Showing papers in "Powder Metallurgy in 1995"

High pressure triaxial cell for metal powder

Abstract: A triaxial press was developed to study the mechanical behaviour of metal powders. This press is numerically controlled and it allows different load paths with stress and/or strain control: isotropic, conventional triaxial tests, and loading-unloading cyclic tests. The confining pressure limit is equivalent to a specimen radial stress of 400 MN m(-2) whereas the maximum axial stress is 1250 MN m(-2). Experiments on an iron powder showed that there was reproducibility of the results and only a small error in measurement. With respect to the mechanical behaviour of this powder, the important influence of deviatoric stress on the densification was observed but no dilatancy effects were observed.

Injection moulding of 316L stainless steel powder using novel binder system

Abstract: Powder injection moulding based on a novel binder system, and using 316L stainless steel powder as a test material, is described. The binder system comprises a major fraction of polyethyleneglycols (PEGs) of various molecular weights and a minor fraction of very finely dispersed polymethylmethacrylate (PMMA) incorporated in the form of an emulsion. The feedstock was mixed as a thick, aqueous slurry at room temperature and was rapidly homogenised by stirring at a low shear force, which should minimise contamination because of a much reduced wear rate of the mixing equipment. The possible demixing during the drying of the slurry was minimised by frequent stirring during this stage and the use of some shear mixing of the dried feedstock. The green strength of the moulded specimens was measured by a three point bend test. In the first stage of debinding, the PEGs were removed by water leaching from the moulded components. This opened up pore channels which allowed much faster removal of the remaining PMMA binder during subsequent thermal debinding. The debinded samples were sintered at different temperatures and the sintered samples were examined by SEM and optical microscopy for pore structure and defects such as cracks

Stress concentration at pores in sintered materials

Abstract: Statistical distributions for the stress concentration factor A k at pores in three sintered iron based materials have been determined using the equivalent ellipse concept. Pore dimensions were obtained by metallographic examination of fracture surfaces produced by impact at liquid nitrogen temperatures. Most of the pores (80%) were characterised by A k < 5; the vast majority of the remainder had values in the range 5–8 and the maximum observed value was 12. The means and standard deviations of the A k distributions were found to vary only marginally with material composition and compaction pressure. On the basis of these distributions, conclusions are drawn concerning the local yielding, deformation, and fracture of porous ductile materials. PM/0691

Surface Reactions During Water Atomisation and Sintering of Austenitic Stainless Steel Powder

Abstract: The surface oxides formed during water atomisation and sintering of austenitic stainless steel were determined using electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). Optical microscopy and electron microscopy (SEM and TEM) were used for structural analysis of powder and sintered material. The materials studied were 304L, 304L + Si, 304L + Al, and 304L + C. All powders were prealloyed except for 304L + C, which was obtained by admixture with graphite. Sintering was carried out in dissociated ammonia and in a vacuum. It is shown that the surface oxidation is strongly affected by the change in cooling rate with particle size. The average oxide thickness increases significantly with increasing particle size, while the surface oxide changes from a silicon rich oxide to an oxide containing more iron and chromium. A strong correlation between the average oxide thickness and the secondary dendrite arm spacing (i.e. the cooling rate) is observed. It was not possible to distinguish any clear effect of increasing the silicon content above 1 % on the surface oxidation. During sintering, the iron and chromium oxides formed during water atomisation are reduced. The silicon oxide forms a continuous layer at 1120°C, while it is broken up into discrete particles at 1250°C. The reduction favours neck growth resulting in improved mechanical properties. For equal final density, the impact strength can be correlated to the relative neck radius. Admixture with carbon before sintering can further enhance the oxide reduction. As a result of a higher sintering temperature or the addition of carbon, sintering is enhanced and improved mechanical properties are obtained. Prealloying with aluminium leads to a highly inferior mechanical strength due to the formation of aluminium oxide on the powder surfaces

Supersolidus Sintering of High Speed Steels: Part 2: Sintering of Tungsten Based Alloys

