Showing papers in "Powder Metallurgy in 2003"

Effect of Fe addition on sintering behaviour of titanium powder

Abstract: The effect of iron on the sintering behaviour of titanium powder was investigated from two aspects: (1) diffusional homogenisation of iron; (2) densification of Ti-5Fe alloy. Under the present proc...

Thermal and electrical conductivities of sintered powder compacts

Abstract: A simple new equation for calculating thermal and electrical conductivities of powder sintered compacts is proposed. In this equation, the effective conductivity of the sintered compact is a function of the solid material conductivity, the degree of compact porosity and the packing porosity of the starting powder. The latter parameter determines to a great extent the compact pore structure, since it depends on powder particle size, shape and distribution. The new equation is physically applicable from zero porosity to the packing porosity. The conductivity equation has been evaluated using sintered compacts of iron, nickel, titanium and aluminium. The compacts were prepared by cold pressing and sintering. Results are in very good agreement with theoretical predictions.

Tensile properties of Fe-3Mn-0·6/0·7C steels sinteredin semiclosed containers in dry hydrogen, nitrogen and mixtures thereof

Abstract: Tensile properties of powder metallurgy 3% manganese-0·8% carbon (content of green compact) steels were determined following laboratory sintering in (nearly) full, semiclosed containers with no getter powders in dry, 0-100% hydrogen-nitrogen atmospheres. Manganese was mixed with the NC 100·24 sponge iron powder as low carbon ferromanganese and carbon as a graphite addition. Dogbone compacts were pressed at 660 MPa, the sintering temperatures were 1120 and 1250°C and cooling rates ~65 K min- 1. In specimens sintered in nitrogen containing atmospheres at 1120°C, final carbon content was ~0·7% and for those processed at 1250°C ~0·6%. Sintering in dry hydrogen resulted in lower carbon and oxygen contents. Independent of the H2/N2 ratio in the furnace atmosphere, however, all the specimens were ductile and exhibited similar strengths. Yield strengths R 0·2 were in the range: 426-464 MPa, tensile strengths Rm were 724-780 MPa and strains to failure were 1·6-2·0% after sintering at 1250°C. The 1120°C si...

Effect of high energy ball milling on titanium-hydroxyapatite powders

Abstract: This paper reports on an investigation of the potential use of high energy ball milling (HEBM) for the production of homogeneous mixtures of titanium/hydroxyapatite powders. This system is of considerable interest for the production of functionally graded material (FGM) components for use in implants in the body. In order for these FGMs to have satisfactory properties homogeneous mixtures of the titanium and hydroxyapatite (HA) powders must be produced prior to consolidation. In this investigation Ti/20 wt-% and 40 wt-% HA powder mixtures were produced by high energy milling for times of 15 minutes and 1 hour. Mixtures were also prepared on a conventional turbula powder mixer for reference purposes. The mixtures were consolidated by either cold pressing, cold isostatic pressing or hot pressing under various conditions. Selected specimens were also subjected to sintering over a range of temperatures (400-1100°C). The resulting microstructures were characterised using scanning electron microscopy, X...

Sintering behaviour of iron-molybdenum steels withtheadditionof Fe-B-C master alloy powders

Abstract: The effect of addition of Fe-B-C master alloy powders on the sintering behaviour of iron-molybdenum sintered steels has been investigated by dilatometric experiments. The addition ofthe Fe-B-C mast...

In situ dimensional change, mass loss and mechanisms for solvent debinding of powder injection moulded components

Abstract: Dimensional change during solvent debinding is linked to defects such as cracking and slumping in powder injection moulded components. Owing to the delicate condition of the compact during debinding, considerable difficulty exists in determining the magnitude and cause of swelling or shrinkage. Previous studies of this dimensional change have used measurement techniques involving contact or force on the sample, which may alter the behaviour. This study observes in situ dimensional change during solvent debinding using a non-contact laser dilatometer. Dimensional change was investigated for iron and stainless steel powder injection moulded bars with wax-polymer binders. The effects of solvent temperature, paraffin wax content, paraffin wax density, particle size and solids loading were analysed. Comparisons between the dimensional change and mass loss observations during solvent debinding suggest a relationship between the two phenomena based on soluble polymer swelling.

