Showing papers in "Powder Metallurgy in 2005"

Improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts

Abstract: The application of microwave technology to a diverse range of materials and processes has resulted in a wide spectrum of materials that are commercially processed using microwaves, from the heating of food to the vulcanisation of rubber to the sintering of specialty ceramics. Microwave sintering of elemental or alloy metal powders has gained significance in recent times as a novel processing method since it offers many advantages over the conventional sintering method. Despite substantial R&D investment in this area in the past two decades, no competitive microwave technology has yet emerged for powder metallurgy (PM) sintering. In sharp contrast, because it is 'obvious' that microwaves are reflected by metals, it is not uncommon to be unable to locate many journal papers or literature, wherein metal powders have been sintered in a microwave field. This paper reports the improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts as compare...

TEM study of PM 2000 steel

Abstract: Transmission electron microscopy (TEM) as well as corresponding analytical techniques, such as high resolution TEM (HRTEM), energy dispersive X-ray (EDX) analysis, electron energy loss spectroscopy...

Experimental and numerical study of die filling, powder transfer and die compaction

Abstract: Experimental and numerical studies are presented of the behaviour of a Distaloy AE powder during filling and transfer (i.e. the parallel movement of punches from their filling position to their starting position prior to compaction). Discrete element simulations provide information about the density distribution in a die after filling and how this is modified during transfer. The transfer process creates a depression in the top of the powder. The discrete element computations explain this behaviour in terms of the circulation patterns generated in the powder as a result of the transfer process. The profile of the powder in the die and the density distribution after transfer provide the initial conditions for a finite element simulation of the compaction process. These computations employed a Drucker–Prager–Cap model, which was implemented in the finite element code ABAQUS/Explicit, using a user material subroutine. A sensitivity study is presented which evaluates how the density distribution and d...

Green strength of powder compacts provided for production of highly porous titanium parts

Abstract: The production route of highly porous near net shape titanium parts includes the green machining of compacts made of metallic and pore former (often ammonium bicarbonate) powder mixtures. Success in green machining greatly depends on compact strength. The aim of the present work was to determine and quantitatively evaluate factors influencing the green strength of titanium–ammonium bicarbonate compacts. The substantial dependence of green strength on the amount of pore former and compaction pressure was demonstrated. Some influence of the particle size of ammonium bicarbonate on compact strength was also observed. A theoretical explanation of these experimental results is given. A sharp decrease of green strength owing to decomposition of ammonium bicarbonate in compacts during holding in air was experimentally found. The factors influencing decomposition, i.e. time, compaction pressure and particle size of pore former, were investigated in detail. The catalytic action of titanium on decomposition...

Surface chemical analysis of prealloyed water atomised steel powder

Abstract: Prealloyed water atomised steel powder was investigated regarding composition, morphology and thickness of the surface oxide. The materials were two varieties of Cr alloyed and one Mo alloyed. The oxides formed on powder surfaces were studied by means of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy, in combination with high resolution electron microscopy and X-ray microanalysis.On all powder surfaces, the oxide formed contained strong oxide forming elements such as Cr, Mn and Si. Still, the dominant oxide on the powder surface was Fe oxide, the relative cation concentration in the surface being about 80%. The surface morphology showed a heterogeneous structure with particulate compounds supposed to be rich in strong oxide formers and an Fe rich thin oxide layer. This heterogeneous surface oxide morphology was more pronounced for the Mo alloyed powder compared with the Cr alloyed varieties. For this alloy, the average thickness of the oxid...

Comparison of Mn, Cr and Mo alloyed sintered steels prepared from elemental powders

Abstract: Molybdenum, chromium and manganese offer considerable potential as alloy elements in sintered steels, especially for PM precision parts used, for example, in automotive engines and transmissions. This holds in particular for recycling and health/safety aspects. Within this work, the influence of these elements as admixed alloy metals on the sintering behaviour and the properties of structural PM steels is discussed. The considerable differences in the homogenisation behaviour during sintering are described as well as the respective advantages and drawbacks. It is shown that for high density PM steels, Cr and Mo are better suited while for conventionally produced mass products Mn is attractive, its affinity to oxygen being less of a problem today than commonly assumed and since Mn addition promotes swelling, this element might be a replacement for Cu. For Mn, admixing is the more attractive route, for Cr, in contrast, prealloying offers advantages, while for Mo both alloying techniques are feasible.

