Interrelationship between the properties of powders,the pressing and sintering conditions,and the structure of porous niobium III. The porous structure of material of sintered niobium powders

Conclusions In the initial stage of sintering of high-porosity compacts of niobium powder the average pore size increases, the fine pores disappear, and coarser pores than in the original compacts appear.There is a linear relationship between average pore radius and the total volume of pores for similar sintering conditions.The relationship between open and closed porosity after sintering is determined completely by the total porosity and does not depend upon sintering conditions.In sintering of high porosity specimens of dispersed niobium powders zonal isolation of the particles occurs.Translated from Poroshkovaya Metallurgiya, No. 7(331), pp. 23–28, July, 1990.     

粉末注射成形奥氏体不锈钢的疲劳和冲击强度

本文的目的是研究粉末性能对粉末注射成形奥氏体不锈钢的力学性能(特别是疲劳和冲击强度)的影响。试样用混有聚酰氨粘结剂系的水雾化(WA)和气雾化(GA)粉末制成。注射坯在空气中脱粘，而后在不同温度、不同保温时间下真空烧结。粉末性能对烧结体的密度、显微组织和力学性能有显著影响。孔和析出物显示出Ostward时效，而孔和析出物的长大满足Lifshitz-Wagner方程。WA和GA粉末试样的疲劳极限分别约为300MPa和310MPa。它们的疲劳强度略低于常规锻材。烧结体的冲击值随密度增加线性增大。     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

Powder bronze with additions of an activated charge

Kinetics of processes of dispersion and agglomeration and the formation of sintered powder materials based on mechanochemically activated (MCA) charges have been studied. Effects of the time of milling on the fractional composition, energy consumption of the milling process, density of the compacts and sintered samples, ultimate strength for radial compression, and the efficiency of strengthening upon the introduction of MCA additions have been shown. A technology of production of a powder antifrictional bronze with additions of MCA charges has been developed, which includes a mechanochemical activation of wastes of compacts, their pressing, and sintering.     

On the swelling of silver powder during sintering

Silver powder compacts may suffer an overall expansion during sintering. A simultaneous analysis of dimensional changes, mass loss and pore closing in the course of sintering showed that volume change results from the competition between classical sintering phenomena inducing densification and material creeping under stresses caused by internal gas pressure, which induces swelling. The internal gas includes air trapped during pressing and oxygen released from particle surface inside the pores that closed during pressing or sintering. This swelling phenomenon is particularly strong with powders with large specific surface area and compacts with high green density. It is less effective during fast sintering, as observed during direct induction heating experiments. Electrical conductivity is strongly affected by swelling. Powder compacts may exhibit a lower conductivity after sintering when interparticle bonding does not balance the density decrease occurring during sintering.     

Multistage sintering process for Ni3Al powder metallurgical products

A multistage sintering process for powder metallurgical products of nickel aluminide intermetallic compound has been investigated. It comprises at least two stages of sintering and interstage cold deformation to collapse and eliminate the sintering pores. Most of all, a thermally absorbing material has to keep in contact with the powder compacts during the preliminary heating stage (650 °C, the first stage of reactive sintering). It depresses the maximum temperature of specimens to develop the useful transient phase Ni₂Al₃. This transient phase is a brittle and especially crispy material with a relatively low melting point (1135 °C). It plays an important role in preventing the development of any significantly large cracks during the pore-eliminating process. The purpose of the second stage of sintering material at an elevated temperature (1200 °C) is to develop a transient liquid phase from the Ni₂Al₃ to heal or eliminate any microcracks, crazes, and collapsed pores from previous steps, as well as to transfer the material to the final Ni₃Al structure. It is beneficial to produce a sound product having a large dimension and excellent mechanical properties. Consequently, the specimens will be further densified by a repeated cycle of thermal mechanical treatment (TMT).     

Character of porosity of compact materials obtained from metalorganics

The possibility of obtaining highly porous (60–80% pores) materials by combining techniques of powder metallurgy with chemical-metallurgical processes of formation of nanodimensional activators of sintering during the thermal destruction of metalorganics is shown. It is noted that, irrespective of the composition of starting charges, the surface porosity of such materials is represented by pores <30 μm, the fraction of which does not exceed 1%. The basis (86–89%) is represented by pores <4 μm. The character of porosity inside the sintered articles is determined by the sizes and shape of the powderlike skeleton material introduced into the starting charge to prevent the outflow of metalorganics during thermal treatment. When using powders with highly developed particle surfaces as such, the character of the internal porosity is similar to the surface porosity. The combination of scale particles up to 100 μm in size and highly dispersed (<10 μm) particles promotes the formation of long porous channels (100–350 μm) 10–50 μm in width.     

