Modeling of hot isostatic pressing of metal powder with temperature–dependent cap plasticity model

In this paper, the coupled thermo–mechanical simulation of hot isostatic pressing (HIPing) is presented for metal powders during densification process. The densification of powder is assumed to occur due to plastic hardening of metal particles. The constitutive model developed is used to describe the nonlinear behavior of metal powder. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, powder plasticity behavior, and frictional contact algorithm. A Lagrangian finite element formulation is employed for the large powder deformations. A modified cap plasticity model considering temperature effects is used in numerical simulation of nonlinear powder behavior. The influence of powder-tool friction is simulated by the use of penalty approach in which a plasticity theory of friction is incorporated to model sliding resistance at the powder-tool interface. Finally, numerical examples are analyzed to demonstrate the feasibility of the proposed thermo–mechanical simulation using the modified cap plasticity model in the hot isostatic forming process of powder compaction.     

Neue Kapseltechnik für Diamantverbundwerkstoffe mittels PVD‐Verfahren

Novel encapsulation technique for diamond composites using PVD‐process For machining of mineral materials diamond tools consisting of a steel body combined with diamond impregnated segments are used. Frequently, these segments are hot pressed. Other process routes are pressureless sintering of green compacts partly combined with hot isostatic pressing and hot isostatic pressing of encapsulated powder mixtures. The compaction effect of hot isostatic pressing require a low porosity of sintered components realized by using ultra‐fine metal powder or an impermeable capsule made of metal or glass. The Institute of Materials Engineering pursues a novel process route by physical vapor deposition of a coating on pressureless sintered composites. The thin coating acts as a capsule and guarantees the pressure transfer in the following hot isostatic pressing process. Although bronze powders with particle sizes up to 90 μm are used, the manufacturing of diamond composites with low porosities is possible. In comparison to conventional encapsulation‐techniques the main advantages of this novel process route are the use of comparatively coarse metal powders and a larger geometric flexibility.     

Consolidation of nanoscale iron powders

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.     

Studies on the electric discharge compaction of metal powders

Consolidation and densification of metal powder are usually attained by compaction followed by prolonged high-temperature sintering or by hot pressing. However, these methods are not suitable where grain growth is to be avoided. So electric discharge compaction is being studied so as to establish a fast, cheap, and reproducible process. In this process, high current pulse for a few millisecond duration is passed through metal powder compacts held under pressure in a die. This paper gives the details of the investigation being carried out to establish the process parameters for the densification of titanium, tin and zinc powders. Remarkable improvements in the density and microstructure are noticed after subjection to the electric discharge.     

Densification behavior of aluminum alloy powder mixed with zirconia powder inclusion under cold compaction

Densification behavior of composite powders was investigated under cold compaction. Experimental data were obtained for aluminum alloy powder mixed with zirconia powder inclusion under triaxial compression. The Cap model with constraint factors was implemented into a finite element program (ABAQUS) to simulate compaction responses of composite powders during cold compaction. Finite element results were compared with experimental data for densification behavior of composite powders under cold isostatic pressing and die compaction. The agreement between experimental data and finite element calculations from the Cap model with the constraint factors was good for composite powders with low volume fractions of inclusions.     

Synthesis of Ag-ZnO composites via ball milling and hot pressing processes

Ag — 8 wt. % ZnO composites were synthesized by ball milling, heat treating and hot pressing of silver and zinc oxide powder mixtures. The crystalline size and microstrain of the milled powders before and after heat treatment were determined by Debye-Scherrer andWilliamson-Hall methods. It was shown that heat treatment resulted in decrease of microstrain and increase in the crystallite size of the milled powders. The effect of uniaxial pressure magnitude and duration of hot pressing at 550 °C on the final density of the powder compacts were investigated. The results showed that both plastic flow and atomic diffusion mechanisms affected densification of the composite powders during the hot pressing process. However, the latter one had more effective role on the density of the hot-pressed samples. The synthesized composites showed homogenous microstructure with relatively high density and hardness.     

