【作者】 蔡超；

【导师】 史玉升； 宋波；

【作者基本信息】 华中科技大学 ， 材料加工， 2017， 博士

【摘要】 高性能构件多服役于苛刻环境,以超高承载、极端耐热、超轻量化和高可靠性等为指标,是高超飞行器、运载火箭、轨道空间站和核聚变装置等重大装备的核心组成部分。由于其性能受多重因素的耦合影响,因而对其材料特性和制造方法提出了严峻的挑战:材料需具备轻质、高强的能力,制造需实现几何尺寸精确可控的一体化整体成形。传统的高性能材料面临制备与成形分离,流程长,灵活度低等技术瓶颈问题。因此,亟需寻求一种高性能材料的制备与成形一体化方法,以保证构件具有超高且可控的综合力学性能,又具备复杂的整体结构。热等静压(Hot isostatic pressing,HIP)成形技术利用高温高压结合模具控形技术,能使粉末材料快速且整体成形制造与锻件性能相当的复杂零件;同时,能利用可控的加载温度与压力,实现材料的制备与组织调控。该技术已被美国、俄罗斯和英国等制造业发达国家视为具有发展潜力的成形制造技术。然而,热等静压成形制造复杂制件时,制件存在包套加工难、表面质量差、疲劳性能弱、后续涂层制备繁琐、成形材料单一等问题。针对上述技术瓶颈问题,本论文主要展开了复合成形方法、型芯材料、工艺、一体化成形方法和高性能材料制备五方面的研究。具体研究工作及成果如下:针对外部复杂包套难制备,异质包套难除去,易污染制件表面等问题,提出利用激光选区熔化(Slective laser melting,SLM)/热等静压(Hot isostatic pressing,HIP)复合整体近净成形复杂零件的新方法,利用SLM制造HIP同质包套(与基体粉末材料相同),克服包套制造难的问题,同时免除异质包套去除的繁琐过程,并通过实验验证了该方法的可行性。研究发现无论热等静压温度如何变化,界面处均没有发现明显的因为元素偏析而成形的扩散层,其主要特征是界面出现了明显的组织突变。在950°C,1000°C和1050°C条件下热等静压成形的制件抗拉强度分别为968.3 MPa,948.3MPa和938.5 MPa,延展率分别为16.1%,15.8%和15.1%,但试样始终都在同质包套端断裂。针对内部控形型芯易变形,制件内表面粗糙度难调控,且难去除等问题,提出采用石墨作为热等静压控形型芯材料,利用石墨材料的常温脆性的特性,采用喷砂的方法快速去除。同时,以传统模具材料H13、T8、Cr12钢作为对比材料研究不同型芯材料对制件表面质量的影响,研究发现各型芯对应的制件表面粗糙度分别是1.0±0.3,9.6±1.3,5.4±0.9和1.9±0.6μm,并阐释了高温高压下粉末-型芯间的作用机理与制件表面成形机制,为控制热等静压制件表面粗糙度提供了一定的理论指导。针对制件内部粉末颗粒边界难消除,疲劳性能无法满足航空航天标准等问题,提出先温后压的异步加载新工艺,系统研究对比了传统同时升温升压的同步加载工艺与新工艺的在表面质量、微观组织和机械性能上的差异。研究发现,与同步加载工艺相比,异步加载制件表面粗糙度从10.8μm提升到1.0μm。制件都是由等轴晶和板条组织构成,但是在同步加载制件中能发现明显的原始粉末颗粒边界。所以导致尽管异步加载制件中的平均晶粒大小为7.02μm高于同步加载制件的5.53μm,但是异步加载制件的疲劳极限为450 MPa,比同步加载制件高出30%。主要因为异步加载工序利用高温先降低了材料的屈服强度,增强了塑性变形能力,使得粉末颗粒表面氧化层充分破碎,减小了氧化层对其冶金结合的影响,极大提升了热等静压制件疲劳性能,进一步推动热等静压整体成形技术在航空航天上应用的可能性。针对复杂零件涂层难制备,提出涂层制备与复杂零件一体化成形的新方法,并以Ti-6Al-4V作为成形材料,石墨作为控形型芯,Ni作为预涂层,研究该方法的可行性与功能涂层的耐磨性能。在热等静压高温高压的作用下,石墨表面的Ni预涂层扩散到Ti-6Al-4V制件表面,形成主要由TiNi与Ti2Ni构成厚度大约150μm致密的扩散层。扩散层表面微观硬度高达6.01 Gpa,而微观硬度由表面向内部逐渐降低。扩散层的平均摩擦系数是0.471,比无扩散层平均摩擦系数(0.537)降低了13.7%。并且,扩散层能显著改善Ti-6Al-4V制件耐磨性能,其磨损率为2.90×10-3 mm3N-1m-1,比无扩散层制件的磨损率1.26×10-3 mm3N-1m-1降低了2.3倍。Ti-Ni扩散层在摩擦磨损过程中主要经历了塑性变形、氧化磨损和黏着磨损。针对目前我国近α型钛合金高温抗氧化性能差,自主研制了一种Ti-Al-Sn-ZrMo-Nb-Ta-Si系近α型高温钛合金,并在具有代表性的热等静压温度(900,950,1000°C)条件下,系统研究了新型近α型高温钛合金的致密化行为、相变、组织演变和高温拉伸性能。结果表明大量棒状S2型硅化物和α2相(Ti3Al)在基体α相中析出。随着热等静压温度的升高,制件的微观特征经历了一个连续的变化:板条状组织+少量等轴晶→等轴晶+少量的板条状组织→全等轴晶。此外,制件的晶粒大小也随着热等静压温度的升高而不断长大。在整个高温拉伸测试温度范围内,等轴晶加少量的板条状组织表现出最高的拉伸强度,与英国近α型高温锻态钛合金IMI 829在600°C下拉伸性能相当。而在延展性方面,制件的延伸率随着热等静压温度的升高而连续增加。针对钛合金抗磨损性差,摩擦系数不稳定,高温强度低,设计并在线制备具有新颖空间网状结构的TiB/Ti-6Al-4V复合材料,系统研究了不同TiB增强相含量对基体微观组织、硬度、摩擦磨损和常高温拉伸性能的影响。实验结果表明,随着TiB2添加含量的增加,TiB-Ti复合材料体系的空间网状结构组织经历了有趣的演变:连续的Ti-6Al-4V基体+非连续的TiB晶须增强相→准连续的Ti-6Al-4V基体+准连续的TiB晶须增强相→不连续的Ti-6Al-4V基体+连续的TiB增强相。随着TiB2添加量的增加,制件的维氏硬度由基体材料的304.9 HV梯度增加到347.3,433.9,566.3 HV。复合材料空间网状结构附近的纳米硬度,随着距离网状结构距离的增加,而逐渐降低,同时在网状结构交点处具有最高的纳米硬度。TiB2添加量为0,3,5和8 wt.%制件的平均摩擦系数分别为0.4823,0.3951,0.3695和0.3191。磨损量分别为5.6,4.6,1.9和1.6 mg。复合材料的常温拉伸性能随着TiB2添加量的增加而增加,但是当TiB2添加量为8 wt.%时,制件抗拉强度却陡然下降。在高温拉伸性能上,复合材料在温度高于基体材料200°C条件下,依然保持与基体同等水平的强度。TiB增强相对Ti-6Al-4V合金基体的强化主要归功于:承载与载荷传递机制、细晶强化机制和位错强化机制。更多还原

【Abstract】 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.更多还原