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.     

The porosity dependence of flexural modulus and strength for capsule-free hot isostatically pressed porous alumina

Structural properties such as flexural moduli and strength have been measured for a range of porous alumina specimens of different initial powder sizes and final porosities, sintered using the capsule-free hot isostatic pressing method. This processing method produces a porous body in which the closed porosity is negligible. The relationship of these structural properties to total porosity has been investigated. The results indicate that both a power and an exponential function could adequately describe the porosity dependence of flexural strength. The strength values obtained were test method dependent, and were significantly lower for specimens with sintering aids. A power law model based on a critical porosity, as proposed by Phani, gave the best fit for the modulus measurement data. No dependence of mechanical properties on particle size was observed. The strength measurement results did not appear to support suggestions that better strength could be obtained by the capsule-free hot isostatic pressing method than conventional sintering, as reported elsewhere.     

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.     

喷射成形多孔材料的致密化工艺综述

介绍了6种常用的喷射成形多孔材料致密化工艺,即热挤压、轧制、锻造、热等静压、楔形压制和陶粒压制工艺,重点介绍了各种工艺的原理、特点、优缺点以及针对某些缺点而采取的一些改进措施.发现热挤压、轧制、锻造、热等静压和陶粒压制工艺能单次使喷射成形多孔材料整体成形致密化,而楔形压制是一种通过局部小变形累积以实现整体成形致密化的工艺;楔形压制在致密化喷射成形大尺寸材料方面具有独到的优势.最后,探讨和展望了今后的发展方向.     

Transformation in metal materials during hot isostatic pressing

The paper presents a review of transformations in a number of metal materials during hot isostatic pressing which result in changes in the shape, density, morphology, and composition of the structural components of some metals, including cast, powder, and composite titanium alloys, heat-resistant nickel alloys, hard alloys of the WC-Co system, and also some aluminum and aluminide materials. Methods for studying the compaction processes using an eddy current dilatometric cell and differential barothermal analysis to determine the baric shift of the characteristic temperatures for some heat-resistant nickel alloys are described. The data on the practical application of hot isostatic pressing are presented, which makes it possible to fabricate the most important parts of gas turbines. It is shown that hot isostatic pressing causing transformations of different physicochemical origin should be considered as a necessary step of the production of metals to ensure their limiting strength characteristics.     

Book review

Abstract

Castings of 70–30 cupronickel (NES 824) often contain an undesirable amount of porosity, which leads to a high rejection rate for components made from this type of alloy. The possibility of removing such porosity by the application of hot isostatic pressing (hipping) to the castings has been investigated. Part of this investigation involved the hipping of artificial porosity specimens of controlled geometry and a comparison of the results obtained with the predictions of certain well known models for pressure assisted sintering. The results obtained were complicated by the effect of the specimen design on the rate of compaction which is not taken into account in any of the models. However, where the pores are truly internal the degree of agreement with both models is reasonable. It is evident that for internal porosity the removal of porosity in a typical industrial hipping operation occurs during the initial application of pressure and temperature rather than during the subsequent isothermal stage. This applies for up to 10% porosity in the form of a single hole but may not be true of large castings with uneven distributions of pores.

MST/3268     

Microhardness as a measure of homogeneity of porous tungsten

An important application of tungsten is as a high current density cathode in porous form. It is used as a dispenser cathode after being impregnated with an electron emissive material. Therefore, porosity distribution in the part is a crucial parameter for its use since a uniform and controlled porosity will lead to a better performance.In this study, microhardness is used in order to determine the uniformity of the pore distribution and homogeneity of the structure. Effects of different pressing techniques on porosity of porous tungsten have been examined. Considerable attention has been given to die and cold isostatic pressed samples. A comparison has been made between the respective porous structures in view of the microhardness measurements. Optical microscopy and SEM are used to relate the results and porous tungsten structure for a better understanding of the applied method.     

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.     

