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.     

THE DRIVING FORCE FOR SHRINKAGE IN COPPER POWDER COMPACTS DURING THE EARLY STAGES OF SINTERING

Abstract

The shrinkage behaviour of compacts from irregular copper powder during the initial stages of sintering has been determined by a dilatometric method. The effects of compacting pressure and of external load during sintering at a constant heating rate of 3°C/min upon shrinkage were observed. The residual stresses present on the surface of compacts heated at the same rate to temperatures of 200, 300, 400, 500, and 600°C were also measured. It was observed that shrinkage starts at temperatures where considerable residual stresses in the surface of the compacts are still present, and that this temperature also depends upon the external stress applied during sintering. Residual and externally applied stresses complement each other in shifting the temperature of start of shrinkage to lower values with increasing stress. It is concluded that, in the low-temperature range up to 400°C, residual and externally applied stresses, rather than surface-tension forces, cause shrinkage.     

Pressing vacuum sintering of multipowders for manufacturing novel engine valve seat on Gleeble 1500 simulator

Abstract

Patented Fe based multipowders for manufacturing novel engine valve seat were compacted and sintered using a Gleeble 1500 thermal simulator system. Microstructures and properties of the sintered alloy have been studied by hardness test, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) analysis. Results indicate that a sintered alloy with a hardness of 38 HRC and a density of 7·23 g cm^?3 can be obtained when the green compacts are sintered at 1240°C for 30 min. It is found that lattice parameters of the alloy matrix increase with increasing sintering temperature. Additionally, the investigation of green compacts' shrinkage during sintering shows that there is a threshold value for the density of the green compacts. When the density of the green compacts is lower than that value, the density of the sintered alloy remains almost constant with increasing density of green compacts. An equation to calculate the threshold density has been suggested as νρ² = C ₁ ρ² + C ₂ m² + C ₃ m ρ.     

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.     

Anisotropic shrinkage during sintering of glass-powder compacts under uniaxial stresses: Qualitative assessment of experimental evidence

The shrinkage anisotropy that occurs during sintering of glass-powder compacts under uniaxial compressive stresses is considered. Available experimental data from the literature are used to calculate a shrinkage anisotropy factor (k) and to study its dependence on the applied stress and progress of densification. The factor k is defined by the ratio between axial and radial strains. The experimental data are interpreted in terms of qualitative trends proposed for the variation of k with the progress of sintering for a constant applied load. It is shown that when sintering densification predominates over viscous deformation induced by an external load at the onset of the process, k increases with the sintering time. The existence of a stage at which the shrinkage anisotropy factor becomes negative is confirmed by the analysis of available experimental data. The trends for the variation of k are also given for the case of the externally induced deformation predominating over sintering densification. However, no sufficient experimental data were found to validate the proposed trends in this case. The prediction of the theoretical model of Scherer for viscous sintering under uniaxial stresses is shown to agree qualitatively with the experimental trends. In connection with the use of dilatometry in sintering kinetic studies, the effect of small stresses on shrinkage anisotropy is highlighted.     

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.     

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.     

Co—WC Pseudobinary Eutectic Reaction

Abstract

Using sphere-plate models made from copper it is shown that during sintering the dislocation density increases considerably in the contact region. Its distribution and time dependent variation can be analysed by means of the Rossel technique and described quantitatively. The same effect is observed during sintering compacts of electrolytic copper powder. The results of positron annihilation spectroscopy show the high dislocation densities generated in the heating phase to be reduced by non-conservative dislocation movement during the intensive shrinkage stage of sintering. Resulting densification mechanisms are discussed. PM/0344     

EQUATIONS DESCRIBING DIMENSIONAL AND DENSITY VARIATIONS IN SINTERED BODIES: APPLICATION FOR PREDICTING OPTIMUM PRODUCTION PRACTICE

Abstract

A system of equations is established describing how variations in powder characteristics and process parameters are related to the quality of the sintered product.

The basic assumption is that a cylindrical body is to be fabricated by cold pressing and sintering. It is also assumed that the allowable variations in the diameter, the bulk density, and/or the linear density of this sintered body are specified.

The powder material is characterized by the slope, K, of the curve of green density vs. sintered dem,ity. Values near unity give least variation in sintered diameter. An important parameter of the process is the variation, B_g, of the green density.

Variation of the parameters, B of bulk density, L of linear density, and D of sintered diameter are dependent on two groups of variables. The total variation may be composed of variations in the powder quality and the sintering conditions (group 1) or of variations in the green density (group 2).

Taking a practical example, equations are derived for the variables controlling the production of ground and unground sintered ceramic nuclear fuel pellets.     

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.     

A study on laser sintering of Fe-Cu powder compacts

The sintering of Fe-Cu powder compacts by laser beams was studied to determine densification and microstructural development. The influence of processing variables such as Cu content, laser output power, sintering time, and green density on the densification and Brinell hardness (HB) were analyzed, and several temperature curves measured during laser sintering (LS) were also discussed in this article. After LS, the samples exhibited considerable shrinkage, which was very different from the effects of conventional sintering (CS). The main reasons for shrinkage are the laser’s very fast heating rates, short sintering times, and relatively high sintering temperatures. As a consequence, insufficient time is available for Fe particles to diffuse in the solid state to form a rigid skeleton. After formation, the liquid penetrates quickly along boundaries and separates the Fe particles, leading to rapid collapse and particle rearrangement, which finally results in considerable shrinkage. An increased Cu content, laser output power, and sintering time can promote shrinkage and hardness, but compact slumping occurs when the laser power is too high.     

