SINTERING KINETICS OF OXIDE-FILMCOATED COPPER POWDER

Abstract

The sintering kinetics of pure copper powder and of copper powder coated with a critical oxide (CU₂O) film thickness (～,500Å), has been studied by following the densification of the compacts as a function of temperature and time in pure dry hydrogen and in vacuum. The activation energy for the sintering of pure copper powder in hydrogen was 55,000 cal/mole, suggesting that the volume self-diffusion mechanism predominates during the sintering process. In the case of the oxide-coated powder the corresponding value was 37,000 cal/mole. The high rate of sintering of the coated powder in hydrogen and in vacuum is explained in terms of an activated sintering mechanism.     

THE SINTERING OF PYROPHORIC POWDERS

Abstract

The room-temperature oxidation behaviour of reactive metal and ceramic powders is reviewed and it is shown that many materials under normal circumstances oxidize to give a limiting film thickness of ～50Å. Since the thickness of oxide films has an important bearing on sintering characteristics the control of oxidation may be essential for some materials and desirable for others.

The oxidation behaviour of uranium carbide and uranium nitride in various concentrations of oxygen and water vapour in argon is described and its effects on the sintering behaviour and composition changes that take place during sintering are considered.

Suitable handling conditions for these materials are provided by glove-boxes filled with argon; there is a brief discussion of the factors that might apply if they were used in other areas of powder metallurgy.

The application of high-purity handling atmospheres in the preparation, handling, and sintering of tungsten powders has given pellets that had densities of 95% theoretical and oxygen contents <10 ppm after sintering in hydrogen at 1550°C.     

Powder reaction mechanism in fabrication of high silicon iron alloy

Abstract

In the present paper, the reaction mechanism of silicon and iron powders under different sintering conditions during the fabrication of high silicon iron sheet (～6·5 mass-%Si) is clarified. It is indicated that the phases, Fe₃Si (Si) and FeSi, play an important role in the reaction between iron and silicon powders. Two temperature regions of the powder reaction are very important for producing commercial high silicon iron sheets: the temperature region of ～1000°C in which the ductile composite structure can be produced, and the temperature region of ～1200°C in which the density and homogeneity can be improved.     

REDUCTION AND SINTERING OF SLIP-CAST IRON-OXIDE/COPPER-OXIDE MIXTURES

Abstract

Experiments are described on the preparation of sintered iron bodies by slip casting iron oxide, followed by reduction and sintering. It is shown that it is possible to produce iron parts of different shapes and sizes by a single reduction + sintering treatment. The introduction of copper by adding copper oxide to the iron oxide overcomes certain drawbacks and improves the properties of the final product.     

DIMENSIONAL CHANGES OF SINTERED Fe-Ni,Fe-Cu,Fe-Ni-Cu ALLOYS

Abstract

Sintered alloys of the Fe-Ni, Fe-Cu, and Fe-Ni-Cu systems have been investigated by using single-pressing, double-pressing and hot-forging techniques. Different iron powders containing 0-6 wt.-% nickel or 0-5 wt.-% copper, also (for the ternary system) 0-5 wt.-% of both nickel and copper, were compacted and sintered and the effect of the additions on dimensional changes was studied. The influence of particlesize, compacting pressure, sintering temperatures, and furnace type on dimensions has been determined.     

STUDIES OF THE SINTERING AND HOMOGENIZATION OF NICKEL-COPPER COMPACTS

Abstract

Studies are described of the progress of sintering and alloying in compacts of similar compositions made from nickel-coated copper, copper-coated nickel, and mixed nickel and copper powders. Density losses observed in the early stages of sintering were lower in magnitude and were more quickly recovered in the case of the composite powder compacts. Alloying by diffusion at both 1900 and 2200°F (1040 and 1205°C) progressed most rapidly in compacts prepared from nickel-coated copper powders, and the probable reasons for this observation are discussed in detail. Electrical resistivity was used to follow homogenization of the compacts, and samples were rendered nearly 100% dense by cold working and annealing before making resistivity measurements. Resistivity / sintering-time curves for dense specimens showed no maxima of the type reported by earlier investigators for porous compacts, which were attributed to alloying effects.     

THE FORMATION OF URANIUM AND OF BERYLLIUM ALLOYS BY THE SOLID-STATE SINTERING OF MIXED ELEMENTAL POWDERS

Abstract

Large volume expansions accompany the formation of binary alloys of beryllium with uranium, thorium, iron, copper, zirconium, titanium, and vanadium, and of uranium with aluminium, during the sintering of the mixed, cold-compacted elemental powders. No expansion was detected during the sintering of binary mixtures of beryllium with aluminium, silicon, and magnesium, or mixtures of uranium with zirconium, molybdenum, iron, nickel, manganese, and chromium.

