【作者】 王跃明；

【导师】 熊翔； 闵小兵；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 钨及钨合金具有熔点高、密度高、强度高、热膨胀系数低等优异性能,随国防工业及国民经济的迅速发展,钨及钨合金薄壁或复杂形状零部件在各部门的应用日益广泛,对其性能及性价比要求也越来越高。然而,经济的高速增长也使得钨资源消耗日益严重,造成了资源的紧缺和价格的升高。此外,传统的成形方法如化学气相沉积、物理气相沉积、自蔓延高温合成、粉末冶金等在制备钨及钨合金薄壁或复杂形状零部件时存在明显的局限性。等离子喷涂成形技术因其具有高温、高速、高效、低成本、操作方便及成形件厚度易控制等技术特点,为钨及钨合金零部件的近净成形提供了一种理想的技术途径。本文采用等离子喷涂技术精密近净成形了钨喷管、钨坩埚、小型钨及钨铼合金发热体、钨药型罩、钨喉衬及95W-3.5Ni-1.5Fe薄壁回转体件,研究了高温烧结、热等静压及旋锻等后续处理对成形件显微形貌、晶粒度、相对密度、显微硬度、拉伸强度、压缩强度及屈服强度的影响规律,并深入分析了各类后处理技术的致密化机制。本文主要研究内容及结论如下：1.系统研究了等离子喷涂工艺参数对钨沉积层显微结构的影响,经优化后的等离子喷涂成形工艺参数为：喷涂功率40kW、喷涂距离150mm、氢气流量50L/h。经检测,等离子喷涂成形各类钨材质异形构件的开孔率、相对密度、显微硬度、拉伸强度及压缩强度分别约为7.5％.88.5％.341.7HV0.025、55.1MPa及581.3MPa。等离子喷涂成形过程中,为防止成形件因其壁厚增加而可能出现多种形式的开裂失效,可采取优化喷涂工艺、间歇式喷涂及气氛保护等有效措施严格加以控制。2.深入研究了等离子喷涂成形钨异形件的热等静压致密化行为及机制。研究发现,随热等静压压力升高及处理时间延长,沉积层中微观缝隙和孔隙逐渐愈合,成形件力学性能逐步提高,其中经二步热等静压处理后成形件致密度、显微硬度、拉伸强度及压缩强度分别增大至96.7%、547.8HV0.025、245.3MPa及1443.5MPa。本文提出了一个由加热、再结晶、层片重排、弹性接触、塑性流动、扩散蠕变等六个阶段组成的热等静压致密化机制。3.深入分析了等离子喷涂成形W/Re合金的“Re效应”。研究发现,微量Re元素的固溶强化作用即可有效抑制高温环境中W晶粒长大,还可改变材料在冷加工过程中的形变机构,不同于纯W构件的脆性断裂,W/Re合金室温压缩过程中出现了明显的屈服现象。此外,等离子喷涂成形W/Re合金的力学性能随烧结时间的延长而增大。经2300℃烧结6h后,W/Re合金显微硬度、拉伸强度、压缩强度及屈服强度分别达490.8HV0.025、384.5MPa、1922.5MPa、947.4MPa,明显高于相同烧结条件下的纯W构件的力学性能。4.采用高温烧结与旋锻加工的技术路线对喷涂成形件进行了致密化处理。结果表明,2300℃高温烧结可使沉积层由层片结构转化为颗粒结构,其相对密度、显微硬度及拉伸强度分别提高至97.9%、381.5HV0.025及228.5MPa。经2道次旋锻加工后钨/钼棒总压缩率达21%,沉积层致密度进一步提高至98.5%,显微硬度显著增大至620.4HV0.025。然而,经旋锻加工后沉积层由表层向内钨颗粒变形程度逐渐减弱,整个样品的变形表现出较大的不均匀性。此外,沉积层经高温烧结后为多面体钨晶粒与孔隙组成的复合体,晶粒内存在少量不可消除孔隙,旋锻加工后沉积层仍达不到全致密。5.等离子喷涂成形钨喉衬进行了地面点火试车实验。钨喉衬抗热震烧蚀性能良好,其线烧蚀率仅为0.08mm/20sec,各项指标均超过同尺寸传统石墨喉衬的使用性能。研究表明,钨喉衬各部位烧蚀形貌及烧蚀机理存在明显差异,收敛段以机械剥蚀为主,热化学烧蚀为辅；喉部则机械剥蚀、熔化烧蚀及热化学烧蚀等三种机理并存,其烧蚀最为严重；而扩散段则为热化学烧蚀,其烧蚀程度最低。6.采用高能球磨、喷雾干燥造粒及感应烧结的工艺路线制备了95W-3.5Ni-1.5Fe复合喷涂粉末,采用等离子喷涂近净成形技术制备了高密度合金薄壁回转体,研究了真空固相烧结及真空液相烧结对其显微结构和力学性能的影响。等离子喷涂成形高密度合金致密度为87.70%、显微硬度为388.0HVo.025、拉伸强度为102.6MPa、延伸率接近零。经1200、1300℃烧结后无液相形成,成形件仍保持原始的层片结构特征；1400℃烧结时仅产生少量液相,难以进行有效的颗粒重排,沉积层仍为层片结构,成形件断口以沿晶断裂为主；1465℃烧结时由于大量液相的产生,颗粒重排迅速发生,沉积层由原始的层片结构转变为类球形钨颗粒与网状Y相组成的两相合金组织,致密度、显微硬度、拉伸强度及延伸率分别提高至98.05%、493.5HVo.025、567.1MPa和5.7%,断口呈现以沿晶断裂为主、穿晶断裂为辅的断裂方式；当温度进一步升高至1485及1500℃时,钨颗粒显著长大,γ相蒸发加剧,成形件断口均以沿晶断裂为主,其显微硬度、拉伸强度及延伸率等性能反而有所降低。提出并验证了一个由早期固相烧结及再结晶阶段、液相生成与渗透阶段、溶解-析出Ⅰ阶段、颗粒重排阶段、溶解-析出Ⅱ、固相骨架形成和晶粒长大阶段等五个阶段组成的等离子喷涂成形高密度合金烧结致密化机制。更多还原

【Abstract】 Tungsten and tungsten alloys are special materials of high melting point, high density, excellent strength, good ductility and toughness. Parts of near-net-shape tungsten and tungsten alloys such as heating element, crucible, rocket nozzle, kinetic energy penetrators, counter weight balances, radiation shields, and electrical contacts have been developed and found wide applications in national defence, chemical processing, mechanical engineering, airplane and aerospace industries. In addition, the consumption of tungsten resource is getting more and more serious for high speed increasing of economy. And now tungsten resource is in short supply, which leads to increasing of unit price. However, conventional industrial methods such as powder metallurgy (PM), chemical vapor deposition (CVD), physical vapor deposition (PVD) and self-propagating high-temperature synthesis (SHS) are difficult to fabricate large-scale or thin-walled W or W-based parts with complex shape due to their ultra-high melting point and