Research On Microstructure And Properties Of Additively Manufactured Ti-6Al-4V Alloy Tailored By Electropulsing

Ti-6Al-4V alloys fabricated by selective laser melting（SLM）and electron beam melting（EBM）have been widely studied and applied.The high cooling rate and complex thermal cycling during SLM produce a martensitic structure with high defect content and make the as-built Ti-6Al-4V alloy exhibit low plasticity.EBM Ti-6Al-4V alloys with coarse structure and high porosity content exhibit low wear resistance and fatigue life.Therefore,Ti-6Al-4V alloy workpieces prepared by SLM and EBM usually need to be post-treated to optimize their microstructure and properties to meet additional service requirements.In this thesis,Ti-6Al-4V alloys fabricated by SLM and EBM are post-treated by monolithic electropulsing treatment,heat treatment combined with monolithic electrical pulse and surface localized electropulsing,and the mechanisms of organization and property evolution of the additively manufactured Ti-6Al-4V alloys under these treatments are investigated,and calculations and simulations are applied to explain the results.Finally,the corresponding theories and mechanisms are proposed.The main findings are as follows:（1）The variant selection behavior of SLM Ti-6Al-4V alloy under the electropulsing was investigated,the reasons for the different variant selection behaviors produced by electropulsing and heat treatment were analyzed,and the mechanism of microstructure evolution of SLM Ti-6Al-4V alloy under the electropulsing was elucidated.The electropulsing induced plastic deformation（introduced dislocations and residual compressive stress）inside the priorβgrains,which changed the nucleation and growth of theαvariants during cooling,resulting in a mixed colony and basket-weave structure.The electropulsing optimized SLM Ti-6Al-4V alloy showed a high strength-ductility synergy（yield strength,～925 MPa;elongation,～15%）because the basket-weave microstructure hindered the movement of dislocations and microcracks.The heat-treated alloy exhibited a low elongation（～12%）due to its full colony microstructure.The thermal stability of the colony and basket-weave mixed microstructure obtained by electropulsing was also investigated,and the factors affecting the thermal stability were analyzed to determine the reason for the high thermal stability of the colony and basket-weave mixed microstructure.During stabilization annealing（973 K/16 h）,the increase in the content of theβphase inhibited the growth ofαlath.The stabilization annealed colony and basket-weave mixed microstructure still maintained a high strength-ductility synergy.（2）The evolution of the bimodal microstructure in Ti-6Al-4V alloy during the super-transus electropulsing treatment was investigated,and a novel bi-lamellar microstructure was prepared,with which the alloy exhibited high yield strength（～1188MPa）,excellent tensile strength（～1264 MPa）and good elongation（～13.9%）.The high current density of the electropulsing induced a rapid heating of the alloy and produced a transient phase transformation,which limited the diffusion of alloy elements and the growth of the prior-β.During water cooling,different phase transformations occurred in the Al or V element-enriched regions of one prior-βgrain,which produced a novel bi-lamellar microstructure.This particular microstructure evolution can significantly refine the microstructure of Ti-6Al-4V alloy,resulting in excellent mechanical properties.（3）The evolutions of the annealed and bi-lamellar microstructures in SLM Ti-6Al-4V alloys during super-transus electropulsing treatment were investigated,and a novel bi-lamellar microstructure was achieved,making the alloy exhibit high yield strength（～1118 MPa）,excellent tensile strength（～1280 MPa）,and good elongation（～11.2%）.All the Al-enriched and V-enriched regions transformed into high-temperatureβ-phase after heated by the electropulsing to 1366 K,the fast heating rate and followed water cooling inhibited the element diffusion.Because the martensitic transformation point in the Al-enriched region（Ms,1149 K）is higher than that in the V-enriched region（983K）,and the former transforms into coarser martensitic laths first,and the latter transforms into finer martensitic laths later during water cooling.During tensile deformation,the V-rich fine martensitic lath region and the Al-rich coarse martensitic lath region deformed first,and the Al-rich coarse martensitic lath deformed later,resulting in the novel bi-lamellar microstructure exhibiting better work-hardening capability than the martensitic organization with uniform element distribution.（4）The effect of sub-transus electropulsing treatment on the alloy element partitioning（AEP）in SLM Ti-6Al-4V alloy was investigated,and the influences of AEP on mechanical and corrosion resistance were analyzed.Compared with the conventional heat treatment at the same temperature,the electropulsing heating to 1173K produced a closeβ-phase content;however,the ultra-fast heating process inhibited the diffusion of elements,which in turn suppressed the AEP process of the SLM Ti-6Al-4V alloy and roughly preserved the elemental distribution state of the initial microstructure.In addition,the diffusion distance of the V atom during the electropulsing treatment,although short（～60 nm）,was sufficient to obtain a significant reduction of the high V atom concentration at the originalβ-phase position,which in turn resulted in a bi-lamellar with a small elemental concentration difference.The electropulsing-treated samples with a more uniform distribution of aluminum elements showed significantly higher yield strengths（952 vs.855 MPa）than the heat-treated samples.The SLM Ti-6Al-4V alloy with a small element concentration difference and a small size ofα_p exhibited high charge transfer resistance(R_ct),stable surface corrosion product film,and fewer corrosion pitting sites.（5）The microstructure evolution of the EBM Ti-6Al-4V alloy with surface electropulsing treatment time was investigated in this study,and the mechanism of phase transformation induced strengthening and hole repair were analyzed.The thickness of the hardened layer increased from 1.3 mm to 1.7 mm with increasing treatment time,while the hardness remained at 320-355 HV_0.2.Mathematical simulation indicated that the high cooling rate（>3279 K/s）in the treated zone was enough to result in a martensite transformation,and the compressive stress induced by thermal stress and electrode loading could cause deformation and furtherly improve the strength.More importantly,the combination of the large-scale compressive stress with the current crowding effect induced compression in the hardened zone is a benefit for repairing the pore.（6）The texture evolution of the surface microstructure in EBM Ti-6Al-4V alloy during the surface electropulsing treatment was studied,and the mechanism for the generation of<0001>_α′//TD texture induced by electropulsing was analyzed.Due to the direction parallel to the（0001）face of martensite（α′）in Ti-6Al-4V alloys had the fastest heat transfer rate,and the building direction（BD）during the surface electropulsing possessed the fastest cooling rate,the lowest energy was required for martensite growth with the（0001）face parallel or nearly parallel to the BD direction provided that the Burgers orientation relationship was satisfied.As a result,surface electropulsing induced more martensitic（0001）surfaces paralleled or nearly paralleled to the BD direction,which in turn produced a strong<0001>_α′//TD texture and some primary martensitic laths that could cross low-angle prior-βgrain boundaries.