Relationship between physical structure and machinability of green compacts

Abstract

The dependence of green machinability on compact density and strength was investigated for room temperature and warm compacted steel powder compacts containing two different types of lubricant. Brazilian disc compression tests were employed to determine green strength, while machinability was assessed in terms of response to drilling.

For the room temperature compacted materials, it was found that high compact densities and strength were not, in most cases, associated with improvements in machinability. Furthermore, it was shown that lubrication (both type and quantity) and compaction pressure plays a critical role in determining the level of breakouts observed. In contrast, the use of warm compaction, in conjunction with specially designed lubricants, has been shown to be a suitable method of producing high density, high strength compacts while retaining good green machining characteristics. Mechanisms responsible for the observed behaviours of both the room temperature and warm compacted specimens have been forwarded in the present paper.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Modelling the high temperature flow behaviour of sintered Cu–28Zn pre-alloyed powder compacts considering the effect of strain

In the field of the deformation process modelling, constitutive equations tend to be utilised as a calculation basis for the estimation of the materials’ flow responses. This holds also for powder metallurgy products. Hence, in this study, the flow behaviour of sintered Cu–28Zn pre-alloyed powder compacts was investigated by a collection of isothermal hot compression tests in the temperature range of 550–850°C at strain rates of 0.001, 0.01, 0.1 and 0.5?s^?1 up to the true strain of 0.6. For the sintered part with a predefined porosity of 9.4%, the impacts of the temperature and strain rate on the deformation behaviour were shown by the Zener–Hollomon parameter in an exponent-type equation. As a result, a model was developed to predict the high temperature flow behaviour of the Cu–28Zn pre-alloyed powder compact. The results indicated that the strain-dependent constitutive equations are in good conformity with the experimental results.     

Measurement of green strength of warm pressed Distaloy PM compacts: influence of specimen geometry and test method

Abstract

It is important that the green strength properties of powder compacts be accurately determined if advanced manufacturing techniques, such as green machining, are to be exploited successfully. In this study, the green strength of Distaloy AE Densmix, a powder mix designed for such applications, was measured using several test methods. Fracture statistics were correlated with compact density and test method employed, and the Weibull modulus calculated for each case. It was found that Weibull analysis accurately predicted the green strength dependence on the test configuration utilised.     

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.     

Bearing Materials by Powder Metallurgy

Abstract

The dependence of green strength on green density and on compacting pressure was investigated for the bidirectional die pressed and isostatically pressed Cu powder compacts. The breaking strength of the pressed Cu compact was found to increase with green density and also with compacting pressure. The green strength seemed to be directly proportional to the contact area between powder particles. A theoretical equation for the relationship between green density and contact area was derived from a geometrical consideration, and agreed well with experimental findings. PM/0272     

316L不锈钢粉末高速压制行为

采用自行设计制造的18m高落锤式高速压机,研究316L不锈钢粉末的高速压制行为.实验结果表明,冲击速度增大可有效提高生坯密度,对室温粉末进行高速压制,当冲击速度从10 m/s提高到18m/s时,生坯密度从7.18 g/cm3提高到7.61 g/cm3.而在同样冲击速度下,对160℃温粉末进行高速压制时,生坯密度从7.33 g/cm3提高到7.76 g/cm3.同时生坯强度随冲击速度的提高而升高,冲击速度从10 m/s提高到18m/s时,160℃压制的生坯强度从72.5 MPa提高到94.1 MPa,室温压制生坯强度从62.1MPa提高到89.3MPa.通过对生坯SEM照片的分析,得知高速压制过程中粉末会发生严重的塑性变形和碎裂现象,孔隙的形状也会发生改变.该文还对高速压制致密化机理进行了探讨,指出在较高的速度压制时,颗粒间的摩擦和绝热剪切作用使粉末颗粒界面的温度升高,有利于粉末颗粒的塑性变形和焊合,从而有效提高了生坯的密度.     

