Effect of liquid content on distortion and rearrangement densification of liquid-phase-sintered W-Cu

Tungsten and copper exhibit negligible solubility for each other, so densification during liquid phase sintering of W-Cu is limited to rearrangement of the W particles and solid-state sintering of the W skeleton. Experiments are conducted to evaluate the effects of Cu volume fraction on liquid phase sintering of W-Cu. Sintered microstructures are quantitatively analyzed and are used to define critical microstructural parameters that prevent distortion and rearrangement densification. Slumping is prevented first by capillary forces, then by formation of a rigid W skeleton at critical values of contiguity and connectivity, which depend on the dihedral angle. A refined expression for the dependence of contiguity on volume fraction that includes the effect of the dihedral angle is developed. An analysis of gravitational, capillary, and bonding forces acting on W particles in liquid Cu explains the ability to achieve high sintered densities through rearrangement despite a lack of distortion with up to 80 vol pct liquid. Capillary forces are sufficient to break weak solid-solid bonds that form during heating, enabling rearrangement to occur even without dissolution of these bonds. At higher solid volume fractions, sufficient particle contacts form to prevent rearrangement by these capillary forces, thus limiting sintered densities.     

多孔W骨架用特殊粉末的制备

采用多孔W颗粒粉末制备的高孔隙度钨骨架中可填充高体积分数的铜,从而在保证钨铜复合材料具有低膨胀系数的基础上进一步提高其热导率.该文作者在以WO3为原料配制的料浆中加入造孔剂碳酸铵((NH4)2CO3)和表面活性剂聚乙二醇(PEG),采用喷雾干燥一还原法制备多孔W颗粒粉末,并研究(Nag)2CO3、PEG的加入量以及WO3含量对W颗粒形貌的影响.使用pH计测量pH值和旋转粘度计测定料桨粘度,利用扫描电镜(SEM)和X射线衍射仪(XRD)分别对粉末的形貌和物相组成进行观察与分析.结果表明:料浆中(NH4)2CO3、PEG和WO3的质量分数分别为10%、4%和40%时,得到的多孔W颗粒孔隙度较高;前驱体为球形的WO3粉末时则可在820℃的氧气气氛中还原90min得到纯W相.     

The effects of ultrasonic vibration on mechanical properties of tungsten particle-reinforced copper-matrix composites

W-Cu micro-powder mixtures usually have poor sinterability due to the relatively low solubility of W in both solid and liquid Cu. In fabricating W-Cu composites, an electroless copper plating process is often used to coat Cu on the W particle surface prior to the sintering process. Due to their small size W particles tend to agglomerate during the plating process, hence the individual particle may not be properly coated with Cu. In this study, ultrasonic vibration is applied in the electroless plating process to break up the agglomerations and restrain the powders from gathering, ensuring a uniform deposition of the Cu on individual W particle. W-Cu composite samples containing pure Cu and 6, 9 and 12 wt-% of Cu-coated W particles, respectively, are fabricated using a standard powder metallurgy technique. It is shown that the application of ultrasonic vibration in the activation and deposition steps of the electroless copper plating process prevents W powder agglomeration and ensures that each W particle is coated with Cu. As a result, the mechanical properties of the W-Cu composites are significantly improved. It is found that the optimal tensile strength and yield strength are obtained using a W reinforcement phase content of 9 wt-%.     

熔渗用多孔钼骨架制备工艺研究

采用2种不同粒度的钼粉,在其中添加不同比例的添加剂,经压制烧结,制备了孔隙率在10%～50%的熔渗钼铜合金用多孔钼骨架。通过SEM对多孔钼骨架的微观形貌进行了观察,并研究了制备及烧结过程中粉末粒度、添加剂含量、压坯致密度及烧结温度对钼骨架孔致密化及孔隙形貌的影响。     

Effects of tungsten particle size and copper content on densification of liquid-phase-sintered W-Cu

Tungsten and copper exhibit negligible solubility for each other, so densification during liquid-phase sintering of W-Cu is limited to rearrangement of the W particles and solid-state sintering of the W skeleton. Experiments are conducted to evaluate the effects of Cu volume fraction and particle size on these densification mechanisms. Dilatometry shows that particle rearrangement is limited for low Cu volume fractions and large W particle sizes. After an initial rearrangement stage, densification rates at low Cu volume fractions decrease to those of solid-state sintering of W. Higher Cu volume fractions enable high sintered densities through rearrangement. The dominance of rearrangement or solid-state sintering is determined by the balance of capillary and bonding forces, which depends on the particle size and the ratio of the neck diameter to the particle diameter.     

