Effect of Zinc Stearate on Apparent Density,Mixing, and Compaction/Ejection of Iron Powder Compacts

Abstract

The behaviour of zinc stéarate (0–4 wt-%) during mixing with iron powder has been studied by monitoring changes of apparent density and comparison with powder prepared by depositing zinc stéarate from solution onto the iron particles. Apparent density was increased by the development of thin surface films at low lubricant contents but was decreased by the formation of thick layers and free lubricant at high contents. Pressure/density data were obtained from floating-die compaction of cylindrical iron powder compacts containing deposited zinc stéarate with and without die-wall lubrication to determine the relative significance of interparticle friction, particle/die-wall friction, and compaction inhibition. An optimum lubricant addition minimized interparticle friction during compaction. This was greater than the optimum observed for loose packing owing to the more arduous conditions prevalent in compaction. Compaction pressure losses associated with interparticle friction were lower than those caused by die-wall friction. The latter was only minimized by lubricant additions which also caused undesirable compaction inhibition. The forces required to overcome particle/die-wall friction were similar for both compaction and ejection.     

润滑剂对Fe基粉末冶金材料温压工艺的影响

通过扫描电子显微镜观察和性能测试研究了硬脂酸锌、乙烯基双硬脂酰胺(ethylene bis stearamide,EBS)、复合润滑剂以及压制温度对Fe基粉末冶金材料温压工艺的影响规律。结果表明:当润滑剂加入量(质量分数)超过0.4%后,Fe基粉末的流动性和松装密度均随润滑剂加入量的增加而降低,其中加入单一EBS润滑剂的影响更大。添加润滑剂后增加了Fe基粉末冶金生坯的致密度,其中添加硬脂酸锌和复合润滑剂的Fe基粉末冶金生坯断口颗粒间结合更为紧密。润滑剂对提高Fe基粉末冶金试样生坯密度、烧结密度及抗弯强度的作用顺序为复合润滑剂硬脂酸锌EBS,Fe基粉末冶金材料的密度和力学性能均随温压温度的升高而增加。在最佳润滑剂加入量0.4%时,120℃温压Fe基粉末冶金试样密度比室温压制Fe基粉末冶金试样的密度提高了0.14~0.21 g/cm~3,硬度和抗弯强度提高了40%~65%。     

Development of a technology of compaction of increased-density parts using gas drainage methods

The influence of the amount and dispersion of technological lubricant (zinc stearate) and solid lubricants (talc, graphite, calcium fluoride) as well as the sintering temperature and time on the physical and mechanical properties of sintered iron is studied. When the stock contains no technological lubricant the porosity of the specimens drops to 5.0–5.2% but their strength is substantially greater than that of SP10 and SP30 steels of the same porosity. The strength of the specimens is highest when the iron powder has the smallest grain size. Solid lubricants in the stock result in the pressed iron-powder parts having poorer mechanical properties, although they do act in part as technological lubricants during compaction. Favorable results may be obtained by complex alloying of the stock, i.e., the simultaneous introduction of several solid lubricants.     

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.     

利用高生坯强度润滑剂开发无线通讯装置用PM软磁零件

软磁烧结铁框是无线通讯装置中的一个重要零件，用其来防止电子元件间的电磁干扰和对人有害的电磁辐射。铁框结构复杂且轻巧，依据电子电路的具体设计，壁厚可薄至0.8mm。传统的金属冲压成形因零件挠曲而不能满足尺寸公差与平直度的要求。用粉末冶金工艺制造铁框面临的难题是因壁薄，截面处易碎而使生坯难以搬运。用常规的硬脂酸锌或EBS蜡之类润滑剂混粉，生坯的废品率高达50％。因此，采取了用新近开发的高生坯强度润滑剂，制造烧结铁框的方法。为使开发的烧结铁框能达到磁性要求，选用了添加0.45%磷的高纯铁粉ATOMET 1001HP。采用冷压、温压及在压制后进行和不进行固化的不同工艺，测试了几种高生坯强度润滑剂纱列，从中选出了一种使压坯强度满足粉末冶金铁框压坯搬运要求的润滑剂系列，从而大大降低了生产成本。     

Characterization,Physical and Mechanical Behavior of Sintered Atomized Iron–Zinc Stearate Composite

Powder metallurgy is an effective method to process the iron component in near net shape. In this paper, the influence of particle size, lubricant and compaction load on the physical and mechanical properties of the sintered iron–zinc stearate composite sample has been investigated. Atomized iron powders of particle size 100–200 and 200–300 mesh with zinc stearate 2.5, 5.0 and 7.5 wt% were used for preparing the samples. Green samples were prepared by cold compaction at various loads of 200, 180 and 160 KN and the sintering is done at 500 °C. The physical properties and the mechanical properties such as density, hardness and compression strength have been measured for the sintered samples. Scanning electron microscope was used to characterize the atomized iron powders and the sintered samples. It were confirmed that the shape of atomized iron powder particles were irregular, dendritic and acicular. Energy dispersive spectroscopy was used to identify the elemental compositions of powders and the sintered samples. Taguchi (L₁₈) method was effectively used to develop the regression model and describe the contribution of the input parameters in compressive strength, density, and hardness. The highest effect on density was powder particle size followed by compaction load and lubricant wt%. The Hardness value increased with increase in the powder particle size and compaction load and decreases with increase in the lubricant wt%. The compression strength increased with increase of compaction load and decrease of lubricant wt% and particle size.     

