The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.     

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.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

High performance tungsten synthesized by microwave sintering method

The W–Ni alloys with varying amount of Ni (0.1, 0.25, 0.5 and 1.0 wt.%) were microwave sintered at 1450 °C for different holding time 5, 15 and 30 min, and their microstructure, grain size, relative density, thermal conductivity, and Vickers microhardness were characterized. Comparing to the addition of Fe and Cr, the Ni addition can greatly improve the relative density and maintain the high thermal conductivity of W at the same time. It was shown that the addition of 1.0 wt.% Ni into W and microwave sintering at 1450 °C for 5 min would be the best conditions to obtain W–Ni alloys with a relative density close to 100% and an average grain size as small as 15 μm. The Vickers microhardness and thermal conductivity of the sintered W–Ni samples range from 370 to 440 and from 90 to 130 W/m K, respectively.     

Effect of heating mode on sintering of tungsten

Microwave heating is recognized for its various advantages, such as time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved properties. The present paper investigates the feasibility of consolidating tungsten powders through microwave sintering. A comparative analysis has also been attempted between the sintering response of pure tungsten powder compact in a microwave and conventional furnace.     

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.     

Hot pressing of tungsten carbide with and without sintering additives

This study presents the results of investigations concerning hot-pressing of submicron WC powders without sintering additives and with the addition of carbon or tungsten or both elements simultaneously. Dense polycrystals of diverse microstructures and phase compositions were obtained. Attempts were made to correlate a microstructure and phase composition of sinters with their thermal and mechanical properties. It was found that the presence of graphite nanolayers on grain boundaries in WC sinters with the addition of carbon favourably influences their thermal conductivity. All produced polycrystals are characterised by a high fracture toughness. The smallest scatter of K_Ic results is observed for compositions activated by a carbon addition. The presence of the graphite nanolayers as well as grain size in WC sinters with carbon additions reduce the polycrystal hardness. All WC polycrystals, regardless of introduced additives, are characterised by high bending strength and by high values of the Young and Kirchoff moduli. The tested polycrystals are not suitable for machining carbon steel of C45 grade.     

Thin intergranular films and solid-state activated sintering in nickel-doped tungsten

Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400 °C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was presumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed equilibrium-thickness intergranular films in ceramics. Assuming they form at a true thermodynamic equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering.     

Effects of low-content activators on low-temperature sintering of tungsten

Tungsten compacts are processed by the activated sintering technique at low temperatures near 1200 °C, whereby either nickel, iron or a combination of the two are employed as additives. Despite very small contents of additives of a few monolayers and below, the processing technique is found viable for providing enhancing effects to the sintering kinetics of tungsten, and the sintered materials remain in the partially densified stage. Nickel is proved to be a superior activator, which induces a reduction of the activation energy as its content increases.     

Consolidation and properties of binderless sub-micron tungsten carbide by field-activated sintering

The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m^1/2 and 2480 kg/mm², respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.     

The modeling of electric-current-assisted sintering to produce bulk nanocrystalline tungsten

Nanocrystalline tungsten has the potential to have superior strength and hardness properties versus conventional tungsten. While tungsten nanopowders are becoming more commonly available, processing through conventional press and sinter techniques induce unacceptable grain growth. As an alternative, electric-current-assisted sintering (commercially known as SPS or PPC) allows extremely high heating rates (>1,000°C/min.) to be achieved which accelerates the consolidation process and preserves the nanocrystalline structure of the material. The high heating rates can, however, lead to non-uniform density and compromised properties. This work employs numerical simulations of the process as a means to understand and reduce these gradients and optimize the process.     

TiC颗粒增强钨基复合材料的烧蚀性能

用自制的氧乙炔烧蚀装置对ＴｉＣ颗粒增强钨基复合材料的烧蚀性能进行了测试,同时用多波长比色高温计对烧蚀试样表面温度和用热电偶对试样背面温度进行了在线监测。复合材料的质量烧蚀率和和线烧蚀率由低到高的排列顺序为：Ｗ〈３０％ＴｉＣｐ／Ｗ〈４０％ＴｉＣｐ＝Ｗ。ＴｉＣ颗粒加入到Ｗ中可明显提高材料的抗烧蚀性能,而且ＴｉＣ颗粒含量越高,材料的抗烧性能越好。ＴｉＣｐ／Ｗ复合材料的烧机理是Ｗ和ＴｉＣ的氧化烧和燃气流的     

超细钨粉的粒度表征

对供给态和研磨态超细钨粉的颗粒粒度进行了表征。将供给态粉和研磨态粉由激光衍射法、FSSS法和BET法测量出颗粒粒度。结果表明 :利用激光衍射法和FSSS法所测量出来的供给态粉和研磨态粉的粒度结果是错误的 ,这是因为测量系统的缺陷和测量原理的不合适导致的。可以使用吸附等温线来获得表面粗糙度的分数维维数D和微孔的表面积St,且用D和St 来修正dBET的计算公式。使用修正公式得到的 4种粉末的平均粒度值与扫描电镜的观测值相一致     

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.     