Large arc voltage fluctuations and droplet formation in electric arc wire spraying

Abstract: The diameter of droplets in electric arc wire spraying is critical in determining the microstructure, porosity and oxide content of the resulting sprayed coatings. Droplet diameter controls dynamic...

Influences of sintering atmospheres on densification process of injection moulded gas atomised 316L stainless steel

Abstract: Densification of metal injection moulded 316L stainless steel is one of the hottest research fields in powder metallurgy in recent years. In the present work, various sintering atmospheres, i.e. N2, N2 + H2, Ar and Ar + H2, were adopted to investigate their influences on the sintered density, pore size, pore shape, grain size and mechanical properties. Relative densities of 98% of the theoretical density (7·84 g cm-3 and 7·85 g cm-3), pore sizes about 2~3 μm and mean grain sizes about 50 μm could be obtained after the specimens were sintered in Ar and Ar + H2 atmospheres. Optimised mechanical properties could be acquired in Ar + H2 atmosphere, i.e. σ b = 630 MPa, σ 0·2 = 80 MPa, δ = 52%, HRB = 71. When sintered in N2 and N2 + H2, the specimens have lower densities than those sintered in Ar and Ar + H2. An ultimate tensile strength of 765 MPa and a hardness of 82 HRB could be obtained respectively in N2 + H2 and N2, atmospheres. The dimensional tolerance of the sintered part is less than ±0·5% wit...

PM processing of Al-Al2O3 composites and their characterisation

Abstract: Aluminium based composites containing 5 to 20 vol.-%Al2O3 as the reinforcing phase were synthesised by powder metallurgy. The process consisted of blending aluminium powder with Al2O3 powder followed by hot compaction and hot extrusion. The microstructural features, mechanical properties and wear characteristics of the composites were investigated with variation in volume fraction of Al2O3 phase. The results indicated a continuous increase in yield and ultimate tensile strength with a marked decrease in ductility of the composites arising from increased Al2O3 content. High temperature tensile test data revealed that the strength of the base material Al and composite materials containing 5 and 15%Al2O3 decreased with increase in temperature. However, the rate of strength reduction was higher for the extruded compact of unreinforced aluminium than the composites. Lower wear rates were observed in the composite materials than the unreinforced aluminium compacts. An increase in Al2O3 content led to a...

Investigation of warm compaction and sintering behaviour of aluminium alloys

Abstract: The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejectio...

Synthesis of transitionmetal nitride/carbonitride nanopowders and their application for modification of structure of hardmetals

Abstract: Nanodispersed powders of nitrides/carbonitrides of IV-VI group transition metals (for example TiN, TiCxN1-x, ZrCxN1-x, NbCxN1-x, Cr3C1·6N0·4) with an average particle size of 20-200 nm have been prepared by the method of plasmachemical synthesis. Owing to the dynamic conditions of synthesis (starting product evaporation or decomposition, mixing and condensation of the end product lasting for some tenths of second), the product is formed in a form of nanodispersed powder, mainly as a monocrystal with a great concentration of crystal lattice defects. This determines high chemical activity of these powders, including increased sintering rate and decreased sintering temperature (for example, in the case of TiN, TiCxN1-x or NbCxN1-x this temperature is 400-600°C lower in comparison with the traditional powders). Addition of transition metal nitride/carbonitride to hardmetal can effect formation of its structure and properties. It was shown during the investigation that addition of 10 wt-% of the TiN na...

Observations on densification of Al-Al2O3 composite powder compacts by pressure cycling

Abstract: The effectiveness of pressure cycling on the consolidation of powder composites is investigated. Mixed Al and various amounts of Al2O3 powders were consolidated under static and cyclic pressure at room temperature in uniaxial consolidation experiments. The results showed that each pressure cycle increases green density and the cyclic effect is stronger when there is a relatively large volume fraction of Al2O3 powder. The compacts produced by pressure cycling have much higher strength than those produced with a single pressure excursion and the process ability of compacts should also improve via pressure cycling. Microscopic observations showed that greater uniformity is obtained in compacts by cyclic consolidation. The origin of the beneficial effect of pressure cycling is related to the deviatoric stresses generated by volumetric mismatch due to the different compressibilities of the phases.

