Sintering of a nano-crystalline tungsten heavy alloy powder

A nano-crystalline Tungsten heavy alloy powder was obtained by mechanical alloying of elemental powders in a jar mill with a high ball to powder ratio. The chemical composition of the primary powder was 93 W-4.9Ni-2.1Fe (wt%). The mechanically alloyed powder had 22 nm sized tungsten crystallites distributed in an amorphous nickel base phase. Mechanical alloying reduced particle size of powders and also yielded to more uniform particles size distribution. Sintering behavior and microstructural development of that powder were studied and compared with a conventionally mixed powder. Mechanically stored energy and better distribution of primary elements in Nano-crystalline powder had decreased motivation energy of sintering and that powders showed more densification at relatively lower sintering temperatures. Sintering at low temperatures can depress grain growth during sintering and provide desirable properties. A transient intermetallic phase was formed in the nano-crystalline powder during sintering that has not been seen in conventionally mixed powders.     

Si粉粒径及其添加量对SiC陶瓷材料结构和性能的影响

将平均粒径为75 μm和48 μm、质量分数为0%~8%的Si粉分别添加到SiC陶瓷材料中,在1550℃下保温3 h烧成,研究Si粉粒径及其添加量对SiC陶瓷材料烧结性能、力学性能和显微结构的影响。结果表明:添加不同粒径及质量分数的Si粉可改善SiC陶瓷材料的显微结构,提高其烧结性能和力学性能;在一定范围内,较小粒径的Si粉更有利于形成均匀、致密的SiC烧结体,大幅提升SiC陶瓷材料的性能;当Si粉粒径为48 μm且添加的质量分数为4%时,SiC陶瓷材料的烧结性能和力学性能较优,其体积密度和显气孔率分别为2.58 g/cm3和13.5%,抗弯强度和洛氏硬度分别为25 MPa和115 HRB。      

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Ni/ZrO_2复合材料力学及介电性能研究

采用热压烧结方法,以氧化钇部分稳定氧化锆(Y-PSZ)粉体及不同粒径的金属Ni为原料,制备了Ni/ZrO2复合材料,研究了其力学和介电性能,探讨了烧结过程中Ni形貌的变化对复合材料介电性能的影响.结果表明,随不同粒径Ni粉的掺入,复合材料的抗弯强度减小,且较大粒径的Ni使材料抗弯强度减小更快.随Ni含量增加,复合材料断裂韧性增强.复合材料的介电常数和损耗与Ni粉含量和粒径有关.Ni含量相同时,较大粒径Ni粉的掺入使复合材料具有较高的介电常数和损耗,这是由于在烧结过程中Ni粉形貌发生变化引起的.     

Rapid processing & characterization of micro-scale functionally graded porous materials

Spark plasma sintering (SPS) is a process that has stimulated worldwide interest for the rapid consolidation of powder-based materials through the combined effects of electric current and pressure. Recently the localization of SPS has been realized through current activated tip-based sintering (CATS) where electric current is selectively applied to small targeted regions of a green compact/powder bed via a precision controlled electrically conductive small tip. The unique tip-specimen geometry allows for locally controlled temperature and current distributions that can result in microstructural modifications on the micro-scale. The present paper presents for the first time the rapid processing and characterization of micro-scale functionally graded materials in relation to porosity content and size. The effects of initial green density and particle size on the developed micro-scale functionally graded material are discussed.     

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.     

