FACTORS AFFECTING THE RADIAL AND AXIAL SHRINKAGE IN SOFT-METAL POWDER COMPACTS

Abstract

The variables affecting the radial: axial (R/A) shrinkage ratio in copper-powder compacts have been investigated. The value of R/A is linearly dependent on compacting pressure, green density, and sintering temperature, and also increases with decrease in the particle size of the powder. The observed variation of R/A is attributed to the differences in density in the green compacts, which result in anisotropic stresses in sintering. Surface-tension forces or residual stresses introduced during compaction cannot alone be regarded as the main driving forces responsible for shrinkage; anisotropic stresses also play an important role in the densification of metal-powder compacts. By proper control of these variables, parts can be produced from the compacts to close dimensional tolerances.     

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.     

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.     

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     

FACTORS AFFECTING THE COMPACTION OF TUNGSTEN POWDERS

Abstract

It is difficult to form tungsten powders into compacts by pressure-forming methods. The brittleness of the powder particles causes them to fracture under pressure instead of producing the typical “point welds” exhibited by more ductile particles. Because of this, the powder characteristics such as particle size, size distribution, and particle shape play a most important role in the compacting of tungsten powders.

Both regular- and irregular-shaped particles of tungsten powder are discussed as regards the formation of strong and dense compacts from these powders. Powders composed of irregular-shaped particles gave stronger, but less dense compacts. The effects of particle size and particle-size distribution are also considered. Each of these factors has individual as well as combined effects. It was found that certain critical particle-size distributions produced the densest compacts.

It is concluded that interlocking of particles, which is brought about by surface irregularities, and interfit, which is determined by correct particle-size distribution, are the determining factors in the compaction of tungsten powders.     

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.     

Metallic Contact of Atomized Aluminium Powder Compacts for Hot-Rolling in Air

Abstract

The effect of oxide film on the surface of an atomized aluminium powder compacted under pressure was investigated in a sintering process by means of measuring electrical resistivity, observation with a scanning electron microscope, and measurements with an X-ray photoelectron spectrometer. Oxidation of the compact in air was also observed. On the basis of these results, hot-rolling of the compact in air was undertaken to produce an aluminium sheet, with successful results under appropriate compacting pressure, heating time and sintering temperature before rolling.     

SINTERED IRON-COBALT-MOLYBDENUM ALLOYS

Abstract

The magnetic properties and certain physical properties of sintered iron-cobalt-molybdenum alloys have been investigated, and a study made of the effect of processing variables, i.e. particle size, compacting pressure, sintering temperature and time. The compacting and sintering conditions which result in optimum properties have been determined. Consideration is also given to the influence on magnetic properties of variations in heat-treatment procedure, involving such factors as quenching temperature and media, ageing time and temperature, &c.

The substitution of ferro-molybdenum powder for elemental molybdenum powder reduces the cost of raw materials substantially without greatly diminishing the magnetic properties. Commercial powders with a normal particle-size distribution give good properties.

It was found that the alloys had somewhat better qualities than those made by casting.     

DETERMINATION OF HOOP STRESSES INDUCED IN A CYLINDRICAL STEEL DIE BY COMPACTING METAL POWDERS

Abstract

Because of the lack of stress data, steel dies for compacting metal powders were designed in the past on the assumption of hydraulic pressure transmission by powders.

True die stresses existing in a hardened steel die during the compacting of various lubricated and non-lubricated powders have been measured by mounting strain-gauges on the periphery of the die at numerous points along its length.

Values of the hoop stress varied from approximately 0·05 to 0·40 of the calculated and measured hydraulic fluid stress. For a particular metal powder, the stress was a function of the average particle size, the powder height, and the amount of lubrication.     

VARIOUS FACTORS AFFECTING THE WEAR OF STEEL DIES USED FOR THE COMPACTING OF IRON POWDER

Abstract

Small cylindrical iron powder compacts have been produced in series of ～ 75 000 specimens, using an automatic compacting press running at a speed of 33 strokes/min. Die-wear rates were determined, as influenced by powder type, compacting pressure, compact density, type and hardness of die steel, and punch/die clearance. Simultaneously, the ejection forces were continuously recorded.

No substantial difference was found between the die-wear rates of common iron powder grades of an atomized or a sponge type, but an electrolytic grade of especially compact particle structure gave a lower rate. Die wear increases in roughly linear proportion both with the number of compacts and with the compacting pressure.

Steel type and hardness of the die had a pronounced influence upon die-wear behaviour, dies of higher hardness yielding lower wear rates. Unsuitable heat-treatment can cause high wear rates even though it might produce high die hardnesses.

A punch/die clearance of 10 μm resulted in the most favourable die-wear behaviour. At a clearance of only 5 μm the punches became stuck in the die very quickly; at clearances of 25 and 45 μm severe cladding of the die-bore surface with iron from the compacts occurred after 40 000-50 000 strokes.

The following, partly interacting, phenomena were found to be contributory factors in the die-wear mechanism: abrasion, cladding, surface cracking, and chipping. It proved impossible to establish a reliable correlation between ejection forces and die-wear rates.     

