The sintering behavior of quasi-spherical tungsten nanopowders

In this work, the sintering behavior of quasi-spherical tungsten nanoparticles was investigated by analysis the sintered compacts obtained at different sintering temperatures and dwell time, and the influence of microstructures on the density and Vickers microhardness of sintered products was also studied. Experimental results show that particle shape and size distribution are critical to the sintering activity and mechanical properties of obtained compacts. 91.3% of theoretical density (TD) of the compact could be obtained at low sintering temperature of 1500 °C, and the highest hardness of 606 VHN could be achieved when sintered at 1100 °C due to formation of uniform, densely packed sintered compacts with grain size of 235.7 nm. Importantly, unusual linear correlation between grain size and relative density was observed in our experiment, and a cut-off point exists at 85.6% of TD. The kinetic analysis revealed that surface diffusion is responsible for the mass transport during the initial sintering stage.     

Self-compacting high density tungsten–bronze composites

Green compacts of W–bronze were encapsulated in shells of bronze powder, placed in a ceramic mold and sintered in alumina tube furnace at 1150 °C. Throughout the sintering cooling stage the differential coefficient of thermal expansion ΔCTE of W–bronze was employed to induce an external compressive densification action. The process included simultaneous sintering, hot isostatic pressing (HIP) and infiltration act to enhance densification. By this technique, pilot sintered compacts of different W50–80 wt.%–pre-mix bronze of 97–99% theoretical density were produced. This process resulted in compacts of higher hardness, higher sintered density and better structure homogeneity as opposed to similar compacts densified by the conventional sintering process. The results showed a gain in hardness by 10–20% and in density by 5–15%. The impact of different cooling rates of 3, 4, 8 and 30 °C min^?1 on sintered density, microstructure and densification mechanisms was examined and evaluated. Low cooling rates of 3 and 4 °C min^?1 gave the best results.     

Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering

Selective laser sintering (SLS) technique is capable of rapidly fabricating customized implants with porous structure. A simple encapsulation process was developed to coat 316L stainless steel (316L SS) powder with ethylene-vinyl acetate copolymer (EVA). Subsequently, porous 316L SS was prepared by SLS preforming of EVA-coated metal powders, debinding and sintering in hydrogen atmosphere. The effects of processing parameters on pore characteristics and mechanical properties were analyzed. The results indicate that the porosity of green body mainly depends on laser energy density, while the pore features and mechanical properties of sintered specimens are largely dominated by sintering temperature. After sintering at 1100–1300 °C, the average pore size and porosity are 160–35 μm and 58–28%, respectively. In addition, the elastic modulus and compressive yield strength are 1.58–6.64 GPa and 15.5–52.8 MPa, respectively. It is revealed that the pore structural parameters and mechanical properties of the as-sintered porous 316L SS can be controlled readily to match with those of cancellous bone by modification of SLS processing parameters and subsequent sintering temperature.     

Application of compression lubricant as final porosity controller in the sintering of tungsten powders

Sintering behavior of two tungsten powders (1.2 μm and 6 μm) was studied for preparing infiltrable porous skeleton. Both powders were compressed by mechanical press (MP) and cold isostatic press (CIP) with and without stearic acid respectively as compaction lubricant. Results showed that presence of solid lubricant powder in addition of its essential effect on soundness of parts, depending on its size and distribution, could mainly affect sintered microstructure. Stearic acid as compaction lubricant in addition of decreasing friction between particles during the compaction, has acted as spacing particles between primary powder particles. In the cases that lubricant particles are much bigger than tungsten particles a big pore remained after evaporation of lubricant. During the sintering, big pores became bigger due to coarsening mechanism and formed an interconnected network of pores and on the other hand small pores shrank or even disappeared due to densification. By exact controlling of the size of tungsten powder and lubricant powder, infiltrable tungsten skeletons with 80% of theoretical density were produced successfully at low sintering temperatures such as 1500 °C.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

Synthesis of nano-sized Fe–Ni powder by chemical process for magnetic applications

Synthesis of nano-sized Fe–Ni permalloy of the composition 20 wt% Fe and 80 wt% Ni took place by electroless chemical reduction method in alkaline tartarate bath using hypophosphite as a reducing agent. The powder was cold compacted at 600 MPa and then sintered at 1050 °C. Metallographic investigations were performed by optical microscope and SEM with EDAX analysis. Hot-stage XRD was performed for the investigated materials to follow the phase transformations in the material. Physical, magnetic and electrical properties were studied for the prepared powder and its related sintered compacts. FeNi powder prepared from the experiments has a 200 nm particle size with 2.4 wt% phosphorus content. The prepared powder has amorphous structure with a low saturation induction (B_s) but by raising the temperature to 500 °C, the FeNi₃ intermetallic appears first and then the cubic FeNi solid solution is formed at 1050 °C, which has the highest saturation induction value.After cold compaction and sintering, the electrical conductivity and the saturation induction increased by increasing the time of sintering but the coercive force decreased and the material becomes softer after sintering. Measurements of the magnetic permeability indicate that the optimum applying field for the investigated sintered material is between 40 and 100 Oe which gives the highest range of the magnetic permeability. From the magneto-resistance measurements, it is shown that the sintered material has a positive magneto-resistance in the field direction but a negative one in the direction perpendicular to the current and the field.     