Fast densification and deformation of titanium powder

Abstract: A coarse titanium powder containing 0.2 wt-% oxygen with an average particle size of 45 micrometers was rapidly densified using the spark plasma sintering (SPS) technique. The fully consolidated s ...

Development of new feedstock formulation based on high density polyethylene for MIM of M2 high speed steels

Abstract: A new feedstock formulation for metal injection moulding (MIM) of M2 high speed steels has been developed. The binder is a multicomponent system based on high density polyethylene (HDPE) and paraffin wax (PW). The compatibility between binder constituents has been studied by dynamomechanical thermal analysis (DMTA) showing a partial miscibility between both components. Viscosity measurements of the different binder mixtures at different shear rates showed that the optimum formulation for MIM was 50 vol.-% HDPE. With this optimised binder, several mixtures were prepared with different powder loadings of M2 grade high speed steel. Torque measurements of the mixtures indicated that the maximum amount of metal to be used was 70 vol.-%. The wide distribution of the metal powder was homogeneously distributed into the polymer matrix. The polymeric part was driven off by thermal debinding using a thermal cycle designed on the basis of a thermogravimetric study of the binder. Finally the vacuum sintering ...

Ceramic comparison of filling behaviour of metallic, ceramic, hardmetal and magnetic powders

Abstract: A model die shoe filling system has been employed to characterise the flowability of a range of commercial powders. The filling rig mimics some features of a typical industrial die filling process. Qualitative and quantitative studies were undertaken using a high speed video system and by determining the variation of the mass delivered to the die as a function of shoe velocity. The critical velocity, i.e. the velocity above which incomplete filling is achieved, was determined for each powder. Experiments were carried out in air and in vacuum to determine the influence of air flow and pressure on the filling behaviour. The size of the die cavity and the height of powder in the shoe are shown to have an influence on the mass–velocity relationship; and therefore the critical velocity. Equations are presented which allow the results generated using a standard die to be extrapolated to other die geometries.

Experiments on the aging of Sn–Ag–Cu solder alloys

Abstract: The microstructural stability of the Sn–3·8%Ag–0·7%Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h, at 150°C. The first change, evident at high magnification after heating for 1 h, occurred from the as cast lamellar plus fibrous form of the Ag3Sn and Cu6Sn5 interdendritic eutectic phases to a particulate form. With further heating, coarsening of the two compound phases occurred, gradually rendering the Sn dendrite pattern less distinct. Due to the very rapid diffusion of Cu in solid Sn, the Cu6Sn5 phase coarsened most rapidly, growing from its originally finely divided (200 nm) size in the ternary eutectic to form many particles up to 3 m m or more in size in a time of 100 h. At that time, nearly 50% of the total Cu was contained in these particles. The Ag3Sn phase coarsened more slowly. Approximate measurements of average particle size as a function of time suggested that coarsening occurs by Ostwald ripening, co...

Microstructural development during liquid phase sintering of Fe and Fe-Mo alloys containing elemental boron additions

Abstract: The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0. 5t o 3.5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7.82 g cm 23 were obtained for these alloys. PM/1128

Microhardness of pore walls in porous titanium prepared with novel powder metallurgy

Abstract: Owing to the adjustable mechanical properties, porous titanium as bone substitute can reduce the effect of stress shielding. Microhardness and its distribution on pore walls are good factors to determine the microstructure of porous titanium related to the wear and corrosion resistance in vivo. In the procedure of novel powder metallurgy, porous titanium was prepared after space holders as the second phase were removed during heat treatment. Through changing the compaction pressure, sintering temperature or holding time, porous bodies with different sintering index were prepared. The effect of the friction between die wall and powders or between powders themselves can be reduced by changing the compaction pressure. The mean hardness H m and homogeneous index HI are used to determine the homogeneity of porous structures. In order to gain specimens with high sintering index or high microhardness, sintering at higher temperature is preferred for longer holding time.

