A Study On Macro/microstructure Evolution And Mechanical Properties Of Wire And Arc Additive Manufactured TC4 Titanium Alloy

TC4 titanium alloy is a medium-strengthα+βtwo-phase titanium alloy with many advantages,such as high strength-to-weight ratio,low density and excellent corrosion resistance.It has become the most widely used and studied titanium alloy in the aerospace field.In recent years,with the rapid development of metal additive manufacturing technology,wire and arc additive manufacturing（WAAM）has lower cost and higher forming efficiency than laser and electron beam additive manufacturing,and has gradually become an important additive manufacturing method for large and complex titanium alloy components.Generally,the typical WAAMed macrostructure of TC4 titanium alloy are the epitaxial coarse columnarβgrains,which leads to strong mechanical anisotropy and greatly hinders the applications of WAAMed TC4 titanium alloy in the aerospace field.Additionally,additive manufacturing is a non-equilibrium process that has experienced rapid heating and cooling.There is a large residual stress inside the formed parts,and the microstructure is usually metastable.The thermal history difference experienced by different locations causes the inhomogeneous microstructure.Therefore,it is of great significance to eliminate coarse columnarβgrains and to stabilize and homogenize the intragranular microstructure.Based on the current problems,this study focused on the control ofβgrain morphology in WAAM,investigated the influence of deposition parameters and deposition strategies on the evolution of the as-deposited macro/microstructure and the mechanical properties,and clarified the formation mechanism of equiaxedβgrain and the effect of thermal history on intragranular microstructure.On this basis,the influence of subtransus heat treatment on the microstructure and mechanical properties was further studied.The mapping relationship between the macro/microstructure and the mechanical properties was clarified by the differences in theβgrain morphology,and the differences in the microstructure characteristics between the deposited state and the heat-treated state.The main results achieved in this study are as follows:（1）The typical macrostructure characteristics of TC4 titanium alloy manufactured by WAAM in this study were as follows:there were a certain range of equiaxedβgrains at the bottom of the WAAMed TC4 titanium alloy wall,and the middle and upper parts were epitaxially grown columnarβgrains whose growth directions were inclined in the opposite direction of the scanning direction.From outside to inside of theβgrains,there were the continuous grain boundariesα(α_GB),Widmanst（?）tten grain boundaryα(α_WGB)and intragranular basketweaveα（α_B）.The face-centered cubicα/βinterface phases were observed.（2）When depositing with a large deposition current（140 A）,the large stress and strain in the bottom of the deposited wall induced the recrystallization and the columnarβgrains transformed into equiaxedβgrains.With the increase of the deposition height,the stress and strain decreased sharply and the columnarβgrains were retained,the texture of equiaxedβgrains formed at this moment was strong.The maximum texture intensity on（100）βphase pole figure was 8.2 times random,and the maximum texture intensity of the columnarβgrains was 20 times random,so the texture intensity of the recrystallizedβgrains was significantly reduced.When depositing with a low deposition current（120 A）,the volume fraction of equiaxedβgrain increased significantly.This was due to the fact that more wire was inserted into the molten pool at this time.On one hand,the unmelted wire residue in the molten pool might induce new nucleation;on the other hand,the insertion of cold wire from the top of the molten pool could also decrease the temperature of the melt at the top of the molten pool,which was conducive to the lateral growth of theβgrains at the top of the molten pool,and thus hindered the epitaxial growth of the columnarβgrains at the bottom.The competitive growth ofβgrains in both directions led to the formation of equiaxedβgrains.The equiaxedβgrains formed at this moment had a strong cubic texture,and the maximum texture intensity on（100）βphase pole figure was 23 times random.（3）The size of equiaxedβgrains obtained by using the typical deposition parameters（Electrical current:120 A,Scanning speed:250 mm/min,Wire feed speed:2000 mm/min）in this study was 1414±793μm,the width of columnarβgrains was 2809±805μm.The size ofβgrains decreased with decreasing deposition current,and increasing scanning speed and wire feed speed.The width ofαlath was 1.03±0.13μm,and it increased with the increasing deposition current,scanning speed and wire feed speed.The tensile strength and elongation of the as-deposited samples were 809±11 MPa and 11.47±2.04%along the scanning direction,and 780±15 MPa and 13.91±2.49%along the building direction.The decreasing depositing current,increasing scanning speed and wire feed speed could improve the strength,but the elongation was decreased.The tensile properties of the deposited walls along the scanning direction and the building direction shown that the anisotropic difference of strength and elongation of columnarβgrains were 2.75–3.95%and 14.37–23.35%,respectively,while the anisotropic difference of strength and elongation of equiaxedβgrains were 0.29%and 9.62%,respectively.The anisotropy was significantly decreased.（4）The bidirectional scanning led to vertically grown columnarβgrains.Increasing the interlayer dwell time led to increasing stress and strain,and promoting recrystallization to form equiaxedβgrains.The layer bands were tilted and horizontal with the bidirectional and unidirectional scanning,respectively.As the interlayer dwell time increased,the tensile strength increased,and the elongation decreased.The optimal tensile strength and elongation along the scanning direction were 853±9 MPa and 15.51±2.0%,respectively.（5）The microstructure obtained by using the typical deposition parameters of this study was not substantially changed after annealing of 600℃/4 h/AC,and theα/βinterface phase was partially decomposed;the intragranular microstructure was coarsened after annealing of850℃/2 h/AC,a very small amount ofα_s was generated,and theα/βinterface phase reformed during air cooling.There were the discontinuous coarseα_GB,α_p,andα_safter solution and aging of 930℃/1 h/AC+550℃/4 h/AC,part of theα_p presented the crab-like morphology,theα_spresented the Widmanst（?）tten structure,and theα/βinterface phase only appeared aroundα_p;by reducing the cooling rate after solution,there were the discontinuousα_GB,α_p,α′martensite and fine dispersed granularα_safter solution and aging of 930℃/1 h/WQ+550℃/4 h/AC,noα/βinterface phase appeared;by increasing the aging temperature,there were the fine discontinuousα_GB,α_p,and dispersedα_safter solution and aging of 930℃/1 h/WQ+800℃/2h/AC,α_s had various crystallographic orientations,theα/βinterface phase located aroundα_pand part ofα_s.The discontinuousα_GB,dispersedα_s and moreα/βinterface phases made the dislocation distribution more uniform during deformation,the ultimate tensile strength and elongation were 886±8 MPa and 16.60±1.56%,respectively,which increased by 4.6%and36.1%compared to the as-deposited state respectively.The heat treatment study confirmed that the break-up mechanisms ofα_pwas the result of the boundary splitting mechanism and termination migration mechanism.