high ductile-to-brittle transition temperature. People have long wished to develop a new and effective fabrication method to produce W or W-based parts of desired shapes and density. Capable of making high quality near-net-shape parts, plasma spray forming (PSF) has then come into play as a potential fabrication method of choice for W or W-based alloys. In this work, thin-walled or large-scale parts with complex shape including W nozzle, W crucible, W and W/Re alloy heating element, W shaped charge liner, W throat,95W-3.5Ni-1.5Fe thin-walled cylinder and so on were fabricated by PSF. The relationships between microstructure and mechanical properties including granularity, relative density, micro-hardness, elongation and ultimate tensile strength (UTS) of PSF deposits before and after vacuum sintering, hot isostatic pressing (HIPing) and rotary swaging were investigated in detail. In addition, densification mechanism of PSF deposits was deeply discussed. The main contents and conclusions of this thesis were shown as follows:1. The influences of spray power and spray distance on microstructures of PSF W deposits were systematically investigated. The optimized spray power, spray distance and hydrogen flowrate were 40 kW, 150 mm and 50 L/h, respectively. The pore ratio, relative density, micro-hardness, UTS and compressive strength of PSF W parts were 7.5 %,88.5%,341.7 HV0.025,55.1 MPa and 581.3 MPa, respectively. During PSF, all kinds of cracks were formed owing to increasing of wall thickness of PSF parts. And spraying parameters optimization, intermittent spraying and inert gas (pure Ar or N2) protection could be used to prevent formation of cracks.2. The behaviors and mechanisms of HIPing of PSF W parts were investigated in detail. The relative density of PSF W deposits increased with HIPing pressure and dwell time owing to mitigation or elimination of micro-pores and gaps which resulting in increase of micro-hardness and UTS. After two-step HIPing, relative density, micro-hardness, UTS and compressive strength of PSF parts were increased to 96.7%,547.8 HV0.025,245.3 MPa and 1443.5MPa, respectively. A six-staged mechanism of HIPing for PSF parts including heating up, recrystallization, lamella rearrangement, elastic contiguity, plastic yielding and creeping was proposed and discussed. However, lamellar structure with distinct layer boundaries was retained in HIPed samples. As a result, HIPing temperature should be increased to mitigate or eliminate these defects.3. "Re effects" of PSF 95W-5Re alloys was deeply investigated. We found out that tungsten grain growth in high temperature environment could be controlled owing to fine-grain strengthening effects of Re addition. Differing from elastic deformation of W parts, W/Re alloy products deformed in an elastic-plastic mode under cold pressing. In addition, mechanical properties of PSF W/Re alloys were increased with sintering time. After vacuum sintering at 2300℃for 6h, micro-hardness, UTS, compressive strength and yield strength of PSF W/Re alloys (increased up to 490.8HV0.025,384.5MPa,1922.5MPa and 947.4MPa, respectively) were much higher than those of W parts.4. The PSF W parts were densificated by vacuum sintering and rotary swaging. After sintering at 2300℃for 