The influence of gravity in sintering

The radial shrinkage during sintering of cylindrical compacts and loose aggregates of copper powder was measured. It was found to be nonuniform from top to bottom of the samples and to depend upon the method of supporting them. The nonuniformity is due to the effect of gravity forces during sintering. Since gravity has an effect in sintering without externally applied stresses, no sharp dividing line can be drawn between conventional sintering and hot pressing.This article was kindly contributed by Dr. Henry H. Hausner.     

Analysis of Dimensional Changes During Sintering of Fe—Cu

Abstract

The dimensional changes of Fe–Cu compacts during sintering are of considerable interest for the production of sintered parts. Previous investigations of the swelling and shrinkage behaviour were mostly based on dilatometric measurements and led to partially contradictory results. In the present investigation Fe and Cu powders of ideal sphericity were compacted to densities of up to 93%, sintered for different times, and then rapidly cooled. The simultaneous measurement of density and microstructural parameters permitted the quantitative distinction of the swelling mechanism of Fe–Cu compacts into four main contributions; I penetration of the melt between Fe particles and II along the grain boundaries; III diffusion of Cu into the Fe particles from the particle surface; and IV diffusion of Cu into the Fe grains from the grain boundaries. Whereas the penetration between the Fe particles is the result of pure capillary forces the penetration along grain boundaries is thought to be a special case of solution-reprecipitation.

Particle rearrangement during sintering of Fe–Cu compacts could be separated into two stages. Primary particle rearrangement, just after the melt phase occurs, leads to rapid densification of loose packed and slightly compacted specimens. In highly compacted specimens it can be neglected. The penetration of the melt along the grain boundaries leads to disintegration of the Fe particles, which enables densification by rearrangement of the individual grains of the Fe particles.     

Distortion in a sintered 7000 series aluminium alloy

Abstract

The 7000 series aluminium alloys processed using elemental powder mixtures are prone to distortion, which is manifest as hourglassing or waisting in cylindrical specimens. By characterising the density distribution using hardness measurements, it is shown that the green density is not evenly distributed through a part, even though aluminium is relatively soft and readily compacted. Because the density equilibrates during sintering, the non-uniform green density leads to distortion. The cause of this distortion is a result of differential shrinkage, which occurs during sintering as well as on solidification during cooling from sintering. Distortion can be controlled by increasing the compaction pressure, which homogenises the green density and does not affect the tensile properties.     

A dilatometry study of the influence of green density on anisothermal and isothermal shrinkage of plain iron green parts

ABSTRACT

The influence of green density between 6.5 and 7.3?g?cm^?3 on the anisothermal and isothermal shrinkage of atomized plain iron was investigated by dilatometry. The geometrical activity deriving from the extension of the interparticle contact areas and the structural activity provided by the defectiveness of the interparticle contacts promote anisotropic anisothermal shrinkage starting from 500°C up to the bcc to fcc iron transformation. It displays a minimum at 6.9?g?cm^?3, resulting from the combined effects of the thermodynamic driving force and of geometrical and structural activity. Anisothermal shrinkage is caused by an anisotropic increase in the internal radius of the neck. Anisotropic isothermal shrinkage is smaller than the anisothermal one and is almost independent on green density. No anisothermal shrinkage was observed on the tapped powder, demonstrating that anisothermal shrinkage in green specimens is due to geometrical and to structural activity introduced by prior cold compaction.     

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe₃Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH _f ^°(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe₃Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe₃Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe₃Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.     

Electrical conductivity and microstructure of sintered ferrous materials: sintered iron

Abstract

The viability of electrical conductivity as a tool for describing the microstructure of sintered iron compacts was investigated, the sintering temperature being varied from dewaxing to high temperature sintering. The relationships between formation of sintered contacts, presence of lubricants, and mechanical properties were evaluated through determination of conductivity and effective load bearing cross-section A_c . The latter parameter was measured via quantitative fractography of specimens impact fractured at 77 K. The role of porosity and sintering temperature on grain growth in iron was also evaluated using quantitative metallography. It was found that the conductivity of pressed compacts increases during the dewaxing stage, while the effect of the sintering parameters at higher temperatures is less conspicuous. In any case, the conductivity can be related to the load bearing cross-section by a logarithmic equation. Using the already established relationships between A_c and the mechanical properties, the latter can be predicted by using the conductivity, which might be helpful in quality control of PM components.     

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.     

DIMENSIONAL CONTROL IN HARD-METAL MANUFACTURE

Abstract

All hard-metal alloys contract ～50% by volume during sintering and, in addition, the continuous network of liquid phase makes them extremely weak. Hence, the two major problems in dimensional control are coping with this large contraction and at the same time preventing distortion during sintering.

Green compacts of closely controlled and very uniform density are essential to ensure that the contraction is both predictable and uniform. Methods of predicting contraction and of powder preparation and pressing to achieve this end are described.

Distortion results not only from mechanical weakness during sintering but also from the carburizing, decarburizing, oxidation, and reduction reactions that occur when the green compact is heated.

In conclusion, indications are given of the order of dimensional accuracy that can be expected in hard-metal manufacture.