Preparation of highly dispersed superfine W-20 wt% Cu composite powder with excellent sintering property by highly concentrated wet ball-milled process

The ball milling process and the CuWO_4-WO_3 precursors were investigated, and a new highly concentrated wet ball-milled process(HWM) was designed. W-20 wt% Cu composite powders with excellent sintering property were synthesized by highly concentrated wet ballmilled process and co-reduction. The powders were characterized by scanning electron microscopy(SEM), X-ray diffraction(XRD), field electron transmission electron microscopy(FESEM) and laser-diffraction diameter tester.The results indicate that particle size of W03-CuO powder mixtures decreases to 390 nm rapidly with the milling time increasing to 5 h. The CuWO_4 precursors promote the microstructural homogeneity of W and Cu. W-Cu composite powders have a highly dispersed and well sintering property. The particle size of W-Cu powders milled by HWM for 5 h is about 680 nm. High-resolution transmission electron microscopy(HRTEM) result suggests that W phase and Cu phase are mixed at nanometer scale. The above W-Cu composite powders reach the relative density of about 99.3%.     

Liquid phase sintering of functionally graded WC–Co composites

Functionally graded cemented tungsten carbide (WC–Co) is an example of functionally graded materials (FGM) in which mechanical properties are optimized by the presence of microstructural gradients such as cobalt gradient and grain size differences within the microstructure. In particular, a cobalt gradient is preferred. However, the manufacture of FGM WC–Co with a cobalt gradient is difficult because the flow of the liquid phase during liquid phase sintering (LPS) would eliminate any initial cobalt gradient built into the powder compacts. In this paper, different factors, which can be used to influence the migration of liquid during sintering, are investigated. These factors include gradients in grain size, carbon and cobalt content, and sintering time. It is shown that a difference in particle size may induce a step-wise profile of cobalt concentration. Initial carbon content differences, however, can be used to obtain a gradient of cobalt during sintering. The effects of these factors are explained based on the roles of capillary force and phase reactions.     

AlON粉体制备及透明陶瓷的烧结

γ-AlON是一种具有优良的光学、力学及化学性能的新型透明陶瓷,可望替代蓝宝石单晶用于军用及民用等多个领域.对AlON单相粉体的碳热还原氮化法制备工艺及块体的烧结工艺进行了研究,通过X射线衍射仪、扫描电镜、分光光度计等分析测试手段探索了工艺条件对相纯度、粉体大小及形貌等因素进而对组织及透光率的影响.结果发现,控制Al_2O_3和碳粉的相对百分比及反应时间即可制得纯相的AlON粉体;经球磨可进一步获得亚微米级的粉体,其颗粒大小及形貌可通过调整球磨时间加以控制;球磨后粉体经1950 ℃氮气氛无压烧结8 h后,可制得光学性质均一的透明陶瓷.其中较细且更为等轴化的粉体烧结后具有均匀、低孔隙率的组织,同时也具有更高的透过率.     

Preparation and Properties of W-15Cu Composite by Electroless Plating and Powder Metallurgy

用化学镀和粉末冶金的方法制备出W-15Cu复合材料。首先用化学方法对W粉表面预处理,然后在其表面化学镀铜。得到的复合粉末用图谱进行表征。发现用化学镀的方法制备出的W-15Cu复合粉末纯度非常高,且W颗粒均匀、被Cu致密的包覆着。呈现出包状结构。这种复合粉末表现出优异的压制性能,压坯分别在300, 400, 500, 600 MPa的压制压力下成形。压坯在1250 ℃温度下保温90 min烧结后,从其断口形貌可以发现W颗粒没有明显的长大,且W颗粒表面特征并没有发生改变,仍然表现出预处理后的表面特征。对烧结体的相对密度、硬度、抗弯强度和电导率同样进行了表征     

SnO_2颗粒大小对Ag/SnO_2(10)材料组织与性能的影响

采用机械混粉、冷等静压成形、烧结、热挤压、轧制、拉拔、中间热处理等工艺集成的粉末冶金方法制备了3种含有不同平均粒径的SnO_2的Ag/SnO_2(10),探讨了SnO_2颗粒大小对Ag/SnO2(10)显微组织、密度、力学性能和电学性能的影响。结果表明,SnO_2粉末越细,表面活性越高,越容易团聚。脆性相SnO_2在加工过程中有破碎现象。随着SnO_2颗粒尺寸的减少,材料的密度、抗拉强度、硬度和电阻率逐渐上升。     