Investigation of rheological behaviour of 316L stainless steel-3 wt-%TiC powder injection moulding feedstock

Abstract: The rheological behaviour of powder injection moulding feedstock comprising of 316L stainless steel and 3 wt-%TiC powders was studied using a capillary rheometer. The flowability and the sensitivity of viscosity to shear rate and temperature of the feedstock were investigated and compared with those of the binder system and the 316L SS PIM feedstock. The general rheological indexes were examined through relevant equations and the influence of TiC addition on the mouldability of the 316L SS feedstock was determined. It was found that all the feedstocks are basically pseudoplastic but the values of flow behaviour index n are influenced by the TiC addition, the solid volume fraction and the temperature. In this context, the results show that TiC addition plays an important role in rheological parameters, i.e. TiC particles decrease the viscosity of PIM feedstock and this effect is more pronounced in the system comprising higher solid volume fraction. Furthermore, the flow activation energy decreases with the introduction of TiC particles, in particular at high shear rates. In other words, the addition of ceramic particles would ease the feedstock flow and suppress the sensitivity of the feedstock to strain rate and temperature variations, if a proper moulding temperature is applied. From the viewpoint of mouldability, the optimum PIM condition for the SS/TiC composite powder containing a wax based binder system was found at 55% solid volume fraction and 70uC. At this condition, the viscosity of feedstock is low enough to fulfill the requirements of a medium pressure, injection moulding process. PM/1102

Comparison of high performance PM gears manufactured by conventional and warm compaction and surface densification

Abstract: The size of planetary gears in heavy truck gearboxes means that PM can be a cost effective alternative to conventional manufacturing from forged blanks. Warm compaction and surface densification offer effective routes to reach high density, and thereby high strength and fatigue properties. Characteristics for PM gears manufactured by these methods are outlined and compared with wrought and machined gears.

Quantitative evaluation of porosity effects in sintered and heat treated high performance steels

Abstract: Many heavy duty components suitable for production by powder metallurgy (PM), such as transmissions and also safety parts, require mechanical properties characteristic of high density. Therefore, the ability to produce PM parts of high density has the potential to expand the use of sintered components beyond its present limitations. The purpose of the present study was to investigate the features of pores, to establish quantitative correlations under different sintering conditions and to compare the effects of heat treatments such as case hardening on the pore distribution in the as sintered state. An approach involving quantitative analysis of the upper 10% of the pore size distribution was also employed because these bigger pores have the greatest influence on mechanical properties (especially the dynamic ones). Pore area, maximum length and roundness were analysed for compacts of several densities. A comparison of average values was made to detect differences in pore characteristics among densi...

Jetting in metal injection moulding of 316L stainless steel

Abstract: The jetting phenomenon in metal injection moulding (MIM) has been investigated. To date, the behaviour of the polymer metallic powder melting during filling stage that causes problems in choosing process parameters has not been adequately described in the literature, the manner of the melt flows inside a die cavity is not easy to predict. In order to better understand the process, an experimental mould with replaceable inserts including a window glass section was designed and manufactured in our lab. The glass window allows monitoring the manner of the melt flows in real time and permit to record it with a change couple device (CCD) camera. In this paper, the injection moulding experiments have been performed to test the filling process and occurrence of jetting. The experiments indicate that there is high probability of conventional jetting depending on process parameters. Various gate cross sections and runner length die cavity were employed as variable parameters in order to investigate their i...

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract: This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.

Solvent debinding kinetics and sintered properties of injection moulded 316L stainless steel powder

Abstract: A stainless steel (316L) powder having a median particle size of ∼25 μm was injection moulded using binders composed of polyethylene glycols (PEG) and polymethylmethacrylate (PMMA). The PEG content was leached in water and debinding kinetics studied as a function temperature. At 40°C, the removal of PEG was controlled by dissolution of the polymer in water and by diffusion of water and PEG for leaching time 60 min, respectively. Water soluble PEG was completely removed within 180 min and the diffusion mechanism controlled the de-binding process at 60 and 80°C. Sintered properties improved with increased sintering temperature and time. The improvement in mechanical properties was less pronounced for a longer soaking time owing to more grain coarsening.

On numerical simulation of powder compaction process: powder transfer modelling and characterisation

Abstract: A numerical model for the powder transfer stage in powder metallurgy cold compaction processes, and the corresponding material characterisation procedure, are proposed. They have been designed on the basis of the following requirements: robust and consistent computational mechanics ingredients, reliability of the obtained results for practical processes in powder metallurgy and industrial viability in the sense that characterisation of any mixture doesn't require either much effort or much time of the enduser. The starting point is a previously developed numerical model for powder compaction, formulated in terms of the large plastic deformation theory, which requires calibration of four parameters controlling the evolution of the yield surface. This calibration, which had been successfully carried out in the past in a range of moderate to high densities, is now extended to very low densities in order to make numerical simulations able to deal with compaction processes involving relevant powder tra...