New method for measuring and characterisation of friction coefficient at wide range of densities in metal powder compaction

Abstract: In order to investigate the friction behavior of powder during compaction, a new method has been developed. Compaction is a complicated process and direct and continuous measurement of the coeffici ...

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract: Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densit...

Comparison of numerical codes for simulation of powder injection moulding

Abstract: In this study the powder injection moulding process was simulated with commercial analysis packages, C-Mold, Moldflow and ProCAST. The simulated melt front advancement, temperatures and pressures were compared with the experimental measurements. The three codes correctly predict the positions of weldlines. C-Mold performs the fastest analysis with good predictions of the temperatures and pressures within the cavity, but it is limited to two-dimensional models. Predictions of the in cavity pressure made by Moldflow are less accurate than those calculated by ProCAST and C-Mold. Predictions of the temperatures made by Moldflow and ProCAST are comparable with each other and agree reasonably well with the experimental observations. However, ProCAST is superior in terms of the number of different output data and visualising capabilities. The disadvantage of ProCAST is that it does not permit simulating the filling and post-fill ingstages of the powder injection moulding process in one simulation run.

Computer aided design of PM high speed steels for vacuum and nitrogen atmospheres

Abstract: Computer aided alloy design has been used to develop new highly sinterable alloys in order to fulfil the requirements for robust processing of powder metallurgy high speed steels. Three alloys (M35IG, AR-K2 and M42HVIG) have been designed and subsequently analysed in terms of the expected sintering behaviour, atmosphere interaction and microstructural features. Predicted optimum sintering temperatures are 1200°C for the vacuum sintered M35IG alloy and 1150°C for the AR-K2 and M42HVIG alloys sintered under a 90N2+9H2+ 1CH4 atmosphere. On the other hand, the expected sintering windows exceed 10°C for all the alloys and the sensitivity to small compositional changes has been minimised guaranteeing the robust processing requirements. Additionally, sintering temperatures of 1150°C allow processing in continuous belt furnaces with the inherent interest for the industrial environment. Thermo-Calc software and the TC-Fe2000 database have been used to obtain the different calculations on the multicomponen...

Influence of SiC particles oncompressive strength of sintered aluminium alloy

Abstract: Room temperature compressive tests have been carried out on metal matrix composites (MMCs) based on an Al alloy (CW67 alloy). The particulate reinforcements used were SiC in three volume fractions (5%, 10%, and 15%) with an average size of 15 μm. Two matrix aging conditions were studied: peak aged and equivalent underaged conditions. The presence and increasing volume fraction of SiC particles increases elastic modulus and yield stress but decreases ultimate compressive strength and ductility of the W67 alloy characterised under compression in both peak aged and underaged conditions. Higher compressive strength and lower ductility have been achieved in the matrix submitted to peak aging in comparison to the matrix that underwent underaging.

Effect of nitrogen on sintered duplex stainless steels

Abstract: In this paper, the authors have studied the effects of the different sintering atmospheres (vacuum, nitrogen-hydrogen, and dissociate ammonia) on 50A-50F duplex stainless steels, fabricated by the powder metallurgy process.The DSS sintered in atmosphere of N2-H2 exhibit a tensile strength of at least 500 MPa with an elongation to fracture smaller than 2%. The presence of nitrides was the main cause of the deterioration in the corrosion resistance.

Netshaping concepts for tungsten alloys and composites

Abstract: The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based ...

Metal powder compacting dies: optimised design by analytical or numerical methods

Abstract: The initial data needed to design metal powder compaction die are: compact shape and density, powder mix composition, compaction and radial pressure, part number and tool materials. The design targets are: diameters of insert and ring, sometimes number of rings and interference or interferences. The constraints include: no tensile stresses on the insert, no risk of relative motion at part ejection, no unwanted alteration of material microstructures and maximum stresses always below the allowable limits. Usually the design is based on engineering experience, company knowhow, and approximated analytical calculations and cost considerations.This study is focused on the use of numerical methods to determine the design parameters of dies for powder compaction. Both room temperature and warm compaction have been investigated. Numerical algorithms, implemented into FEM calculation codes, enable one to optimise the common diameter of insert and ring, corresponding to the lowest stresses on both items, or...

