In situ strength evolution during the sintering of bronze powders

Powder metallurgy (PM) allows the fabrication of complex net-shaped components. Accurate design specification of these components requires precise prediction of the compact’s response to sintering process parameters. Nonuniform sintering responses such as strain gradients can result in process failures such as permanent deformation and cracks. To avoid these types of process failures without costly trial-and-error design, the most important response to predict is the compact’s strength as it evolves during the sintering process. A device and method have been developed to characterize the in situ strength evolution as a function of various sintering process parameters. The specific strength parameter investigated and modeled in this article was transverse rupture. This strength was precisely determined for 90Cu-10Sn bronze in response to various combinations of temperature, heating rate, and heating time. The consequence of this work is to identify thermal cycles that minimize distortion and otherwise improve dimensional tolerances.     

Dynamics of interparticle reactions in spherical metal powders during electric sintering

Conclusions The dynamic characteristics of the process of interparticle neck formation depend not only on current density (raising the current density by 1.7 times increases interaction rates by an order) but also on the positions of the particles relative to the direction of passage of current (particles lying in a plane parallel to the direction of passage of current interact 3.4–5 times more rapidly than particles lying in a plane perpendicular to this direction). The kinetics of powder particle sintering by an electric current is due to the most intense sintering mechanisms, such as viscous flow, transport through the gaseous phase, and spreading of liquid metal over the solid surfaces of particles. The strongest influence on the kinetics of mutual particle sintering is exerted by a transport mechanism with n< 0.5. Such a mechanism may be provided by electroerosion transport in the interparticle gap.Translated from Poroshkovaya Metallurgiya, No. 9(297), pp. 25–29, September, 1987.     

Diffusion processes in the sintering of nickel alloy powders with spherical particles under pressure

Conclusions Hot gasostatic pressing and subsequent annealing at 1200C result in the formation of 160–200-m diffusion zones between particles, which extend over practically the whole particle widths. During sintering under conditions of hot hydrostatic pressing, which is characterized by a shorter period of annealing at any given temperature and pressure, the widths of the diffusion zones after sintering and annealing attain 50–65m. The optimum diffusion zone width in the sintering of particles making good contact with one another (pure and even surfaces) is about 30–40m. However, the presence of oxides, carbide phases, and more complex compounds on the surfaces of particles substantially increases the optimum width of this zone. In such a case the optimum width is determined by the need for diffusion to cross interparticle contacts with diffusion-inhibiting barriers. In view of this, for particles of the nickel alloys investigated the optimum diffusion zone width may be taken to be 100m. At this width there are no areas in which the diffusion zone width is less than 40m, i.e., the minimum thickness at which strong bonds can be expected to form between particles.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 38–41, May, 1982.     

Study of the sintering of nickel powders produced by different methods

Conclusions It was established in this investigation that, in the temperature range 500–700°C, carbonyl nickel powder is subject to more intensive sintering than electrolytic nickel powder. The theory of diffusional viscous flow is unsuitable for this case, and needs revision.It is shown that crystal lattice distortions, as determined by x-ray diffraction methods, do not cause an increase in the rate of sintering. Hypotheses are put forward for the possible mechanism by which diffusional mobility is increased in the case of carbonyl nickel.     

Properties and sintering behaviour of fine spherical iron powders produced by new hydrogen reduction process

Abstract

The most important use of fine spherical iron powders is for metal injection moulding (MIM). For many applications, the high costs of powder based on the carbonyl or atomising production route are a limiting factors. An alternative two-step hydrogen reduction process using a granulated hematite powder, which is a recycling product from steelmaking, has been developed to produce <25 µm spherical powder. The morphology and properties of the powder have been found to depend strongly on the second temperature step of the reduction process. A further important step is enclosed powder processing by milling and sieving to remove agglomerates. The powder properties and sintering behaviour as a function of heat treatment and processing parameters are reported and discussed.     

不同方法制备的球形铸造碳化钨粉末的性能研究

球形铸造碳化钨粉末是一种新型的超耐磨陶瓷颗粒材料,对比研究了不同方法制备的球形铸造碳化钨粉末的化学成分、显微形貌、微观组织、相结构、显微硬度、流动性以及松装密度等特性。结果表明:相对等离子重熔球化法和感应重熔球化法而言,等离子旋转电极雾化法制备的球形铸造碳化钨的总碳含量与共晶碳含量理论值的偏差最小、游离碳和杂质含量最低;粉末颗粒内部更致密、共晶组织更细密、显微硬度最高。感应重熔球化方法制得的粉末流动性最好、松装密度最大。     

Device for electric-pulse sintering of powders under pressure

The construction and energy parameters of an experimental device for electric-pulse sintering of powder under pressure are described. The properties of material prepared from Nibon-20 powder by electric-pulse sintering technology are given. Translated from Poroshkovaya Metallurgiya. Nos. 1–2(411), pp. 125–128. January–February, 2000.