Abstract: The effect of carbon content on the vacuum sintering characteristics of annealed and cold compacted water atomised type Tl high speed steel powders is reported. It was found that increasing carbon content from 0·8 to 1·4 wt-% resulted in significant improvements in sinterability; optimum sintering temperatures were lowered from 1320 to 1240°C whereas the sintering window expanded from ∼10 to ∼40 K. Compositions with carbon contents outside this range were unsinterable owing to the formation of either undesirable M6C eutectic structures before attaining full density (0·6%C) or skeletal structures of an Fe–Cr rich phase on reaching full density (1·7%C). Sintering behaviour and microstructures are discussed with reference to phase diagrams. It is shown that optimum processing requires heating to temperatures within the austenite + carbide + liquid phase field with the width of the sintering window being determined by the temperature interval separating the phase boundaries which define this region. P...

Characterisation of Composite of TiC + TiB2 Bonded with Nickel Based Binder Alloy: Mechanical and Microstructural Properties

Abstract: Studies of. the bonding mechanisms in transition metals and hardmetals based on transition metals have been conducted to examine the current understanding of the binder-hardmetal interfacial reactions in cemented hardmetals. The studies led to the proposal of a binder selection procedure to be used, in conjunction with thermodynamic and phase equilibria data, for binder selection based on the number of d state electrons in the constituent transition metals. The binder selection procedure developed was usedto select a preferred binder composition for a TiC + TiB2 composite. An experimental investigation of the selected binder for the TiC + TiB2 composite led to an improvement in densification after sintering, three point bend strength (modulus of rupture values), and indentation fracture toughness K IC values. Electron probe microanalysis and scanning transmission electron microscopy investigations revealed the segregation of one of the constituent metal additives in the composites which indicated ...

Production of Porous Sintered Co–Cr–Mo Alloys for Possible Surgical Implant Applications: Part 1: Compaction, Sintering Behaviour, and Properties

Abstract: The compaction and sintering behaviour of a coarse water atomised cobalt based alloy and a bimodal mixture of ball milled ASTM F75 plasma rotating electrode process powder with the cobaIt alloy powder is reported. The techniques employed were intended to generate a porous substrate, using cold compaction techniques, on which a polymeric surface layer could be mechanically bonded. Suitable combinations of compaction pressure and sintering temperature were shown to generate porosities in the bimodal mixture equivalent to those obtained with coarse water atomised powder, producing considerable improvements in green strength over the latter. Sintering in different atmospheres caused variations in surface chromium, nitrogen, and carbon content. Atmospheres that prevented decarburisation, e.g. argon, encouraged liquid phase sintering to occur at high temperatures (> 1340°C) via a carbide eutectic reaction; while nitriding and decarburisation effects also caused differences in hardness. The measured effe...

Production of Porous Sintered Co–Cr–Mo Alloys for Possible Surgical Implant Applications: Part 2: Corrosion Behaviour

Abstract: In the first part of this study, data were presented for the compaction, sintering, and mechanical properties of a water atomised cobalt based alloy and a bimodal mixture of ball milled ASTM F75 plasma rotating electrode process powder blended with the cobalt based alloy powder. In the present study, the corrosion of these two powders was compared for a range of compacts of varying porosity between 6 and 35% total porosity and between 0·5 and 26% interconnected porosity under conditions which simulated in vitro body fluid conditions. Differences in microstructure and surface composition caused by reactions with three different sintering atmospheres, e.g. loss of chromium, decarburisation, or Cr2N formation, altered the corrosion behaviour. The worst corrosion resistance occurred in compacts sintered in molecular 75H2–25N2 (by volume) gas, while argon atmosphere sintered compacts were the most resistant to corrosion. Pore size and pore geometry also had a considerable influence on the corrosion be...

