FAST DIFFUSION ALLOYING FOR POWDER FORGING USING A LIQUID PHASE

Abstract

Prealloyed powders are dominant items in the economics of powder forgings today. Present estimates predict a maximum powder consumption that is too low to bring about essential reductions in the cost of these materials. Long homogenization discourages the use of plain iron powders with admixed alloying elements, especially where induction heating is employed, and the use of cheap alloying elements such as manganese is hampered by oxidation problems. A way of overcoming these difficulties is described. If the alloying additions needed for hardenability are made in the form of a low-melting master-alloy powder, diffusion times can be very much reduced. A condition is that the molten prealloy wets the iron particles, reducing the diffusion distance to the order of one particle radius. It is also desirable that the master alloy should penetrate quickly along the grain boundaries of the iron, further reducing the diffusion distance. Low-melting alloys of manganese with copper satisfy these conditions. Forgings can be produced from plain iron powders with copper-borne manganese additions without excessive oxidation of the manganese and preforms can be sufficiently homogenized within a heating time of a few minutes to give hardenability and tensile properties similar to those of conventional quenched and tempered steels. Since only small amounts of master-alloy powder are needed, ‘solid-liquid alloyed’ plain iron powders appear to offer great flexibility in alloy composition at a cost substantially below that of conventional prealloyed powders.     

Improvement of fatigue properties of PM steels by shot peening

Abstract

Shot peening was used for improving the fatigue properties of Fe–2Cu–0·5C PM steel. The steel is generally used for production of high performance sintered parts such as small connecting rods for cooling system compressors. To distinguish the effects of each alloying element, Fe, Fe–2Cu, and Fe–0·5C were also investigated. Optimum shot peening intensities are: 25A for pure iron and Fe–2Cu, 30A for Fe–0·5C, and 35A for Fe–2Cu–0·5C. For these intensities, improvements of the fatigue strengths of the materials investigated are as follows: 31% for pure iron, 48% for Fe–0·5C, 46% for Fe–2Cu, and 38% for Fe–2Cu– 0·5C. From the experimental results it could be concluded that both of the alloying additions, carbon and copper, are contributing to the fatigue strength improvements by shot peening.     

Effect of Compacting Pressure and 0·5%Mo on Properties of Sintered Iron Powder and Manganese Steels

Abstract

An investigation has been carried out on the effect of compacting pressure, in the range 150–600 MPa, and of the addition of 0·5%Mo on the properties of sintered Hametag iron powder and manganese steels. Higher sintering activity compared with standard iron resulted in higher density and higher mechanical properties in the presence of manganese vapour. The addition of 0·5%Mo in the form of ferromolyb-denum caused an increase in density and strength properties in the Fe–C and Fe–Mn–C steels. PM/0157     

Microstructural characterisation of nickel rich areas and their influence on endurance limit of sintered steel

Abstract

Nickel is an often used alloying element in powder metal steel to achieve high hardenability. However, when nickel is added, the slow diffusion rate between iron and nickel leads to the formation of nickel rich areas (NRAs). Two steel alloys were studied: a Fe–6·4Ni–0·7Mo–0·7C with standard sized nickel powder additions and a Fe–2·4Ni–0·7Mo–0·7C with a finer sized nickel powder. Microstructural characterisation of the parts revealed that sufficient hardenability was achieved for both materials, but that NRAs were observed when standard sized nickel is used. X-ray energy dispersive spectrometry and electron diffraction show that the NRAs are composed of martensite and austenite under rapid cooling conditions. Three-point bending fatigue tests were carried out on both alloys to evaluate the effect of these soft austenitic areas on the fatigue properties of powder metal steel parts. The analysis of the endurance limit results shows that NRAs are not a governing factor.     

Separation of copper from steel

Abstract

Copper has been separated from iron and steel by chlorine–air mixtures at 800°C. It was found that, contrary to predictions based upon thermodynamics, cupric chloride was the favoured copper product rather than cuprous chloride. This was due to the high vapour pressure of cupric chloride. It was found that in order to prevent the reaction between iron and cupric chloride it was necessary to preoxidise the iron to form an impervious oxide film. Copper contents lower than 0·05 wt-% were readily obtained after 10 min exposure to the gas, even when starting with several per cent of copper mixed with the iron or steel.     

