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     

THE EFFECT OF INTERPARTICLE CONTACT AREA ON THE STRENGTH OF COLD-PRESSED ALUMINIUM POWDER COMPACTS

Abstract

Measurements of the tensile strength of spherical cold-pressed aluminium powder, pressed to various densities up to the theoretical maximum, have shown that compaction is a two-stage process. At some high, intermediate pressure, interparticle sliding occurs in a way that does not itself increase densification but makes it easier for further deformation to occur. It is likely that the pressure at which this sliding takes place is dependent on the work-hardening rate of the powder as well as the powder size and morphology. In any case, it is shown to be important to the densification and strength reached by the compact.

It is concluded that the strength of a green compact is dependent upon the interparticle metallic contacts made during compaction. However,the green strength is well below that of wrought aluminium, probably due to the presence of broken-up oxides,which act as stress-concentrators at the interparticle boundaries.     

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.     

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.     

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.     

Mechanical properties of metal injection moulded 316L stainless steel using both prealloy and master alloy techniques

Abstract

Stainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of ? 16 μm and ? 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed ? 16 μm and the ? 22 μm powders. Also, the viscosity of the mixtures was higher for the ? 16 μm material and the master alloy mixtures than for the –22 μm gas atomised prealloyed powders.     

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.     

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.     

Numerical simulation of metal powder die compaction with special consideration of cracking

Abstract

Powder die compaction is modelled using the finite element method and a phenomenological material model. The Drucker–Prager cap model is modified with the goal to describe the formation of cracks during powder transfer, compaction, unloading, and ejection of the parts from the die. This is achieved by considering the cohesive strength and the cohesion slope, which characterise the current strength of the powder compact in the Drucker–Prager model, as state dependent variables. Evolution equations are formulated for these variables, so that the strength increases by densification and decreases by forced shear deformation. Some of the parameters appearing in the evolution equations are determined from measured green strength values. An iron based powder (Distaloy AE) is used for the experiments. Examples are shown to demonstrate that the density distribution can be calculated accurately as compared with an experiment, that cracking can be modelled at least qualitatively correctly, and that the compaction of complex 3D parts can be simulated.     

Mechanical properties distributions of PM manganese steels analysed by Gaussian and Weibull statistics

Abstract

Powder metallurgy (PM) parts acceptance is determined by compositional and processing parameters and their controls. Statistical procedures are used for assessment; normal distribution and 'six sigma' appear to predominate. For fatigue of metallic materials and strengths of ceramics, fibres and composites, however, Weibull probability of survival analyses are widely used. The original analysis considers a threshold stress at which the probability of failure is zero. This stress is frequently taken to be zero, simplifying the analysis to two parameters. The yield stress has been suggested for the threshold stress, probably not sufficiently conservative for less ductile PM materials. A new three-parameter Weibull analysis, in which it is taken to be the fracture strength minus six standard deviations, is presented. Powder metallurgy manganese steels are under commercial consideration and this approach is applied to 12 variants of such laboratory processed specimens. It is compared with the two-parameter Weibull and Gaussian, the least conservative, analyses.     

Study of warm compaction of Alumix 123 L

Abstract

Although powder metallurgy (PM) material is dominated by ferrous alloys, there is a growing interest in Al PM. The usage of Al PM in automotive applications depends on the development of higher density and improved dynamic properties. Several approaches have been proposed to increase density of sintered parts. Warm compaction process of Al powder was used to achieve high density. In this study the authors focused on the effect of warm compaction on Alumix 123 L (ECKA Granules) powder blend. It has been found that warm compaction at 110°C led to a reduction in the ejection force by 27·9%, increased green density to 94% of theoretical density and increased sintered strength to 315 MPa as compared to those pressed at room temperature.     

The Effect of Density Distribution on the Radial/Axial Shrinkage Ratio of Iron Powder Compacts

Abstract

The effect of compacting pressure on the radial/axial (R/A) shrinkage ratio for iron powder compacts was studied. It was observed that R/A and compacting pressure have a linear relationship. The effect of density distribution on R/A ratio inside the green compact was determined. The observed changes in R/A are attributed to the change in density distribution in the green compact.     

