Electrical conductivity and contact phenomena in porous metallic materials

The change in electrical resistance of copper fiber and powder compacts upon heating from room to the sintering temperature was studied. It was found that different regimes of heating in vacuum did not affect the temperature dependence of electrical resistance. A relationship between electrical conductivity and the relative size of interparticle contacts was derived for a model fiber body. This was in good agreement with data on the tensile strength of real fiber materials.     

Study on microstructure and mechanical properties of spark plasma sintered Alumix 431 powder

Spark plasma sintering (SPS) has been used to successfully densify a 7075 aluminium alloy obtained from Alumix 431 powder. Sintering experiments were conducted at the temperature of 450 and 500°C for 2.5, 5 and 10?min. All the presented results confirm the excellent sinterability of Alumix 431 powder in vacuum during the SPS process and clearly show the effect of sintering temperatures and holding times on the densification, microstructure and mechanical properties of the obtained sintered compacts. The best results of hardness (1412?±?39?MPa), tensile strength (345?±?15?MPa) and compressive strength (618?±?4?MPa) were obtained by the compacts sintered at 500°C for 5 min.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

压制方式和烧结工艺对置氢Ti6Al4V粉末烧结体组织和性能的影响

采用磁脉冲成形和模压成形2种方法对置氢Ti6Al4V粉末进行轴向压制,然后在保护气氛下烧结,研究压制方式和烧结工艺对烧结体真空退火后组织/性能的影响.结果表明:磁脉冲压实的不同氢含量粉末坯体烧结并真空退火后的相对密度、硬度和抗压强度分别比传统模压500 MPa下压制的高8%～13%、9～17 HRA和254～1033M...     

Explicit finite element method simulation of consolidation of monolithic and composite powders

The explicit finite element method (FEM) has been used to simulate the compaction of monolithic and composite powder compacts. It is concluded that with the proper FEM model and appropriate loading speed, explicit FEM can be used to simulate powder compaction with satisfactory accuracy. The simulated pressure-density curves for four periodic powders are in reasonable agreement with experiments using model powders consisting of rods. The effects of the friction coefficient, Poisson’s ratio, and hardening exponent on densification are investigated. Powder compacts consisting of particles with larger Poisson’s ratio, larger interparticle friction, and larger hardening exponent are more diffcult to consolidate in monotonic compaction. Compaction of multiparticle arrays is also simulated to assess the effects of packing randomness and particle rearrangement. The results reveal that local packing details affect the compaction behavior and, in general, the more heterogeneous the powder mixture is, the more difficult it is to consolidate the powder compact. Networking of hard particles significantly increases the densification resistance.     

Electrical resistivity of green powder compacts

Abstract

Different parameters affect the electrical resistivity of green specimens. This paper presents the effect of the particle size distribution, the compacting pressure, and the oxidation of the powder on the electrical resistivity of green specimens fabricated with different powders (Fe, Zn, Ni, and Cu). The results show that the electrical resistivity increases when the compacting pressure decreases, the particle size is reduced and the oxidation increases. It indicates that the electrical resistivity is sensitive to powder surface characteristics and particle interfaces in green compacts. Electrical resistivity may therefore be used to study particle interfaces, evaluate green powder compact characteristics, and monitor powder oxidation.     

Sintered copper–graphite powder compacts forindustrial applications

Abstract

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.     

Gas permeability of powders solidified at high and ultrahigh cooling rates

Conclusions An expression has been derived for determining the duration of vacuum treatment of compacts from a powder of flake-lamellar particle shape, produced by solidification at ultrahigh cooling rates. The expression can be employed also for calculating the times of degassing of spherical powders. The coefficient of air permeability of a compact from a given fraction of a powder of flake-lamellar particle shape is independent of the height (thickness) of the compact. Air permeability varies as a function of compact density. An assembly of several compacts has, at any given relative density, the same resistance to evacuation as a single compact of the same thickness. (The height of the compacts investigated was smaller than their diameter, and their density therefore did not vary significantly along their vertical axes.)Translated from Poroshkovaya Metallurgiya, No. 6(222), pp. 22–27, June, 1981.     

