| High-performance components(HPC)often serve in a harsh environment and possess the indexes of super high bearing capacity,extreme heat-resisting,ultralight and high reliability.They are the core components of hypersonic vehicle,carrier rocket,orbital space station and nuclear fusion device.Due to their performance influenced by multiple factors,material characteristics and manufacturing method for HPC have to face severe challenges: material needs to have the ability of light and high strength,and manufacture needs to realize integral forming with precisely controllable geometrical dimension.Traditional highperformance materials face some technological bottleneck,such as the separation between material preparation and forming,long process and low flexibility.Therefore,it is urgent to find an integral method for the material preparation and forming,which can ensure controllable high mechanical properties and complex-shaped monolithic structure.Near-net shape hot isostatic pressing(NNS-HIP),combined with tooling development,has been widely applied to fabricate fully dense parts with complex geometry from metal powders,exhibiting excellent mechanical properties comparable to those of parts fabricated by forging technology.Moreover,it can realize material preparation and controllable microstructure by adjusting the temperature and pressure.Manufacturing developed countries such as the United States,Russia and the United Kingdom have chosen this technology as a strategic reserve technology.However,HIP technology exists some problems during the fabrication of complex-shaped parts,e.g.,complex-shaped capsule is hard to be fabricated,surface qualities of as-HIPed parts cannot meet the requirement of industry application,the fatigue properties of as-HIPed parts are low,process of subsequent coating preparation is complicated and less kind of materials.In order to solve the abovementioned technical bottleneck,five aspects,e.g.,compound forming method,new controlshaped core material,new process,integral forming method and new materials,are systematically investigated in this thesis.The research work are concluded as follows:For the problem that complex-shaped capsules were hard to be fabricated,removed and easily pollute the surface of as-HIPed parts,slective laser melting(SLM)/ hot isostatic pressing(HIP)hybrid method was proposed to integral fabrication of complex-shaped parts.The new method used SLM technology to manufacture the capsule that its material is the same as the HIPed powder,which can overcome the problem complex-shaped capsule hard to be fabricated and removed.In this study,it has verified the feasibility of the hybrid method.It was found there is obvious interface between capsule and part due to no element segregation over the entire HIP temperature range.The principal character was that there was a sudden microstructure change around the interface between capsule and part.The tensile strength of samples fabricated in 950 °C,1000 °C and 1050 °C is 968.3 MPa,948.3 MPa and 938.5 MPa,and the elongation is 16.1%,15.8% and 15.1%,respectively.But all the samples were fractured on the capsule side.For the problem that control-shaped cores were easily deformed,hard to control the surface roughness and remove,graphite as the tooling material for HIP process was proposed in the first time,and traditional tooling materials T8、Cr12、H13 were chosen for a comparison.The surface roughness(Ra)of Ti-6Al-4V parts fabricated by H13,T8,Cr12 steel and graphite internal core was 9.6,5.4,1.9 and 1.0 μm,respectively.For the net-shape hot isostatic pressing(HIP)process,control of the internal surface roughness of as-HIPed parts remains as a challenge in the practical engineering.the evolution mechanism of the internal surface between Ti-6Al-4V compact and tooling materials,as well as the surface of as-HIPed parts during HIP process were revealed,which provides a theoretical guidance for adjusting and controlling surface roughness of the as-HIPed parts.For the problem that fatigue properties of as-HIPed parts were hard to meet the aeronautical and astronautical standards due to the existence of powder particle boundaries,asynchronous loading HIP process(elevating temperature first and then raising pressure)was proposed,the effects of two different procedures(simultaneous loading: elevating temperature and raising pressure at the same time,asynchronous loading: elevating temperature first and then raising pressure)on the surface qualities,microstructure and mechanical properties of Ti-6Al-4V alloy part during hot isostatic pressing were investigated.It was found that,compared to SL process,the surface roughness significantly improved from 10.8 μm to 1.0 μm.Both of microstructure were composed of equiaxed grains and lamellar structure,but the prior particle boundaries(PPBs)were clearly observed in SL process.It is the reason why the fatigue limit of parts fabricated from