颗粒增强铝基复合材料热等静压近净成形有限元模拟

目的 建立可靠的模拟方法,以更高效地预测颗粒增强铝基复合材料(PRAMC)粉末热等静压中的形状变化和不同部位致密度的差异,解决传统实验试错方法适用性差且费时费力的问题,满足批量应用的需求。方法 以45%(体积分数)SiCp/6092Al复合材料为研究对象,构建了能预测粉末热等静压成形过程的有限元模型。使用Gurson-Tvergard-Needleman(GTN)模型作为粉末本构模型,建立了粉末尺度的代表性体积单元(RVE)对GTN模型进行修正。结果 通过对比GTN模型计算结果与实验结果,发现修正后的GTN模型能更准确地预测模型的最终变形尺寸,与修正前相比,相对误差降低了1.6%~2.9%。使用修正后的GTN模型对杯形回转体零件的热等静压成形过程进行预测,最终形状的计算结果与实验结果的相对误差仅为0.2%~3.1%,致密度分布的相对误差在0.5%以内。在探究包套厚度对热等静压过程的影响时发现,随着包套厚度的增大,热等静压过程中的屏蔽作用增强,内部粉体致密度下降。结论 为PRAMC热等静压近终形制备的形状和致密度控制问题提供了有限元预测工具,辅助优化了热等静压工艺和包套设计,降低了颗粒增强铝基复合材料热等静压近净成形过程开发的试错成本。     

Effects of lubrication procedure on the consolidation, sintering and microstructural features of powder compacts

The role of lubrication procedure on the consolidation behavior of metallic powders and subsequent microstructural development during sintering was investigated. Iron powder and iron–0.8 w/o graphite powder mixture were used as model materials. The effects of die wall lubrication procedure were compared to the traditional admixed powder lubrication method. The influences of manufacturing parameters such as the compacting pressure in the range of 150–800 MPa and the sintering temperature from 400 to 1300 °C were studied. It was found that the lubrication procedure has a great influence on the consolidation and microstructural features of the materials investigated. Admixed lubricant aids the densification in the low-pressure region but limits the maximum density at high pressure. On the other hand, die wall lubrication offers the possibility of achieving the required density in single pressing for parts made by conventional double pressing and double sintering route or by warm compaction technique. The method also results in the formation of more metal/metal contacts during compacting, which leads to better green strength. Moreover, during sintering at moderate temperatures the area of metallic contacts is more and stronger compared to the powder lubricated specimens. Consequently, better mechanical properties are obtained. However, after sintering at a high temperature (>1000 °C) only less total porosity of the unlubricated compacts attributes to higher performance.     

Physical vapour deposition coating processes for encapsulation of powder metal components before hot isostatic pressing

Abstract

Application of coatings by plasma vapour deposition involving electron beam evaporation and ion plating onto green powder metal compacts has been studied as a potential method for encapsulating powder metal products before hot isostatic pressing. The deposition of defect free coatings is essential if this concept is to provide a reliable encapsulation technique. Coating structures are therefore discussed in terms of the plasma processing conditions and surface roughness of the powder substrate. It is shown that the most promising approach is a combined coating and sinter–hot isostatic pressing cycle, which enables defects within the coating to be removed by the formation of a transient liquid phase.

MST/1455     

Modelling pressure-assisted densification by power-law creep

Densification of ceramic powders by power-law creep during pressure-assisted compaction is analysed. The proposed densification model is based on two existing power-law creep densification models: one for a relative density up to 0.9 (stage I) and the other for densities above 0.9 (stage II). Using these two models independently in their respective density ranges for predicting hot pressing of homogeneous alumina powder results in a discontinuity in the densification rate time history curves as well as in the radial and hoop stress time histories in the compact. To eliminate these discontinuities a novel method of combining the two models into a single unified model is presented. Blending of the models is based on the assumption that porosity changes gradually from being completely open at the beginning of compaction to completely closed at full density. Experimental data generated by hot pressing homogeneous alumina cylindrical compacts at two different temperatures of 1400 and 1450°C at different pressures were used to obtain the material creep constants that were employed in the unified model. This revised version was published online in November 2006 with corrections to the Cover Date.     

Reaction synthesis of titanium aluminides

The formation of titanium aluminides from the elemental powders has been investigated. A traditional powder metallurgy route of compaction (by cold isostatic pressing, hot pressing or hot extrusion) followed by heat treatment was compared with the novel technique of hot extrusion reaction synthesis (HERS). The products from these different production methods were characterised by x-ray diffraction and microscopy (light and scanning electron). The intermetallic compound formed under most processing conditions wasTiAl₃. Only when there was a rapid increase in temperature to high temperatures, as found in induction heating of compacts or in HERS, were the compounds Ti₃Al and TiAl formed.     

