| The component manufactured by hot isostatic pressing have a uniform microstructureand good mechanical properties,which is widely applied in the special field, such asaerospace engineering. The hot isostatic pressing can be used to achieve the near net shapeof complex structural components, combining with Die and powder metallurgical process. Under high temperature and high pressure during the hot isostatic pressing, metalpowder can be densified into complex parts with certain shape in a short process. But as inthis process, there is large contraction and irregular deformation of powder, therefore,applying the numerical simulation method to predict deformation is crucial to the controlof forming precision and performance. This paper theoretically analyzes and demonstratesthe storage and the solving ways of the large sparse equations in the simulation of the nearnet shape through hot isostatic pressing, and thoroughly discusses the densification ofpowder and deformation law in the near net shape of titanium alloy powder through hotisostatic pressing.Two kinds of shape-control molds, namely conformal and inner cores are used in thenear net shape of titanium alloy powder into thin-wall cylindrical cartridge types of partsthrough hot isostatic pressing formation. Simulation and experiment showed that whenconformal mold was used, the overall contraction occurred, the dimensional deviationreached about8%as the cylinder parts offseted inward in a holistic way due to the wallpressure difference. When the internal core mold was used, there were no deformation andmovement inside the parts, therefore the dimensional deviation was lower than1%. Thedensity of parts could reach as mush as99.8%when shaped by near net forming throughhot isostatic pressing. The SEM analysis results revealed that there existed equiaxialphase with thickness of about100um near the container wall, and strip+β phase for therest structure, the strip phase was2-8um wide and20-30um long, and the tensilestrength of parts in the same furnace reached1000Mpa. By comparison, it showed thatwell-designed controlled mold can be made into thin-walled tubular casing made byapplying near net forming through hot isostatic pressing. Studies also showed that it is dueto the controlled mold’s good control of powdered shrinkage in the hot isostatic processunder certain dimensional accuracy and the absence of dimensional deviation that therequirements of near net forming of shape were met by the high accuracy of dimensions ofparts using the internal core mold.The paper also studies the microstructure and macroscopic mechanical properties ofthe finished piece made by titanium alloy powder in near net shape through hot isostaticpressing with two kinds of technologies respectively(910°c,110Mpa&1100°c,110Mpa). When preserved for3hours under the setting of110Mpa and1100℃, micro-organizationof the finished piece was in the form of the relatively thicker strip of+β organizationwhich were uniform, aeolotropic, hence leaving the piece no obvious direction, no holes,no crack or other flaws. The size and length of grain were20um and30um respectivelyand the tensile strength reached920Mpa. The macro mechanical performance exceededcasting level and approached forgings level. When preserved for3hours under the settingof110Mpa and910℃, the microstructure of the finished piece, small and uniform,consisted of some equiaxed+β organization and small, stripy+β organization, withphase-only about15μm long, tensile strength up to1130Mpa, and the intensityincreased by22.8%than the former technology, generally reaching or even exceeding thelevel of forging.Mainly due to the hot isostatic pressing process under high temperature of1100°c, there were over two phases transition temperature and sharp grew up in grains,with the result of mechanical properties decrease.The paper also analyzes the tensile fracture surface and studies the failure mechanismof turning the titanium alloy powder into finished piece through hot isostatic pressingunder the condition of uniaxial tension. Fracture morphology displayed that finished piecemade by titanium alloy powder through hot isostatic pressing had typical ductile fracture,and obvious necking in the fracture and inside had fiber zone, divergence region and shearlip area. Large-sized dimples, with19.2%of elongation as well as contraction of area for46.4%, were found under high power microscopic, which ensured fine plasticity andtoughness of the finished piece made through hot isostatic pressing parts. As the powderparticles produced large local deformation in hot isostatic pressing process, and therecrystallization behavior made powder-particle boundary form equiaxial-phase. At thesame time, the oxide film of powder-particle boundary decomposed under hightemperature, powder original boundary vanished, and the piece made by powder throughhot isostatic pressing gained high ductility and great tensile strength. |
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