Investigation On Electrically Assisted Superplastic Forming/Diffusion Bonding And Its Mechanism Of Ti₂AlNb Alloy

When the human society and industrial civilization have entered the 21 st century,sustainable development becomes an important theme that many countries are making great efforts on track.Against this background,energy conservation and lightweight have been two critical goals for global auto industry and aerospace industry among great powers.Consequently,lightweight materials and green energy saving technologies become significant directions for Chinese researchers and engineers to delve into.Recently,many lightweight and high-performance materials have been developed.Among them,Ti₂AlNb alloy,as one representative aerospace alloy,is considered as a potential high-temperature material to replace the nickel-based superalloy in the near future.Meanwhile,more and more environmentally-friendly material processing technologies have been developed and applied to commercial production.Among the novel technologies,electrically assisted heating technology boasts its flexible technical compatibility and fast heating feature,and hence the technology has been utilized to combine with many conventional processes（such as stamping,rolling,incremental forming,gas bulging,friction stir welding and pressure welding）.To further overcome the intrinsic defects of superplastic forming（SPF）and diffusion bonding（DB）,and extend the applicable field of Ti₂AlNb alloy,electrically assisted superplastic forming/diffusion bonding（EASPF/DB）process was proposed and investigated in this study.the macro and micro behavior of Ti₂AlNb alloy sheet during electrically assisted heating,electrically assisted superplastic forming and electrically assisted diffusion bonding processes of Ti₂AlNb alloy were systematically studied.Finally,a double layer structure of Ti₂AlNb alloy was trial-fabricated to experimentally validate the electrically assisted SPF/DB.Firstly,this study focused on the static heating of Ti₂AlNb alloy sheet.The results showed that the electrically heating property of Ti₂AlNb alloy was superior.The material can reach the highest equilibrium temperature of over1000 o C under a low electric current density（9.20 A/mm²）.The temperature distribution of the sheet was affected by electric current density and the contacting condition between electrodes.The temperature distribution complied with the general rules of electric current heating,i.e.high temperature in the central zone and low temperature in the surrounding zone.The electric current heating experiment on as-received alloy and B2+O basketweave microstructure under 6.70 A/mm² was conducted,and the results of the comparison with furnace heating showed that high-frequency,low-density electric current showed light impact on O-phase and microstructure of Ti₂AlNb alloy sheet.Secondly,free gas bulging experiments of Ti₂AlNb alloy were conducted using electrically assisted superplastic forming（EASPF）and furnace-heating superplastic forming（FHSPF）,and the EASPF-based product exhibited an excellent high aspect ratio of 0.64 while that of FHSPF-based sample was0.18.The thickness distribution of EASPF-based sample was quite different from the conventional gas bulging sample as the maximum thinning rate of the former was located at the bottom.The gas bulging features were possibly attributed to the continuous changes in temperature and microstructure.The electric current density and dissipated condition during the EASPF process were both complicated and continuously varied in line with the increased forming height or changed shape of the sample.The finite element analysis and actual measurement both showed that the temperature on the top was lower than that on the bottom.Meanwhile,the quantity and size of EASPF-based O-phase evidently increased from the top to bottom and the basket-weave microstructure was distributed in undeformed area.It could be concluded in this study that showed that varied temperature during the process of EASPF facilitated the changes in O-phase such as the precipitation of O-phase or instable microstructure,which finally induced inconstant temperature superplasticity.Meanwhile,this study presented an adaption of electrically assisted diffusion bonding（EADB）process that had been formerly based on spark plasma sintering technology and house-built the novel experimental setup in laboratory and this EADB process can be commercially applied to large-size sheet metal with reduced electric current strength and shorter vacuuming time.Specifically,Ti₂AlNb alloy sheets were completely bonded（including the surrounding zone）at 4.58 A/mm² with the highest heating temperature at over1200 o C.The bonding interface and base material were comprised of large-volume B2-phase grains and lamellar O-phase,and the microstructure was strongly influenced by cooling method.When electric current flew through the micro-cavity,the density and temperature were high at the tip of the cavity and then the micro-cavity transformed to spherical shape.Affected by B2-phase solution strengthening and O-phase precipitation strengthening at4.58 A/mm²,the shear strength of the bonded sample was 587 MPa and the average Vicker hardness was 447 HV.Moreover,to effectively reduce the bonding temperature during the process of EADB and enhance the property ofdiffusion bonding for peripheral zone,commercially pure titanium（CP-Ti）foil was placed between Ti₂AlNb alloy sheets before the experiment.The addition of CP-Ti foil interlayer enabled the interface to be bonded without any cavity and crack subject to 4.17 A/mm².The interlayer zone,transition zone and base material microstructure were B2/β+lath-like α-Ti,B2/β and single B2 grains,respectively.The diffusion speed increased as the electric current density was elevated.The highest average values of shear strength and hardness for the bonded sample with CP-Ti foil interlayer were 641 MPa and493 HV,respectively,both higher than those without interlayer.Finally,the double-layer structure of Ti₂AlNb alloy at a laboratory scale was fabricated,which proved the practicability of EASPF/DB process.The nonuniform heating profile induced gradient temperature during the process of EASPF/DB,markedly affecting the thickness distribution even if it was similar to that formed at uniform temperature.As for the microstructure,lath-like α-Ti precipitates were distributed in diffusion bonded zone and fine needle-like O-phase precipitates were distributed in gas bulging zone.The substrate microstructure of Ti₂AlNb alloy double-layer structure was large grains of B2-phase.At last,the compression test at room temperature for Ti₂AlNb alloy double-layer structure included three stages,i.e.,stable deformation,crack growth and complete fracture.