Simulation study of effects of initial particle size distribution on dissolution

Dissolution kinetics of γ′ particles in binary Ni–Al alloys with different initial particle size distributions (PSD) is studied using a three-dimensional (3D) quantitative phase field model. By linking model inputs directly to thermodynamic and atomic mobility databases, microstructural evolution during dissolution is simulated in real time and length scales. The model is first validated against analytical solution for dissolution of a single γ′ particle in 1D and numerical solution in 3D before it is applied to investigate the effects of initial PSD on dissolution kinetics. Four different types of PSD, uniform, normal, log-normal and bimodal, are considered. The simulation results show that the volume fraction of γ′ particles decreases exponentially with time, while the temporal evolution of average particle size depends strongly on the initial PSD.     

烧结作用对CVD法超细铼粉性能的影响

研究了一种化学气相沉积(CVD)制备超细铼粉的新方法,即以NH4ReO7为原料,通过将其分解为Re2O7后气相输运至还原区,经氢气还原生成超细铼粉.对不同还原温度下制备的超细铼粉样品,采用XRD、SEM、激光粒度分析进行表征,实验揭示了烧结作用对晶粒尺寸、形貌、表面状态及粒度等粉末性能的影响规律.结果表明,随还原温度升高,烧结作用增强,制备的超细铼粉晶粒尺寸增大,具有更好的球形度,表面趋于光洁,平均粒径增大.     

The Effect of Sintering on the Properties of Ultrafine

研究了一种化学气相沉积(CVD)制备超细铼粉的新方法,即以NH4ReO7为原料,通过将其分解为Re2O7后气相输运至还原区,经氢气还原生成超细铼粉。对不同还原温度下制备的超细铼粉样品,采用XRD、SEM、激光粒度分析进行表征,实验揭示了烧结作用对晶粒尺寸、形貌、表面状态及粒度等粉末性能的影响规律。结果表明,随还原温度升高,烧结作用增强,制备的超细铼粉晶粒尺寸增大,具有更好的球形度,表面趋于光洁,平均粒径增大。     

Microstructures and properties of Cu/Ag(Invar) composites fabricated by powder metallurgy

The Ag (Invar) composite powder prepared by ball milling was used to fabricate the Cu/Ag (Invar) composites. Microstructures and properties of the composites were studied after sintering and thermo-mechanical treatment. The results indicate that during ball milling, micro-forging weld and work-hardening fracture result in that the average particle size of the Ag (Invar) powder increases rapidly at first, and then decreases sharply, finally tends to be constant. Compared with the Cu/Invar ones, the sinterability of the composites is greatly improved, resulting in that the pores in them are smaller in amount and size. After the thermo-mechanical treatment, the Cu/Ag (Invar) composites are nearly fully dense with the optimum phase composition and element distribution. More importantly, Cu and the Invar alloy in the composites distribute continuously in a three-dimensional (3D) network structure. Cu/Invar interface diffusion is effectively inhibited by the Ag barrier layer, leading to a great improvement of the mechanical and thermal properties of the Cu/Ag (Invar) composites.     

Effect of Particle Morphology and Size Distribution on Cold-Sprayed Pure Titanium Coatings

The effects of commercially pure titanium particle morphology (spherical, sponge, and irregular) and size distributions (mean particle sizes of 20-49 μm) on the cold spray process and resulting coating properties were investigated. Numerous powder and coating characterizations were performed including: powder oxygen and nitrogen contents, powder flowability, powder compressibility, coating microhardness, coating porosity, LOM/SEM analyses, and XRD. Compared to spherical powders, the sponge and irregular CP-Ti powders had higher oxygen content, poorer powder flowability, higher compression ratio, lower powder packing factor, and higher average particle impact velocities. XRD results showed no new phases present when comparing the various feedstock powders to corresponding coatings. A higher particle temperature was also obtained with larger particle size for all feedstock powder morphologies processed with the same set of spray parameters. A spherical powder with 29 μm mean particle size was found to have the lowest porosity coating and best cold sprayability. The relationships of several as-cold sprayed coating characteristics to the ratio of particle impact and critical velocities were also discussed.     