6h, the deposits with significant grain growth transformed from lamellar into granular structure. And relative density, micro-hardness and UTS were increased to 97.9%, 381.5HV0.025 and 228.5MPa, respectively. The total compression ratio of molybdenum based W deposits was 21% after two step deformation. After vacuum sintering and rotary swaging, relative density and micro-hardness of PSF W deposits increased to 98.5% and 620.4HV0.025, respectively. In addition, the deformation content of W deposits was decreased from the exterior to the interior. As a result, microstructure of W deposits after rotary swaging was inhomogeneous.5. The resistance of thermal shock and ablation behavior of PSF W nozzle thorat was investigated by firing test onΦ118mm solid rocket motor (SRM). After firing test, the specimen has entire form with no macro-cracks being formed, which indicates that W nozzle throat has good resistance to thermal shock and ablation. The line ablation rate of throat was only 0.08mm/20sec. Ablation morphologies and mechanisms of convergence, laryngeal and dilation segments are different for different temperature, velocity and concentration of propellant. The primary and secondary ablation mechanisms of convergence segment were mechanical erosion and chemical ablation, respectively. Ablation of laryngeal segment was the most serious for combinative results of mechanical erosion, melted ablation and chemical ablation. In addition, ablation of dilation segment was the slightest owing to being dominated by chemical ablation only.6.95W-3.5Ni-1.5Fe composite feedstock was fabricated by high energy ball milling, spray drying and induction sintering. Then tungsten heavy alloy parts such as thin-walled open cylinders were fabricated by PSF. Influences of vacuum sintering on microstructure and mechanical properties of PSF deposits were studied.Relative density, micro-hardness and UTS of PSF parts (with no measurable elongation) were 87.70%,388.0HV0.025 and 102.6MPa, respectively. Without liquid phase formation, initial lamellar structure was remained in tested samples after sintering at 1200 and 1300℃. At 1400℃, which is located between solidus-liquidus temperature with certain amount of liquid phase attending sintering, initial lamellae with intergranular rupture was presented in fracture surface. While sintering at 1465℃, initial lamellar structure disappeared within 5 minutes and turned into two phase composites with coarsened spheroidal W grains dispersed and embedded in y phase. Relative density, micro-hardness, UTS and elongation were increased to 98.05%,493.5HV0.025,567.1MPa and 5.7%, respectively. And the fracture surface was dominated by intergranular rupture and ductile avulsion of y phase. The decrease of micro-hardness, UTS and elongation were attributed to quick vaporization of y phase and W particles coarsening as sintering temperatures increasing up to 1485 and 1500℃. And fracture surfaces were dominated by intergranular rupture again.A five-stage sintering mechanism including solid state sintering and recrystallization, liquid phase formation and infiltration, solution and precipitation, particle rearrangement, solution-precipitation and coalescence was proposed and double checked by experimental results.更多还原