Investigation into the influence of speed characteristics of compaction on the quality of wares made of uranium dioxide

Results of investigations into the influence of nonuniformity (discontinuity, the step structure) and speed of pellet compaction on the consolidation kinematics of uranium dioxide powders and quality of pellets in regards to cracks are presented. The stress field over the compact volume is plotted. The tendency of uranium dioxide powder to crack is shown to be associated with the instability of speed characteristics and the stress state in the powder volume under the nonuniform compaction. It is found that the imperfection in regards to cracks of pellets made of uranium dioxide powder with a dry binder is lower than that one with a liquid binder. A mechanism is proposed which ensures the uniform compaction mode and reduces waste in regards to cracks for compacts with liquid and dry binders.     

THE SINTERING OF PYROPHORIC POWDERS

Abstract

The room-temperature oxidation behaviour of reactive metal and ceramic powders is reviewed and it is shown that many materials under normal circumstances oxidize to give a limiting film thickness of ～50Å. Since the thickness of oxide films has an important bearing on sintering characteristics the control of oxidation may be essential for some materials and desirable for others.

The oxidation behaviour of uranium carbide and uranium nitride in various concentrations of oxygen and water vapour in argon is described and its effects on the sintering behaviour and composition changes that take place during sintering are considered.

Suitable handling conditions for these materials are provided by glove-boxes filled with argon; there is a brief discussion of the factors that might apply if they were used in other areas of powder metallurgy.

The application of high-purity handling atmospheres in the preparation, handling, and sintering of tungsten powders has given pellets that had densities of 95% theoretical and oxygen contents <10 ppm after sintering in hydrogen at 1550°C.     

Dilatometric analysis of thermal debinding of injection moulded iron compacts

Abstract

The less than desired tolerance control of powder injection moulded compacts is a result of inconsistent dimensional changes in the compacts accumulated during moulding, debinding, and sintering. This study investigated the in situ length changes and their causes during thermal debinding on compacts which have been solvent debound. The dilatometric analysis showed that the specimen shrank in the early stage between 250 and 370°C, not because of sintering, but through the loss of N, C, and O in the carbonyl iron powder. At temperatures between 370 and 450°C, the specimen expanded owing to the carburisation of the iron powder. The length change was also influenced by the heating rate, debinding atmosphere, and the amount of the backbone binder. These dilatometric results are helpful in establishing the guidelines in designing binder compositions and debinding schedules.     

Effect of nickel boride and boron additions on sintering characteristics of injection moulded 316L powder using water soluble binder system

Abstract

A new metal injection moulding system for 316L stainless steels has successfully been derived and tested. A mixture of small water atomised powder (average size 15 µm), larger gas atomised powder (average size 75 µm), and sintering additives has been coupled with a new water soluble binder system for economical powder injection moulding. The details for each process step and the effect of sintering additives are described. The binder system consists of poly (2-ethyl-2-oxazoline) as the leachable polymer, polyethylene as the backbone, and stearic acid as a surfactant and plasticiser. This binder system provides satisfactory mixture stability, excellent mouldability, and reasonably fast water leaching and thermal debinding rates. The optimum powder/binder compositions were determined using torque and capillary rheometry. Densification was by persistent liquid phase sintering through additives, such as nickel boride and boron. This 316L powder system was sintered to 7·9 g cm ^-3 (98·75% of theoretical) at 1285°C using nickel boride addition and at 1245°using boron addition. nickel boride additions are particularly effective at increasing the tensile strength and ductility. In contrast, the boron additions only increase the tensile strength and decrease ductility. Based on microstructure evaluations, this effect is traced to a continuous boride phase on the grain boundaries of the boron doped samples and a discontinuous boride phase on the grain boundaries of the nickel boride samples.     

THE EFFECT OF SURFACE CHARACTERISTICS ON THE SINTERING OF URANIUM AND BERYLLIUM POWDERS

Abstract

Studies of the sintering of uncompacted uranium powders in vacuum have shown that sintering behaviour is markedly affected by the nature of the surface films on the powder. Of the variables in the powder production route that might affect the nature of the surface films, the leaching stage is the most important.

The deleterious effect of surface contaminants on the vacuum-sintering behaviour of beryllium is demonstrated. During the development of a technique for the production of a stable powder from electrolytic flake, the nature of the leaching treatment again proved to have a marked influence on the sintering behaviour of the resultant powder.

Both sets of investigations were hampered by lack of methods for identification of surface films on powders.     