Effect of particle composition on consolidation of hot briquetted iron

Abstract

The influence of the carbon concentration of directly reduced iron (DRI) powders on the compressibility and fracture strength of hot briquetted iron (HBI) has been studied. Industrially produced DRI, pure iron powder and Fe–C alloy powders (synthetic DRI) were used in the study. It was found that the mechanism of compaction could be attributed to pure yielding. The pressure required to attain a given density increased proportionally with the carbon content. The morphology and phases present in DRI powder had a significant influence on the compressibility. The fracture strength of the compacts increased with increasing carbon content of the DRI powder. These observations are discussed with reference to the current understanding of the mechanisms of compaction and fracture of compacted particulate materials.     

Model for uniaxial compaction of ceramic powders

Abstract

A phenomenological yield criterion has been proposed for modelling the uniaxial compaction processes of various ceramic powder compacts on the basis of continuum mechanics. It includes three parameters to characterise the geometric hardening of the powder compact and the strength of the base material. The model was applied to uniaxial compaction of three ceramic powders which possess different particle size distribution, particle morphology, and base material property. The values of parameters in the yield criterion were determined through the uniaxial compaction experiment. Using the yield criterion, the elastoplastic finite element calculation was carried out to analyse the compaction of the three ceramic powders.     

Observations on densification of Al-Al2O3 composite powder compacts by pressure cycling

Abstract

The effectiveness of pressure cycling on the consolidation of powder composites is investigated. Mixed Al and various amounts of Al₂O₃ powders were consolidated under static and cyclic pressure at room temperature in uniaxial consolidation experiments. The results showed that each pressure cycle increases green density and the cyclic effect is stronger when there is a relatively large volume fraction of Al₂O₃ powder. The compacts produced by pressure cycling have much higher strength than those produced with a single pressure excursion and the process ability of compacts should also improve via pressure cycling. Microscopic observations showed that greater uniformity is obtained in compacts by cyclic consolidation. The origin of the beneficial effect of pressure cycling is related to the deviatoric stresses generated by volumetric mismatch due to the different compressibilities of the phases.     

THE PROPERTIES OF POROUS NICKEL PRODUCED BY PRESSING AND SINTERING

Abstract

The effects of compacting pressure and of sintering temperature and time on the properties of porous sintered nickel compacts have been studied, using three carbonyl and two reduced nickel powders. For all five powders, the density of the green compacts and the porosity of the sintered compacts were linearly related to the log compacting pressure. Similar relationships with pressure were observed for strength and electrical conductivity.

Photomicrographs of sections through the sintered compacts made from the reduced nickel powders show that there are pores in two different size ranges, originating from the porosity between the original powder particles and the pores within the particles. It is concluded that sintered compacts from all five powders containing 40–50% porosity have adequate strength and conductivity for use in fuel-cell electrodes.     

Improving iron compact green strength using powder surface modification

Abstract

In powder metallurgy, green strength has important consequences for part production rates and product end quality. Mechanical interlocking and interparticle cold welding are the main mechanisms responsible for green strength. These mechanisms are affected by compaction pressure, temperature, amount of lubricant and additives admixed to the powder, and surface characteristics of the powder. The present paper describes the effect of iron powder surface modification on the green strength of compacted specimens. The green properties of compacts fabricated from iron powder treated with diluted sulphuric acid and coated with copper by a non-catalytic displacement plating method are presented. The results indicate that surface modifications strongly influence the green strength of the compacts.     

Effect of compaction process sequence on axial density distribution of green compacts

Abstract

During powder compaction, frictional forces between the compact and the tool elements are developed. This causes pressure gradients in the compact, which produces, at the same time, undesirable density differences. This work presents a study about the effect of the compaction process sequence (CPS) and the ejection process on the axial density distribution during the uniaxial double ended pressing process of a high slenderness bushing. Different pressing processes were selected and carried out with a hydraulic press at laboratory and industrial levels (differences in the extraction method). Compacts were presintered and cut into thin discs in order to measure the local densities along the part, using the Archimedes' method. The obtained results show that the compaction process sequence has an important effect on the axial density distribution in a green compact, mainly in the case of compacts of high slenderness (>5). The ejection causes in the compact a post-compaction modification of the axial density distribution. Therefore, it is necessary to optimise the pressing process to obtain an adequate density distribution for each specific part.     