New technological approach to fabrication of high density PM parts by cold pressing sintering

Abstract

A new technological approach to the fabrication of high density powder metallurgy (PM) parts via single pressing sintering, allowing cold compaction to be performed without admixed lubricants, has been studied. The influence of in pore gas on the compacts' green density and their sintered properties were evaluated. A mathematical expression relating in pore gas pressure in the compacts to the green density was developed. The expression showed that in order to reduce the negative influence of gases trapped in the pores it is necessary to ensure effective air drainage from the compaction zone. In order to ensure sufficient air evacuation during cold compaction, a new design of porous die was developed. The behaviour of powder mixes with different lubricants during cold compaction in porous die was investigated. All the test conditions were evaluated in terms of green and sintered properties, including the ejection force, green and sintered densities, tensile strength and surface hardness. In the context of the experimental work, compaction in porous die promoted the improved combination of green and sintered properties compared with compaction in conventional dies.     

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

Abstract

A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640^°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ～98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.     

THE CORRELATION BETWEEN RAW MATERIALS,PREPARATION CONDITIONS,AND PROPERTIES OF SINTERED IRON

Abstract

Three plain iron powders of different types (sponge-iron, atomized and electrolytic iron powder) were studied with respect to their sintering behaviour and to the influence of manufacturing parameters—i.e., compacting pressure, sintering temperature, and sintering atmosphere—on the microstructure and the properties of sintered compacts. The changes of length, electric conductivity, and strength during sintering are explained in physical and chemical terms. Technical sintering diagrams are presented. The influence of sintering atmospheres on the mechanical properties of sintered compacts is shown for the three types of powder. The correlation between pore structure and strength is discussed; analytical relationships are developed which are in agreement with the experimental results.     

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.     

IMPORTANCE OF STEARATES IN THE FABRICATION OF UO2 AND (U,Pu)O2 PELLETS

Abstract

Eighteen qualities of metallic soaps have been studied, either as lubricants or as additives for the fabrication of UO₂ and (U,Pu)O₂ pellets. Compounds of the same family, such as zinc stearates, differ in their effectiveness as lubricants. This property is strongly dependent on the particle size of the powder and its purity. The behaviour of the dry lubricants when used as admixtures in UO₂ and also when employed for the direct lubrication of the pelletizing tools is discussed.

The efficiency of metallic soaps as additives to lower the density of the sintered pellets is related to the nature of the metallic ion. Thus, zinc stearate has little influence on the fired density of UO₂, whereas barium stearate and other barium compounds are very effective. Barium inhibits sintering, but zinc, which is eliminated at low temperature, does not diminish the sinterability of the oxide powders. In the latter case, the decrease in density is caused by thermal decomposition of the organic molecule.     

The effect of nitrogen flow rate on Acrawax C decomposition and its removal during sintering of Alumix 431D grade powder

Alumix 431D pre-alloyed powder (ECKA Granules GmbH) containing 1.5 mass% of Acrawax C was used to study the effect of nitrogen flow rate on delubrication and sintering evolution. Mass loss of compacts during heating was controlled by the TG method using a STA Netzsch apparatus coupled with a mass spectrometer. The latter was used to identify the volatile lubricant’s decomposition products. Macro- and microstructural observations of sintered compacts were also performed. The results documented a strong influence of nitrogen flow rate on delubrication and thus on the sintering behaviour of examined powder. High nitrogen flow rate is required to produce the desired sintered compacts. In contrast, at low nitrogen velocity, the lubricant removal is not complete, which leads to the formation of carbonaceous residuals in the form of soot, which in turn significantly impedes densification and deteriorates the sintered products.     

IRON-COPPER-TIN SINTERED COMPACTS

Abstract

A wide range of copper and tin powder additions to iron powder sintered compacts hasbeen studied. From mechanical-property tests it has been shown that when using sinteririg temperatures of 900–1100°C in nitrogen/10% hydrogen atmospheres there is an optimum copper: tin ratio of 15:2. The mechanical properties obtained from compacts pressed from iron mixed with 4% copper+tin in this ratio and sintered at 900°C were similar to those obtained from iron ?l0% copper powder compacts sintered at 1100°C. Moreover, the iron-copper-tin components showed improved dimensional accuracy.

