Showing papers in "Powder Metallurgy in 1996"
TL;DR: In this paper, the authors developed a new material consisting of a titanium-based surface layer, a tough cemented carbonitride core containing Bl type and WC phases, and an intermediate layer with graded composition.
Abstract: Functionally graded materials consisting of two different materials with a graded interlayer have attracted much interest for their ability to exhibit the characteristics of the individual materials plus new functionality. This concept was applied to the field of hard materials. The developed material consists of a titanium based ceramic surface layer, a tough cemented carbonitride core containing Bl type and WC phases, and an intermediate layer with graded composition. The surface region is characterised by a high compressive residual stress of 0·8 GN m−2. When applied in the form of cutting tools, the material was found to have an abrasion resistance and fracture toughness much higher than those of a conventional cermet of uniform composition. Because of the graded composition, the ceramic surface layer has high adhesive strength, providing longer life than coated tools when spalling of the film can occur easily. The new material is produced by controlled sintering of a green compact of uniform ...
44 citations
TL;DR: In this article, preliminary work on the preparation process and some properties of VC-WC-Co alloys containing approximately 10 wt-%VC and 10 Wt-Co are reported.
Abstract: Preliminary work on the preparation process and some properties of VC–WC–Co alloys containing approximately 10 wt–%VC and 10 wt–%Co are reported. Sinterability of the alloys proved to be better tha...
43 citations
TL;DR: In this paper, a metal matrix composites based on a sintered M3/2 high speed steel (HSS) and containing either single additions of TiC, NbC, or MnS particles or combined additions of MnS + TiC or NMS+NbC were subjected to a conventional hardening heat treatment cycle of austenitisation, quenching, and double tempering.
Abstract: Metal matrix composites based on a sintered M3/2 high speed steel (HSS) and containing either single additions of TiC, NbC, or MnS particles or combined additions of MnS + TiC or MnS + NbC were sintered to full density and subjected to a conventional hardening heat treatment cycle of austenitisation, quenching, and double tempering. Relatively poor hardnesses and strengths were obtained in any composites that contained MnS particles, principally because of the weakening effects of MnS particles in the structure but also as a result of decarburisation of the steel matrix when oxygen introduced by the MnS particles reacted with carbon. Composites that contained TiC were changed in structure so that matrix M6C carbides were replaced by the less soluble MC carbide. They consequently showed slightly reduced hardening response compared to the baseline M3/2 HSS after heat treatment. Ball milling was necessary in powder mixtures that contained NbC to break up particle agglomerates. The resultant fine gra...
27 citations
TL;DR: In this article, a simple change in powder injection molding sintering procedure resulted in a product with superior tensile properties and in reduced operating costs, by processing in a nitrogen gas environment instead of in an inert gas environment.
Abstract: A simple, inexpensive change in powder injection moulding sintering procedure resulted in a product with superior tensile properties and in reduced operating costs. By processing in a nitrogen gas environment instead of in an inert gas environment nitrogen was alloyed to 316L stainless steel powder during the sintering and densification step. Interstitial nitrogen concentration of 0·4 wt-% increased the yield strength by over 100%. Graphs of tensile properties versus temperature of powder injection moulded stainless steels with and without nitrogen were found to be comparable with those observed for similar as cast alloys. PM/0692
25 citations
TL;DR: In this article, the authors studied the sintering shrinkage of titanium in its high temperature beta phase by isothermal dilatometry and found that the shrinkage did not follow the normal time exponent type of behaviour, instead being described by the equation y = Kt(m)/[1-(A+Bt)(2)], where m = 1.93 +/- 0.07, with an activation energy of 62-90 kJ mol(-1).
Abstract: Sintering of titanium in its high temperature beta phase was studied by isothermal dilatometry. The sintering shrinkage y did not follow the normal time exponent type of behaviour, instead being described by the equation y = Kt(m)/[1-(A+Bt)(2)], where m = 1.93 +/- 0.07, with an activation energy of 62-90 kJ mol(-1). A detailed analysis of these results, based on the 'anomalous' diffusion behaviour reported for beta titanium, is carried out. It is shown that the generation of a high density of dislocations during the alpha --> beta phase transformation, coupled with sluggish recovery at the sintering necks, enables sintering mass transport by pipe diffusion through dislocation cores from sources of matter within the particles to become dominant.
