Quasi-isostatic Pressing Research Articles

A low-cost granular-medium hot quasi-isostatic pressing method for enhancing compressive properties of 3D printed structures

3D printed structures suffered weak bead-to-bead interfacial bonding due to the layer-by-layer manufacturing process. The hot isostatic pressing (HIP) method was effective in improving the micro-structures and enhancing the mechanical properties of 3D printed structures, but it was limited due to the high-cost device and process. Inspired by the HIP method, and aimed at improving the compressive properties of 3D printed thin-walled structures, this work proposed a low-cost granular-medium hot quasi-isostatic pressing (GMHQIP) method realized by the homemade equipment. The hot quasi-isostatic ambient pressure could be produced by pressing the granular medium in the high-temperature environment. The thin-walled structures treated by the GMHQIP method showed a more regular progressive deformation mode than non-treated structures. Compared to the non-treated structure, the specific energy absorption of thin-walled structures treated at 200 °C with 2 MPa for 8 h significantly increased by 170.4 % to the value of 4.95 J/g. The results indicated that the strength and toughness of 3D printed materials improved significantly after the GMHQIP treatment. The interfacial bonding of 3D printed structures was enhanced by using the GMHQIP method. This work offered a low-cost and efficient post-processing method for 3D printed structures to improve compressive properties.

High-purity corundum grinding balls, obtained by quasi-isostatic pressing

The influence of the composition of press powders based on alumina CT 3000 LS SG, quasi-isostatic pressing and firing parameters on the structural and mechanical characteristics of grinding balls with a diameter of 40 mm is shown. Process parameters of production of high-density corundum balls are optimized on the basis of revealed regularities. The results of complex studies of physical and mechanical properties of ceramic articles are given. The determining effect of the particle size of raw material component on the hardness, density and operational characteristics of the grinding bodies was revealed.

Effect of soft-mould pressing method on anisotropy of the graphitic matrix spheres: Dry-bag isostatic vs. Quasi-isostatic

The anisotropy of graphitic matrix in fuel elements for high-temperature gas-cooled reactors (HTR) has an important effect on the distribution of thermal stress and irradiation stress, which should be strictly controlled. In this study, the graphitic matrix spheres were prepared through quasi-isostatic pressing and dry-bag isostatic pressing. The anisotropy was compared from the elastic modulus, crushing load, coefficient of thermal expansion (CTE) and thermal conductivity in different directions. The causes of anisotropy were investigated from the aspects of raw material microstructure, charging, pressing, as well as heat treatment. The graphite microstructure and the preferred orientation of graphite particles in the production process were found as the main reasons for the anisotropy of thermal and mechanical properties. The dry-bag isostatic pressing method can effectively reduce anisotropy and increase the production capacity compared with the quasi-isostatic method, which is consistent with the development trends of high quality and low cost fuel elements.

Some features of the method of quasi-isostatic pressing of ceramic and refractory products

Some features of the method of quasi-isostatic pressing of ceramic and refractory products are described. The distinctive features of the press forms of isostatic pressing are indicated; a scheme of one-step pressing of grinding bodies is given. Ill. 3. Ref. 6.

The method for quasiisostatic pressing of ceramic and refractory products

The method for the quasiisostatic pressing of ceramic and refractory products, frst developed in theUSSRby the PhD M. I. Timokhova is described. The stages of development of quasiisostatic pressing and its scheme are given, the range of products obtained by this method is indicated.Ill.1. Ref. 6.

Mechanically activated combustion synthesis of molybdenum silicides and borosilicides for ultrahigh-temperature structural applications

Molybdenum silicides and borosilicides are promising structural materials for gas-turbine power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase is considered as an attractive material. In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated SHS. Use of SHS compaction (quasi-isostatic pressing) significantly improves oxidation resistance of the obtained MoSi2−T1 composites. Combustion of Mo−Si−B mixtures for the formation of T2 phase becomes possible if the composition is designed for the addition of more exothermic reactions leading to the formation of molybdenum boride. These mixtures exhibit spin combustion, the characteristics of which are in good agreement with the spin combustion theory. Oxidation resistance of the obtained Mo−Si−B materials is independent on the concentration of Mo phase in the products so that the materials with a higher Mo content are preferable because of better mechanical properties.

Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

ABSTRACTMolybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase and alloys based on this phase are promising materials.In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo−Si−B materials is independent on the concentration of Mo phase in the products. The “chemical oven” technique has been used to obtain a single Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases.