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract: Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering This was made possible by the use of the Fe-P-C system with or without Cu Without post-sintering treatment, a brittle microstructure was obtained By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided In this way, iron phosphide is not formed on grain and/or particle boundaries Presence of pores is confirmed to be beneficial for grain growth control

Consolidation methods for spherical rhenium and rhenium alloys

Abstract: The development of high density spherical rhenium and spherical tungsten-rhenium powders has enabled the use of advanced consolidation techniques for the manufacture of refractory metal components. The investigated consolidation techniques are powder metal injection moulding (PIM) and vacuum plasma spraying (VPS); both produce net shape components. The required particle size distributions for these applications vary. VPS uses a large powder particle size (<44 μm) while PIM requires a fine particle size (<20 μm). The major advantages of spherical powders over traditional powders in plasma spraying are the high density of the powder particles and its good flow characteristics. These two factors combine to produce high density sprayed formed parts. PIM requires that the powder particles be dense as well as fine. Powder particles must be small enough to be entrapped in the binder but without being so small (<0·5 μm) that the body forces dominate. Binders for PIM are tailored to the powder's characteri...

Investigations on friction behaviour of treated and coated tools with poorly lubricated powder mixes

Abstract: Friction at the interface between tools and powder plays a significant role in the cold die compaction process in both compression and ejection steps. In this work, the influence of tools materials and coatings as well as lubricant content in the powder is investigated with a sliding piece device. Three iron powders with different amounts of lubricant, from 0 wt-% to 0·6 wt-% (all percentages are expressed in weight), are tested on eleven parallelepipedic slabs (different couples of material and coating), which simulate tools surfaces. Results are presented in terms of friction coefficient evolution, friction coefficient values and surface analysis. Then they are compared with previous work to check their validity and consistency. Finally an assessment of the friction aptitudes of each slab will be made to open prospects of improving industrial processing conditions.

Effects of Ti powder size and ignition method onproducts of SHS reaction in Cu-Ti-C system

Abstract: The influences of titanium particle size and ignition mode on the products of SHS reactions in the Cu-Ti-C system have been investigated. Ignition by thermal explosion mode results in Cu-TiC composites with higher levels of porosity and coarser TiC particles than ignition by combustion mode, owing to higher reaction temperatures facilitated by preheating of the reactants. The use of coarse titanium results in incomplete reaction and lower reaction temperatures, irrespective of the ignition method, as a result of increased carbon rich and carbon depleted reacting regions brought about by less homogeneous mixing of the reactants. The lower reaction temperatures result in finer TiC particles and reduced porosity. Incomplete reactions result in higher levels of titanium dissolved in copper and TiC particles of lower stoichiometry. This improves wetting between the two phases and results in a more even distribution of the carbides in the Cu matrix.

Comparison of flow and shear band structures in oriented, columnar tungsten, single crystal tungsten-tantalum ands intered tungsten heavy alloy ballistic penetrators

Abstract: The penetration of long rods (L/D10) into standard RHA targets differs with rod failure and flow, which produces erosion and deceleration. Oriented, columnar grained W and [001] single crystal W-4%Ta rods exhibit dense shear/shear band flow phenomena consisting of overlapping bands ofpredominantly dynamically recrystallised (DRX) grain structures observed by optical metallography and transmission electron microscopy. This flow is contrasted withWHA (93%W, 5%Ni, 2%Fe) sintered rod penetration characterised by penetrator nose failure in blocks which move on narrow shear bands between the blocks and function like microstructural lubricants. From these comparative observations of residual, penetrated rods, strategies to promote rod penetration seem to involve the development of wide, overlapping bands or layers of equiaxed or preferentially oriented, refined microstructures, which facilitate material flow at high strain rates. Microstructural precursors, such as deformation twinning or the enhancement...