19 citations
TL;DR: In this article, the solidification range of alloys of Mn-Ni-Cr-Mo-Fe and Mn-Cu-Ni was determined and a system for liquid phase alloying during sintering was investigated.
Abstract: Systems for liquid phase alloying during sintering were investigated. The solidification range of alloys of Mn–Ni–Cr–Mo–Fe and Mn–Cu–Ni was determined. Alloys with the lowest and narrowest melting range were prepared and atomised in nitrogen. Admixtures of master alloys to water atomised, forging grade, pure iron powder were sintered at 1232°C. After hot forging, these PM steels exhibited harden abilities 75–90% of theoretical levels calculated from the factors for conventional steels. Alloying efficiency was further improved to 85–100% of theoretical hardenability when additions of approximately 2%Si and 1% rare earth mischmetal, used to enhance diffusion and sintering, were made to the master alloys. The steels obtained by premixing and sintering master alloys with pure iron powder were substantially homogeneous, had excellent microstructures, and exhibited very good tensile and impact properties. It was observed that molybdenum was the slowest diffusing element of those investigated and that ma...
14 citations
TL;DR: In this article, a review of far-from-equilibrium techniques for synthesis of nanostructured grains, amorphization, and synthesis of intermetallic compounds is presented.
Abstract: Synthesis by ‘far from equilibrium’ processes allows development of novel crystal structures and microstructures, leading to achievement of enhanced physical and mechanical properties. Three far from equilibrium techniques are reviewed (with emphasis on the amount of departure from equilibrium): mechanical alloying, plasma processing, and physical vapour deposition. Consideration is given to solubility extension, formation of nanostructured grains, amorphisation, and synthesis of intermetallic compounds. Examples are drawn from systems based on the light metals aluminium, magnesium, and titanium. PM/0715
14 citations
TL;DR: In this paper, the sintering behavior and mechanical properties of injection moulded Fe-Cu parts made from composite and elemental powders were studied, and the best properties obtained were 95·2% density, 420 MN m−2 tensile strength, 72 HRB hardness, and 18% elongation.
Abstract: The sintering behaviour and mechanical properties of injection moulded Fe–Cu parts made from composite and elemental powders were studied. Composite powders showed improved properties compared with mixed powders. The differences in the results of sintering are explained by the microstructure and the wetting behaviour of copper in each system. Dilatometry analyses indicated that the α–γ phase transformation temperature decreased to 850°C with the addition of 8%Cu. No swelling was observed when copper melted because of the presence of a large number of small interparticle pores. Most of the densification occurred in the α phase range and above the melting point of copper. The best properties obtained were 95·2% density, 420 MN m−2 tensile strength, 72 HRB hardness, and 18% elongation. These improved properties are attributed to the solution strengthening and grain refinement contributed by copper. PM/0712
13 citations
TL;DR: In this article, fine tin powders were produced in a pilot plant gas atomiser, where Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards.
Abstract: Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards. A range of metal flowrates from 0·864 to 1·425 kg min−1 was studied at a melt temperature of 450°C. Powders were sized using dry sieving down to 45 μm and wet sieving for smaller sizes. The Sauter mean diameter of the powders varied from 9·01 to 10·28 μm, depending on the rate of production. The size distribution was bimodal (albeit not very well defined) with the peak separation at ∼44 μm. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Comparison of the tin powders with copper powders from another study, AA 2014 aluminium alloy powders, and magnesium and zinc powders from previous work showed that the differences in mean diameter and standard deviation are small among these common metals at a given volumetri...
10 citations
TL;DR: In this article, a novel method of pulse electric discharge resistance consolidation with temperature gradient control has been developed for the design and fabrication of functionally graded materials (FGMs), which makes it possible to select combinations of constituents with greatly different densification rates, controlling densification up to full density to produce a nanoscaled structure.
Abstract: A novel method of pulse electric discharge resistance consolidation with temperature gradient control has been developed for the design and fabrication of functionally graded materials (FGMs). The powder processing route employed makes it possible to select combinations of constituents with greatly different densification rates, controlling densification up to full density to produce a nanoscaled structure. A die with a specially designed outer shape makes it possible to achieve desired temperature profiles along a graded composition. Using a stepped graphite die with a temperature difference of more than 700 K within a width of 7 mm, a five layered material graded from TiAl intermetallic to partially stabilised zirconia (PSZ) can be fabricated. This structure shows full densification throughout its cross-section without any discontinuity and an increasing hardness from 650HV at the TiAl nanocrystalline layer to 1560HV at the PSZ layer, varying according to a mixture rule. PM/0716
10 citations
TL;DR: In this paper, the influence of alloying additions such as Cr, Mo, Nb, Zr, Ti, Si, Mn, and B was studied in order to improve the ductility at room temperature and mechanical strength at elevated temperatures with other properties retained or improved.