Numerical simulation of powder effect on solidification in directed energy deposition additive manufacturing

An integrated simulation of powder effects on particle temperature and microstructural evolution in laser directed energy deposition additive manufacturing process was carried out. The spatial distribution of the flying powder particles was simulated by the discrete element method to calculate the energy for the flying powder particles under the laser–particle interaction with electromagnetic wave analysis. Combined with the phase field method, the influence of particle size on the microstructural evolution was studied. The microstructural evolution is validated through comparison with experimental observation. Results indicate that the narrow particle size distribution is beneficial to obtaining a more uniform temperature distribution on the deposited layers and forming smaller equiaxed grains near the side surfaces of the sample. Appropriate powder particle size is beneficial to the conversion of the electromagnetic energy into heat. Particles with small size are recommended to form equiaxed grains and to improve product quality. Appropriate powder flow rate improves the laser energy efficiency, and higher powder flow rate leads to more uniform equiaxed grains on both sides of the cross-section.     

YBCO超导体先驱粉的制备新工艺研究

研究了一种高温超导氧化物YBCO先驱粉末BaCuO2(011粉末)的制备新工艺,在烧结过程中把烧结和研磨工艺结合应用,对不同烧结时间的粉末进行了XRD和DTA分析及SEM观测。结果表明,这种新的粉末烧结工艺制备BaCuO2先驱粉只需18h就可以保证BaCO3分解得比较完全,制得的(011)粉末粒度更细,熔点有所降低。因此这种新的BaCuO2先驱粉制备工艺在制备YBCO超导体工艺中具有实用价值。     

Microstructure dependent erosion in Cr3C2-20(NiCr) coating deposited by a detonation gun

Cr₃C₂-20(NiCr) coating powder was deposited on a low carbon steel by pulsed detonation spray gun technique. The coated samples were heat-treated at 600 °C for 11/2 h and allowed to cool in air. A systematic microstructural study was carried out using SEM and TEM to understand the microstructural changes. The mechanical properties like hardness, indentation fracture toughness and adhesion strength of the coating in the as-sprayed and heat-treated conditions were also determined. The change in solid particle erosion of the coating was correlated with the microstructural and subsequent mechanical property changes. It was observed that exposure of the as-sprayed coating to elevated temperature improves the wear resistance. It is concluded that crystallisation of the amorphous phase into nanocrystalline composite combined with better bonding between the adjacent splats through sintering contributes to improved hardness, fracture toughness and wear resistance of the heat-treated coating. The deformation characteristics of the binder phase, amorphous vis-à-vis crystalline, also influence the wear behaviour of the coating.     

高密度纯钨的低温活化烧结工艺及其致密化行为

采用溶胶喷雾干燥-煅烧-氢热还原法制备了BET粒径为0.21μm的超细纯钨粉末,并利用球磨处理进一步活化粉末。研究了超细纯钨粉末形貌及其性能随球磨时间的变化特征,探索了未球磨、球磨5h及球磨10h3种超细纯钨粉末烧结致密工艺,此外还详细研究了纯钨烧结体组织形貌、晶粒尺寸及显微硬度等性能随烧结温度及球磨时间的变化规律。结果表明,球磨处理对超细纯钨粉末的烧结起到了极大的活化作用,由球磨10h粉末制成的压块在1900℃下烧结2h其致密度即可达97.3%,比传统微米级纯钨粉末制成的压块达到相同烧结致密度的温度降低了600℃以上。同时,球磨处理可以大幅降低钨粉的起始烧结温度和再结晶温度,获得组织更加均匀细小、力学性能(硬度)更加优良的钨烧结体。