Synthesis and consolidation of W–Cu composite powders with silver addition

Homogeneous and nanostructured W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag composite powders were prepared via a chemical precipitation method, with the aim of surveying the effect of silver on the properties of tungsten–copper composites. For this purpose, ammonium metatungstate, copper nitrate and silver nitrate with predetermined weight proportion were separately dissolved in distilled water. Furthermore, W–20 wt.%Cu composite powders were provided for comparison. The initial precipitates were obtained by reacting a mixture of the mentioned solutions under certain pH and temperature. The precursor precipitates were then washed, dried, and calcined in air to form oxide powders. In the next step, the reduction was carried out in hydrogen atmosphere to convert them into the final nanocomposite powders. The resulting powders were evaluated using X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscopy (SEM) techniques. The effect of sintering temperature was investigated on densification and hardness of the powders compacts. The results showed that at all sintering temperatures, by increasing in the amount of silver, powders showed better sinterability compared to W–20 wt.%Cu powders. Maximum relative densities of 97.7%, 98.2% and 99.6% were achieved for W–20 wt.%Cu, W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag compacts sintered at 1200 °C, respectively. Moreover, maximum hardness of 359, 349 and 255 Vickers were resulted for W–20 wt.%Cu, W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag compacts sintered at 1200 °C, respectively.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

Microstructure and mechanical properties of a spark plasma sinteredTi–45Al–8.5Nb–0.2W–0.2B–0.1Y alloy

A high Nb containing TiAl alloy from pre-alloyed powder of Ti–45Al–8.5Nb–0.2B–0.2W–0.1Y was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The optimized conditions yield high densities and uniform microstructure. Specimens sintered at 1100 °C are characterized by fine duplex microstructure, leading to superior room temperature mechanical properties with a tensile strength of 1024 MPa and an elongation of 1.16%. Specimens sintered at 1200 °C are of fully lamellar microstructure with a tensile strength of 964 MPa and an elongation of 0.88%. The main fracture mode in the duplex microstructure was transgranular in the equiaxed γ grains and interlamellar in the lamellar colonies. For the fully lamellar structure, the fracture mode was dominated by interlamellar, translamellar and stepwise failure.     

The role of the activator rich-W interboundary layer on liquid phase sintering of W–pre-alloy bronze composites of Fe and Co additions

In this study, the effects of 1–3 wt.% Fe and Co additions on the sintering of W 40–80wt.%–pre-alloy bronze Sn 10 wt.%–Cu compacts were examined. The isothermal part of the sintering process was conducted at temperatures ranging from 920 °C to 1300 °C for 3 h. Relative sintered densities in the range of 70–90% were achieved. The gain in the sintered densities due to activator addition was 5–15%. The sintering activation effects started at temperatures as low as 600 °C below the bulk eutectic temperature. SEM, XRD and EDX tests proved that Fe and Co-rich crystalline interboundary layers completely wet the tungsten grain boundaries in the solid state and act as a short-circuit diffusion path for mass transportation. These outcomes seem to follow the classical activated sintering model and contrast with some other recently proposed models, whereby a detected nanometer-thick, activator-enriched disordered film at W grain boundary is considered fully responsible for the solid-state activated sintering.     

Development of non-eroding rocket nozzle throat for ultra-high temperature environment

The powder processing methods including powder metallurgy (P/M) and powder injection molding (PIM) techniques for tungsten (W)–rhenium (Re) were employed to produce a W–Re rocket nozzle. The composition of W–Re was determined by 25 wt.% of Re to avoid the formatting brittle sigma (σ) phase. The samples for analysis of the densification behavior on sintering were prepared by die pressing and cold isostatic pressing (CIP). The feedstock for the PIM process was produced by mixing the W–25 wt.% Re powder and binder system based on a wax-polymer with an optimum solid loading through the twin-extruder mixer. The injection molded specimens were debound to extract and decompose the binders via the solvent and thermal debindings. The debound samples were sintered in a hydrogen atmosphere. After sintering, hot isostatic pressing (HIP) was carried out in an argon atmosphere to enhance the density.The dilatometry experiments were performed to analyze and predict a densification behavior during sintering. The master sintering curve (MSC) model was used to characterize the densification behavior with a minimal set of preliminary experiments. The mechanical properties were evaluated through microstructure and chemical composition measured by EDX–SEM and X-ray diffraction (XRD).Finally, the eroding test was conducted using the W–25 wt.% Re rocket nozzle produced by PIM under the high temperature. After carrying out erosion tests, the erosion rate, hardness and microstructure were evaluated.     