Alumina particulate/Cu matrix composites prepared by powder metallurgy

Abstract: Two Cu-20 wt-% alumina composites were fabricated by powder metallurgy (PM), one was prepared from Cu coated alumina powders and the second was consolidated from a mixture of Cu and alumina powders. Both composites were cold pressed at different pressures, ranging from 150 to 600 MPa, in a floating die using a hydraulic press, and only compacts pressed at 600 MPa were sintered in pure hydrogen atmosphere at 900°C. The effect of consolidation parameters on powder compressibility was also studied.Microstructure investigation revealed that a continuous uniform Cu layer was precipitated around each alumina powder. When the investigated powders were cold pressed, it was noted that compacts made from Cu coated alumina have higher green density than those fabricated from mixed powders. Moreover, the sintered Cu coated alumina composites showed higher density and more uniform alumina distribution, and continuous interconnected Cu skeletal structure. However, in the case of sintered composites made from po...

Effect of tool coatings on ejection characteristics of iron powder compacts

Abstract: Powder metallurgy (PM) part makers heavily rely on part density as a mean of controlling part performance. Higher compaction pressures may be used to obtain higher densities and better properties. However,ejection stresses usually increases with compacting pressure. Those stresses may affect significantly part quality (surface finish, formation of cracks and lamination) and tool wear.Different methods may be used to minimise ejection stresses, such as the use of admixed lubricant, die wall lubrication and the modification of tool surfaces. This paper presents an approach to evaluate the effect of tool coatings on the ejection of ferrous compacts. The method consists of evaluating the ejection characteristics of core rods with different coatings. The results obtained show that ejection characteristics are sensitive to tool coatings. Coating the surface of the core rods yields important variations of the stripping pressure (2×) and ejection energy (1·6×). No clear correlations between the ejection c...

Effect of basic powder particle properties on densification of porous tungsten

Abstract: Porous tungsten is employed extensively for components such as dispenser cathodes in electronic valves and light technology. Normally parts are produced by die pressing or cipping and sintered in a reducing atmosphere at temperatures • 2000°C. In this study, the aim was to look critically at the behaviour of tungsten powder at compaction and sintering so that the distribution of porosity could be better controlled and optimised. Attention was focused on the basic powder properties, such as particle size and shape, surface area and apparent density. The effects of these parameters on the relative density of the parts are described. A small quantity of aluminium was added to the tungsten powder to serve as a binder and a potential sintering aid.

Effect of production parameters on cooling rates of AA2014 alloy powders produced by water jet cooled, rotating disc atomisation

Abstract: In this study, a novel type of rotating disc unit was designed and constructed and was used to produce rapidly solidified AA2014 alloy powders. Copper and stainless steel were used as the disc material and the temperature of the cooling water was selected as 0°C and 18°C. Effects of the production parameters, such as disc material, cooling water temperature, superheat of liquid metal and disc speed on the microstructure and the cooling rate of the powders, have been investigated.The microstructure of the produced powders was cellular and changed to cellular-dendritic with increasing powder size. It was found that cooling rates were relatively higher using a copper disc and 0°C cooling water temperature. The results indicated that cooling rates of 25 μm powders produced with a copper disc were estimated as 1·01×106 K s-1 and 9·02×105 K s-1 for 0°C and 18°C cooling water temperatures respectively. Decreasing the superheat of the liquid metal and increasing disc rotating speed also increased the coo...

Effect of pulsed current on reactively synthesised TiB2 consolidated by plasma pressure compaction

Abstract: The effect of pulsed current on TiB2 formed by reactive consolidation between titanium and boron is reported in this paper. This consolidation was performed using the plasma pressure compaction (P2C) technique. A comparison between the pulsed and control samples reveals that pulsed current reduces grain growth (pulsed samples had an average grain size of 2·79 μm compared to 5·99 μm) while increasing sintering rates (pulsed samples were on average 15·5% more dense). The reduced grain growth and increased densification is due to the removal of adsorbed oxygen from the surface of the powder.