When it occurs, expansion is anisotropic, being greatest in the direction of compacting; the degree of anisotropy varies with the constituents and the composition of the alloys. In systems undergoing expansion, the volume expansion/composition graphs exhibit maxima. For a given system the magnitude of the maximum is a function of the shape of compact, the particle size of the powders, and the sintering time and temperature; the composition at which the maximum occurs is sensibly unaffected by these latter variables.

These experimental observations, together with those of other investigators, can be satisfactorily interpreted on the hypothesis that volume expansion is due to the formation of diffusional porosity during sintering.     

OBSERVATIONS ON THE SINTERING OF COMPACTSFROM A MIXTURE OF IRON AND COPPER POWDERS

Abstract

Three grades of iron powder-an atomized steel powder, a sponge iron powder reduced from magnetite with carbon, and a powder reduced from mill scale with hydrogen were mixed with 3% of copper powder and pressed into compacts. The diametral dimensional changes of the compacts during sintering below and above the melting point of copper were measured, their microstructures examined, and both related to the characteristics of the powders, particularly their specific surface. During sintering below the melting point of copper, compacts of all three powders shrank. Micrographic examination showed that the copper is transported by solid-state diffusion along the surfacesand grain boundaries of the iron powder particles. During sintering above the melting point of copper, compacts of the atomized and the MH-100 sponge iron powders grew while those of the hydrogen reduced mill-scale powder shrank. This phenomenon is related to the different mode of penetration of liquid copper in the compacts from the three powders, observed in the microstructures of the compacts.     

METHODS OF EXTENDING THE APPLICABILITY OF SINTERED STEELS

Abstract

It is a common opinion among users of structural parts that applications for sintered steels are limited to those where requirements for strength are low to moderate. Furthermore, sintered steels of moderate strength are thought to be very brittle. It is the object of this paper to draw attention to significant improvements which have been achieved in the last few years. These are basically a result of powder developments which are based partly on traditional alloying additions, such as Cu, Ni, Mo, and C, and partly on unique combinations of iron powders and phosphorus or on combinations of iron powders, phosphorus, carbon, and/or copper. Unusually favourable combinations of strength and ductility can be achieved with diffusion-alloys based on iron and phosphorus. Components of high-duty sintered steels capable of replacing components of conventional wrought steels can be produced from partially prealloyed combinations of iron, copper, nickel, molybdenum, and carbon. For many applications these materials can also be an alternative to powder-forged steels. All the above powder combinations show consistent and low dimensional changes during sintering so that close tolerances of intricately shaped components can be maintained. Material and processing costs are such that the improved properties can be achieved economically.     

THE STRUCTURE AND PROPERTIES OF SOME INTERNALLY OXIDIZED ALLOY POWDER COMPACTS

Abstract

The advantages of internally oxidized structures produced by powder-metallurgical techniques are briefly reviewed and discussed. A metallographic survey of the structures of some internally oxidized copper alloy and nickel alloy powder compacts is presented, and the effect of pressing and sintering variables upon the density and hardness of the product is established. Hot-hardness data up to 800°C, and also some tensile data up to 620°C, are presented for certain alloys.

It is concluded that although dispersion-hardened structures can be prepared by the pressing, sintering, and internal oxidation of appropriate alloy powders, and although the methods described offer a valid comparison of the properties of the various alloys studied, optimum mechanical properties are undoubtedly developed only after the powders are hot worked. The latter treatment densifies the product, removes any porosity in the structure, and increases the stored energy in the material.     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

SINTERING OF METAL POWDER COMPACTS CONTAINING CERAMIC OXIDES

Abstract

The dimensional change during sintering of cobalt and nickel powders containing an oxide dispersion has been studied. Generally, the presence of MgO or CaO in cobalt and nickel retards the densification of the metals, and this retarding influence has a maximum for a certain proportion of oxide in the metal. The wettability between the dispersed phase and the metal may explain differences in sintering behaviour.     

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.     

CONTINUOUS COMPACTION BY CYCLIC PRESSING

Abstract

A process recently developed produces wide, thick bars of unlimited length from powder, granular, or sponge raw materials by a cyclic pressing operation. The process can be adapted readily for laboratory study or for large-scale production.

Many materials have been pressed by this technique, and in all cases compaction was similar to that experienced in conventional pressing. Work on the pressing, sintering, and subsequent working of various nickel powders and of a nickel-iron-molybdenum magnetic alloy has been carried out. Carbonyl nickel powder produces nickel strip of high quality. A high-permeability alloy containing nickel 79, iron 17, and molybdenum 4% can be made into strip with good magnetic properties by compacting the powders with the cyclic-pressing technique and then sintering and rolling the pressed bar into strip.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

THE INFLUENCE OF MATRIX IMPURITY CONTENT AND ATMOSPHERE ON THE MOVEMENT OF ALUMINA PARTICLES IN NICKEL-ALUMINA ALLOYS

Abstract

Specimens containing 2 wt.-% alumina as a disperse phase and whose matrices consisted of either pure nickel or nickel + 1 wt.-% iron have been produced by powder-metallurgy techniques and subsequently worked to form thin sheets. The sheets were annealed in the range 1380–1420°C, under oxygen partial pressures in the range 5 × 10^–11 to 10^–18 atm, for times between 15 and 1200 min.