Increased Fatigue Strength of Powder Forged Connecting Rods by Optimised Shot Peening

Abstract: Shot peening is a surface treatment commonly used to improve the fatigue life of aircraft, automotive, and other highly stressed structural components. This improvement is attributed to the formation of compressive residual stress in the surface layer of the material by the impingement of spherical media (shot). The compressive residual stress usually decreases the tensile stress created in the component by ‘in service’ external forces and, therefore, increases the fatigue strength of the part. To quantify the improvement resulting from shot peening, the fatigue behaviour of powder forged connecting rods and laboratory test bars from the base material (2%Cu steel), in both the stress free (unpeened) and surface treated (shot peened) condition, were compared. Fatigue data were correlated with the residual stress generated at the surface. The stress magnitude and depth were determined using X-ray diffraction analysis. Optimum processing parameters for shot peening were also determined. Results show ...

Sintering of M3/2 High Speed Steel Modified by Additions of Copper Phosphide and Titanium Based Ceramic Compounds

Abstract: Composite materials were produced by adding TiC, TiN, or TiO 2 particles to a base mixture of AISI M 3/2 high speed steel powder containing 7 wt-%Cu 3 P powder and 0.5 wt-% graphite. The composites were sintered in the temperature range 1140-1170°C, by an activated sintering mechanism that involved an iron phosphide rich liquid phase. Sinterability increased in the order TiO 2 < TiN < TiC for the composites, which was identical to the wettability of the ceramic particles by the liquid phase formed during the sintering process. Ceramic particles were well bonded to the matrix in the sintered microstructures, with a precipitation of MC type carbides from the liquid phase along the ceramic/matrix interfaces. The composites containing TiO 2 , however, were very porous and the hardness of the particles decreased significantly due to their interaction with the matrix, making these additions unsuitable

Influence of Microstructure on Impact Behaviour of Sintered Ferrous Materials

Abstract: In this paper results of a series of impact tests on ferrous sintered materials, characterised by a variety of microstructures obtained by alloying with different elements or varying the final heat treatment, are presented and discussed. Tests were conducted at room temperature, with an instrumented Charpy pendulum, which allows the possibility of measuring, during impact, the absorbed energy and the force applied on the specimen. Mechanical parameters obtained from load-deflection and energy-deflection curves were analysed and correlated with significant microstructural features and with fractographic information in order to describe the deformation and fracture of these materials under impact loading. In particular, the influence of the strong coupling between porosity and microstructural characteristics was analysed and discussed. PM/0665

Rolling Compaction, Magnetic Properties, and Microstructural Development During Sintering of Fe–Si

Abstract: Mixtures of atomised iron powder and FeSi17 powder have been consolidated in a rolling mill into strips containing 3%Si overall, 0·8 mm in thickness and 50 mm wide. The effect of sintering temperature and cold reduction ratio on the magnetic properties of Fe–Si strips was investigated. The relationship between the magnetic flux density and the density of the sintered strips was almost linear. Coercive force decreased with increasing sintering temperature owing to the intense sintering and the resultant grain growth. Residual induction decreased with increasing sintering temperature. Cold reduction and subsequent annealing improved the magnetic properties of the sintered strips. The microstructural development of elemental Fe–Si powder compacts and the Fe–FeSi17 powder system during sintering at between 900 and 1250°C was studied by optical microscopy. PM/0634

Dimensional Changes During Sintering of Iron Based Powders

Abstract: A study of dimensional changes in a compact of Fe–2·0Cu–0·8C—0·75 zinc stearate was carried out by simulating the operating conditions in a practical sintering furnace. The abnormal expansion which occurs in the α-Fe range after delubricating is caused by gas carburising and has a considerable effect on sintering behaviour. The dimensional changes tended to be in the form of shrinkage due to carburisation during heating but in the form of expansion due to carburisation during cooling. PM/0636

In Situ Measurement of Sintering Shrinkage in Powder Compacts by Digital Image Correlation Method

Abstract: An In Situ system for monitoring sintering shrinkage of powder compacts using digital image correlation has been developed. This system will provide completely automated non-contact measurement of two dimensional deformation, and thus enables In Situ quantification of non-uniform and anisotropic shrinkage, if any, of a powder compact during sintering. A brief description is given of the procedure for identifying micro-areas on a specimen surface before and after deformation by the digital image correlation method. A new digital image processing algorithm has been developed to determine the two-dimensional distribution of shrinkage rate in powder compacts during sintering. A measurement of the shrinkage non-uniformity in a die pressed carbonyl nickel powder compact during sintering from room temperature to 1573 K is presented. The results showed successful identification of non-uniform and anisotropic shrinkage arising from powder filling inhomogeneities and the neutral zone effect of die wall fric...

