Showing papers in "Powder Metallurgy in 1985"

Compaction and Sintering Phenomena in Titanium—Nickel Shape Memory Alloys

Abstract: Ti–Ni specimens of the equiatomic composition have been processed by conventional powder metallurgy techniques. The effects of compaction pressure, sintering temperature, and powder particle size on dimensional changes of sintered compacts are reported. During sintering, anisotropy of dimensional change occurs, with expansion in the radial and contraction in the axial direction. Densities decrease during sintering. From metallographic evidence it is suggested that these observations are connected with the difference in interdiffusion rates of Ti and Ni and the segregation of powder particles in the green compacts. The sintered compacts demonstrate a well defined shape memory behaviour, with martensite nucleation being enhanced by the presence of pores. PM/0358

The Osprey Preform Process

Abstract: Description de l'etat actuel du developpement du procede Osprey de production d'ebauches, procede permettant d'obtenir, directement a partir du bain, des ebauches denses par projection et depot. Preparation d'un certain nombre d'alliages, comprenant des superalliages a base de Ni, ayant des proprietes mecaniques aussi bonnes que celles des alliages obtenus par des techniques plus classiques

Fatigue Crack Initiation in a Porous Steel

Abstract: Porosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of th...

Investigation and Analysis of Liquid Phase Sintering of Fe—Cu and Fe–Cu–C Compacts

Abstract: Mixtures of Fe–10Cu elemental powders were compacted at pressures between 141 and 680 MPa and sintered at 1140°C for 60 min. It was found that as a result of liquid copper penetration into interparticle boundaries and pores inside Fe grains, the volume of the compacts changed. The extent of volumetric growth after sintering was dependent on the internal porosity of the Fe powder. With a low micropore volume, the effects of compacting pressure variation were insignificant. However, with a large micropore volume, volumetric growth increased with compacting pressure. The addition of 1% graphite to the Fe–10Cu compact reduced volumetric growth at all compacting pressures. Increasing the graphite addition incrementally from 0 to 1% at a constant pressure, 283 MPa, served to reduce progressively the volumetric growth. The effect of graphite is to concentrate the copper at the grain corners by increasing the dihedral angle. Quantitative metallographic analysis showed that the amount of undissolved copper...

Metal matrix composites produced by spray codeposition

Abstract: A new technique for producing metal matrix particulate composites, consisting of the spray deposition of the metal matrix and the particles, is described The deposit is removed from the substrate and hot rolled to produce composite strip for examination and testing It is shown that up to 36 vol-% of SiC, Al203, chilled iron, graphite, and sand particles, and mixtures of these, 75–120 μm in size, can successfully be incorporated in aluminium and Al–5Si alloy matrixes The friction properties of some of the composites were shown to be particularly promising High coefficients of friction of ∼0·6 were obtainable under dry contact conditions, and these were remarkably constant with time The friction properties compared very favourably with conventional asbestos based and sintered friction materials which showed lower coefficients of friction that increased with time PM/0310

Present State of Liquid Phase Sintering

Abstract: A review on the present state of liquid phase sintering in ceramic and metallic systems is presented. It is shown how densification via rearrangement and centre approach can occur by the elementary mechanisms of particle disintegration, Ostwald ripening, and shape accommodation. PM/0352

Surface Analysis of Gas Atomized Ferritic Steel Powder

Abstract: The surface of gas atomized ferritic powder, Fe–25Cr, was studied by ESCA and Auger spectro- scopy. During atomization reaction products are formed on the surface of the powder. Due to the very low oxygen potential in the atomizer only oxides of the strong oxide forming elements Cr, Mn, and Si are formed during cooling of the metal particles. These oxides are precipitated as particles on the surface of the metal powder. At low temperature, when the sample is exposed in air, Fe oxide is formed between the oxide islands. The average thickness of the Cr and Mn oxide is estimated to be about 15 nm. The thin Fe oxide is found to be about 3 nm. From the surface analyses, distribution of the metal particle size, and measurements of the total oxygen content it was found that 60% of the total amount of oxygen is bound in the surface oxide. Carbides and nitrides are also formed on the surface. It is suggested that these products occur as carbonitride. These particles are coarser than the oxide particles but...