LEACHING RATES OF ICEL-YAVCA DOLOMITE IN HYDROCHLORIC ACID SOLUTION

Additives can give rise to obvious, step-wise changes both in the oxidation process and in the sintering process. Therefore, the oxidation and sintering characteristics measured in dried pellets prepared from pure magnetite concentrates can not be representative for those characteristics in dried pellets containing additives. The oxidation and sintering characteristics of magnetite iron ore pellets balled with a novel complex binder (namely MHA) were mainly investigated by batch isothermal oxidation measurements in this research. Combined results reveal that the thermal decomposition of MHA binder influences the oxidation and sintering processes of dried pellets. Oxidation rate of pellets increases obviously with increasing the oxidation temperature in the range from 800°C to 1000°C. And the remaining FeO content declines gradually when separately heated for 10 min at low temperature (<1000°C). However, the oxidation rate of pellets decreases distinctly when oxidation temperature is higher than 1000°C. In addition, when oxidation temperature increases from 1000°C to 1250°C, the FeO content of pellets goes up obviously, particularly at 1250°C. The FeO content in the core of sintered pellets heated at 1250°C can even reach 29.68%. SEM spectrum analysis demonstrate that some iron appears in forms of wustite in sintered pellets, which indicates that the reduction reaction of iron oxide occurs during the high temperature sintering process. This is explained by the occurrence of reducing atmospheres because of the pyrogenic decomposition of MHA binder.     

LUBRICATION METHOD FOR ROTARY-PRESS TOOLS

Abstract

A new method for lubricating the tools of a pelletizing press is described, based on the use of dry lubricants mixed with polystyrene balls as a carrier. The solid lubricant is introduced into the die, applied to the walls of the compacting chamber by the movement of the punches, and then ejected following the normal pelletizing cycle. Hence, a very thin film of lubricant remains on the walls and makes it possible afterwards to pelletize a uranium dioxide powder. The process has been applied to a double rotary press in which one half is used to “pelletize” the lubricant and the other half to obtain the required production of UO₂ pellets. Pellets have been fabricated from a loose virgin powder which had not been granulated.

The process does not lead to contamination of the UO₂ pellets because the deposit of lubricant is very thin and uniform. It is applicable whatever the shape and the dimensions of the pellets may be, owing to the good flow rate of the polystyrene balls. Moreover, the polystyrene compact splits as soon as the applied pressure is released since the balls are elastic; a closed cycle can therefore be operated. Other advantages of the process are also described. The reproducibility of the UO₂ pellets fabricated by this technique indicates its efficiency.     

IMPORTANCE OF STEARATES IN THE FABRICATION OF UO2 AND (U,Pu)O2 PELLETS

Abstract

Eighteen qualities of metallic soaps have been studied, either as lubricants or as additives for the fabrication of UO₂ and (U,Pu)O₂ pellets. Compounds of the same family, such as zinc stearates, differ in their effectiveness as lubricants. This property is strongly dependent on the particle size of the powder and its purity. The behaviour of the dry lubricants when used as admixtures in UO₂ and also when employed for the direct lubrication of the pelletizing tools is discussed.

The efficiency of metallic soaps as additives to lower the density of the sintered pellets is related to the nature of the metallic ion. Thus, zinc stearate has little influence on the fired density of UO₂, whereas barium stearate and other barium compounds are very effective. Barium inhibits sintering, but zinc, which is eliminated at low temperature, does not diminish the sinterability of the oxide powders. In the latter case, the decrease in density is caused by thermal decomposition of the organic molecule.     

Sintering of maraging steel with phosphorous additions

Abstract

The aim of this study was to develop a steel powder system for rapid tooling applications. The properties required are rapid densification, dimensional precision, high mechanical strength and corrosion resistance. To this end, the densification and microstructural development of a loose packed 200 grade maraging steel powder sintered with ferrophosphorous additions was examined. Liquid initially formed from a reaction of the Fe₃P and carbon, which was a residue of the polymeric binder used to shape the powder compact. This liquid caused a burst of sintering which ceased as the liquid dissipated. On further heating, a phosphorous rich supersolidus liquid appeared at triple points and grain boundaries leading to rapid densification and a sintered density of 98%.     