EFFECT OF ADDITIONAL ALLOYING ELEMENTS ON THE PROPERTIES OF SINTERED MANGANESE STEELS

Abstract

Sintered alloys based on the Fe-Mn system have been investigated by using single-pressing and double-pressing techniques. Fe-Mn (Mn up to 8 wt.-%) and Fe-Mn-C (C up to 1·4 wt.-%) alloys were prepared both with manganese as an electrolytic powder and with a Fe-Mn master alloy. The influence of sintering temperature and sintering time on mechanical properties and homogenization is discussed. The effect of the additional alloying elements Cr, Mo, eu, and of their combinations on mechanical properties has been determined. Further investigations were carried out with a Fe-Mn-Cr-Mo-C master alloy. The optimum single-pressed and double-pressed alloy (Fe with Mn 0·8, Cr 0·8, Mo 0·8, and total C 0·6%) has a tensile strength (σ_B) of >700 N/mm². Optimum alloys of all investigated systems were hot-forged and their mechanical properties are compared with those of single- and double-pressing techniques. The alloys were heat-treated and their tempering behaviour determined. Jominy standard tests were carried out to determine hardenability of the porous sintered materials.     

Effect of boron on vacuum carburising depth of iron powder metallurgy compacts

Abstract

The work was aimed at determining the effect of boron on vacuum carburising of iron compacts with density over 7·2 g cm^–3. An attempt was made to determine the effectiveness of boron on carbon diffusion rate into the material of compacts with no additional effect of interconnected porosity. Vacuum carburising of compacts made of iron powder with an addition of boron was carried out at 1050°C in a laboratory vacuum furnace.

The effect of boron content within 0·005 to 0·02% on the vacuum carburising depth was analysed. It was found that the boron addition up to 0·01% increased the carburising depth by ～0% in comparison with the compacts of pure iron.     

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.     

Development of forging quality high strength low alloy steels

Abstract

The effect of alloy composition and heat treatment on the structure and properties of a set of high strength low alloy (HSLA) steels has been investigated. By addition of a relatively high dose of niobium (0·17–0·23%)along with nickel (0·2%), chromium (0·4–0·6%), and manganese (1·5–1·8%) to 0·2%steels, it is possible to develop high strength forging grade steels having baintic or autotempered martensitic matrixes. The microstructure and mechanical properties of these steels are sensitive to cooling rate and heat treatment.     

DEHYDRIDING OF Ti–6AI–2Sn–4Zr–6Mo HYDRIDE POWDER

Abstract

A systematic investigation of temperature/pressure/time variables for dehydriding titanium alloy (Ti–6AI–2Sn–4Zr–6Mo) powder was made to determine practical processing conditions for reducing the hydrogen content of the powder to levels below 100 ppm. Studies made between 650·C (1200·F) and 871·C (1600·F) established the following conditions for obtaining the desired hydrogen contents:

Analyses of the experimental results of tests made between 650°C (1200°F) and 816°C (1500°F) indicate that at 1–0·088 μm partial pressure of hydrogen the dehydriding process is controlled by the second order reaction: 2H (gas)= H2 (gas) with an activation energy of 21 760 ± 1840 cal. At <0·088 μm partial pressure of hydrogen the process appears to be controlled by the rate of removal of H₂ molecules clinging to the powder surfaces. The type of vacuum system used for dehydriding is believed to be the major factor governing removal of these molecules.     

PERMANENT MAGNETS FROM ELONGATED SINGLE-DOMAIN PARTICLES

Abstract

A process is described for producing elongated single-domain (ESD) fine-particle magnets. The 150-Å. ESD iron or iron–cobalt alloy particles are prepared by controlled electrodeposition into mercury, followed by thermal growth and treatment with a third metal to attain optimum particle shape and magnetic properties. The particles are then aligned by a magnetic field, compacted under pressure, freed of mercury by vacuum distillation, and embedded in a suitable matrix. This is ground to a coarse powder and fed into automatic presses for realigning and compacting to the final magnet shape. The factors controlling each step of the process are discussed, and the advantages of magnets with artificial microstructures synthesized by this approach are pointed out. The process described produces commercial ESD iron and iron–cobalt magnets with energy products of 2·2 and 3·5 million gauss-oersteds, and laboratory ESD iron and iron–cobalt magnets of 4·2 and 5·0 million gauss-oersteds.     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