In situ synthesis of high density magnetic ferrite spinel (MgFe2O4) compacts using a mixture of conventional raw materials and waste iron oxide

A mixture of magnesite ore and waste iron oxide (mill scale) was used to synthesize high density magnesium ferrite compacts. The effect of different mixture composition (weight percentage of magnesite ore to mill scale) as well as the sintering temperatures on the phase change, compressive strength, physical and magnetic properties of sintered compacts was investigated. The results indicated that a single phase ferrite spinel is obtained in a compact produced from a mixture consisting of 40 wt% magnesite ore and 60 wt% mill scale and sintered at different temperatures. On the other hand, this mixture composition produced compacts possessing low porosity and high saturation magnetization of 6% and 31.89 emu/g, respectively when it is sintered at 1550 °C for 2 h.     

Ni–NiO composites obtained by controlled oxidation of green compacts

Ni–NiO composites have been obtained by the thermally induced oxidation of metallic green compacts at temperatures between 300 and 450 °C and further sintering. Thermogravimetric studies showed that oxidation process in air follows a quadratic dependence with time for temperatures between 300 and 400 °C allowing the control in the metal to oxide ratio. Microstructural analyses of compacts sintered in inert atmosphere reveal a homogeneous distribution of phases. In the mechanical tests the metal to ceramic ratio variation is evident in the ductile to brittle transition of the fracture, making this method suitable to fabricate compacts with tailored mechanical properties.     

Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

Sub-micron binderless tungsten carbide sintering behavior under high pressure and high temperature

Pure tungsten carbide (WC) compacts of about 200 nm grain size were prepared by high pressure and high temperature (HPHT) method. The best property sample with high relative density (99.2%), high Vickers hardness (2925 kg·mm^− 2) and high fracture toughness (8.9 MPa·m^1/2) was obtained in the condition of 1500 °C temperature and 5 GPa pressure. By means of scanning electron microscopy (SEM) and transmission electron microscope (TEM) observations, a large number of twins and stacking faults appeared in sintered samples, and the grain size of sintered samples maintained in the initial range. The XRD patterns of bulk samples reveal that there is a phase transition from WC to W₂C with the increasing of temperature. Moreover, the effect of HPHT condition for sintering kinetics, microstructure evolutions, and mechanical properties of the sintered samples were also discussed.     

Microstructure and properties of copper composite containing in situ NbC reinforcement: Effects of milling speed

This paper presents a study on the effects of milling speed on the properties of in situ copper-based composite produced by mechanical alloying followed by cold pressing and sintering. A powdered mixture of copper, niobium and graphite with the composition of Cu–30%NbC was milled at various speeds (100, 200, 300 and 400 rpm). The NbC phase started to precipitate in the as-milled powder after 30 h milling at 400 rpm and the formation was completed after sintering at 950 °C. Enhancements of NbC phase formation with a reduction in Cu crystallite size were observed with the increase of milling speed. Density, hardness and electrical conductivity of the sintered composite were evaluated. An increase in milling speed resulted in an increase in sintered density and hardness but a reduction of electrical conductivity. The changes in the properties were correlated to the formation of NbC phase and refinement of copper and niobium carbide crystallite size since higher milling speed is associated with higher kinetic energy per hit.     

Stainless steel bonded NbC matrix cermets using a submicron NbC starting powder

The microstructure and mechanical properties of 316 L and 430 L stainless steel bonded NbC cermets were assessed. NbC starting powder mixtures with 15 and 30 vol% steel binder were pressureless vacuum sintered for 1 h at 1420 °C. The liquid forming temperature and shrinkage behaviour of the green powder compacts were investigated by differential scanning calorimetry and dilatometry. Microstructural and compositional analysis were conducted by electron probe microanalysis (EPMA) and XRD to investigate the effect of the steel binder on NbC grain growth and Cr-rich carbide precipitation. Rapid NbC grain growth was observed and the average NbC grain size decreased with increasing binder content. The residual Cr-rich carbide located at NbC grain boundaries can be eliminated by the addition of carbide forming metal precursors such as TiH₂ or by a thermal annealing process of the sintered NbC cermets at 1200 °C. The hardness and fracture toughness of the NbC-steel cermets was influenced by the steel binder type and content. A maximum hardness of 13.6 GPa was measured for the NbC-15 vol% 430 L cermet, combined with a modest fracture toughness of 7.3 MPa m^1/2.     

Microstructure and mechanical properties and of pulse plasma compacted WC - Co

Tungsten carbides-based inserts have been considered as one of the dominant hard materials in the cutting industry, receiving great interest for their excellent combination of mechanical properties. Pulse plasma compaction (PPC) process has been applied to a series of WC-Co samples with varying sintering temperature, initial particle size and sintering pressure in order to study the mechanical and microstructural behaviour. The quality of the products, as well as the mechanical properties and microstructural features this process yields, are commendable and worth looking into. A high hardness of more than 2000 HV has been achieved while a maximum fracture toughness of 15.3 MPa √ m was recorded in samples that were sintered at 1100 °C and 100 MPa. Microstructural features like grain growth and other properties are discussed with respect to the varying parameters. While grain size shows an incremental pattern with increasing temperature, it was still possible to limit them to a great extent ensuring high mechanical properties. The effect of sintering pressure in the range of 60–100 MPa, while keeping sintering temperature constant, was found to be almost negligible.