Solid Solution Additives and The Sintering of Ceramics

Abstract: Trace additives have long been known to be capable of greatly enhancing the sinterability of ceramic powder systems. Explanations for this effect have generally been sought either in the ability of the additive to prevent abnormal grain growth or in its ability to enhance the extent of densification in comparison to that of coarsening in response to the surface energy driving forces. Recent studies of sample microstructures and of sintering dilatometry lend weight to the first explanation, particularly in systems where a degree of microstructural inhomogeneity is to be found. PM/0356

Dependence of Green Strength on Contact Area Between Powder Particles for Spherical Copper Powder Compacts

Abstract: Model 2 The geometrical considerations used here are, in principle, similar to those of Model 1, with respect to the contacting pairs of spheres considered in the densification process. Now, considering the two contacting spheres shown in Fig. 11 (see Appendix 1), the contact area will be increased necessary for evaluating the experimentally obtained data. The relative density of the powder compact can be experimentally determined and this can be related to the contact area between particles. Another aim of the present work was, therefore, to derive a simple geometrical equation describing the relationship between the relative density of the powder compact and the contact area. A model experiment was carried out on an ideal packing system to confirm the validity of this equation.

Liquid Phase Sintering in Ferrous Powder Metallurgy

Abstract: A review is presented of liquid phase sintering in ferrous powder metallurgy. The application of the concept to the Fe–Cu and Fe–Cu–C systems is described. The effect of additions such as tin, boron, and copper based masteralloys are discussed, as is the contribution of iron powder type, particularly in relation to the use of very fine powders. Fe–P and Fe–P–C materials are discussed. The contribution of liquid phase sintering to the development of masteralloys is highlighted. Examples of the industrial application of liquid phase sintering techniques are described. Structural engineering component production by masteralloy techniques is considered, together with its contribution to the assembled composite camshaft concept. Finally, the problems inherent in making more industrial use of the liquid phase sintering systems discussed are outlined and some recommendations for further study are made. PM/0334

Density Distributions in Complex Shaped Parts Made from Iron Powders

Abstract: A series of specially shaped punches, operated in pairs, made possible the manufacture of 37 different, but related, shapes. Four iron powders with four levels of zinc stearate (0·5, 1·0, 1·5, and 2·0 wt-%) were compacted into these shapes to densities in the range 5·3–7·05 Mg m−3, using an instrumented industrial press. The effects of the following variables on the density distribution within each component were examined: iron powder type and characteristics, lubricant type and content, size (height to diameter ratio), shape, density, and compaction pressure. The information obtained for the series of shapes enabled conclusions to be made on the way that material characteristics, pressing conditions, and restrictions on shape control the density distribution within the final component. These data gave clear indications that will be useful in designing a component for efficient production. PM/0314

Trapping of Pores and Liquid Pockets During Liquid Phase Sintering

Abstract: The trapping of pores and liquid within growing grains during liquid phase sintering has been studied in a 97Mo–3Ni alloy. The growth patterns of grains around pores and liquid drops are revealed by an etching technique, following a sintering treatment involving periodic cooling to 1300°C from the sintering temperature of 1460°C. Pores can be trapped within adjacent growing grains when they are not eliminated by being filled with liquid. Liquid drops can form during grain growth by Ostwald ripening. The droplets may form within small pores which become filled by liquid while being trapped within adjacent growing grains. Coalescence of grains is not observed during pore and liquid entrapment. PM/0332

Effect of Carbon Additions on Sintering to Full Density of BT1 Grade High Speed Steel

Abstract: The cold pressing and vacuum sintering characteristics of two batches of BTl high speed steel water atomized powder were investigated. The powder particle sizes, shapes, and microstructures before and after annealing were compared. Relationships were established between compacting pressure and green density, die fill, and linear shrinkage after sintering. Carbon was mixed with both batches before sirttering, 0·3% to batch 1 and 0'15%' to batch 2. The sintered densities of both batches with and without added carbon, are reported for a'range of sintering temperatures between 1230 and 1340°C together with typical sintered microstructures and grain and carbide size 'measurements. Carbon, oxygen, and nitrogen contents were estimated at major steps in the process. The differences found between the sintering characteristics of two batches of the same grade of high speed steel having only slight differences in alloy content were larger than expected. The addition of carbon, however, was found to promote sintering in several ways as well as bringing the final composition up to the specified value: the residual porosity in both batches was eliminated, the temperature range extended over which maximum density was achieved, and the microstructure at the optimum sintering temperature improved. PM/0328

Sintering and Heat Treatment of Steels Made from A Partially Prealloyed Iron Powder

Abstract: Compacts of partially prealloyed steels, containing 1·75%Ni, 1·5%Cu, 0·5%Mo, and 0, 0·3, or 0·5%C, with initial porosities of about 6 or 15%, were sintered at 1150°C for times ranging from 0·25 to 128 h and cooled through the critical range at about 30 K min −1. The chemical heterogeneity of the compacts, which was very marked after short sintering times, gradually decreased with increasing sintering time, but complete homogeneity was not achieved in 128 h. The changes in microchemistry with elapse of sintering time brought about complex microstructural changes in the relatively slowly cooled compacts. These caused the mechanical properties of the compacts to vary in a complex manner with sintering time. Optimum properties of sintered and relatively slowly cooled compacts were obtained after sintering times of ∼ 1 h, when the compacts were still very heterogeneous. Other compacts that had been sintered for 1 h were subjected to conventional austenitizing, quenching, and tempering treatments. The ...