In a further series of experiments, it was shown that tin additions to iron-copper alloy compacts increased the solubility of iron in the liquid phase at the sintering temperature and simultaneously decreased the rate of diffusion of copper into the iron particles. At the same time, tin improved the wettability of the liquid, reducing its surface tension and allowing it to disperse more completely throughout the matrix. The mechanical properties of compacts containing larger amounts of tin were decreased by the presence of brittle compounds, although the sintering rate was increased. It is concluded that the optimum properties of iron-copper-tin compacts are obtained by making correct additions of copper and tin to the iron powder and giving careful consideration to the sintering atmosphere.     

硅黄铜预合金粉末在金刚石工具中的应用

采用气水耦合雾化法制备了含有质量分数0.4%稀土Ce的硅黄铜预合金粉末, 在680~720 ℃热压烧结预合金粉末, 获得了致密烧结体。通过洛氏硬度计测试烧结体硬度为HRB62~65, 使用万能力学实验机测试烧结体抗弯强度为530~550 MPa, 利用扫描电子显微镜观察烧结体基体微观组织为α相黄铜, 组织中还存在蠕虫状及花状的弥散银灰色(β'+γ)相。选择添加质量分数20%的硅黄铜预合金粉末配方(铁粉+铜粉+锌粉+预合金粉末)制备花岗岩切割用金刚石刀头, 与单质粉末混合配方(铁粉+铜粉+锌粉)制备的刀头相比, 添加硅黄铜预合金粉末的刀头硬度增加22%, 抗弯强度降低8%, 刀头锋利度和使用寿命整体提升, 胎体磨粒夹杂减少, 并出现大量排屑沟槽。     

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.     

Effects of lubricants and compacting pressure on the processability and properties of aluminum P/M parts

Lubricants are generally admixed to metal powders to ease compaction and minimize the interaction between the tooling and the compact during ejection. The type and concentration of the lubricant have a significant impact on the processability and the characteristics of the material. While extensive work has been performed with iron-based mixes, relatively few studies have been published on the effects of lubricants and shaping conditions on the processing behavior and characteristics of aluminum (Al) components fabricated by powder metallurgy (P/M).This paper presents the effects of the lubricant type (ethylene bisstearamide vs. polyethylene wax), the lubricant concentration (0.7–1.5 wt.%), as well as the compacting pressure (138–413 MPa) on the powder characteristics and the properties of press and sintered 2xxx series aluminum alloy specimens (AMB-2712). Compaction and ejection characteristics, surface finish, green density and strength, dimensional change during sintering, sintered density and mechanical properties are presented and discussed.     

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.     

Mechanical properties of fully densified injection-molded carbonyl iron powder

The mechanical properties of carbonyl iron powder shaped by injection molding techniques are affected by the grain size, sintered density, and carbon content. Control of the carbon level depends on several factors, including the binder composition, debinding approach, and sintering conditions (atmosphere, temperature, time, and furnace design). Sintered compacts were densified by containerless hot isostatic pressing, giving smaller grain sizes and superior properties than were possible by pressureless sintering at a high temperature. A quick hot isostatic pressing route (gas forging) with a peak pressure higher than 500 MPa for 1 minute helps retain carbon and results in excellent properties due to a high final density and small grain size. This approach resulted in a final strength of 732 MPa with extensive ductility (23 pct reduction of area) for injection-molded carbonyl iron.     

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.     

Production and properties of brass-base P/M constructional materials. A review

Conclusions The best processing properties are exhibited by brass powders manufactured by the diffusional impregnation technique, using a zinc powder, brass swarf, or a copper-zinc master alloy as a point source. However, as this is a very labor-intensive process, normally preference should be given to melt atomization as a method of manufacture of brass powders. Brass P/M parts produced by the conventional method consisting of pressing a powder and sintering the resultant compacts have porosities of not less than 7–10%, and consequently this method is not widely used for the production of constructional brass parts. The sintering of compacts from copper and copper-zinc master alloy powders gives more stable zinc contents compared with the sintering of compacts from copper and zinc powders; the greatest stability of chemical composition is exhibited by sintered compacts from a homogenized brass powder. The formation of diffusional porosity accompanying the evaporation of zinc may be prevented by performing sintering in the presence of a liquid phase (which appears in the presence of a phosphorus or lead addition), saturating the sintering atmosphere with zinc vapor, and adding carbonates or halides of alkali and rare-earth metals to starting powders. The mechanical properties of materials can be markedly improved by eliminating their porosity. This may be achieved by subjecting porous preforms to hot forging, which enables brass P/M parts to be obtained whose mechanical properties are comparable to those of cast parts.Translated from Eoroshkovaya Metallurgiya, No. 3(255), pp. 56–64, March, 1984.