Cold Compaction of Copper Powders Under Mechanical Vibration and Uniaxial Compression

Physical experiments were carried out to study the cold compaction of copper powders under uniaxial compression using our self-designed equipment. Two kinds of copper powders with different particle sizes and distributions were considered. One-dimensional vibrations were utilized before compaction to systematically study the effect of parameters such as vibration frequency ω, amplitude A, and vibration intensity Г on the initial packing density. The macro-property and corresponding microstructures of compacts obtained from initial packings with and without vibrations were compared and analyzed. The results show that higher packing density can be obtained in the compaction of coarse powders with broad size distribution when other experimental conditions are fixed. For each powder, the evolution of packing density vs pressure takes on exponential correlation with high R ² value. Much denser and more uniform compacts can be realized with the aid of vibration which can improve the particle rearrangement and result in the filling of macro pores formed in initial packing, and the characterization on the microstructure identifies that the particles inside the compact become polyhedrons with regular shape and uniformly distributed.     

Solid-phase sintering of ultrafine W(Mo)-Cu composite powders

Conclusions The solid-phase sintering of W-Cu composites can be markedly intensified by using ultrafine starting powder charges, the degree of intensification being a maximum with charges produced by reduction of oxide mixtures subjected to prior annealing in air. In this way it is possible to obtain virtually nonporous W-Cu composites with copper contents of 10–35%. The composition dependence of shrinkage in the solid-phase sintering of ultrafine W(Mo)-Cu powder composites is nonmonotonic in character. An anomaly is observed which would appear to be linked with the phenomenon of zonal isolation.Translated from Poroshkovaya Metallurgiya, No. 1(253), pp. 19–25, January, 1984.     

用机械活化与化学活化方法制备 W-Cu 合金

为了改进制备工艺和提高W－Cu合金的性能,对原材料粉末做了机械活化和化学活化处理,通过成形和烧结制备了W－Cu合金,考察了经机械活化和化学活化后粉末的变化,观察了烧结合金的组织、测试了合金的密度等性能。结果表明,机械活化可以使粉末颗粒变细,至亚微米乃至纳米级、比表面增大、缺陷增多,并使铜在钨中具有一定溶解度。化学活化可以在粉末颗粒表面形成微量合金元素的较均匀分布,并通过反应形成高活性层。二者都能使粉末的活性提高,对经活化处理的粉末施以烧结可以获得较高密度和性能的W－Cu合金。     

成形压力与粉末粒径对钨铜复合材料烧结性能的影响

为进一步提高钨铜合金的致密度和简化制备工艺,研究了粉末粒度与成形压力对无压烧结制备的W-15Cu复合材料致密度的影响.发现随着球磨时间延长,钨铜粉末发生明显的细化和圆化,粉末分布更为均匀,烧结活性有较大提高,合金性能更加优异,组织结构更加良好,致密度相应提高.通过对烧结试样密度和铜含量的测定,得到不同成形压力下材料致密度和铜含量随烧结温度的变化曲线,发现随着成形压力增大,材料的烧结致密度升高,铜流失的现象得到一定的控制.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

Effect of cobalt addition on the liquid-phase sintering of W-Cu prepared by the fluidized bed reduction method

A new process, fluidized bed reduction (FBR) method, was applied for fabrication of uniform W-Cu sintered material. Liquid-phase sintering was carried out to obtain fully densified W-Cu composite, and the effect of cobalt addition on the sintering behavior was investigated. It was found that fully densified material could not be obtained even after sintering at 1200 °C for 4 hours in the case of 75W-25Cu, while more than 96 pct density could be obtained as soon as the sintering temperature reached 1200 °C when 0.5 wt pct cobalt was added prior to the sintering. It has been found that the wetting angle of the liquid copper is reduced significantly by the addition of cobalt, and the formation reaction of Co₇W₆ intermetallic compound at the surface of the tungsten powder is mainly responsible for the enhancement of the densification process.     

