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.     

Volume changes accompanying the sintering of materials of the copper-tin-ferrochromium system

Conclusions In the sintering of Cu-Sn-FC composites their volume shrinkage depends on the compact porosity: It grows with increasing starting porosity. Cu-Sn-FC composite materials of the compositions investigated, unlike materials of the Cu-Sn system, exhibit no compact growth during sintering. In low-porosity composites the ferrochromium inclusions, which undergo no deformation, prevent the formation of isolated drops of tin, thereby eliminating factors giving rise to specimen growth during sintering. In high-porosity Cu-Sn-FC composite compacts the solid ferrochromium inclusions, which are not wetted by liquid tin, do not dissolve in it, and do not react with dopper, prevent a regrouping of particles during sintering, thereby arresting shrinkage. With increasing concentration of FC inclusions their influence on the shrinkage of the composite materials grows.Translated from Poroshkovaya Metallurgiya, No. 6(282), pp. 31–34, June, 1986.     

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.     

Volume changes exhibited by Cu-Al compacts during liquid-phase sintering

Conclusions The volume changes occurring during the liquid-phase sintering of Cu-Al compacts obey Eq. (1), which reflects the fact that diffusion from the liquid phase into the solid precedes and then accompanies the migration of the solid phase into the melt. In dilatometric investigations into sintering in the presence of a liquid phase, compacts were found to grow in volume. Such growth is due to diffusion of atoms into the solid phase from the melt, and precedes shrinkage linked with dissolution of particles in the liquid phase. When the solid phase constitutes about one-third of the whole volume of a compact, the latter's shrinkage during the formation of the liquid phase takes place without prior growth. This shows that in principle it is possible for the rigid skeleton to be destroyed and for shrinkage to occur under these conditions as a result of a regrouping of particles without a substantial change in compact shape.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 31–37, May, 1982.     

Mathematical modeling and computer simulation of the rotating impeller particle flotation process: Part I. Fluid flow

Removal of unwanted particles from molten metal by flotation is one of the most useful melt cleansing techniques used by the foundry industry. An effective way of flotation of particles in a melt relies on purging a gas into the molten metal through holes in a rotating impeller. Impeller rotation creates turbulence inside the melt, which helps agglomerate the impurity particles and, thereby, enhances their removal from the melt. In addition, turbulence increases the probability of particles attaching to the rising gas bubbles and, therefore, enhances the chance of their removal from the molten metal. A mathematical model has been developed to simulate the turbulent multiphase flow field inside the flotation treatment furnace. Simulations based on the model were used to demonstrate the effect of the various process parameters on the performance of a batch-type rotating impeller particle flotation process.     

Gas-Sprayed Tin Powders

A gas-dynamic technology has been developed for obtaining tin powder by spraying molten metal and equipment has been built for this purpose. The powder (grain size <100 m) is distinguished by the stability of its parameters, including the granulometric composition. This is achieved by keeping the technological parameters stable throughout the metal spraying process. Those parameters have been calculated and then refined on the basis of results obtained from experimental research on the process of spraying liquid metal. A constant molten metal consumption is ensured by a special dosing device. Its design and operation are described and the parameters of the powders produced are given.     

Study of meniscus behavior and surface properties during casting in a high-frequency magnetic field

Surface quality of continuously cast metals can be improved by imposing a continuous high-frequency magnetic field from the outside of a mold. Newly proposed concepts of “soft contacting solidification” and “slow cooling solidification,” which is tightly related to the mechanism of improving surface quality, were confirmed in model experiments by using molten gallium and tin. The meniscus motion of the molten gallium accompanied by a mold oscillation and magnetic pressure was measured by a laser level sensor. The shape variation of a meniscus and the process of ripple formation in an oscillation cycle were directly visualized by an optical fiberscope camera. Moreover, molten tin was continuously cast and the relationship between the surface quality and the meniscus motion was studied. A mechanical model for predicting the space between the oscillation marks is proposed. The casting process using intermittent highfrequency magnetic field was developed. New functions of this field were investigated regarding the control of initial solidification. It was found that the surface quality of the continuously cast metal can be improved by the intermittent high-frequency magnetic field as well as the continuous high-frequency magnetic field. Formerly Undergraduate Student, Department of Materials Processing Engineering, Nagoya University,     

The removal of solid particles from molten aluminum in the spinning nozzle inert flotation process

The removal of solid particles from molten aluminum by flotation was investigated based on theoretical fluid dynamics. The energy spent for stirring the melt in the SNIF process accelerates the agglomeration of small particles into larger particle aggregates which can be removed from the metal by gas bubbles during the short residence time of the melt in the refining unit. Theory suggests that supplementation of thermal agglomeration of the particles with turbulent agglomeration and small gas bubbles are the major factors which can lead to high particle collection efficiencies in molten aluminum.     