Fabrication and properties of hollow powder porous structures

Low-cost steel porous structure materials have been prepared from hollow sphere mill-scale powders by using a simple method. Hollow spheres of mill-scale material were fabricated by coating polystyrene spheres, using pelletizer disc. The coated powders were mixed with 1 wt.% inorganic binder diluted in 4 wt.% water, and then were uniaxially pressed at 0, 100, 200 and 400 MPa in a die to produce rectangular shape compacts (30×15×15 mm), calcined at 550 °C for 30 min (to burn off the organic materials). After calcination, the compacts were then sintered in hydrogen atmosphere for 40 min at 1150 °C.The resulted steel porous materials have different relative densities varied from 32.3% to 50.89%, depending upon the previous compaction pressing before sintering. It is noted that the relative density increased with the increase of compaction pressing, whereas the porosity content decreased. Increase of properties, such as hardness, transverse rupture strength, and compression yield stress, occurred due the increase of compaction pressing of the compacts before sintering.     

Inhomogeneous flow and the effective pressure concept in pressure sintering

Many of the models proposed to describe pressure sintering of solids with a randomly distributed porosity have implicit in them the effect of porosity and it is incorrect to incorporate any extra pressure intensification factor. However, usually, pores are segregated into zones which leads to inhomogeneous deformation. A consequence of this inhomogeneity is to enhance densification rates by dislocation creep and particle sliding resulting in an anomalous pressure intensification factor. Some observations on microstructural development during hot isostatic pressing are also made.     

Microstructural and mechanical properties of biodegradable iron foam prepared by powder metallurgy

Research into biodegradable porous materials has been increasingly focused on iron-based materials because such materials possess suitable properties for orthopedic applications. In this study, we prepared porous iron with porosities of 32–82 vol.% by powder metallurgy using ammonium bicarbonate as a space-holder material. We studied the influence of initial powder size and compacting pressure on sample microstructure, contamination and mechanical characteristics. The experimental results were analyzed as well, using Gibson–Ashby model and this analysis showed a good agreement in theoretical and experimental data. Whereas increasing compression pressure decreased porosity, the use of finer iron powder led to an increase in porosity. Increasing the amount of space-holder material in the initial mixture increased the total porosity, improved compressibility and consequently decreased the number of pores originating from imperfect compaction. A higher compacting pressure and the use of finer powder enhanced both the flexural and compressive properties. Even the most porous samples prepared from the fine iron powder possessed mechanical properties comparable to human cancellous bone. Based on these results, we can claim that the use of fine initial iron powder is necessary to obtain highly porous iron, which appears to be suitable for orthopedic applications.     

Comparisons of grain size-density trajectory during spark plasma sintering and hot-pressing of zirconia

Spark plasma sintering (SPS) and hot-pressing (HP) of a granulated stabilized zirconia powder have been investigated for a fixed macroscopic compaction pressure of 100 MPa and a fixed heating rate (25 °C/min for HP, 50 °C/min for SPS). The “relative density/grain size” trajectories have been established for both sintering methods.HP is shown to be similar to SPS for the manufacturing of polycrystalline TZ3Y materials with a final grain size well below the micrometer. Independently of the sintering technology employed, it is interesting to note that three kinds of microstructures are obtained depending on the experimental parameters: porous materials (opened porosity, relative density between 61 and 90%) with a nanometer grain size (around 75-80 nm), dense materials (closed porosity, relative density between 90 and 98%) with a nanometer grain size (around 75-80 nm), fully dense material with a submicron grain size (around 160 nm using SPS and around 105 nm using HP).     

Effect of hot isostatic pressing on mechanical properties and microstructure of 70–30 cupronickel castings

Abstract

The present work is part of an investigation into the use of hot isostatic pressing to recover 70–30 cupronickel castings. These alloys have particularly good corrosion resistance and, when strengthened with silicon and chromium, produce a material capable of use in very severe conditions of stress and massive corrosion. However, it is not possible to recover such castings by the application of repair welding, because of the possibility of reduced corrosion resistance in the vicinity of the weld. Hot isostatic pressing represents an alternative method of casting recovery. The results reported in the present work refer to the effect of hot isostatic pressing on mechanical properties, microstructure, and the level of segregation in the alloys. Hot isostatic pressing may be used to remove casting defects in the form of fine pores up to total porosity of 5%. However, in cases where porosity takes the form of very large defects, the mechanical properties of the recovered region are inferior to those of the originally sound material. This effect is probably associated with the presence of very finely distributed oxide particles in the originally defective parts of the casting. The optimum hot isostatic pressing temperature for the best overall combination of properties was 950°C.