Abstract: Iron aluminides based on Fe3Al were produced by extrusion of gas atomised powders. The influence of alloying additions such as Cr, Mo, Nb, Zr, Ti, Si, Mn, and B was studied in order to improve the ductility at room temperature and mechanical strength at elevated temperatures with other properties retained or improved. Examinations were made on extruded material in the as extruded as well as the heat treated condition. Heat treatments were performed in the temperature range 500–850°C. Microstructural studies show a strong extrusion texture and that most of the alloys recrystallise below 750°C. Alloying additions affect the ordering transformation (D03 to B 2) temperature, which has been studied by differential scanning calorimetry. Transformation temperatures from approximately 550°C to 650°C have been obtained. Hot hardness was investigated up to 700°C. At room temperature the hardness values for the different alloys are in the range 280—420 HVO·5. At 300°C the hardness values are in the range 21...
TL;DR: In this article, a statistical analysis based on the McLean-Anderson design method was conducted to study the influence of each binder component on the flow properties of iron feed stocks for powder injection molding.
Abstract: A statistical analysis based on the McLean-Anderson design method was conducted to study the influence of each binder component on the flow properties of iron feed stocks for powder injection moulding. The formulations were composed of carbonyl iron powder (FE), polyethylene resin (PE), polyethylene wax (PEW), and stearic acid (SA). The viscosities of 15 different formulations were measured using a capillary viscometer. Fluidity and pseudoplasticity were evaluated and the effect of each component on these properties was analysed. The studied composition ranges of FE, PE, PEW, and SA were 90—92, 3—4·5, 2—4, and 0—1·5 wt—% respectively. It was found that lower iron powder content or higher binder content improved the fluidity of the feedstock. The relative effectiveness of binder ingredients in increasing fluidity is SA>PEW>PE and in enhancing pseudoplasticity PE > PEW > SA. Feedstocks with binder phases containing about 50 wt-%PE have the greatest fluidity with flow index n in the range 0·6—0·8. P...
TL;DR: In this article, the authors present a review of methods for the measurement of the physical and mechanical properties of hard materials, together with good industrial practice, including both routine procedures and research requirements.
Abstract: Methods for the measurement of the physical and mechanical properties of hard materials are reviewed. Current standards are assessed together with good industrial practice, including both routine procedures and research requirements. It is shown that key future development directions for test procedures include a requirement for improved ability to discriminate clearly between products under conditions similar to those in which the product will be used. PM/0741
TL;DR: The extrusion of bulk FGMs, that is FGMs in which the changes occur over distances of many millimetres throughout the crosssection, is described in this article, where the extrusion region is increased by adopting a conical stacking method and a pure aluminium surface layer is required to reduce the friction associated with the high silicon carbide contents.
Abstract: The composition, structure, and properties of functionally graded materials (FGMs) vary as a function of position within the material. The extrusion of bulk FGMs, that is FGMs in which the changes occur over distances of many millimetres throughout the crosssection, is described. Graded materials containing up to 30 vol.-%SiCp in a matrix of aluminium alloy Al 2124 were successfully extruded from preforms produced using a cylindrical stacking method employing concentric, cylindrical aluminium foil tubes. However, the extent of the steady state extrusion region was increased by adopting a conical stacking method. For both stacking methods a pure aluminium surface layer was required to reduce the friction associated with the high silicon carbide contents. PM/0729
TL;DR: In this article, an ultrasonic method of monitoring the compaction of iron powder is presented, and the precision of this technique is discussed and its sensitivity to changes in powder properties is investigated.
Abstract: A novel ultrasonic method of monitoring the compaction of iron powder is presented. The precision of this technique is discussed and its sensitivity to changes in powder properties is investigated. The technique is then applied to the problem of the compaction of a multilevel compact. The technique proves capable of independently determining the density of each level continuously during the compaction. Consequently, the technique is a more direct probe of the compaction process than commercially available systems. PM/0721
TL;DR: In this paper, the effect of extrusion pressure P of ram speed V (0·5-4 mm s−1), extrusion ratio R (14:1−28:1), and extrusion temperature T (300−450°C), median powder particle diameter (11−141 μm), and melt superheat during atomisation (150−300 K) has been studied for extrusion of argon atomised Al-5Cr-2Zr (wt-%) alloy powder.