Thermophysical properties of roll-compacted nickel sheet for high-density infrared sheet fabrication

This work is focused on the analysis of the high-density infrared (HDI) sheet fabrication process of powder compacts. Measurements of material properties and distribution of incident heat flux on processed powder sheet surfaces have been conducted with the aim of obtaining a complete set of data that can be used as input in computer simulation software. It was found that these materials exhibit significant anisotropy in thermal conductivity. Indirect measurements indicate that there are small variations in density across the thickness of the powder compacts. Temperature data were obtained from thermocouples placed on the backside of the sheet. The evolution of thermal profile during a static pulse was investigated by using a three-dimensional finite volume model. Numerical simulation results are very sensitive to the surface emissivity. Numerical simulation results agree very well with experimental results for the case in which no liquid pool was formed during HDI processing.     

Mechanical properties of unsintered pressing. I. Phenomenological relations for unsintered pressing strength

Theoretical and experimental relationships are considered for the strength of green compacts in relation to various properties of the powder (particle material strength, particle size and shape, and apparent powder density), together with the formation conditions (pressure) and test conditions. The effects of the powder properties, in particular the apparent density, may be examined from simple relationships of the form “strength - pressing density” with the use of a correction coefficient. One needs to use ideas from the mechanics of compressible media in considering the state of strain in the pressing and the test conditions.     

SOME OBSERVATIONS ON THE SHRINKAGE BEHAVIOUR OF COPPER COMPACTS AND OF LOOSE POWDER AGGREGATES

Abstract

The relative shrinkage in the radial and in the axial directions of both conventionally pressed copper powder compacts and of loose copper powder aggregates has been determined. Irregular-shaped electrolytic copper powder and flake copper powder were used. The structures of the green and of the sintered compacts and aggregates were examined metallographically. The results led to the tentative conclusion that the theories previously advanced to explain the differences in the axial and radial shrinkage of compacts and aggregates are not correct. The pores of green and sintered compacts of irregular copper powder are not necessarily disc- or lens-shaped, but rather equiaxed, and the observed difference in shrinkage cannot be attributed to the shape of the pores. On the other hand, the ratio of axial to radial shrinkage of irregular-shaped powder and of flake powder aggregates is nearly the same, in spite of the fact that the pores in the flake powder aggregates are much larger in the radial than in the axial direction. The possibility that forces other than surface-tension forces have an influence upon shrinkage behaviour is discussed.     

Effect of a rigid component (tungsten alloy) on the process of formation of mixtures based on copper powder and the properties of heterogeneous unsintered compacts

We have analyzed the process of structure formation during pressing of copper-tungsten composites. We have established the effect of the amount of the rigid phase (tungsten alloy) in the mix on the pattern of structure formation and the properties (Young’s modulus and bending strength) of unsintered articles made from heterogeneous powder material based on copper. We have experimentally established that despite the substantially higher porosity and high volume fraction of the rigid phase, specimens of composition Cu — 50% W_alloy have higher green strength and higher modulus of elasticity. Analysis of the true stress — true strain curves of the porous cold-pressed articles compared with the results of microstructural studies allowed us to determine the effect of the structure formation pattern on the strength properties of articles made from copper-based powder material containing tungsten alloy. By studying the structural features of the porous articles, we determined the mechanism for creation of interparticle contact which determines the strength characteristics of unsintered compacts of heterogeneous composition based on a plastic matrix with different amounts of the rigid phase.     

Processing and microstructural evolution of powder metallurgy Zn-22 Pct Al eutectoid alloy containing nanoscale dispersion particles

The present article deals with the processing and microstructural evolution of powder metallurgy (PM) Zn-22Al pct eutectoid alloy. The powder material was produced through inert gas atomization and then cryomilled in liquid nitrogen. The milled powder particles were consolidated by hot isostatic pressing (“hipping”) followed by thermomechanical treatment, resulting in a two-phase microstructure. The microstructures were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The principal processing factors and microstructural characteristics associated with the major processing steps, including spray atomization, mechanical milling (MM), consolidation, and heat treatment, were evaluated and discussed. Hot isostatic pressing and extrusion followed by heat treatment to produce the superplastic structure (Al-rich phase and Zn-rich phase) are effective in elimination porosity. A TEM examination of the microstructure of the alloy after processing reveals the presence of nanodispersion particles that are not uniformly distributed. The formation of the dispersions was attributed to the interaction between the powder material (primarily Al phase) and environmental elements such as oxygen and nitrogen during milling. Moreover, the size and distribution of the dispersions present in the bulk material met the anticipated requirements for serving as inhibitors for grain growth and barriers for dislocation movement. The TEM observations on crept specimens reveal extensive dislocation/dispersion interactions.     

Investigation on microstructure and high strain rate behaviour of pure tantalum prepared by hot press and subsequent annealing treatment

Among refractory metals, tantalum has long been the primary material of use in high strain rate applications. Hot pressing (HP) is a technique to produce specimens with high density and homogeneous microstructure. The individual effect of the HP or annealing heat treatment on high strain rate properties of tantalum powder compacts has been investigated, but little attention has been paid to the combined effect of subsequent annealing. In this study, the effect of annealing on the microstructure and mechanical properties of tantalum subjected to hardness and Split Pressure Hopkinson Bar (SHPB) test was investigated. After annealing at 1100°C, the grains are coarsened, thereby hardness and upper yield strength decreased to value 199 VHN and 1833?MPa, respectively. Moreover, an obvious increase in plasticity and toughness is detected at this temperature, indicating subsequent annealing is beneficial to improve the deformation properties of HPed powder compacts; the maximum plasticity of tantalum was 31.74% after annealing.     