Characteristics of rapidly solidified Cu–10%Sn alloy powders produced by water jet cooled rotating disc atomisation

Abstract

In the present work, an experimental water jet cooled rotating disc centrifugal atomiser was designed and constructed and used to produce rapidly solidified Cu–10%Sn alloy powders. The characteristics of rapidly solidified Cu–10%Sn alloy powders have been investigated with respect to powder size and disc surface condition. Uncoated and ZrO₂ coated copper discs were used to investigate the effect of disc surface conditions on the microstructure and cooling rate of the powders. The produced powders appeared in the shape of sphere, rounded, ligament, irregular and flaky, depending on the particle size. The powders exhibited fine grained microstructure, cell size increased with increasing powder size and higher cooling rates were obtained using uncoated disc. The results indicated that cooling rates of 20 μm powder produced with uncoated and ZrO₂ material coated discs were estimated as 5·82×10⁵ and 1·44×10⁵ K s^?1 respectively.     

SOME MECHANICAL PROPERTIES OF SAE 1045 STEEL JOINTS BONDED WITH SINTERED COMPACTS

Abstract

The physical and mechanical properties of plain steel (SAE 1045) butt joints, bonded with a sintered alloy as a filler material, are described. Sintered alloys were made from two types of iron powders–electrolytic and Swedish sponge– with various additions ranging from 7 to 20 wt.-% of an electrolytic copper powder, a prealloyed bronze powder (90% Cu+ 10% Sn), and a mechanically mixed elemental bronze powder.

The results showed that the tensile strength was not reduced as drastically as that of the copper-brazed joints, when the thickness of the bonding material (sintered alloy) was increased. The highest bond strength in tensile and fatigue testing was obtained with the sintered alloys comprising iron powder and 10 wt.-% pre alloyed bronze powder. The results indicate that there is a practical possibility of brazing steel using sintered compact as a filler material under a neutral or a reducing atmosphere.     

Fabrication of W–20%Cu composite powder from copper and tungsten salts

Abstract

An investigation has been made to prepare homogeneous powders of CuWO₄ and WO₃ from ammonium paratungstate and copper nitrate to prepare nanosized W–Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water; while the content of the beaker was being stirred at a certain speed to reach the desired composition of W–20 wt-%Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W–Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W–Cu oxide powder and then were ball milled and reduced in H₂ atmosphere to convert it into W–Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W–Cu with particle size of ～100 nm and a nearly spherical shape could be obtained by this process.     

On enhancement of hypoeutectic aluminium–silicon powder metallurgy alloy

Abstract

Currently, there exists a considerable demand within the automotive industry for the development of aluminium–silicon alloys that can be processed through powder metallurgy (PM) technologies. As such, a study aimed at the development of a new hypoeutectic aluminium–silicon PM alloy (Al–6Si–4·5Cu–0·5Mg–0·2Sn) was recently undertaken. Although the preliminary data were encouraging, it was postulated that a refinement of the bulk alloy chemistry could yield additional gains. In the present work, the sintering response mechanical properties of this alloy were enhanced by making controlled adjustments in composition. The influence of the amount and raw powdered sources of copper, magnesium and tin were the principal variables investigated. Using XRD and metallographic examination, it was determined that a decrease in the copper content to 3·0 wt-% improved ductility significantly. Furthermore, the incorporation of magnesium as an elemental powder versus an Al–Mg master alloy improved sintering behaviour significantly, while tin additions up to 0·2 wt-% increased yield stress and hardness. The optimised alloy exhibited a threefold increase in ductility as well as a 25% gain in yield strength.