Effect of aluminum particle size on the volume changes experienced by compacts from a mixture of aluminum and copper powders during liquid-phase sintering

Conclusions The process of liquid-phase sintering of aluminum-copper powder compacts comprises two main stages — growth and shrinkage. Growth is due to diffusion of copper atoms from the liquid phase into the solid, with the formation of liquid interlayers at the grain and subgrain boundaries. Compacts from powder mixtures containing large aluminum particles exhibit increased growth in the first stage of sintering, which may be due to nonuniform reaction of the liquid phase on the peripheries of the particles, resulting in the appearance of local strains and further separation of the solid-phase particles.Translated from Poroshkovaya Metallurgiya, No. 9(285), pp. 23–27, Septmeber, 1986.     

Mechanical properties of green compacts. II. Effect of powder relative bulk density on the strength of compacts with different forming temperature conditions

Mechanisms of strength for green compacts made from powders of iron, nickel and its alloys, copper, tin, and zinc are analyzed. The strength of green compacts prepared from metal powders of medium fineness with a relative bulk density (RBD) from 0.119 to 0.568 by two-way compaction in rigid dies with homologous temperatures from 0.15 to 0.59 (pressure from 200 to 800 MPa, powder deformation rate 10^?2–10^?3 m/sec) is studied. Compact strength is determined by diametric compression of cylindrical compacts. The dependence of strength on compact porosity is studied by the Bal’shin equation. The possibility is demonstrated of using this relationship in order to describe hot compaction and formally describe cold compaction of powders with RBD up to 0.40. The effect of homologous temperature and powder RBD on compact strength is determined. The homologous temperature for transition from warm to hot compaction and the effect of compact density (degree of deformation) on this temperature is studied. It is shown that linear approximation is possible for the dependence of compact strength on powder RBD according to the equation σ _f.c = 87–217?RBD.     

Dynamics and control of solidification of molten metal flows

A solidified layer on the inside of a cooled flow channel can be used to control the flow rate of molten material through that channel. This concept can be used for flow rate control of molten furnace products in the metallurgical industry. In this study, internal solidification of molten metal flows has been modeled mathematically for both steady-state and dynamic cases. The model predicted solidified layer thickness and metal flow rate. Experimental verification of the mathematical model was obtained using molten tin. Novel design features of the experimental apparatus included the use of boiling heat transfer and the vertical mounting of the cooling section. Engineering knowledge regarding the design, operation, and control of a pilot scale (24 kg/s) molten metal circuit was obtained during the construction, commissioning, and operation of the experimental apparatus. Experimental results for tin flow rate from the experimental apparatus were within experimental error of the predictions of the mathematical model.     

Comparison of Surface Tension Measured Values for Molten Tin at Different Oxygen Potentials

The surface tension of molten tin has been determined by a set of a self‐developed digital equipment with the sessile drop method at an oxygen partial pressure of 1.0×10^?6 MPa at different temperatures. The dependence of surface tension of molten tin on temperature was discussed as well. Based on the summarized relationships of the surface tension of molten tin to temperature and oxygen partial pressure reported in the literature, the reasons for the differences in those reported data were analysed. The emphasis was placed on the comparison of surface tension of the same molten tin sample measured by using different equipments according to the sessile drop method. Results of the comparison indicate that the measurement results obtained with the sessile drop method under similar experimental conditions are coincident, and the self‐developed digital equipment for surface tension measurement has a higher stability and accuracy. The relationships of surface tension of molten tin and its temperature coefficient with temperature and oxygen partial pressure were also elucidated from the thermodynamic analysis in this paper.     

Effect of porosity on the volume changes experienced by Al-Cu compacts during liquid-phase sintering

Conclusions In the Al-Cu system a linear relationship between the end and starting porosities exists only at porosities exceeding 20%. At a smaller pore content the linearity is disturbed by the presence of an irreducible oxide phase on the aluminum particles. Decreasing the starting compact porosity results in greater growth in the first stage of sintering and smaller shrinkage in the second. The extent of the compact growth preceding the shrinkage may markedly exceed that due to copper and aluminum atom diffusion under conditions of uniform reaction of the liquid phase on all the surfaces of the particles. The anomalously large compact growth in the first stage of sintering is due to a negative regrouping of particles resulting from an uneven Kirkendall flow of material inside the particles, bringing about a change in their shape. The extent of shrinkage is not apparently linked with the structure forming during sintering, but depends on the starting porosity. Pores in a compact affect grain growth during sintering by inhibiting it.Translated from Poroshkovaya Metallurgiya, No. 7(295), pp. 22–26, July, 1987.     