Study of certain factors in the wear of grinding balls during service

New studies have shown that the main factors which affect the service life of grinding balls are the method and parameters used to form them - quasi-isostatic pressing, the composition of the ceramic material, and its fineness after comminution. Another important factor is the regime used to bake the balls; the regime should ensure the formation of a fine-crystalline structure in the baked products. A single-stage method is developed for the quasi-isostatic pressing of grinding balls.

Towards better combustion of lunar regolith with magnesium

It has been shown recently that lunar regolith forms combustible, thermite-type mixtures with magnesium. Combustion of these mixtures could be used for the production of construction materials on the Moon. The present paper focuses on decreasing magnesium content in mixtures based on JSC-1A lunar regolith simulant and increasing the density and strength of the combustion products of these mixtures. Combustion of JSC-1A/Mg mixture pellets was studied in argon gas and in silica powder. Preheating to 100°C decreased the minimum concentration of magnesium, required for combustion, from 10wt.% to 8wt.%. Submerging the pellets into silica increased the product strength. SHS compaction experiments were conducted, which involved quasi-isostatic pressing of the products immediately after the end of the combustion process. This resulted in a significant increase in the product density. The compressive strength exceeded 10MPa, i.e., was higher than that of common bricks (5MPa).

Isostatic pressing assisted indirect selective laser sintering of alumina components

PurposeThe purpose of this paper is to assess a new powder metallurgy process to make alumina parts through indirect selective laser sintering (SLS). Density measurements, some geometrical assessments and scanning electron microscopy (SEM) microstructural analyses are performed after each stage of the process, allowing an objective overview to be provided of the challenges and possibilities for the processing of high density technical ceramic parts through SLS of ball milled alumina/polyamide powder agglomerates.Design/methodology/approachThe powder production by ball milling, SLS, cold isostatic pressing (CIP) or quasi isostatic pressing (QIP), debinding and sintering (FS) stages of the powder metallurgy process were sequentially investigated.FindingsAlumina parts with a density up to 94.1 per cent could be produced by a powder metallurgy process containing an SLS step. Microstructural investigation of the sintered samples reveals an alumina matrix with a grain size of ∼5 μm and two different kinds of pore morphologies, i.e. long elongated pores, which stem from the intergranular spacings during SLS, and intermediate pores, which likely originate from larger polyamide agglomerates in the ball milled powder. Also, QIPing at elevated temperatures is found to be a promising alternative for CIPing at room temperature to increase the final part density.Research limitations/implicationsCracks, long elongated pores and intermediate pores remained in the sintered parts. Homogenizing the microstructure of the parts through optimizing the composite starting powder, the deposition during SLS, the SLS parameters and QIPing parameters is essential to overcome these limitations.Practical implicationsHomogenizing the starting powder mixture and the microstructure of the SLS material is the key issue for producing ceramic parts through indirect SLS.Originality/valueIndirect SLS of ceramics has hardly been reported and the combined use of SLS and QIPing is innovative in the field of indirect SLS of ceramics.

Some properties of elastic press components in powder material quasiisostatic pressing

An elastic press buffer is a basic element in quasiisostatic pressing, so a major aspect of developing such pressing is to choose a suitable constructional material for the elastic pressing elements to provide the necessary conditions for consolidating a powder and providing reliable operation of the pressing mold in the industrial production of pressed items [1, 2]. Various elastomers have been tested: gelatin, some resins, technical rubber, and natural and synthetic rubbers. The gelatin pressing elements were made by pressing a specially prepared solution in metal molds of appropriate configuration. However, the low hardness made it undesirable. Buffers have also been made of technical rubber with medium hardness, which is usually supplied as sheets of thickness 30 – 60 mm, and which was used by cementing together previously cut blanks and then processing with a cutter or abrasive wheel. The preparation labor and inhomogeneity of some compound press buffers has forced one to seek other materials. Some polyurethane elastomers have been tested in molds for pressing ceramic materials based on Al2O3, which have been found to be promising [3, 4]. Table 1 gives the basic physicomechanical characteristics of polyurethane elastomers [5]. These tests showed that the best material is SKU-7l material prepared by casting. This elastomer if prepared by other technologies such as pressing shows a deterioration in the parameters of the pressed components. The SKU-7l material provides components equally dense throughout the volume, and with more even and less rough surfaces than when other elastomers are used. The SKU-7l has exceptionally high wear resistance and capacity to work at pressures exceeding 1000 MPa. Its strength is 6 – 8 times higher than that of rubber, and the wear resistance exceeds that of St3 steel by a factor 3, and of epoxide resin by a factor 60. Most ordinary elastomers lose their properties on attaining a Shore hardness of A85. Out of all known elastomers, only polyurethane ones preserve their elasticity in a wide range of Shore hardnesses ( A95). The unique combination of physicomechanical properties means that they can be used for pressing powders. They as constructional materials not only replace metals but also are better than the latter as regards working properties. Press buffers were made of SKU-7l polyurethane by firms that specialized in making polyurethane components by pouring a solution into aluminum molds we supplied and subsequent heat treatment. The resulting buffers did not require mechanical processing, but with that technology it was difficult to provide tools for the pouring, so it was decided to be economically justified to make buffers of appropriate shape by the mechanical processing of polyurethane blanks provided by the specialized firms. Much of the research was directed to methods and modes of mechanical processing. In the mechanical treatment it must be remembered that polyurethane is an elastic material that has a tendency to deflect away from the cutting tool, so the latter should be very sharp. The elasticity means that the clamps holding an item for treatment should not distort it too much. It is found that in any mechanical treatment operations one needs a high speed for the cutting tool and a slow feed for the workpiece to avoid overheating. Water-soluble oils and emulsions can be used in cutting to prevent excessive heating and are usually recommended for the softer polyurethanes. Satisfactory results have been obtained with the following parameters: cutting tool speed Refractories and Industrial Ceramics Vol. 50, No. 3, 2009