Présentement, il existe une grande demande dans l’industrie de l’automobile pour le développement d’alliages aluminium silicium qu’on peut obtenir par les techniques de métallurgie des poudres (PM). Ainsi, on a récemment entrepris une étude visant au développement d’un nouvel alliage hypoeutectique aluminium silicium (Al-6Si-4·5Cu-0·5Mg-0·2Sn) par PM. Bien que les données préliminaires soient encourageantes, on a postulé qu’un raffinement de la chimie de l’alliage brut pourrait produire des gains additionnels. Dans le présent travail, on a amélioré les propriétés mécaniques de la réponse au frittage de cet alliage en faisant des ajustements contrôlés de la composition. L’influence de la quantité et l’influence des sources de poudres brutes de cuivre, de magnésium et d’étain étaient les principales variables examinées. En utilisant la XRD et l’examen métallographique, on a déterminé qu’une diminution de la teneur en cuivre à une valeur de 3·0% en poids améliorait significativement la ductilité. De plus, l’incorporation du magnésium comme poudre élémentaire par rapport à un alliage maître d’Al-Mg améliorait significativement le comportement de frittage, alors que des additions d’étain jusqu’à 0·2% en poids augmentaient la limite élastique et la dureté. L’alliage optimisé exhibait une augmentation triple de la ductilité ainsi qu’un gain de 25% de la limite d’élasticité.     

INFLUENCE OF IRON POWDER TYPE ON THE PROPERTIES OF SINTERED IRON AT VARIOUS DENSITY LEVELS

Abstract

The properties of various commercial and experimental iron powder types and of compacts made from them in the density range 6·8–7·87 kg/dm³ by single-pressing, double-pressing, and hot-forging techniques have been determined. It was shown that the ductility in all cases was more adversely affected than the tensile strength by the presence of porosity. However, it was also shown that at any particular density level or with a given processing schedule the mechanical properties varied widely, depending on the iron powder used. On the basis of the mechanical-property results, the powder types to be preferred at different density levels are indicated.     

Dispersion-Strengthened Copper Alloys with Useful Electrical and Mechanical Properties

Abstract

Dispersion-strengthened alloys have been made which combine a high level of tensile strength at temperatures up to at least 600°C with an electrical conductivity better than that of most precipitation-hardened copper alloys. The reverse gel precipitation process has been used to co-precipitate hydroxides which were then selectively reduced in hydrogen, consolidated under an atmosphere of pure argon, and finally hot-extruded to bar. Copper?3 vol.-% zirconia alloys were prepared in which all the particles were <150 nm dia., while copper–1·5 vol.-% thoria and copper–3 vol.-% thoria alloys were prepared with most particles <50 nm dia. Although the dispersion in the Cu–zirconia alloys was somewhat inferior to that obtained in the Cu–thoria alloys, useful properties were obtained. The Cu–zirconia alloys were as strong as the commercial alloy Cu–1 wt.-%Cr at 500°C and twice as strong at 600°C. There was little difference in the strength of a Cu–1·5 vol.-% thoria alloy and the Cu–zirconia alloys but the former was more ductile. The most interesting properties were obtained from Cu–3 vol.-% thoria alloys which exhibited an electrical conductivity in excees of 90% IACS at 20°C and tensile strength five times that of Cu–1%Cr at 600°C, even after annealing at 600°C for 1 h. The Cu–3 vol.-% thoria alloys were readily cold-worked, exhibited exceptional stability, and were resistant to recrystallization up to 900°C. Grain sizes were of the order of 1·5 μm for unalloyed copper, 1 μm for Cu–1·5% thoria, and 0·5 μm for Cu–3% zirconia or Cu–3% thoria. Grain growth was severely restricted by the dispersions.     

Influence of minor Zr and Ti on microstructures and properties of Al–8·6Zn–2·5Mg–2·2Cu alloys

Abstract

The effect of minor Zr and Ti elements on microstructures and properties of Al–8·6Zn–2·5Mg–2·2Cu alloys was investigated. The results show that there is no significant change in the grain size by simultaneous addition of Zr and Ti as compared to individual additions of Zr or Ti to Al–8·6Zn–2·5Mg–2·2Cu alloy. The ability of restraining recrystallisation of Al–8·6Zn–2·5Mg–2·2Cu–0·16Zr–0·04Ti alloy is weaker than that of alloy Al–8·6Zn–2·5Mg–2·2Cu–0·16Zr alloy, since Ti and Zr have a poisoning interaction. The corresponding tensile properties, hardness and stress corrosion cracking resistance of the Al–8·6Zn–2·5Mg–2·2Cu–0·16Zr alloy were decreased by addition of Ti.     

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.