Microwave Heating, Isothermal Sintering, and Mechanical Properties of Powder Metallurgy Titanium and Titanium Alloys

This article presents a detailed assessment of microwave (MW) heating, isothermal sintering, and the resulting tensile properties of commercially pure Ti (CP-Ti), Ti-6Al-4V, and Ti-10V-2Fe-3Al (wt pct), by comparison with those fabricated by conventional vacuum sintering. The potential of MW sintering for titanium fabrication is evaluated accordingly. Pure MW radiation is capable of heating titanium powder to ≥1573 K (1300 °C), but the heating response is erratic and difficult to reproduce. In contrast, the use of SiC MW susceptors ensures rapid, consistent, and controllable MW heating of titanium powder. MW sintering can consolidate CP-Ti and Ti alloys compacted from ?100 mesh hydride-dehydride (HDH) Ti powder to ~95.0 pct theoretical density (TD) at 1573 K (1300 °C), but no accelerated isothermal sintering has been observed over conventional practice. Significant interstitial contamination occurred from the Al₂O₃-SiC insulation–susceptor package, despite the high vacuum used (≤4.0 × 10^?3 Pa). This leads to erratic mechanical properties including poor tensile ductility. The use of Ti sponge as impurity (O, N, C, and Si) absorbers can effectively eliminate this problem and ensure good-to-excellent tensile properties for MW-sintered CP-Ti, Ti-10V-2Fe-3Al, and Ti-6Al-4V. The mechanisms behind various observations are discussed. The prime benefit of MW sintering of Ti powder is rapid heating. MW sintering of Ti powder is suitable for the fabrication of small titanium parts or titanium preforms for subsequent thermomechanical processing.     

INFLUENCE OF TIN POWDER CHARACTERISTICS ON THE SINTERING OF IRON-TIN-COPPER POWDER COMPACTS

Abstract

In view of increasing industrial interest in the use of tin additions as an aid to the sintering of iron-based powder compacts, an examination has been made of the influence of the characteristics of the tin powder on sintering performance.

The effect of additions of narrow size-range fractions of atomized tin powder on the dimensional changes and tensile properties obtained on sintering Fe-Sn-Cu compacts made with –100 mesh (–152 μm) or – 300 mesh (– 53 μm) sponge iron and – 300 mesh (– 53 μm) atomized copper powders has been determined. The compacts contained tin and copper in the ratio 2:3. The narrow size fractions were separated from – 300 mesh tin powder by air elutriation. It was found that the use of coarse tin powder reduced the tensile strength of – 300 mesh iron-based Fe–1% Sn–1 ½% Cu compacts, but had no influence when this mixture was based on –100 mesh iron powder, or when the mixture composition was Fe–2% Sn–3% Cu. The effects have been examined in relation to the sintering mechanism by scanning electron microscopy and by X-ray microanalysis.     

STUDIES OF THE BINDER PHASE IN WC-Co CEMENTED CARBIDES HEAT-TREATED AT 650°C

Abstract

The binder phase of WC-Co cemented carbide is a Co-rich alloy in which tungsten and carbon are in solid solution. The binder phase is supersaturated with respect to tungsten even after slow cooling from the sintering temperature. In this study the binder phase contained 6% W in solid solution before heat-treatment at 650°C (923 K). Transmission electron microscopy on thin foils of binder phase showed that a finely dispersed. phase, α′, precipitated in the cubic binder phase. After long ageing times Co₃W could be identified by X-ray diffraction methods. As no discontinuous , ‘cells’ of Co₃W and ?-Co could be observed, the following reaction is suggested.

α-Co(W, C)→α-Co+α′→α-Co (C)+Co₃W (needles)

The precipitation of α′ was accompanied by an increase in hardness and a decrease in transverse rupture strength. The effects observed are consistent with those found during annealing of Co-rich cobalt-tungsten-carbon alloys (> 85% Co).     

A review on recent developments in binder jetting metal additive manufacturing: materials and process characteristics

ABSTRACT

Binder Jetting Metal Additive Manufacturing (BJ-MAM), known also as metal 3D-printing, is a powder bed-based additive manufacturing technology. It consists of the deposition of liquid binder droplets to selectively join powder particles to enable the creation of near-net shaped parts, which subsequently are consolidated via sintering process. This technology is known for its capability to process a wide range of different materials and for its orientation towards large volume production series. Binder Jetting has recently been drawing the attention of both the research sphere as well as several industrial sectors. The present review study encompasses the various and most remarkable aspects of BJ-MAM part fabrication. The review covers the material selection and characterisation considerations, followed by the manufacturing process features and the parameter effect on different part properties. It concludes with an overview concerning the most recent case studies with regards to diverse metal alloy developments.     

WHAT CONTROLS THE COSTS OF SINTERED POROUS MEDIA?

Abstract

Various factors which have an important economic bearing upon the production of sintered porous media are reviewed. In particular, reference is made to the manufacture of porous media from: (a) spherical powder, (b) irregular powder, (c) wire, (d) fibres, and also combinations of these materials. Processes considered include: (1) gravity sintering, (2) pressure sintering, (3) press compaction and sintering, (4) roll compaction and sintering. Consideration is also given to the fabrication of porous media.