Combustion synthesis/quasi-isostatic pressing of TiC–NiTi cermets: processing and mechanical response

TiC-NiTi composites were produced by a technique combining self-propagating high-temperature synthesis (SHS) of elemental powders of Ni, Ti, and C with densification by quasi-isostatic pressing (QIP). In order to create a one-step synthesis/densification process, the Ti ? Ni ? C reactant material was surrounded in a bed of graphite and alumina particulate before initiation of the combustion reaction. The sample was ignited within the particulate and subjected to a uniaxial load immediately after passage of the combustion wave. The constitutive response, composition and resulting structures of the composites with varying volume fractions of NiTi are characterized. Powder mixtures prepared anticipating the formation of stoichiometric TiC result in the formation of composites with a eutectic matrix of Ni3Ti and NiTi. This titanium impoverishment of the matrix is consistent with the formation of nonstoichiometric TiCx during the com- bustion reaction. The Ni3Ti phase can be suppressed by anticipating the formation of TiC0.7 and adjusting the chemical content of the reactant mixture to include addi- tional titanium. These cermets combine the high hardness of the ceramic phase with the possible shape memory and superelastic effects of NiTi. Introduction and objectives

Densification of porous bodies in a granular pressure-transmitting medium

Densification is a critical step in the manufacture of near-net-shaped components via powder processing. A non-isostatic stress state will in general result in shape distortion in addition to densification. In the quasi-isostatic pressing (QIP) process the green body is placed into a granular pressure-transmitting medium (i.e. PTM), which is itself contained in a rigid die. Upon the application of a uniaxial load, the PTM redistributes the tractions on the green body, thereby creating a stress state that is quasi-isostatic. The character of the deformation of the PTM is studied using model experiments on pressing of the PTM in a rigid die and a scanning electron microscopy analysis of the PTM powder. An important problem of the optimization of the PTM chemical composition enabling the maximum densification of a porous specimen with the minimum possible shape distortion is solved. The results of modeling agree satisfactorily with the experimental data on cold QIPing Ti and Ni powder samples and hot QIPing TiC–TiNi cermet composites.

Effect of Fe on the phases and microstructure of TiC–Fe cermets by combustion synthesis/quasi-isostatic pressing

Abstract Fully dense TiC–Fe cermets ( x = 10, 20, 30, and 40 wt.%) were produced from Ti–C–Fe powder mixtures by combustion synthesis with quasi-isostatic pressing. The effect of Fe content on combustion temperature, combustion wave velocity, and final product density was investigated. The final product was characterized by XRD, SEM, and TEM. The combustion temperature and wave velocity decreased with increasing Fe content. Product density increased with increasing Fe content (96% at 30 wt.%). X-ray diffraction and transmission electron microscopy (TEM) revealed the final product to contain TiC, Fe phases, lath martensite, and Fe 2 Ti. The TiC particle size decreased with increasing Fe content. In addition, a low density of dislocations was observed in both the TiC particles and Fe binder, indicative of annealing and recrystallization, respectively.

