| TC11 titanium alloy due to its high specific strength,strong corrosion resistance,good heat resistance,long-term service at high temperature of 500℃ and other excellent characteristics,is widely used in precision equipment with harsh working conditions,such as aircraft engine blades,pressure plates and other structural parts.However,due to the high deformation resistance of titanium alloy,sensitive to casting temperature and strain rate and other physical properties,its mechanical processing and casting hot forming are relatively poor.Therefore,the laser-MIG arc composite fuse additive manufacturing method of 3D printing TC11 titanium alloy can realize high efficiency,rapid forming and control of microstructure and mechanical properties,greatly shorten the industrial production cycle,ensure manufacturing accuracy and improve manufacturing efficiency.However,there are few researches on microstructure control and coupling mechanism between laser and arc in this new composite additive manufacturing technology,and there is no mature theoretical guidance.In this thesis,the laser-MIG composite additive manufacturing process and microstructure properties of TC11 titanium alloy were studied.The mechanism of the main process parameters on the droplet transition behavior was observed and analyzed by means of high-speed photography method,and the process parameter window of additive manufacturing was optimized.The influences of different process parameters on the macroforming,microstructure,texture orientation characteristics and mechanical properties of additive thin-wall were studied.Finally,the "solution + aging" heat treatment was carried out on the thin wall,and the influence of different solution temperature and solution cooling rate on the microstructure transformation and mechanical properties was investigated.The main conclusions are as follows:The influence of the main process parameters of laser-MIG composite additive manufacturing on the formation of sedimentary layer was studied by the control variable test method.The laser power increased from 500 W to 2500 W,the keyhole effect was enhanced,the penetration depth increased from 1.9mm to 4.7mm by 147.4%,and the droplet transition frequency increased by 93.3%.When the wire feeding speed increases from 4.5m/min to7.0m/min,the morphology of the deposited layer increases proportionally,the droplet transition frequency increases by 89.6%,and the droplet size decreases by 50%.Scanning speed is too large or too small,will lead to uneven spread of sedimentary surface.When the laser power 1000～2000W,wire feed speed 5.0～6.5m/min,scanning speed 0.5～0.7m/min,optical fiber spacing 2mm,the deposition layer forming is better.The optimized process parameter window was used to realize the additive manufacturing of thin-wall TC11 titanium alloy with unequal thickness.The surface of the deposition layer was smooth,the droplet transition process was stable,no obvious splash particles,no metallurgical defects such as unfused,cracks,and the deposition efficiency ηDE could reach a maximum of 4.5kg/h.As the total heat input of Q increased from 4.48 k J/cm to 6.6k J/cm,the deposition efficiency increased,the average melting width increased by 57.5%,the average story height reached 2.79 mm,but the convexity of the side wall also increased by64.3%.When the ratio of Q arc heat input to Q laser heat input is large,the side wall convexity is relatively high,all greater than the average Ra=0.54.When the process parameters are P=1500W,Vf=6.5m/min,Vs=0.6m/min,the deposition efficiency can be ensured,ηDE=4.37kg/h.And the convexity of side wall is only 0.5,the comprehensive forming is the best.The microstructures of TC11 sediments were observed and analyzed by OM and EBSD.The results show that the walls are mainly composed of primary α phase,slatted β phase and secondary acicular martensite α’ under different heat input.With the increase of heat input,the secondary acicular α phase preferentially precipitates at the grain boundary of β phase and grows intragranular,forming a large number of lamellar Weil structures.In the wall,the structure distribution is uneven at different deposition heights.The top layer is dominated by Weil’s structure and acicular α’ with large length and width,while the bottom layer is dominated by α+β basket structure and α clusters.In the overlapping area between the wall layers,a large number of fine basket structures are distributed,which are closely arranged with each other,accompanied by acicular martensite α’ phase cross distribution.Alpha is more[1 21 0] and [011 0] grain orientation,and given priority to with large Angle grain boundaries,smaller values of the notes,small residual stress,dislocation density is low.With the decrease of heat input,the average grain size decreases from 4.7μm to 3.1μm,the grain size decreases by 34% and the texture strength increases by 26.2%.OM,SEM,EDS and XRD were used to analyze the microstructure of different "solution+ aging" heat treatment processes.The results show that the fusion lines between layers gradually disappear after heat treatment,the microstructure morphology of overlapping area and sedimentary layer at different heights is more uniform,and the microstructure is decomposed.The microstructure is mainly netted.As the solution temperature increased from950℃ to 990℃,the content of α phase decreased by 49.4% from 47.26% to 23.93%,and a large number of α to β changes,and the average thickness of α phase increased from 0.62μm to 2.34μm.When the solution cooling rate decreased,acicular secondary α’ phase was precipitated in β phase,which was arranged in parallel.After heat treatment,the hardness distribution is more uniform,with an average distribution between 340HV-380 HV,and the longitudinal hardness fluctuation of the wall is reduced by 72% compared with the deposition.The maximum tensile strength is 811 MPa at 950℃1h+AC-530℃6h+AC heat treatment,which is 18.9% higher than that of the deposition.When the heat treatment process is990℃1h+WQ-530℃6h+AC,the plasticity of the heat treatment state is the best,and the shrinkage of the section is increased by 44.6% compared with that of the #A deposition state.After heat treatment,the maximum impact toughness can reach 50.3J/cm2,which is increased by 71.1% compared with the deposition,and the impact anisotropy is decreased by 63.9%.The comprehensive properties are obviously improved,and the structure composition and properties are homogenized. |
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