New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Abstract

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.

The precipitation of β-FeC₂O₄.2H₂O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.

Acicular shaping of the β-FeC₂O₄.2H₂O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.     

润滑剂含量对钛粉高速压制性能的影响

采用高速冲击压机压制钛粉,研究润滑剂含量对压坯性能的影响.结果表明:加入适量的润滑剂可以提高钛粉成形时的质量能量密度,从而获得更高密度的压坯.当润滑剂加入量为0.3%(质量分数)时,钛粉成形的最大质量能量密度为0.192 KJ/g,压坯密度为4.38 g/cm3,相对密度为97.4%.此外,适量的润滑剂能提高钛粉压制过...     

温压压制压力强化因子及压制曲线的唯象分析

介绍了一种使用钢铁粉末室温压制回归方程来建立其温压压制方程的分析方法。粉末和模具温度、装粉高度通过压力强化因子影响温压生坯密度。温压压制方程描绘的压制曲线与实验数据相符合。曲线表明装粉高度增加时,粉末和模具加热温度应当降低。对于某些装粉高度温压失去有效性。     

Characteristic of interconnected porosity in ferrous PM compacts

Abstract

The purpose of this work was to quantitatively describe the interconnected porosity in iron compacts, both in macro- and microscale. Size and volume fraction of micro-, meso- and macropores were examined in the compacts with density within 5˙6–6˙4 g cm^?3, made in laboratory conditions of two iron powders: NC100˙24 and ASC100˙29 manufactured by Höganäs Company. The interconnected porosity was determined using the method based on measuring the sorption isotherms of CO₂ and benzene at T=25°C in static conditions in a high vacuum gravimetric appliance equipped with McBain–Bakr weighers.

Volume distributions of individual size classes of micro- and mesopores in the compacts made of both iron powders with fixed density were compared.

Relationships between density and the interconnected micro- and macroporosity of the examined compacts were determined.     

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.     

处理工艺对Fe–Al合金粉末成形性及生坯压溃强度的影响

为了提高Fe–Al合金粉末的压制成形性和生坯压溃强度,分析了氢气还原和真空退火工艺对Fe–Al合金粉末形貌和性能的影响,研究了原始粉末和经过处理后粉末所制管坯的成形率。结果发现,经氢气还原和真空退火处理后,粉末形貌变化不大,粉末性能提高,生坯压溃强度增强,成形性得到改善;尤其是经真空退火粉末的性能得到极大提高,氧的质量分数由0.5%低到0.2%,松装密度由1.82 g·cm?3降低到1.64 g·cm?3,显微硬度由HV 260降低到HV 158,压缩比由63%降低到56%,生坯压溃强度由2.0 MPa提高到2.7 MPa,粉末成形性得到极大改善。在批量化压制长管坯时,对原始粉末预先进行真空退火处理,长管生坯数量成品率由50%提高至100%,产品成本降低。     

Experimental investigation of yield behaviour of metal powder compacts

Abstract

In this paper the compaction and yield response of two steel and two copper powders are examined. These were chosen to determine how the material response depends on the type of material and the morphology of the powder particles. Experiments were conducted in a computer controlled triaxial cell. Here, concentration is on the response during simulated, frictionless closed die compaction, whereby the radial stress is controlled so as to keep the radius of the sample constant. The compaction process was stopped at regular intervals and a series of probing paths were followed in stress space to construct the yield surface for the compact.

The experimentally determined yield surfaces are compared with yield surfaces predicted by empirical models and micromechanical models of the Fleck type, which assume that the compact consists of monosized spherical particles. During the early stages of compaction the form of the yield surfaces for spherical powders are consistent with Fleck's micromechanical model, but the surfaces become less elongated in the direction of loading at high densities. The yield surfaces for irregular shaped powders are significantly different from the predictions of the Fleck micromechanical model. A modified anisotropic Cam-Clay model is proposed, which is able to predict yield surfaces for the four powders at all densification levels.