Effects of bond strength on densification and flow of powder compacts

Abstract

Plastic flow surfaces for metal and metal/composite powder compacts with variable cohesive strength are derived using the Beltrami total strain energy criterion, modified to permit asymmetric yielding. The present theory thus includes the domains of both soil mechanics and powder metallurgy, covering both granular and porous microstructures of varying bond strengths over all possible densities. A quantitative theory of the expansion of non-bonded particle compacts under certain combinations of applied shear stress and pressure is given. Physical models of the failure mechanisms are provided, their applicability depending on the particle interface strength. If the particle interface lacks strength, the flow surface is identical to that of the ‘critical state’ criterion of soil mechanics. The flow ellipse lies entirely within the negative pressure domain and failure occurs at the particle interface by various mechanisms including frictional slip. At the other extreme, if the particle interfaces are as strong in shear as the particles, failure occurs by plastic shear of the particles and the flow ellipse is centred at the origin. Density–stress and density–aspect ratio maps are shown which define these domains. Theoretical predictions compare well with the results of data compiled from the literature as well as data from tests performed in this study on cold pressed Al and Al/SiC powder compacts.     

INFLUENCE OF TIN POWDER CHARACTERISTICS ON THE SINTERING OF IRON-TIN-COPPER POWDER COMPACTS

Abstract

In view of increasing industrial interest in the use of tin additions as an aid to the sintering of iron-based powder compacts, an examination has been made of the influence of the characteristics of the tin powder on sintering performance.

The effect of additions of narrow size-range fractions of atomized tin powder on the dimensional changes and tensile properties obtained on sintering Fe-Sn-Cu compacts made with –100 mesh (–152 μm) or – 300 mesh (– 53 μm) sponge iron and – 300 mesh (– 53 μm) atomized copper powders has been determined. The compacts contained tin and copper in the ratio 2:3. The narrow size fractions were separated from – 300 mesh tin powder by air elutriation. It was found that the use of coarse tin powder reduced the tensile strength of – 300 mesh iron-based Fe–1% Sn–1 ½% Cu compacts, but had no influence when this mixture was based on –100 mesh iron powder, or when the mixture composition was Fe–2% Sn–3% Cu. The effects have been examined in relation to the sintering mechanism by scanning electron microscopy and by X-ray microanalysis.     

Nature of the growth of Al-Cu powder compacts during liquid-phase sintering

Conclusions Arresting the process of liquid-phase sintering of aluminum-copper system powder compacts by rapid cooling does not affect the character of the volume changes experienced by them during subsequent sintering under the same temperature conditions. In the growth stage a decrease in the crystal lattice parameter of aluminum and an appreciable broadening of an x-ray line have been observed, caused by the formation of aluminum base solid solutions. These findings bear out the hypothesis that the growth of aluminum-copper powder compacts above their eutectic melting point is mainly due to diffusion of copper from the liquid phase into aluminum particles.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 16–19, May, 1988.     

Some characteristic features of the sintering of binary systems

Conclusions The main process controlling the magnitude and sign of the volume changes experienced by compacts from powders of binary systems during sintering is alloy formation by the mechanism of diffusional mixing of components. The character of the volume changes of compacts is determined by the direction of migration of atoms in the course of heterodiffusion, which in turn depends on the structure of the constitution diagram of the system being sintered. A diffusional flux of atoms migrating toward particles of the main component of a compact causes its growth or hinders its shrinkage. Preferential diffusion toward alloying addition particles stimulates shrinkage provided that at the same time the bulk of excess vacancies in the particles of the main component are annihilated on dislocations. The sign of the volume change of a compact does not depend on the state of aggregation of its alloying addition. However, its passage into the liquid state intensifies alloy formation and the resultant volume change. The so-called particle regrouping mechanism, which is usually linked with the appearance of a liquid phase in a compact, cannot be reconciled with the compact growth phenomenon, since its ope ration would be expected to prevent this phenomenon rather than promote it.Published to stimulate discussion.Translated from Poroshkovaya Metallurgiya, No. 7(211), pp. 62–69, July, 1980.     

Fabrication of Al-3.7?Pct Si-0.18?Pct Mg Foam Strengthened by AlN Particle Dispersion and its Compressive Properties

Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al₃Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al₃Ti particles is a possible cause of this smooth plateau stress region.     

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.     

Kinetics of disilicide layer growth at the interface of tungsten with molten copper, silver, and tin saturated with silicon

The kinetics of WS₂ layer growth at the interface of tungsten with molten metals saturated with silicon is studied. Research is performed at 1200°C using melts based on copper, silver, and tin. It was established that WSi₂ layer growth in these melts obeys a “parabolic” rule but the corresponding growth rate constants differ markedly, i.e., from 3.4·10^?11 m²/sec (melt based on copper) to 1.5·10^?13 m²/sec (melts based on silver and tin). The reasons for this difference are discussed.