Nanocrystalline and amorphous structure formation in Ti-Al system during high energy ball milling

Abstract: The aim of the present work was to study the structural evolution of Ti–48Al (at.-%) powder blend during mechanical alloying. Because milling parameters play a vital role in achieving the desired structure/phase, milling was carried out with optimised parameters. Both powder handling and milling operations were performed under high pure argon atmosphere to prevent oxidation of the powder blend. Mechanically alloyed powder blend was then characterised by scanning electron microscope (SEM), X-ray diffraction (XRD) and differential thermal analyzer (DTA). Mechanical alloying induced severe plastic deformation resulting in cold welding, powder particle refinement and narrowing of powder particle size distribution as is evidenced from SEM micrographs. XRD analysis indicated complete dissolution of aluminium in titanium at 20 h of milling and achieving nanostructure before amorphous phase formation. DTA scan indicated the disappearance of one of the exothermic peaks, and a gradual drop in crystallisatio...

Microstructure and tensile properties of 3%Cr–0·5%Mo high carbon PM sintered steels

Abstract: Tensile properties of 3%Cr–0·5%Mo sintered steels vary greatly depending on the sintering temperature, compaction method and cooling rate. Given the high cooling rate and carbon content, the microstructure is either fully martensitic or bainitic–martensitic, with different amounts of bainite. Microhardnesss of the sintered samples is very high, ranging between 650 HV0·05 and 950 HV0·05, therefore, materials have a tendency to be brittle and their properties improve after a low temperature tempering. The best mechanical properties were obtained on the bainitic–martensitic materials sintered at high temperature.The present work demonstrates the possibility of obtaining excellent tensile properties using a 3%Cr–0·5%Mo prealloyed powder to form a dual phase microstructure, comprising of martensite and lower bainite in equal amounts.

Effect of silicon, vanadium and nickel on microstructure of liquid phase sintered M3/2 grade high speed steel

Abstract: Liquid phase sintering of M3/2 grade high speed steel (HSS) was carried out at 1270°C in high vacuum reaching near full density starting from loose packed powder. Focus is placed on the study of the effects of the addition of Si, Ni and V as elemental powder and cooling rates on the as sintered microstructure, the main objective being improving M6C characteristics and control of pearlite appearance. Slow cooling from the sintering temperature and Si addition in wt% resulted in a completely fine pearlitic matrix with less elongated and more uniformly distributed M6C precipitates. Adding V or Ni in wt-% quantities decreased the amount of pearlite owing to MC formation and delayed pearlite formation. The study involved the use of thermodynamic modelling and sintering cycle optimisation as well as the evaluation of sintered material by means of optical and scanning electron microscopy, X-ray diffraction and hardness testing.

Metal injection moulding of scissors using hardenable stainless steel powders

Abstract: The production of hairdressing scissors, which are a high value added product, by a metal injection moulding route, using a 440 type hardenable stainless steel powder and a PEG/PMMA composite binder system has been investigated. Previous research has highlighted a number of problems associated with the PM processing of 440 type stainless steels; however, the present study has ascertained that, with careful control of the sintering conditions, a microstructure with finely distributed, spheroidal chromium carbide and high hardness could be attained following subsequent heat treatment.

Microstructural and mechanical characterisation of composite materials consisting of M3/2 high speed steel reinforced with niobium carbides

Abstract: Fully dense composite materials of M3/2 tool steel reinforced with 5 and 8 vol.-% of niobium carbide were developed using the powder metallurgy route. The consolidated materials exhibited a fine and uniform microstructure consisting of a fine dispersion of carbide particles in a matrix of ferrite/martensite. The 0·2% yield strength up to 600°C was evaluated in compression. The slight increase observed after reinforcement by NbC particles in as hipped materials suggests that the martensite has the major contribution to the strength. After tempering, the reinforced materials showed a moderate increase in yield stress at room temperature with respect to the unreinforced M3/2. This increase is attributed to reinforcement mechanisms associated with niobium carbide. PM/1175

Evaluation of copper powders for processing heat sinks by metal injection moulding