Effects of tungsten particle shape and size on dynamic deformation and fracture behavior of tungsten heavy alloys

Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The double-sintered tungsten alloy specimen whose tungsten particles were very coarse and irregularly shaped showed cleavage fracture in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and thein situ fracture test results,i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the serf-sharpening effect and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

Anisotropic shrinkage induced by particle rearrangement in sintering

A microscopic model is presented which describes the anisotropic shrinkage induced by particle rearrangement during sintering. Two types of topological transitions in the rearrangement, that is the formation of a new contact, are illustrated in three-dimensional simulation of sintering of four spheres arranged in a rhombus. The shrinkage rate is shown to become anisotropic as the particle coordination number changes with rearrangement. A micromechanical principle is discussed with the use of sintering force and effective viscosity. The anisotropic shrinkage is caused by the additional sintering force acting on the new grain boundary formed at the rearrangement.     

强化石灰烧结法中硅的行为研究

针对我国铝土矿资源特点和我国烧结法生产氧化铝的现状,对强化石灰烧结法中硅的行为进行研究。采用铝硅比为3.84的矿石,按照钙铝比为1.3~1.5进行配料,在1270℃以上进行熟料烧成。实验结果表明:熟料烧成无困难,熟料中氧化铝含量达到了48%~53%,熟料中主要物相为CaO.Al2O3和2CaO.Al2O3.SiO2,烧结熟料溶出性能好;烧结熟料溶出后粗液中硅含量仅为20mg/L~30mg/L,硅量指数可达1000以上,可省去专门的脱硅工序;提出用"纯氧化铝溶出率"作为烧结法的经济指标,强化石灰烧结法用A/S比为3.84的矿物达到了强化碱石灰烧结法A/S7的指标,且碱耗更低。     

Enhancement of densification and sintering behavior of tungsten material via nano modification and magnetic mixing processed under spark plasma sintering

In the present study, the influence of nano additives (Ni, Fe) and different mixing (turbular and magnetic) on the densification, microstructure and micro-hardness of the tungsten material under spark plasma sintering is analyzed. After turbulent mixing the nanoparticles are distributed widely in the W interparticle gaps but after magnetic mixing the nanoparticles are distributed not only on the gaps of the W particles but also on the broken surfaces. Ni incorporated tungsten materials achieved the maximum density of 98.3% at 1400 °C (turbular mixing) and 97.9% at 1300 °C (magnetic mixing). Fe incorporated tungsten material showed density of 97.7% at 1600 °C and 97.2% at 1400 °C after turbular and magnetic mixing. The influence of nanoparticles in the densification process was explained by Laplace force, boundary slip and Agte-Vacek effect. The microstructural analysis showed that nano-modification reduced the degree of porosity, and provides a compact material at low temperatures. X-ray fluorescence analysis reveals that magnetic mixing shows more uniform distribution of nanoparticles than turbular mixing. The nanoparticles incorporation increased the micro hardness of tungsten material. Hence, it is clear that magnetic mixing and nano modification greatly improved the densification and sintering behavior of the tungsten material.     

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y₂O₃ dispersed tungsten alloy were investigated as functions of the Y₂O₃ content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y₂O₃ content increases, the sintered density increases at a given temperature. The transition temperature (T_tr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y₂O₃ particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.     

Fabrication of sintered tungsten by spark plasma sintering and investigation of thermal stability

Tungsten has been considered as the most promising candidate for plasma-facing materials (PFMs) in a next generation fusion reactor. It is well known that commercialized ITER (International Thermonuclear Experimental Reactor) grade tungsten is manufactured by the mechanical processing at high temperature after sintering to ensure a high density with an improved structural stability. In this study, in order to obtain the high-density sintered tungsten with more enhanced structural stability, spark plasma sintering (SPS) method was employed, where a pulsed direct electric current was applied during heat treatment of powders with a pressure in the specimen. It is well known that by utilizing SPS, high-density sintered materials at a relatively lower temperature for a shorter time could be achieved compared to the other conventional sintering methods. In particular, in this study, reduction in H₂ atmosphere and two-step sintering were introduced to remove the residual oxygen and achieve the full densification with suppressed grain growth at relatively low operating temperature. In an optimized condition, a fully densified sintered tungsten with a relative density of 99.9% and an average grain size of 4.4 μm was fabricated. The thermal stability of tungsten specimens was evaluated by high heat flux (HHF) test, where the surface temperature was set up to 2300 °C by nitrogen plasma. Then, the microstructural changes of the specimen surface have been examined after the HHF test. As a result, it was confirmed that the high-density sintered tungsten samples fabricated by SPS show an excellent microstructural stability for PFMs.