The significance of various economic factors is discussed in relation to end-product properties and applications.     

Development and characterisation of direct laser sintering multicomponent Cu based metal powder

Abstract

Recent advances in direct metal laser sintering (DMLS) have improved this technique considerably; however, it still remains limited in terms of material versatility and controllability of laser processing. In the present work, a multicomponent Cu based metal powder, which consisted of a mixture of Cu, Cu–10Sn and Cu–8·4P powder, was developed for DMLS. Sound sintering activities and high densification response were obtained by optimising the powder characteristics and manipulating the processing conditions. Investigations on the microstructural evolution in the laser sintered powder show that liquid phase sintering with partial or complete melting of the binder (Cu–10Sn), but non-melting of the cores of structural metal (Cu) acts as the feasible mechanism of particle bonding. The additive phosphorus acts as a fluxing agent to protect the Cu particles from oxidation and shows a concentration along grain boundaries owing to the low solubility of P in Cu and the short thermal cycle of laser sintering. A directionally solidified microstructure consisting of significantly refined grains is formed, which may be ascribed to laser induced non-equilibrium effects such as high temperature gradient and rapid solidification.     

DIMENSIONAL CONTROL IN HARD-METAL MANUFACTURE

Abstract

All hard-metal alloys contract ～50% by volume during sintering and, in addition, the continuous network of liquid phase makes them extremely weak. Hence, the two major problems in dimensional control are coping with this large contraction and at the same time preventing distortion during sintering.

Green compacts of closely controlled and very uniform density are essential to ensure that the contraction is both predictable and uniform. Methods of predicting contraction and of powder preparation and pressing to achieve this end are described.

Distortion results not only from mechanical weakness during sintering but also from the carburizing, decarburizing, oxidation, and reduction reactions that occur when the green compact is heated.

In conclusion, indications are given of the order of dimensional accuracy that can be expected in hard-metal manufacture.     

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

Abstract

The microstructural transformations and the dimensional evolution of green specimens obtained by pressing mixtures of austenitic and ferritic stainless steel powders have been investigated by sintering at 1120 and 1240°C. Dilatometry experiments show that the linear shrinkage is influenced by the amount of ferritic powder. Moreover, during sintering Ni diffuses into the ferritic grains causing austenite destabilisation and the formation of a mixed constituent, whose constitution has been investigated by means of EDXS and interpreted on the basis of the Schaeffler diagram. Sigma phase also forms during sintering of the duplex mixtures.     

IRON-COPPER-TIN SINTERED COMPACTS

Abstract

A wide range of copper and tin powder additions to iron powder sintered compacts hasbeen studied. From mechanical-property tests it has been shown that when using sinteririg temperatures of 900–1100°C in nitrogen/10% hydrogen atmospheres there is an optimum copper: tin ratio of 15:2. The mechanical properties obtained from compacts pressed from iron mixed with 4% copper+tin in this ratio and sintered at 900°C were similar to those obtained from iron ?l0% copper powder compacts sintered at 1100°C. Moreover, the iron-copper-tin components showed improved dimensional accuracy.

In a further series of experiments, it was shown that tin additions to iron-copper alloy compacts increased the solubility of iron in the liquid phase at the sintering temperature and simultaneously decreased the rate of diffusion of copper into the iron particles. At the same time, tin improved the wettability of the liquid, reducing its surface tension and allowing it to disperse more completely throughout the matrix. The mechanical properties of compacts containing larger amounts of tin were decreased by the presence of brittle compounds, although the sintering rate was increased. It is concluded that the optimum properties of iron-copper-tin compacts are obtained by making correct additions of copper and tin to the iron powder and giving careful consideration to the sintering atmosphere.     

High temperature sintering and its effect on dimensional and geometrical precision and on microstructure of low alloyed steels

ABSTRACT

The effect of high-temperature sintering (1180°C and 1250°C) on the dimensional stability, geometrical precision, density and microstructure of rings made of five different low alloy steels was studied. Density and dimensional shrinkage slightly increase with temperature, but the dimensional and the geometrical precision of parts, do not depend on sintering temperature. Sintering temperature maybe therefore increased up to 1250°C without impairing the dimensional and geometrical precision of the investigated rings. An estimation of the effect of the high sintering temperature on tensile properties is presented, based on the microstructure and on the fraction of the load-bearing section. A significant increase in both tensile strength and tensile ductility may be expected, in particular when the sintering temperature is increased up to 1250°C.