Abstract: The effect on extrusion pressure P of ram speed V (0·5–4 mm s−1), extrusion ratio R (14:1−28:1), extrusion temperature T (300−450°C), median powder particle diameter (11−141 μm), and melt superheat during atomisation (150−300 K) has been studied for extrusion of argon atomised Al–5Cr–2Zr (wt-%) alloy powder. Mean powder particle size and T had the major influence on P; R and V had less influence and melt superheat during atomisation had no influence. The absence of a breakthrough pressure, attributed to the low values of V used, allowed extrusion at values of T as low as 300°C, retaining the integrity of the rapidly solidified microstructure. The relative importance of work of consolidation compared to that of homogeneous deformation, as demonstrated by a value of 4·6 for the ratio a/b in the equation P = a + b ln R at 350°C, was larger than reported for 7090 and 7091 alloys but smaller than for Al–2·4Fe (wt-%) at a comparable T. The magnitude of the temperature dependence of P was similar to tha...
TL;DR: In this paper, the authors used a toolset consisting of a thin walled die in a fairly rigid metal, with a rubber sleeve capable of transmitting high radial pressure, to reduce ejection forces in a metal powder compaction process.
Abstract: Dramatic reduction of ejection forces in a metal powder compaction process has been achieved using a balanced die method. The approach uses a toolset consisting of a thin walled die in a fairly rigid metal, with a rubber sleeve capable of transmitting high radial pressure. By maintaining a pressure balance between internal compaction pressure and external ‘bolstering’ pressure across the thin wall die, the ejection forces were reduced to almost zero. With limited equipment available, the compaction process was carried out in incremental stages to produce cylindrical metal billets of 32 mm diameter by 20 mm height. Green densities of 6·49 Mg m−3 were achieved for NC100 iron powder supplied by Hoganas at much lower compaction pressure than those reported elsewhere. PM/0711
TL;DR: In this article, the manufacturing of bulk metal matrix composites (MMCs) by direct hot pressing of blended prealloyed powder (AA 6061) with 15 vol.-% of SiC particles (SiCp) and/or Saffil fibres, in an aluminium degassed can, is considered.
Abstract: The manufacture of bulk metal matrix composites (MMCs) by direct hot pressing of blended prealloyed powder (AA 6061) with 15 vol.-% of SiC particles (SiCp) and/or Saffil fibres, in an aluminium degassed can, is considered. Microstructure, mechanical properties, and wear resistance are compared with those of monolithic 6061 alloy and with selected examples from the literature. Good dispersion of reinforcement was achieved in the SiCp MMCs which, when heat treated to the T6 condition (solution treatment, quench, age), possessed 0·2% proof stresses of 400 MN m−2 with UTSs of ∼420 MN m−2 and elongations to failure of ∼2·5%. The Saffil and Saffil + SiCp MMCs had proof stresses of ∼350 MN m−2, UTSs of ∼380 MN m−2, elongations between 0·5 and 1·30%, and K Q values of ∼15 MN m−3/2. Pin on disc tests showed the sliding wear resistance of the SiCp MMC to be greatly superior to that of monolithic AA 6061 and also better than those of the Saffil and hybrid composites. PM/0673
TL;DR: In this article, the authors measured the flow field of a free fall gas atomiser in three dimensions using hot wire anemometry and a pitot tube for confirmation of axial flow speeds.
Abstract: The flow field of a free fall gas atomiser has been measured in three dimensions using hot wire anemometry and a pitot tube for confirmation of axial flow speeds. A finite difference based fluid dynamics package has then been used to model the gas flow from the atomiser geometry. Overall a close fit for the velocity magnitudes was obtained for predictions parallel to and along the central axis of the atomiser. An exception was a broader but lower peak along the central axis. Hot aluminium droplets of a distribution reproduced from experimental atomisation were injected into the gas flow at the confluence of the gas jets along the central axis. The modified gas flow pattern is specifically altered, when compared to the original single phase flow, close to the axis where the droplets travel. The resulting mass flowrate at the axial peak flow corresponds to that expected from the law of conservation of momentum. PM/0714
TL;DR: In this paper, the machinability of sintered stainless steel 430LHC has been investigated under dry drilling conditions, varying sintering atmosphere, manganese sulphide content, drill diameter, feed, and cutting speed.