Effect of Carbon on Microstructure of Mixed Powder System Cooled at Moderate Rates after Sintering

Abstract: Mixed low and high alloy steel powders were pressed and sintered with varying carbon contents. This paper describes the microstructural development as a function of admixed carbon content and the percentage high alloy powder in the mixture. As such materials can be employed in the as sinter cooled state, the phase stability and microhardness of the various mixtures has been monitored. The amount of retained austenite increases dramatically at high carbon contents giving rise to unstable structures

Influence of MnS on Fatigue Fracture Surface Morphology of Nickel Alloyed PM Steel

Abstract: The influence of pore fractions of 0·05 and 0·09 and 0·5%MnS on the fatigue fracture surface morphology of two nickel alloyed powder metallurgy steels, was investigated. The alloys were produced by diffusion bonding and a combination of prealloying together with diffusion bonding respectively. Fatigue fracture of smooth rectangular specimens was obtained by testing at constant amplitude and zero mean stress, at a frequency of 30 Hz. Fractographic examination showed that the same fracture mechanisms are found in alloys with and without MnS addition. All fatigue cracks initiate from pores located at the surface or just below it. Fatigue crack growth occurred by four propagation modes associated with: trans particle striated and serrated mechanisms, interparticle neck separation from microvoid coalescence, and transparticle cleavage fracture. The occurrence of various combinations of these fracture mechanisms depends on the porosity and the applied stress amplitude. PM/0656

Model for Predicting Buildup of Cylindrical Billets in Osprey Preform Process

Abstract: A detailed numerical model to predict the buildup of cylindrical preforms in the Osprey process is described. A sensitivity study confirmed the importance of chamber design factors such as the direction of spray on to the end of the preform, scanning amplitude, and scanning frequency with respect to preform rotation. The nature of the scanning motion was also found to have a profound influence on the shape of the preform end. PM/0672

Electroless Nickel Coating of Sintered Iron Compacts

Abstract: Sintered iron specimens with a 9% porosity level were successfully coated by electroless nickel deposition using a hypophosphite bath The problems encountered during the deposition process are discussed Some important deposit characteristics such as composition, thickness, hardness, and adhesion strength of the deposit are also reported for both as deposited and coated specimens heat treated at 400°C for 1 h in a vacuum PM/0650

Constitution, Microstructure, and Hardness of Water Atomised and Hot Isostatically Consolidated Copper Alloy Powders

Abstract: Copper based alloys within the Cu–Cr–Zr system produced in powder form by water atomisation were consolidated to full density by hot isostatic pressing (hipping) at temperatures below 0·5T m. After hipping, materials of a mean grain size between 2 and 10 μm were obtained. The increased capacity for solid solubility of chromium and zirconium in copper, produced on atomisation, resulted in enhanced strengthening by the precipitation of coherent chromium particles during hipping. This approach allowed heat treatment after processing as was the case for the extrusion-heat treatment route. The peak hardnesses obtained were either equal to (Cu–Cr–Zr alloys) or slightly lower than (Cu–Cr alloys) the respective values obtained after extrusion followed by heat treatment. PM/0618

Influence of Alloying Elements on Mechanical Properties of Sintered High Speed Steel Powders Diluted with Highly Compressible Powders