Dislocation-Activated Sintering Processes

Abstract: On sphere-plate models made from copper it is shown that during sintering the dislocation density increases considerably in the contact region. Its distribution and time-dependent alteration are able to be analysed by means of the Kessel-technique and described quantitatively. The same effect is observed during sintering compacts of electrolytic copper powder. The results of positron annihilation spectroscopy show the high dislocation densities generated in the heating phase to be reduced by non-conservative dislocation movement in the stage of intensive shrinkage. Resulting densification mechanisms are discussed.

Relationships Between Processing, Microstructure, and Tensile Properties of Co–Cr–Mo Alloy

Abstract: Stellite alloy no. 21 was produced via rapid solidification processing (RSP) in a variety of particulate morphologies (coarse and fine powder, fibres, and ribbons), the various RSP forms showed similar fine microstructures with only slight differences in the scale of the microstructural features. These RSP particulates were consolidated by extrusion, dynamic compaction, and rapid omnidirectional compaction (ROC). Dynamic compaction proved to be unacceptable for this alloy because of non-uniform porosity and the inability to develop a metallurgical bond between particulates. A plot of elongation against yield strength depicted two yield strength-ductility relationships for the Co–Cr–Mo type alloys. As-ROC samples had a low yield strength-ductility relationship. However, after hot forging, the relationship shifted to a level comparable to similar material processed by conventional extruding and forging operations. Atomized powder size also affected the strength-ductility relationships of the extrude...

Figure of Merit for Activated Sintering

Abstract: A quantitative figure of merit for assessing the potential of various elements for activation sintering has been derived by combining the relevant heats of solution, surface energies, and the energy of formation for vacancies in the activator. Good agreement has been obtained between calculated values and the accepted qualitative ranking for activators used to accelerate the sintering of tungsten and molybdenum. The formalism should be capable of extension to multicomponent systems. PM/0351

Influence of Process Parameters on Precision of PM Parts

Abstract: Powder metallurgy is normally considered a very good technology for producing mechanical parts with high precision. This is generally confirmed by comparison with other mass production technologies. In the present study, typical precision levels achieved by various fabrication operations are considered. Since the final precision is determined by the various contributions of each production stage, each contribution has been considered separately. The influence of various chemical compositions (always on an iron base) has also been considered. Another important consideration is the dimensions of the part to be produced. For simple geometries, it is demonstrated that the ISO system of classifying tolerances is not suitable for classifying correctly the precision levels achievable on parts of the same shape but with different dimensions. The problems of dimensional tolerance increase as the part dimensions increase. The theoretical evaluations are based on information supplied by the producers of iron...

Homogenization in Sintering and Its Influence on Mechanical Properties of Fe–Cr–C and Fe–Ni–C Steels

Abstract: During the sintering of Fe–Cr–C and Fe–Ni–C steels produced from mixtures of powders with different chemical compositions, homogenization takes place. The degree of uniformity can be evaluated from the standard deviation of histograms of alloying element concentration distributions. Such histograms were obtained both by modelling the diffusion processes in sintering and from experimental data. The theoretical assumptions have been shown to give good agreement with the experimental results. Mechanical properties such as ultimate tensile strength, elongation, reduction in area, hardness, and impact behaviour are shown to improve gradually with sintering and homogenization. PM/0333

Sintering with Nitrogen Based Atmospheres

Abstract: The use of nitrogen based atmosphere systems for sintering is now well established both worldwide and in the UK. On the basis of this experience, the atmosphere systems that can be used to replace conventional endothermic and cracked ammonia atmospheres are briefly described and their practical implementation reviewed in terms of advantages and limitations. In particular, the recent application in the UK of a nitrogen-methanol atmosphere system as an alternative to endothermic gas is discussed. In addition to the practical experience gained, a number of laboratory development programmes have been carried out to gain a better understanding of the effect of nitrogen based atmospheres on sintered components. The results of work on high temperature sintering, on the effect of hydrogen level on sulphur content in ferrous compacts, and on nitrogen pick-up in ferrous compacts are reviewed. PM/0345

Application of Advanced Powder Process Technology to Titanium Aeroengine Components