MST/1732     

热等静压技术在镍基铸造高温合金领域的应用研究

镍基铸造高温合金是航空发动机与燃气轮机生产制造过程中应用的主要材料之一,在航空航天、能源工业、船舶舰艇等领域有着广泛的应用。现代航空工业的飞速发展离不开高温合金综合性能的快速提升,而热等静压技术在镍基铸造高温合金领域的应用对镍基铸造高温合金综合性能的改进方面发挥了举足轻重的作用。本文介绍了热等静压技术的工作原理与应用发展历史,总结了热等静压技术在镍基铸造高温合金领域的研究应用现状,重点阐述了热等静压技术对铸造高温合金的致密化作用机理与组织性能影响、热等静压对长期服役镍基铸造高温合金组织修复研究以及实现两种镍基高温合金扩散连接的应用优势与研究成果。同时指出热等静压技术研究中存在的一些问题及国内热等静压技术在镍基铸造高温合金领域的发展趋势。     

Hot consolidation and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying

The present study is aimed to investigate the consolidation behaviour and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying. The consolidation was achieved by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques. The microstructure and mechanical properties were evaluated. The hardness and compressive strength of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after vacuum hot pressing are 9.50 and 2.19 GPa and those after hot isostatic pressing are 10.04 and 2.83 GPa, respectively. The wear resistance is found to be higher than the commercially used materials such as Ni-hard faced alloy.     

Study on fabrication and mechanism in of porous metals by spark plasma sintering

Porous stainless steel with regular spherical particles were fabricated by spark plasma sintering (SPS) and compared with those obtained by conventional hot pressing (HP). The effects of the parameters during sintering processes on properties, such as porosity and compressive strength, were investigated. Microstructure studies showed that the regular spherical stainless steel particle could form porous ingots with regular pores in the way of close sphere package. Results indicated that the compressive properties of the SPS samples were superior to those of the HP samples. The study on relative diffusion coefficient in SPS and HP specimens indicated that SPS enhanced neck growth and accelerated the atom diffusion. Porous metals could be superfastly prepared by SPS under the conditions of higher heating rate, shorter holding time and lower sintering temperature.     

Fabrication of bulk materials from fine particles of PbTe-SnTe solid solutions prepared through thermal decomposition of salts

A process has been proposed for the fabrication of microcrystalline thermoelectric materials based on PbTe-SnTe solid solutions by compacting and hot-pressing fine powders prepared through the thermal decomposition of lead acetate trihydrate and tin oxalate in the presence of tellurium powders. We analyze the mechanism of the thermal decomposition of the powder mixtures, resulting in the formation of Pb_{1 − x} Sn _x Te particles, and examine the influence of cold compaction, heat treatment, and hot pressing on the grain composition, electrical conductivity, and Seebeck coefficient of materials prepared by pressing the powders.     

Effect of hot rolling on the enhancement of mechanical properties of low density Cu–Cr–Nb sintered alloy

Studies have been carried out on the effect of hot rolling on the enhancement of mechanical properties of sintered low density Cu–8 at.% Cr–4 at.% Nb alloy made from gas-atomised powders by vacuum hot pressing using varying pressure and temperature. The pressure is varied from 10 to 30 MPa whereas, 800, 900 and 1000 °C are three hot-pressed temperatures. Though hot rolling of porous low density hot-pressed samples results in near theoretical density, the hot-pressed sintered material with higher initial porosity shows lower strength and ductility after rolling. Hot-pressed compacts below certain density could not be rolled and the sample has cracked during rolling. The higher density material could be easily rolled and significant improvement in mechanical properties has been noticed. This has been attributed to the conversion of porosities into microscopic cracks which could not be healed due to the presence of Cr₂Nb precipitates. Finally, structure property correlation has been established.     

Preparation of materials from aluminum oxide nanopowders using modern methods of consolidation

Processes of hot pressing of aluminum oxide nanopowders and special features of their compaction have been discussed. The effect of hot pressing parameters on the structures of materials from aluminum oxide nano- and submicron powders has been studied. The kinetics of grain growth of nano- and submicron powders during hot pressing and the dependence of the resultant materials density on hot pressing parameters have been analyzed.     

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.