of welds was influenced by heat input,the average tensile shear strength reached 180 MPa under the current of 80 A,which was equivalent to 80%that of Al5052 base metal.The fracture occurred in welded seam caused by pores and micro crack.After HESP process,the surface microhardness in seam enhanced obviously.In the seam surface,the maximum residual compressive stress located in weld centerline.Along the sample thickness through the weld centerline,the maximum residual compressive stress value reached¹40 MPa and this zone spaced³0μm below the top surface.Under the peening pressure of 0.3 and 0.4MPa,the average tensile shear strength remained at 195 MPa,which was 92.8%of the Al base metal.Due to the introduction of residual compressive stress and reduction of welding flaws,the fracture occurred in Al5052 under tensile.It is believed that the zinc activation in Al-Si alloy decreased the two-phase interfacial tension and wet angle between filler metal/Ti,the spreadability of filler metal was improved,so the better seam quality was achieved.After HESP treatment,these broken Si（Al-Si）particles contributed to the dislocation multiplication and acted as dislocation barrier.The residual compressive stress was induced,and the subgrain boundaries formed,thus the surface microstructure evolution was promoted.The work hardening and fine-grained strengthening occurred in matrix of seam surface enhanced the comprehensive mechanical properties.Meanwhile,the reduction of pores and cracks in seam surface changed the fracture behavior of joints.