Hot isostatic pressing of Y-TZP powder compacts

A fundamental study of hot isostatic pressing (HIPing) was performed on Y-TZP powder compacts, and the effect of the various HI Ping process variables (time, temperature and pressure) was examined on the densification behaviour. The results were analysed using Ashby's HIPing model, which was found to be applicable for this system. The densification was found to be governed by the grain-boundary diffusion of cations. The use of grain-boundary transport characteristics determined in this study enabled HIP maps to predict the densification behaviour of HIPing within an accuracy of a factor of three for all stages of densification, except for the results at low HIPing pressure and the region of near full density.     

Mechanisms of Polycrystalline CVD ZnS Densification during Hot Isostatic Pressing

The densification kinetics of CVD ZnS are studied during hot isostatic pressing by monitoring the porosity of the material. The process is shown to be accompanied by significant microstructural changes. The results indicate that, during hot isostatic pressing, both the diffusional coalescence and plastic deformation mechanisms are operative.     

Dynamic consolidation/hot isostatic pressing of SiC

Shock consolidation is a method that presents a bright potential but has been limited by inevitable cracking of compacts, especially for ceramics. In an effort to eliminate cracking while retaining the unique features of shock consolidation, three novel approaches have been implemented: (1) the use of local shock-induced reactions to increase the temperature of particle surfaces and to provide a bonding phase (reaction products); (2) shock densification at a low pressure (just above the threshold for pore collapse) followed by hot isostatic pressing; (3) shock consolidation of pre-heated specimens. These techniques were applied to silicon carbide. Reduction of cracking was observed with interparticle melting and reactions. Microstructural results, mechanical properties and advantages and limitations of these approaches are discussed. It is shown that shock consolidation of ceramics is inherently limited because shock-induced cracks are introduced into the process, damaging the particles. A criterion for the plastic deformation versus fracture of ceramic powders under shock consolidation is proposed.     

Processing and properties of zirconia-toughened alumina ceramics

Al₂O₃:ZrO₂ ceramics have been prepared from physically mixed pure oxide powders. The results indicate that careful processing of the starting powders and a two-stage sintering process can avoid expensive processing methods like hot pressing/hot isostatic pressing used for achieving high densification. The mechanical properties were measured and the resultant microstructure studied to explain the toughening behaviour of this material.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Effect of Phase Transformation on Densification During Hot Isostatic Pressing

Densification of mixed phases which undergo phase transformation at elevated temperatures during hot isostatic pressing is described. The effect of the phase transformation on the densification process is shown relative to the densification characteristics of the major phase component of the mixed system. The densification of a mixed phase system undergoing phase transformation is compared using the mechanism maps for hot isostatic pressing of the single phase material. Densification of mixed phase systems undergoing phase transformation, such as homogenization with Kirkendall Effect, is considerably slower than its constituent single phase material. The tendency to generate Kirkendall pores due to homogenization of chemically different materials opposes the normal densification process, thus slowing down the densification process.     

Effect of Ni powder characteristics on the consolidation of ultrafine TiMoCN cermets by means of SPS and HIP technologies

Fully dense titanium carbonitride cermets have been consolidated from Ti(C,N)–Ni–Mo₂C–TiAl₃ powder mixtures either by spark plasma sintering or hot isostatic pressing techniques. Carbonyl Ni powders enhance the densification of the cermets produced by SPS (spark plasma sintering), a phenomenon likely related to a more efficient dissolution of Mo₂C additions and the possible precipitation of α″ phase. Both SPS and HIP (hot isostatic pressing) processes lead to materials with a bimodal Ti(C,N) grain size distribution containing a considerable fraction of nanometric grains. Unlike SPS, HIP induces significant graphite precipitation which could be explained by the destabilization of the carbonitride phase under high isostatic pressures at high temperature. Optimized compositions processed by SPS exhibit a combination of hardness and toughness close to the range covered by ultrafine WC–Co hardmetals of similar binder contents.     

超细/纳米粉末改进Ti(C,N)基金属陶瓷性能研究进展

综述了近几年超细或纳米粉末改进Ti(C,N)基金属陶瓷性能的方法,简要分析了含超细或纳米粉末Ti(C,N)基金属陶瓷的致密化问题.总结了真空烧结 热等静压处理和放电等离子烧结的特点,并分析了微波烧结和等离子活化烧结制备Ti(C,N)基金属陶瓷的可能性.