VIBRATORY COMPACTING OF METAL POWDERS

Abstract

The vibratory compacting of copper powder has been studied using a mechanical vibrator. The major factors influencing the green density of the compacts were the amplitude and frequency of vibration, and the applied pressure. A minimum time of 10 sec on the vibrator was necessary to achieve the maximum density value. Other factors examined were the effects of vibration on blended powders with constituents of widely different densities, and the suitability of this method to compact various materials. Vibratory compacting produced compacts of improved uniformity and green density.     

INVESTIGATION OF THE DIMENSIONAL CHANGES IN SOFT METAL POWDER COMPACTS

Abstract

The variables affecting the radial/axial (R/A) shrinkage ratio in compacts made from spherical copper powder have been investigated, also the linear dependence of R/A on compacting pressure and sintering temperature. The values of R/A for spherical powder are higher than those for irregularly shaped powder. The effect of particle shape and height/dia. ratio of the compact on R/A have been studied. The differences in green-density distributions have been determined, together with the effect on these of pressure and height/dia. ratio of the compact. The observed variation of R/A is attributed to differences in density distribution in the green compacts, resulting in anisotropic stresses during sintering.     

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.     

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.     

EFFECTS OF PARTICLE SIZE ON THE SINTERING KINETICS IN TUNGSTEN POWDER

Abstract

An experimental study has been made of the effects of initial particle size on sintering kinetics in tungsten powder within the temperature range 1100–1500°C. Particle size, compacting pressure, sintering time and temperature all influence the rate of sintering. Isothermal changes in density and volume have been measured. The results indicate grain-boundary diffusion as the mechanism principally responsible for material transport in the case of particle sizes <4 μ Surface diffusion appears to bethe mechanism of material transport in compacts with particle sizes of 14– 16 μ The temperature-dependence of the rate of sintering is characterized by activation energies of 101± 2 and 72± 2 kcal/mole for fine particles (< 4 μ) and coarse particles (14–16 μ), respectively.     

Stiffness variation of porous titanium developed using space holder method

Abstract

The excellent properties of Ti have resulted in its generalised use for bone implants. However, Ti is very stiff in comparison with human cortical bone, and this creates problems of bone weakening and loosening of the implant. This article discusses the mechanical properties (flexural and compressive strength, and stiffness) of porous Ti–6Al–4V specimens developed using the space holder method. These properties are examined relative to the production process parameters: compacting pressure and sintering time, as well as temperature, and the addition of spacer and its particle size. It is seen that when spacer is added, compressive strength decreases with the application of compacting pressure and that these are the most influential parameters. The developed pieces show a closed and unconnected porosity. Small additions of spacer (25 vol.-%) reduce stiffness to around half of that shown by the solid material, and the resulting pieces are strong enough to be used as bone substitute.     

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.     

氢化钛粉末及压坯的脱氢规律

通过热重分析(TGA)研究TiH2粉末粒度对其脱氢温度及脱氢量的影响,采用热膨胀仪研究粉末粒度对TiH2压坯收缩率的影响,同时利用真空烧结炉研究成形压力和温度对TiH2压坯烧结脱氢的影响。结果表明:TiH2粉末粒度越细,起始脱氢的温度越低;与粒度约为45μm的原料TiH2粉相比,经过球磨的粉末脱氢量减小;球磨30 min后的TiH2粉末压坯,烧结线收缩率和收缩速率都显著增大;原始TiH2粉末压坯和球磨30 min后粉末压坯的最大收缩率分别为5%和9.5%,最大收缩速率分别为2.4×10-4和7.30×10-4μm/℃;成形压力越大,TiH2压坯脱氢峰值温度越高,650℃保温1 h,TiH2压坯失重率达到3.572%(理论含氢量为4.01%)。     

PowderMet2009: prospects improving after automotive debacle

Abstract

A homogeneous powder and binder distribution in the green body in powder injection moulding (PIM) is important. In the present study, the mould filling model of PIM has been developed, based on the multiphase fluid theory, viscosity model of feedstock and powder-binder drag force model. The particle Reynolds number is influenced by the particle size and density, resulting in the different drag force between powder and binder. Furthermore, the varied velocity of binder and powder will be obtained with numerical calculation of the continuity equations, leading to the change of green body homogeneity. CFX was used to simulate the mould filling in PIM. The results showed that the homogeneity of green bodies was relative to the filling patterns, which varied with different powder densities. The powders were not suitable for PIM when the particle size was bigger than 20 μm, and the fine powders were beneficial to improve the homogeneity.     

New insights into influencing variables of water atomisation of iron

Abstract

Trace amounts of surfactants have an acute influence on measured surface tension of melts and may influence viscosity. A water atomisation experiment was performed to investigate if variations of these elements could affect quality. Effects of water pressure, melt superheat, and sulphur content, iron scrap oxygen content, and aluminium content were studied. Responses studied were particle size distribution, apparent density, flow, powder chemistry, morphology, green density, and dimensional change. A large sulphur addition reduced the particle size, as a result of a reduction of surface tension, but the largest effect came from changing water pressure. Higher water pressures also resulted in powders with lower apparent density, lower flowrate, and reduced swelling during sintering. An empirical water atomisation model is proposed. Aluminium additions reduced the powder size standard deviation and increased the carbon content of the powder. A reduced powder size standard deviation was seen also for melts with raised superheating.