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.     

Variability of powder characteristics and their effect on dimensional variability of powder injection moulded components

Abstract

In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations in powder characteristics such as surface area, tap and apparent density, and chemistry have little effect on dimensional stability.     

DEFORMATION AND DENSIFICATION DURING THE ROLLING OF METAL POWDERS

Abstract

The deformation of particles and the general process of densification during the roll-compacting of strip from metal powder have been determined by photomicrographic and QTM studies. Observations were made on the expansion of the compacted strip after it had passed the plane joining the roll axes. The effect was related to elastic recovery of the material and the expansion of gases entrapped in the pores between the particles.

The production of satisfactory green strip was found to be restricted to a range of thicknesses obtained between certain maximum and minimum roll gaps. These limits were related to roll pressure and strip density. It was also restricted by a maximum rolling speed that was governed by powder flow to the compaction zone.

Density variations that occurred across the width and through the thickness of green strip were also determined.     

Influence of Lubricants on Dimensional Changes and Mechanical Properties of Sintered Ferrous Compacts

Abstract

The influence of admixed zinc stéarate on the shrinkage of uniaxially pressed iron powder compacts has been studied. For pressing conditions which caused inhibition of compaction the removal of the stéarate during sintering produced an increase in shrinkage parallel to the pressing axis and in direct proportion to lubricant content. Additions of stearic acid (varying particle size), zinc stearate, lithium stearate, stearamide, and Cosmic 64 wax were used to investigate the influence of lubricant on mechanical properties of green and sintered iron powder compacts. Green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing. Vapour from the rapid initial decomposition of lubricants which reduced green strength could have a deleterious physical influence on the tensile strength of dewaxed or sintered Fe compacts. Decomposing lubricants also produced undesirable chemical effects. These arose from reactions between lubricant decomposition products and the matrix or by these products interfering with reactions between matrix and sintering atmosphere.     

ASPECTS OF THE VOLUME PRODUCTION OF GREEN POROUS BEARING COMPACTS

Abstract

The history and method of manufacture of self-lubricating porous metal bearings are briefly outlined. The importance of achieving appropriate quality at each stage of manufacture, and particularly during compacting, is emphasized and the qualities desired in a green bearing compact are detailed. Various factors relating to the economic large-scale manufacture of green bearing compacts are then considered. The virtues of elemental powders and the influence of lubrication upon powder mix properties and pressing and ejection of compacts are discussed. The special powder flow problems associated with long, thin-walled bearings and the advantage, in this respect, of a moving core rod are described. The particular difficulties encountered in compacting such bearings are stated and special consideration is given to the ejection problems involved. It is concluded that limitations of bearing length and wall section must be accepted.     

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.     

Some Mechanical Properties of Powder-Forged Iron/Alloy Mixtures

Abstract

An investigation was made of the feasibility of producing alloy forgings (nominal composition Fe–0·5C–0·6% Mn), from a powder prepared by mixing the alloying additions with iron powder. The carbon was added as microcrystalline graphite and the manganese as elemental powder and as ferro-manganese powder. Additions of copper and ammonium chloride powders were also made for the purpose of assisting the manganese to alloy with the iron. The copper addition improved the tensile strength but lowered the ductility. The ammonium chloride had little effect except for an apparent lowering of hardness. It was found that useful mechanical properties could be obtained in forgings made from mixtures of the alloying ingredients.     

THE PREPARATION AND SOME PROPERTIES OF CHROMIC OXIDE-CHROMIUM CERMETS MADE BY PARTIAL REDUCTION OF THE OXIDE

Abstract

Compacts of chromic oxide/carbon mixtures have been sintered in vacuum to produce oxide-metal cermets. The effect of carbon addition, type of carbon, purity of oxide, compacting pressure, and sintering temperature on the green and sintered density has been studied, and this has been supplemented by tests of hardness and compressive strength.

Considerable densification can be attained by a small addition of carbon to the powder mixture, and this is accompanied by an increase in compressive strength to 20 tons/in², compared with 3 tons/in² for the pure oxide sintered to the same temperature.