Combustion synthesis and quasi-isostatic densification of powder cermets

Self-propagating High-temperature synthesis (also known as SHS or combustion synthesis) presents a bright potential for the synthesis of compounds with high degree of purity. However, for many reactions, the product is highly porous, and it must either be pulverized for subsequent densification, or densified while it is still hot and ductile. Densification has successfully been applied to a number of ceramic and metal–ceramic systems by (a) a high-speed forging technique, and (b) by a quasi-static pressing technique using a granular pressure-transmitting medium (PTM). The reactive mixture and PTM are placed in a piston and cylinder setup and the system is pressurized by uniaxial compression at a pre-established time after reaction completion. The state-of-stress is close to isostatic and the process is therefore termed as quasi-isostatic pressing (QIP). Modeling of the densification was carried out using the Skorohod constitutive equation; comparison of the results with experimental distortion obtained in indentation experiments enables obtaining equation parameters. The distortion undergone by the combustion synthesis products during QIP densification was modeled using the appropriately verified constitutive equation and assuming a linear elastic response for the granular PTM.

Model of Quasi-Isostatic Hot Pressing of SHS Products in the System Titanium ― Carbon ― Nickel. Part 2. Results of Modelling and Experimental Studies

Results are provided for modelling and experimental studies of hot quasi-isostatic pressing of SHS products of the system Ti ― C ― Ni. Good conformity is demonstrated between experimental data and calculated results using a rheological model for nonlinearly-viscous synthesis products. The model considers presence of a melt and the local nature of solid phase particle flow. Quantitative estimates are obtained for compaction of different zones of the loose shell, the force characteristics of the process, and the nonisostatic coefficient. This information is of interest for production planning and optimization of SHS product compaction.

Model of Quasi-Isostatic Hot Pressing of SHS Products in the System Titanium ― Carbon ― Nickel. Part 1. Structural State and Rheological Model

Quasi-isostatic hot pressing of the products of self-propagating high-temperature synthesis (SHS) is considered. The temperature regime, and the aggregate and structural state of the SHS products of the Ti ― C ― Ni system are studied. A block model of the deformed volume is presented; parameters of the virtual field of plastic flow velocities are determined and an energy equation in the form of a variation functional is determined. Rheological models of the non-linearly viscous SHS products and the rigidly-plastic loose shell are considered within which the dual mechanism of compaction and the local nature of dispersed material particle deformation are taken into account.

Research and Development of Fuel Element for Chinese 10 MW High Temperature Gas-cooled Reactor

The fuel elements for Chinese 10 MW High Temperature Gas-cooled Reactor (HTR-10) are spherical all- ceramic fuel elements (SFE). TRISO (Tri-isotropic) coated fuel particles (CP) are uniformly dispersed in the graphite matrix of the fuel element. All radiological fission products are almost completely retained inside the SiC layer of the intact CP. The fabrication of SFE includes UO2 kernel preparation by sol-gel method, pyrolytic carbon (PyC) and SiC coating on the UO2 kernels by Chemical Vapor Deposition, manufacture of SFE by the quasi-isostatic pressing and the inspection of over 30 kinds of properties. This paper describes research and development (R & D) and design specification of the HTR-10 SEF, summarizes the fabrication technology and quality control mastered in R & D for HTR-10 SFE.

In-situ combustion synthesis and densification of TiC– xNi cermets

The combustion synthesis reaction was combined with quasi-isostatic pressing (QIP) technique to fabricate full density TiC– xNi composites in a single processing operation. Combustion wave velocity and temperature of Ti–C–Ni were measured and the microstructure of the product was characterized by X-ray diffraction and scanning electron microscopy. With increasing Ni content in TiC– xNi, both the combustion wave velocity and temperature decrease. The Ni additive, mainly as a diluent and the binder of TiC grains in a matrix, formed a quasi-continuous phase enveloping spheroidal TiC particles and brought about a grain size decrease from 9 to 1 μm. TiC-20 wt% Ni cermet produced by the combustion synthesis/quasi-isostatic pressing process under 160 MPa for 20 s show near full density, high hardness and transverse rupture strength (1024.2 MPa).

Quasi-isostatic pressing of saggers

The main method of shaping saggers in insulator factories operating in the electrical engineering industries is static pressing in metal molds. In this method, ensuring singlesided monoaxial compression of the material being pressed, there are typically high losses of pressure as a result of the existence of external friction of the body on the wall of the matrix and internal interparticle friction, especially for articles ofsubstantial height, for example, for saggers. In this case we note a reduction in the forming pressure over the height of the product, which reduces its density upon removal from the pressing plunger. Thus, saggers shaped by monoaxial pressing have an irregular density and strength in different directions. Therefore the working factors of saggers do not fully satisfy the requirements.