Effect Of Trace Nb On Microstructure And Properties Of Aluminum Alloy Joint With Oscillating Laser-CMT Hybrid Welding

With the development of lightweight structure,aluminum alloy is widely used in aerospace,high-speed train,automobile industry and other fields due to its low density and high specific strength.Belonging to Al-Zn-Mg-Cu heat-treated strengthened aluminum alloy series,A7204P-T4 aluminum alloy is widely used in high-speed train structural components due to its high strength and good extrusion properties.However,welding porosity and softening of the welded joint are the wellknown challenge for aluminum alloy welding.Butt joint of A7204P-T4 aluminum alloy sheet with 6mm thickness was welded by fiber laser-cold metal transition(CMT)arc hybrid welding and oscillating fiber laser-cold metal transition(CMT)arc hybrid welding.Based on optimized welding process,the microstructure porosity and mechanical properties of hybrid welding joints with different niobium(Nb)contents,which the niobium foils with different sizes(laser-CMT:0.74%and 1.36%of weight fraction;oscillating laser-CMT:0.43%0.83%and 1.27%of weight fraction in butt joint by cross section calculation)preset between the butt joint faces were comparative investigated and the mechanism of the softening in the weld zone and the hardening in heat affected zone near the seam were investigated as well.Meanwhile,the metallurgical mechanism of niobium and the influence of niobium on the evolution behavior of tissue properties were analyzed.The experimental results show that well-formed joints were obtained by optimizing welding process parameters.The weld structure is mainly composed of fine crystal regions,columnar crystal regions and equiaxed dendritic regions.The columnar crystals and dendrites disappeared from weld seam after adding niobium in laser-CMT hybrid welding.Furthermore,the addition of niobium can reduced the porosity of the weld effectively.However,certain amount of unfused metal niobium was found in the laser-CMT hybrid welding seam,which volume and quantity increased with the increasing of the addition amount.The average tensile strengths of three types of weld joints with no niobium,0.74%of niobium and 1.36%of niobium were 325 MPa,334.5 MPa and 328 MPa respectively.Compared with the no niobium added,the elongation at break of the sample with 0.74%of niobium increased by about 71%.On the other hand,the plasticity of joint reduced again when increase the niobium adding content as a large amount of niobium got unfused in weld seam.In the oscillating laser-CMT hybrid welding process,the laser oscillating promotes the transformation of columnar grains to equiaxed crystals near the fusion line and the uniformity of microstructure of the weld center.At the same time,the laser oscillating promotes the uniformity of the Mg and Zn elements in the weld and effectively reduces the segregation of the metal elements.The research on the precipitation phase of the weld joint shows that the?'(Mg Zn₂)strengthening phase in the weld is significantly reduced and the harmful precipitation phase is coarsened,which results in a reduction in the microhardness of the weld zone.The precipitation strengthening of a large number of Al₁₂Mn precipitation phases and the precipitation in the GPI region make the microhardness of the near weld heat affected zone higher than that of the base metal.Statistics on the porosity of the weld seam show that the oscillating laser-CMT hybrid welding process can effectively reduce the porosity of the welded joint.The average tensile strength,average yield strength,and average post-break elongation of welded joints were 333.3 MPa,261 MPa and 6.3%,respectively,which were 87.8%,85.3%and 41.2%of the base metal's.Compared with non laser beam oscillating welded joints,elongation after breaking of the joint increases by about 80%thanks to its lower porosity.Furthermore,use of oscillating laser effectively promoted the melting of niobium in the weld.When 0.43%of niobium was added to the weld,the average grain size of the fusion zone and the center of the weld reduced to 12?m and 18?m.After adding0.83%of niobium,the average grain size of the fusion zone was reduced to 10?m further.Analysis of the precipitated phase revealed that the initial crystalline phase Al₃Nb is formed in the weld.Heterogeneous nucleation of Al₃Nb and segregation of niobium solute promoted nucleation.Meanwhile,Nb Al₃diffusion distribution suppresses the growth of recrystallized grains in deformed aluminum alloy and promotes the refinement of weld grain.When adding 0.43%and 0.83%of niobium,the average tensile strength of welded joints increased from 333 MPa to 336 MPa and341 MPa respectively.