Abstract: The influence of carbon and nickel on the mechanical properties of high speed steel (HSS) powders diluted with a highly compressible powder (e.g. pure iron or low alloy steel powder 4401) has been investigated. The aim was to optimise the mechanical properties and microstructure in the sintered steel which is used for wear resistant applications. The results show that the carbon content is a critical factor in determining the mechanical properties and the amount of retained austenite in the sintered steel. Nickel additives enhance mechanical properties and do not significantly affect the amount of retained austenite at contents lower than 2%. The 4401 low alloy steel is a more suitable matrix than pure iron in this respect. The influence of retained austenite on the micro hardness of the prior HSS particles does not show any direct relationship with the mechanical properties of the sintered composite. PM/0668

Improved properties by warm compaction

Abstract: Warm compaction is a new technology resulting in increased densities of 7.2-7.5 Mg m -3 in a single step. By this process it is possible to obtain density levels and mechanical properties comparable with double pressing and sintering. However, the improved properties achievable by warm compaction must be combined with tolerances of the same order as for the conventional PM processing route. Some characteristic properties of Distaloy AE, a low alloy PM material, after warm compaction and sintering have been investigated, illustrating the robustness of the process. The weight scatter for a component produced using warm compaction in trials under productionlike conditions is also compared with that for conventional compaction.

Role of Nickel in Fe3Al Intermetallic Compound Formation

Abstract: The effect of nickel on the microstructure and mechanical properties of Fe3Al alloys containing 25 at.-%Al has been studied. Specimens were prepared from elemental powders. It has been determined that the addition of nickel helps densification during sintering and results in a refinement of final grain size. Near full density samples can be obtained. Mechanical tests at room temperature show that hardness, ductility, and tensile strength increased as the nickel content is increased. The fracture mode in tension is a mixture of transgranular cleavage and quasicleavage. The proportion of quasicleavage increases with nickel content. PM/0641

Characteristics of Short Cast Iron Fibres and Their Application for Diamond Cutting Wheels

Abstract: A study on the effects of compaction and sintering parameters on microstructures and properties of sintered compacts of short cast iron fibres has led to the development of a diamond grinding wheel bonded with a matrix of short cast iron fibres. When compared with high quality bronze bonded diamond grinding wheels the new type of wheel was found to be more suitable for the high efficiency grinding of hard and brittle materials such as ceramics. When Si 3 N 4 was ground, a trebling of the grinding rate and a sixfold increase in grinding ratio was achieved

Optimisation of Post-Densification Cold Rolling Schedule for PM Copper Strip

Abstract: Preparation of fully dense copper strip from Cu2O powder via an integrated powder technology route is briefly described. Full density strip obtained by such a route has been found to contain 0·5 vol.-% of Cu2O inclusions, a somewhat higher level than that found in strip prepared from atomised copper powder. Internal degradation of full density PM copper strip by inclusion-matrix decohesion and/or inclusion fragmentation during cold rolling has been studied as a function of thickness reduction. Coarse inclusions ≥10 μm in size undergo fragmentation/decohesion at relatively small thickness reductions of the order of 20%.In contrast, smaller inclusions in the submicrometre range do not exhibit decohesion/fragmentation after thickness reductions of as high as 60%. Cold rolling thickness reductions up to 20 and 40% were found to be safe for strip prepared from Cu2O and atomised copper powders respectively. For strip produced from Cu2O powder and cold rolled to 20% thickness reduction an annealing treat...

Influence of Binder Content and Particle Size on Green Strength of WPP Parts

Abstract: Wet powder pouring (WPP) was developed as an alternative near net shape process to metal injection moulding (MIM). The amount of binder, which is necessary to obtain a sufficient green strength, is relatively low (2–5 vol.-%) compared to the amount used in the MIM process (about 30 vol.-%). This is advantageous with respect to short debinding times and low residual carbon and oxygen contaminations. A further reduction of the binder content or the application of the WPP process to new materials would be promoted by an understanding of the dependence of green strength on particle characteristics and binder content. In the present investigation the green strengths of stainless steel WPP parts were determined for different particle size distributions and different binder contents (2–8 vol.-%). The green strength was in the range 3 to 7 MN m−2 and the fracture mode was always brittle. A simple fracture mechanical model was developed, which was able to explain the observed relations between binder conte...