Abstract: Powder characteristics that enable metal injection moulding of high conductivity copper heat sinks are investigated Gas atomised, water atomised, oxide reduced and jet milled copper powders are characterised in terms of particle size, packing density, mixing torque and impurity content The sintering kinetics and rate of oxide reduction are investigated using die pressed samples Densities of 93 to 96% of theoretical can be achieved for all of the powders Oxide reduction is complete by about 900°C One of the water atomised copper powders is injection moulded to produce a demonstration heat sink Thermal and electrical conductivities are measured and related to iron content and porosity Overall impurity contents for typical powders are about 0·15 wt-%, resulting in thermal conductivities of 280 to 320 W m−1 K−1 for MIM copper

Strain hardening behaviour of powder metallurgy alloys

Abstract: A model to describe the strain hardening behaviour of porous alloys, produced by powder metallurgy, is presented. It accounts for the influence of the matrix strain hardening ability, the initial porosity content and the pore geometry, and its predictive ability is verified with reference to sintered iron and an Fe–0·3%C sintered alloy. The necessity to introduce in the model a parameter able to account for the internal notch strengthening effect exerted by pores is emphasised. This parameter is found to be dependent on the sintering degree of the materials.

Production of Al–Al3Ti powders by mechanical alloying and annealing

Abstract: Mixed powders of Al and Ti (10 wt-%) have been mechanically alloyed in an attritor mill under vacuum or nitrogen atmosphere. Pure aluminium powders have also been prepared, in the same conditions, for comparison. After milling for 10 h, a metastable solution of Ti in an Al matrix is obtained, with ∼9 wt-%Ti dissolved in the matrix. The evolution of these powders during milling is reported. Their thermal stability has been studied using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), identifying the observed changes by X-ray diffraction (XRD). Annealing of these powders at different temperatures, up to a maximum of 625°C, produces the precipitation of new phases, such as Al4C3 and different structures of Al3Ti, as well as grain growth. The appearance of these second phases, and their influence on powder microhardness, has been characterised as a function of the selected heat treatment temperature.

Processing of Mn-Zn ferrites using mould casting with acrylic thermosetting binder

Abstract: In this paper, a simple manufacturing process for Mn–Zn ferrite powder is described, which can be considered as a modified powder injection moulding process. This method uses acrylic thermosetting resin as the binder. The moulding is carried out at room temperature by directly pouring the slurry (resin and ferrite) in the mould. The mixture is heated at the curing temperature (70°C) of resin to permit polymerisation and cross linking of the polymer. In order to optimise the moulding step, different volume fractions of powder with resin were mixed. The optimal powder load was 50 vol.-%. The best thermal debinding cycle was determined by means of thermo-gravimetric analysis. Sintering was performed according to oxygen partial pressure equilibrium curves at 1330°C for 3 h. Magnetic properties were compared with those obtained by uniaxial compacted parts.

Fracture toughness of PM alloy steels

Abstract: Macroscopically valid plane strain fracture toughness specimens of porous sintered alloy steels will generally fail in a microscopically ductile manner because the plane strain requirement is violated in the rather thin sinter necks between pores and a dimple fracture develops from microvoid coalescence. Therefore, in ordinary sintered steels, valid K Ic values cannot be determined because a spontaneous brittle fracture does not occur on exceeding a critical crack opening displacement. This favourable behaviour is maintained in the as sintered condition with steels containing 0·5 or 0·65%C up to densities of 7·5 g cm−3. In order to come to valid K Ic values, it is necessary to establish plane strain conditions between the pores, e.g. by approaching full density, or to embrittle the steel, e.g. by suitable heat treatments or alloying techniques. In the present study 0·3 or 0·6% phosphorus was added to a diffusion bonded alloy steel which is known to turn brittle in the presence of phosphorus when s...

Particulate gold as money and currency material

Abstract: In ancient times, alluvial placer gold deposits were an important source from which particulate gold was derived by panning, and such naturally occurring gold has been widely used as currency throughout the world. Examples, from ancient to pre-modern times, from Western Africa, the USA and Asia are presented. The Ghana administration was unique in becoming so committed to particulate gold currency that by the late seventeenth or early eighteenth century, existing solid gold currency was being converted into particulate gold. As is discussed, particulate gold was preferred to counter the practice of embedding base metal inside the solid gold currency.