| 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 on?118mm 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. |
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