AL process was higher than that of parts produced from SL process,although the average grain size of sample fabricated by AL process is 7.02 μm is coarser than that of one fabricated by SL process.For the problem that functional coating was difficult to form on the surface of complexshaped parts,in-situ integrated fabrication of complex-shaped parts with a functional coating during hot isostatic pressing was creatively proposed.Ti-6Al-4V and graphite were chosen as material and tooling material,and Ni is deposited on the surface of graphite as pre-coating for verifying feasibility of the integrated fabrication method and studying the wear properties of functional coating.Under the action of high temperature and pressure,Ni pre-coating diffused to surface of Ti-6Al-4V part during hot isostatic pressing and a dense coating with thickness of approximately 150 μm was formed,which mainly consisted of TiNi and Ti2 Ni phases.The microhardness corresponding to the outermost coating is 6.01 GPa,and the microhardness presents a gradient descent from the coating surface to the interior of the HIP-fabricated Ti-6Al-4V compact.The average friction coefficient of sample with coating is 0.47,which decreased 13.7 than that of sample without coating(0.537).which is much lower than that(0.54)of Ti-6Al-4V compact without Ti-Ni coating.Moreover,The Ti-Ni coating could significantly improve the wear resistance of the Ti-6Al-4V compact.The wear rates of the Ti-6Al-4V compact without and with coating were 2.90×10-3 and 1.26×10-3,respectively,suggesting the wear rate with Ti-Ni coating can be decreased by 2.3 times.For the problem of poor high temperature oxidation-resistance properties for our country’s near-α titanium alloy,a kind of near-α-high-temperature titanium alloy Ti-Al-SnZr-Mo-Nb-Ta-Si was designed and prepared,and the densification behavior,phase and microstructure development,high temperature tensile performance of the novel near-α hightemperature titanium alloy fabricated by hot isostatic pressing(HIPping)at representative temperatures were comprehensively studied.The results indicated that numerous rod-like S2 silicides((TiZr0.3)6Si3)and α2 phase(Ti3Al)precipitated from α matrix.The microstructural characteristics of HIP-fabricated parts experienced a successive change on increasing the HIPping temperature: lathlike structure + little equiaxed grains → equiaxed grains + little lathlike structure → fully equiaxed grains.In addition,the grain size of samples unceasingly growed up with the increase of HIPping temperatures.Over the entire tensile tests temperature range,the equiaxed grains plus little lath-like structure(HIP-B)exhibited the highest tensile strength,which is equivalent to tensile properties of forged IMI 829.As to the ductility,the elongation of specimens increased successively with increase of HIPping temperatures,due to the type of microstructure and the diffusion-metallurgical bond.For the problem of poor wear resistance,unstable friction coefficient and poor high temperature strengths,in-situ synthesized titanium borides reinforced Ti-6Al-4V composites with novel reinforcement architecture were successfully prepared by hot isostatic pressing(HIPing)starting from TiB2/Ti-6Al-4V powder system.The effect of volume fraction on phase transition,crystallographic microstructure,hardness,wear resistance properties,as well as room and high temperature tensile properties of in-situ synthesized TiB/Ti-6Al-4V composite are systematically investigated.The experiment results indicate the microstructural features of the composites experience an interesting evolution with the volume fraction of TiB reinforcement increasing as follows: continuous Ti-6Al-4V matrix + quasi-continuous TiB reinforcement→quasi-continuous Ti-6Al-4V matrix + continuous TiB reinforcement→discontinuous Ti-6Al-4V matrix + continuous TiB reinforcement.With the increased of TiB2 addition,the microhardness of samples increased from 304.9 HV to 347.3,433.9,566.3 HV.The nanohardness showed a gradient decreased tendency from the boundary to the center of network microstructure,and crossover point at the network structure possessed the highest microhardness.The average COF was 0.4823,0.3951,0.3695,0.3191 and the wear mass loss was 5.6,4.6,1.9,1.6 mg for the samples with 0,3,5 and 8 wt.% TiB2,respectively.At room temperature,the tensile strength of Ti Bcontaining samples increases and then hastily decreases when the addition content of Ti B2 reaches 8 wt.%.As to the high temperature properties,the titanium boride reinforced Ti-6Al-4V alloy composites could be increased by over 200 °C while retaining the same tensile strength accompany with a suitable elongation.The strengthening mechanism was mainly attributed to the load-bearing transformation,grain refinement and dispersion strengthening of in-situ formed TiB needles network structure. |
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