Abstract: Current production processes for the manufacture of titanium aeroengine components such as compressor blades, discs, and static parts are well proven. However, many of these processes incur very low material utilization, up to 85% of the input material being lost as machining swarf which has little subsequent value. Powder metallurgy offers the possibility of making close-to-form components in titanium, thus markedly improving material utilization. Development studies have been undertaken at Rolls Royce in which" a PM direct-hot' isostatic pressed (hipped) route has been used to make two components normally produced from machined forgings. Prealloyed Ti-6AI-4V powder was used to give optimum mechanical properties in a complex static part, and in a separate programme elemental powder of this same alloy was used to make a simple static part at minimum cost. Both routes involved a ceramic mould to contain the powder, which gives much better shape definition than a metal mould. Technical aspects of these routes are examined with regard to the critical need for a consistent process and comparisons are made with the product of the current route. PMj0305

Microstructural Features of Creep and Shrinkage in Molybdenum Containing Minor Additions of Nickel

Abstract: Microstructural changes occurring during the sintering of molybdenum compacts containing minor additions of nickel were compared with changes during the creep of thin molybdenum foils in the presence of nickel vapour. Evidence from micro-structural, shrinkage, and creep rate studies indicates that there exist three stages of sintering: an intrinsic stage during heating, in which the dislocation structure caused by cold compaction is still maintained; a transient stage, in which recovery and recrystallization occur; and a post-recrystallization stage, in which grain boundary diffusion is the dominant shrinkage mechanism. PM/0353

Comparison of Heat Treatment Response of Wrought and Sintered BT1 Grade High Speed Steel

Abstract: A comparison was made of the secondary hardening characteristics of a conventionally wrought and two batches of directly sintered BT1 high speed steel. Minor differences in hardness between the types were observed after austenitizing between 1050 and 1280°C and tempering from 400 to 600°C. The recommended hardening temperature for BT1 was considered too high in at least two types by as much as 30 K. The wrought material had the finest microstructure throughout, e.g. 8 μm grain diameter for 1200°C austenitization treatment. Considerable grain refinement, however, was achieved in both directly sintered products by a post-sinter transformation anneal. The grain size after quenching was 2–3 times smaller than for the as-sintered, e.g. 15 and 19 μm compared to 46 and 36 μm respectively for the same austenitization temperature. PM/0331

Sintering of Titanium Alloyed Steels

Abstract: An investigation into the manufacture of sintered steels containing titanium as an alloying element is reported. Transient liquid phase sintering has been used. Titanium may be used to generate three types of alloy system: (i) microalloyed, (ii) solid solution strengthened, and (iii) precipitation hardened. Structural alterations and shrinkage during sintering, as well as mechanical properties developed following heat treatment, are presented and discussed. Ultimate tensile strength and elongation to failure were found to be ∼500 MN m−2 and 5%, 400–450MN m −2 and 25–20%, and 600–700 MN m−2 and 3–2% for alloys of types (i), (ii), and (iii), respectively. PM/0316

Controlled Rolled Low Alloy Powder Forged Steels

Abstract: Two low alloy powder forged vanadium steels of varying forged blank densities were controlled rolled in the austenitic range. Grain refinement of ferrite was aimed at, by processing the steels below the recrystallization temperature where nucleation takes place extensively on the elongated prior austenite grain boundaries. Effects of initial forged density and deformation on transformation were studied. The cooling rate effect was also examined by using four different cooling media (vermiculite, air, air blast, water) following the deformation. Rolling introduced deformation bands where ferrite nucleation occurred, which aided refinement of the transformed micro-structure. Also, densification achieved with increasing deformation led to almost pore free structures and improved grain refinement. Gas bubble formation observed in pore free zones in these hydrogen annealed powder forged steels was ascribed to a hydrogen gas reaction. Varying initial forged densities had no effect on specimen cooling ra...

Production of Engineering Materials from Rapidly Solidified AA7050 Al–Zn–Mg–Cu Powders

Abstract: The microstructures of rapidly solidified 7050 alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy and compared with those observed in a cast and homogenized ingot of the same material. The powder alloy was then consolidated by cold compaction followed by extrusion, the resulting structures being compared with those of extruded cast alloys. Finally, the structure and properties of the two materials were compared after heat treatment. It is shown that metastable phases not previously reported are present in the powder particles and that localized melting occurs in the powder alloy at a lower temperature than in the cast alloy; this is the primary cause of blistering, but appropriate modification to the heat treatment will prevent it occurring. Properties comparable to or better than the cast material may be produced by designing a more suitable heat treatment to avoid liquation of this metastable phase. The evidence presented su...