Abstract: The machinability of sintered stainless steel 430LHC has been investigated under dry drilling conditions, varying sintering atmosphere, manganese sulphide content, drill diameter, feed, and cutting speed. The results are presented as tool life–cutting speed diagrams and converted to cost–cutting speed relationships for a realistic set of production parameters. The tool life is related to tribological aspects, hydrogen sintering and the effect of admixed manganese sulphide in improving machinability. The experimental results allow production costs to be minimised in a quantitative way. PM/0717
TL;DR: In this paper, the authors developed an optimized process for manufacturing silicon nitride components and investigated the effects of solid content, binder type, debinding solvent, and thermal debinding heat profile.
Abstract: The major objective of this study was to develop an optimised process for manufacturing silicon nitride components. The effects of solid content, binder type, debinding solvent, and thermal debinding heat profile were investigated. Injection parameters such as pressure, speed, and temperature were also investigated and were found to have substantial influence on the mechanical properties of products. Solvent debinding before thermal debinding substantially reduced the debinding time through the formation of open channels in the binder. A debinding model is proposed based on experimental observations. The defects were considerably reduced and strength was enhanced by cold isostatic pressing without causing significant dimensional changes in the sintered body. Complex shaped silicon nitride blades with density 97% of theoretical and 880 MN m−2 in bend strength were made successfully. PM/0704
TL;DR: In this article, the suitability of different iron-based powders and their effect on microstructure and mechanical properties have been investigated, and it is shown that the ball milling time required for highest sintered density is heavily dependent on the milling conditions.
Abstract: Iron based composites, with alumina or niobium carbide as a hard phase, have been developed in which the hard phase is dispersed finely in the iron matrix by means of mechanical alloying in either an attritor or a horizontal ball mill. These alloys consist of 10 vol.-% hard phase, carbon, either boron or phosphorus as a liquid phase activator, and (in one case) metallic molybdenum. Using liquid phase sintering between 1150 and 1250°C, depending on the alloy, relative composition densities of 94–99% have been achieved, providing wear behaviour and mechanical properties comparable to high speed steel. It is shown that the ball milling time required for highest sintered density is heavily dependent on the milling conditions. The suitability of different iron based powders and their effect on microstructure and mechanical properties have been investigated. PM/0688
TL;DR: In this article, the reasons for shrink fitting and the procedure of assembling liners and rings for PM dies are surveyed and the importance of checking heating temperature for rings, to avoid dangerous decrements of yield strength, is emphasised.
Abstract: The reasons for shrink fitting and the procedure of assembling liners and rings for PM dies are surveyed. Some basic principles for calculations are presented and the importance of checking heating temperature for rings, to avoid dangerous decrements of yield strength, is emphasised. Available methods of calculation or the judgement of experienced personnel in PM and toolmaking companies have proved to be reliable for normal compaction conditions. However, the significant innovation represented by the new method of warm compaction requires that safe running conditions are ensured at temperatures that could exceed 150°C. A critical analysis of the changes from room to warm compaction temperature of elastic parameters, such as elasticity moduli and Poisson's ratio, of the materials involved has been made. It has been shown that the requirements of warm compaction do not modify the basic considerations and the corresponding evaluation of the heating limit for the rings. On the basis of an assumed rat...
TL;DR: In this paper, a simpler method for the assessment of the critical packing volume concentration (CPVC) of a given powder by monitoring temperature during mixing is proposed, and the effects of powder characteristics and operational parameters on the CPVC estimation are studied.
Abstract: Powder forming of a successful product requires initial selection of powder with appropriate packing characteristics. Packing estimations from particle characteristics are erratic, and estimations from apparent density and tap density measurements are only moderately good because of poor powder flow as a result of inter particulate friction. Use of a wetting fluid in the torque measurement method avoids friction and allows particle rearrangement, but the monitoring of torque or viscosity changes with powder binder mixing requires costly equipment. A simpler method for the assessment of the critical packing volume concentration (CPVC) of a given powder by monitoring temperature during mixing is proposed. The CPVC values of different powders are measured with the new method. The effects of powder characteristics and operational parameters on the CPVC estimation are studied. The packing values arrived at by this method are found to agree with those from the torque measurement method. PM/0703