Sintering Characteristics of Al–Pb/Fly-Ash Metal Matrix Composites

Aluminum–lead/10 wt% fly-ash powder mixtures containing 0–20 wt% lead (Pb) were prepared. These powder mixes were compacted in the pressure range of 200–400 MPa by single action die compaction process. The prepared compacts were sintered in the temperature range of 500, 530, 560 and 590 °C in an argon gas atmosphere for duration of 45 min. For the sintered compacts, the sintered density, hardness and compressive strength were reported. Sintered density, hardness and compressive strength increased with the increase in compaction pressure. Sintered density increased whereas the hardness and the compressive strength decreased with the addition of Pb.     

水雾化不锈钢粉末注射成形坯的溶剂脱脂技术

采用多组元聚合物蜡基粘结剂体系,进行水雾化316L不锈钢粉注射成形,研究溶剂脱脂温度、粉末装载量、试样厚度、粉末粒度等工艺参数对溶剂脱脂率的影响.结果表明,脱脂温度越高、粉末装载量越低、试样厚度越薄、粉末粒度越小,则粘结剂脱除率越大.文中首先分析讨论了脱脂可能的速率控制步骤,再通过实验结果对速率控制步骤进行判定,认为脱...     

Direct Observation on the Evolution of Shear Banding and Buckling in Tungsten Fiber Reinforced Zr-Based Bulk Metallic Glass Composite

The evolution of micro-damage and deformation of each phase in the composite plays a pivotal role in the clarification of deformation mechanism of composite. However, limited model and mechanical experiments were conducted to reveal the evolution of the deformation of the two phases in the tungsten fiber reinforced Zr-based bulk metallic glass composite. In this study, quasi-static compressive tests were performed on this composite. For the first time, the evolution of micro-damage and deformation of the two phases in this composite, i.e., shear banding of the metallic glass matrix and buckling deformation of the tungsten fiber, were investigated systematically by controlling the loading process at different degrees of deformation. It is found that under uniaxial compression, buckling of the tungsten fiber occurs first, while the metallic glass matrix deforms homogeneously. Upon further loading, shear bands initiate from the fiber/matrix interface and propagate in the metallic glass matrix. Finally, the composite fractures in a mixed mode, with splitting in the tungsten fiber, along with shear fracture in the metallic glass matrix. Through the analysis on the stress state in the composite and resistance to shear banding of the two phases during compressive deformation, the possible deformation mechanism of the composite is unveiled. The deformation map of the composite, which covers from elastic deformation to final fracture, is obtained as well.     

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。     

Effect of storage on the properties of ultrafine nickel and copper powders

Conclusions Storage of ultrafine powders under unfavorable conditions, in particular in air, adversely affects their processing properties, bringing about marked changes in their compressibility and sinterability. Storage in argon of technical purity does not offer effective protection to particles against surface oxidation. Storage of UFPs in the form of compacts of relative density 0.5–0.6 almost completely inhibits the powder oxidation process, and granulated powders produced by disintegrating compacts possess superior processing characteristics A simple method is proposed for estimating the oxygen content of a powder from the electrical resistivities of compacts.Translated from Poroshkovaya Metallurgiya, No. 10(274), pp. 74–78, October, 1985.     