Metallographic examination shows that the particles do not grow in size but that they move to fill the grain boundaries and eventually to form large clumps at grain-boundary corners. The particle movement is suppressed by the control of oxygen partial pressure and/or the addition of iron to the matrix. A tentative mechanism to explain this movement has been put forward, based on the flow of nickel atoms round the particles, resulting in particle movement in the contrary direction.

Further experiments, on wires of nickel matrix containing 1 wt.-% additions of iron, cobalt, chromium, or copper, but without the disperse phase, have shed further light on the way in which nickel atoms can move in these environmental conditions.     

EFFECTS OF PROCESSING AND POWDER CHARACTERISTICS ON THE WEAR AND FRICTION PROPERTIES OF SOME NICKEL-BASE MATERIALS

Abstract

Pure nickel and nickel-based binary and ternary materials containing tungsten carbide and graphite dispersions have been studied. Three types of carbide powder and six types of graphite, different in size, shape, and structure (one in each group being nickel-coated) were studied to varying degrees. The smallest carbide powder (0·35 μm) possessed the best wear-resistance and the lowest coefficient of friction. The larger and coated types of graphite are somewhat superior; however, considerable amounts (～ 40 vol.-%) are needed to improve wear-resistance substantially and reduce the coefficient. Use of the coated type of carbide leads to rather high coefficients (0·58-0·76) under dry testing conditions. Several effects of the presence of a small amount of oxygen in the sintering atmosphere are discussed; a most interesting result is the marked improvement in the wear-resistance of pure nickel treated to produce a dispersion of nickel oxide. With some powders blending can lead to reduced wear-resistance, while with others increasing the blending time has no effect. Several of the findings confirm the interpretation of the results of the previous study regarding the dependence of wear on the ratio of the volume fraction of tungsten carbide to that of graphite, based on microstructural considerations. Wear-resistance cannot be correlated with high hardness.     

THE PREPARATION AND PROPERTIES OF COPPER,NICKEL, AND IRON CONTAINING A DISPERSED OXIDE PHASE

Abstract

Three methods are described for the preparation of a stable oxide in copper, nickel, or iron. The first, which has been applied to all three metals, requires a reaction between the matrix metal oxide and the refractory oxide at high temperatures such that subsequent reduction will yield a dispersion of oxide in the matrix metal powder. In the second method, copper hydroxide or carbonate is precipitated in the presence of an aluminium salt. Ignition of the precipitate to the oxide followed by hydrogen reduction gives a copper powder containing a dispersion of alumina. In the third process, a salt that can be decomposed to a stable oxide is dissolved in molten nickel nitrate, which on rapid cooling eitherretains the additive in solid solution or as an intimate dispersion. Nickel powder containing the dispersed oxide is obtained on subsequent decomposition.

The results on sintered and worked materials show a substantial improvement in mechanical properties compared with the pure metals. Improvements are also achieved in the sintered nickel alloys, which can be sintered to high densities. The effect of the dispersion on the structure and recrystallization behaviour of these materials is discussed.     

PREPARATION AND SELECTED PROPERTIES OF CERTAIN DISPERSION-STRENGTHENED LEAD-BASE ALLOYS

Abstract

Dispersion-strengthened lead-copper and lead-aluminium alloys have been prepared by atomizing the alloys from a temperature at which the copper or aluminium was dissolved in the liquid lead. The particles of the resultant powders consisted of a very fine dispersion of a second phase in a lead matrix. The powders were compacted and extruded. The mechanical properties of the extrusions at room and elevated temperature have been determined and their microstructure examined.

At room temperature the alloys had a combination of tensile strength and ductility superior to most other lead alloys, including lead alloys strengthened by a dispersion of lead oxide. The strength of the leadcopper alloys increased with increasing copper content; this was attributed to the closer spacing of the,dispersed copper particles in the extrusions.

The creep of the alloys was investigated at room and at elevated temperatures up to 100°C. Lead-aluminium alloys containing 0·55% aluminium had steady-state creep rates of < 1%/year at stresses up to 2500 lb/in² at 25°C and up to 1200 lb/in² at 100°C. The creep-resistance of the lead-copper alloys was somewhat lower. Their steady-state creep rate at low stresses was proportional to the applied stress, while at high stresses it was proportional to the fourth power of the stress.