Healing Behavior of Micropores in Powder Metallurgy 316L Stainless Steel during Hot Forging and Heat Treatment

The healing behavior of micropores in powder metallurgy （P/M） 316L stainless steel during hot forging and subsequent heat treatment was studied. The results showed that hot forging can improve the homogeneity of the pore size and enhance the relative density of material in varying degree due to different forging temperatures. As a re- sult of deformation and diffusion bonding at high temperature, the irregular pores were spheroidized and finally turned into stable inner grain pores. The comparison of compression behavior between P/M and wrought dense mate rials has shown that the pores can either be the obstacles of dislocation movement or be the nucleation sites accelera- ting the reerystallization according to the difference of deformation temperatures.     

粉末冶金齿毂模具裂纹分析与解决方法

从压制阴模模具材料、热处理制度、金相组织等方面研究了粉末冶金压制阴模键子处裂纹形成原因,讨论了粉末冶金齿毂模结构及压制模具设计结构的合理性,通过分析数据得到了开裂的原因并加以改正。结果表明,压制阴模本身的材质选择和热处理制度没有问题,导致裂纹产生的主要原因是整体模具结构设计不合理,导致压制阴模键子损坏。改进原有模具结构可以提高压制模具寿命,降低模具使用成本,保证生产顺利进行。     

原位合成制备AlN弥散强化2024铝合金的研究

通过在氮气气氛下部分氮化Al-Mg-Cu合金压坯,再进行烧结和热挤压致密化的新工艺制备了Al N弥散强化2024铝合金。对Al N弥散强化2024铝合金的微观组织和力学性能进行了初步研究。透射电镜和X射线衍射表明,Al-Mg-Cu合金压坯经过氮化后生成了Al N颗粒。这种合金经过热挤压和热处理后表现出优异的力学性能。含有7.95%(体积分数)Al N弥散强化的2024铝合金经过挤压后并热处理的抗拉强度和屈服强度分别达到665 MPa和553 MPa。     

Effect of heat treatment of the densification behavior of porous bodies from fine tungsten powders during sintering

Conclusions It has been established that the fall in the activity of a tungsten powder produced by reduction at 950°C and annealed, before pressing, at a temperature of 1700°C or higher is due not to growth of its particles during the annealing but to a decrease in the internal porosity of the particles and their agglomeration and spheroidization. With single-fraction as-reduced and annealed powders their is a correlation between the maximum size of pore channel constructions in a compact and the letter's sintering behavior. Prior heat treatment of tungsten powders produced by reduction at an end temperature of 700°C exerts a nonmonotonic influence on their sintering activity. Shorttime (=30 sec) sintering of compacts from such tungsten powders has been found to result in the formation of a new porous structure, with enlarged maximum pore channel constrictions, responsible for their sintering behavior. A correlation has been discovered between the volume shrinkage of compacts and the maximum size of constrictions in the enlarged capillaries.Translated from Poroshkovaya Metallurgiya, No. 11(227), pp. 25–31, November, 1981.     

Phase and structure changes during the sintering of compacts of high-speed steels obtained from powders with various rates of solidification

An investigation was made of the phase and structure transformations in compacts of high-speed steel R6M5F3 powder fractions ?630+50 μm at the temperatures of solid-phase (1160–1220°C) and liquid-phase (1240°C) sintering. The structure of the compacts was crystalline or quasicrystalline, depending on the degree of superheating and cooling rate of the melt during powder spraying. This was related to the presence of a cellular structure in particles of a critical size in the powder. Compacts of powder with the cellular structure experienced higher shrinkage during solid-phase sintering as a result of structre relaxation and recrystallization. The driving force of these processes is the change in chemical potential arising from the decomposition of highly supersaturated metastable solid solutions, and the excess grain-boundary energy of the quasicrystalline matrix structure.     

Pressing behavior of atomized iron powders

Conclusions The optimum zinc stearate content, ensuring the greatest compressibility of a powder at a given pressing pressure, is determined mainly by the volume of interparticle pores. The higher the density of a compact and the smaller the size of its pores, the smaller is the amount of zinc stearate it can hold. The weakening action of zinc stearate, which manifests itself in stress relief at interparticle contacts during pressing, substantially increases the density of compacts and at the same time decreases their strength. This phenomenon is particularly pronounced with compacts from powders of poor compactibility. After pressing at any given pressure, the strength of a compact with zinc stearate will be higher than that of a compact without a lubricant when the decrease of the contact surface brought about by the elastic aftereffect in the stearate is counterbalanced by the increase of this surface resulting from better compressibility of the powder. The addition of an optimum amount of zinc stearate to an iron powder is always more effective than lubrication of the die walls. A new mechanism of densification of iron powder with zinc stearate is proposed, in which the key factor is an intensification of stress relief at interparticle contacts by the solid lubricant.Translated from Poroshkovaya Metallurgiya, No. 5(221), pp. 16–22, May, 1981.