Powder Metallurgy Composite Materials Strengthened with Fibers

The extrusion dynamics of high-strength powder metallurgy composite materials was studied. The extrusion parameters for porous compacts of copper fibers and Cu ― Mo composites were optimized. It was shown that orientation of fibers is possible only when they are sufficiently widely dispersed in the powder matrix and also when the fiber length is much greater than its diameter l ? d. The mechanical properties of the composites was investigated. A practically pore-free structure was obtained at a degree of reduction λ = 4-6. With this the strengthening effect of fibers in the matrix was fully realized.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

Transport and thermoelectric properties of bismuth and Bi-12 at. pct Sb alloy powder compacts

The transport and thermoelectric properties of bismuth and Bi-12 at. pct Sb alloy powder compacts, varying in particle size from 100 to 400 mesh, have been determined over the temperature range 300 to 77 K (liquid nitrogen). From the measured values of resistivity, Seebeck potential, magnetoresistance effect and Hall coefficient, the thermal conductivity, figure of merit, and the charge carrier mobilities and concentrations are determined for the compacts. The values of the thermoelectric parameters, a andZ, for the multicrystalline powder compacts approach but do not exceed the highest values obtained for the single crystals.     

Volume changes experienced by Al-Zn compacts during liquid-phase sintering

Conclusions The sintering of compacts from aluminum powders with zinc additions in the presence of a liquid phase is accompanied by their volume growth and a corresponding increase in their porosity. The volume growth of compacts from Al-Zn powder mixtures during liquid-phase sintering is mainly due to the Kirkendall effect, which manifests itself during the formation of a solid solution on the aluminum particles as a result of the diffusion of zinc atoms from the melt to the particles preceding their dissolution in the liquid phase. In general, the porosity of sintered compacts is satisfactorily described by Eq. (1). When, however, the zinc content of a compact does not exceed its limit of solid-phase solubility in aluminum at the sintering temperature, the process of dissolution of aluminum in the melt may be ignored. In such a case the end porosity of compacts is described by Eq. (2) with a correction for shrinkage due to a regrouping of particles. The extent to which the volume of compacts from an Al-Zn powder mixture grows during sintering increases with increasing mean aluminum powder particle size.Translated from Poroshkovaya Metallurgiya, No. 10 (238), pp. 11–16, October, 1982.     

Effect of HIP post-treatment on the HIPed Ti6Al4V powder compacts

Hot isostatic pressing (HIP) is an ideal fabrication technique for products with complex structure, but also a reliable post-treatment method. The individual effect of the HIP or HIP post-treatment (HIPPT) on the mechanical properties of powder compacts have been investigated, but little attention has been paid to their combined effect. In this study, the effect of HIPPT on the densification, microstructure and mechanical properties of HIPed powder compacts were investigated. The relative density shows a regional increase after HIPPT. The interlayer spacing of lamellar α phase is increased, equiaxial grains are coarsened and high-angle grains are decreased. Moreover, an obvious increase in elongation is detected, which demonstrates that HIPPT is beneficial to improve the properties of HIPed powder compacts.     

Activation of the sintering process of iron powders by introducing nanodispersed additives

The results of studying of the effect of additions on nanodispersed iron powder (Fe-NP) obtained by the electrical explosion method on the formation and sintering of industrial coarse-grain iron powder are presented. The effect of amount of Fe-NP added to the charge material on compactibility and moldability of compacts, properties, microstructure, and phase composition of sintered material is analyzed. Electrically explosive Fe-NP is shown to be low-technologic. Charges based on the coarse-grained iron powder that contain up to 20% Fe-NP have good compactibility and moldability. It is established that introduction of Fe-NP into the charge causes activation of sintering and is favorable for obtaining the sintered samples with fine-grain crystalline structure and increased physicomechanical characteristics. The effect of amount of the Fe-NP addition to effective activation energy of sintering is evaluated.     

Powder Metallurgy Composite Materials Strengthened with Fibers

The extrusion dynamics of high-strength powder metallurgy composite materials was studied. The extrusion parameters for porous compacts of copper fibers and Cu ― Mo composites were optimized. It was shown that orientation of fibers is possible only when they are sufficiently widely dispersed in the powder matrix and also when the fiber length is much greater than its diameter l ≫ d. The mechanical properties of the composites was investigated. A practically pore-free structure was obtained at a degree of reduction λ = 4-6. With this the strengthening effect of fibers in the matrix was fully realized.