液相烧结制备电力开关用W-7Cu-xNi合金微观组织及性能分析

为了提高电力开关用W-7Cu合金的综合性能,通过添加Ni元素方式对其进行加强,以液相烧结、机械球磨方式制备得到包含不同Ni含量的W-Cu合金。通过实验测试手段对其微观组织及物理性能进行了施压测试分析。研究结果表明：逐渐提高Ni含量后,形成了更大尺寸W颗粒,相邻颗粒间距降低。W-Cu-4%Ni合金界面形成更优润湿角,获得具有连续网状分布的Cu相,改善了W-Cu组织的分布均匀性,此时Ni元素已经完全与Cu相相溶。当Ni含量提高后,W-Cu合金获得了更大的硬度与致密度,热导率下降,相对密度增加。当加入4%的Ni时,致密度达到了95.6%,热导率从161 W/(m·K)降低为96.4 W/(m·K),获得了致密度更大合金。     

轧膜成形制备W-Cu层状功能梯度材料及其性能研究

以W粉和电解Cu粉为原料,聚乙烯醇缩丁醛(PVB)为粘结剂,通过有机基轧膜工艺制备出3种组成的单层生坯(Cu质量分数分别为25%、50%、75%),再叠层共轧,制备出了具有不同粘结剂含量的W-Cu层状梯度材料生坯,之后在H2气氛中烧结,获得了W-Cu层状梯度材料,考察了粘结剂含量与制备工艺条件对材料显微组织和性能的影响。结果表明,通过单层轧制、叠层共轧共烧可以制得层状梯度W-Cu复合材料;粘结剂含量对W-Cu层状梯度材料的致密度和性能有着明显的影响。当粘结剂质量分数为6%时,轧膜坯有较好的成形性,且成形坯的孔隙率较低;所得多层生坯经1 150℃烧结后相对密度达93.11%;所得梯度W-Cu材料有良好的物理、力学性能。     

W-Cu材料室温强度和组织均匀性的影响因素

用浸透法对孔隙度为12%～17%的烧结钨进行了渗铜试验 ,研究了影响低铜含量的W -Cu材料室温бb及渗铜组织均匀性的因素 ,研究结果表明 ,铜对钨骨架的强化和韧化机制对提高W -Cu材料的室温бb 有重要作用 ;钨骨架具有合适的孔隙度是W -Cu材料获取均匀一致的渗铜组织的重要条件。     

铜粉对MIM钨铜合金组织和性能的影响

采用金属注射成形（MIM）技术制备了钨铜合金,定量表征了铜粉的粉末粒度和粒形,重点研究了铜粉粒度和粒形对MIM钨铜合金组织与性能的影响。通过对比铜粉的粒径、粒度分布宽度、长宽比、粗糙度、赘生物指数和钝度等特征参数,破碎铜粉与水雾化铜粉颗粒呈枝晶状,粒径远小于还原铜粉,但破碎铜粉粒度分布宽,微观结构上的规则度、表面光滑程度以及分散程度最佳。破碎铜粉混合钨粉为原料,通过MIM技术制备钨铜注射生坯致密度高、缺陷少,烧结后钨铜合金的组织与性能最优,致密度为96.2%,硬度为235HV,抗弯强度为1 200 MPa,热导率为128 W/(m·K),电导率为30%IACS。     

Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part II: Influence of Processing Conditions on Densification and Properties

In this study, nanostructured Al 5083 powders, which were prepared via cryomilling, were consolidated using spark plasma sintering (SPS). The influence of processing conditions, e.g., the loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties were studied. Additionally, the mechanisms that govern densification during SPS were discussed also. The results reported herein suggest that the morphology and microstructure of the cryomilled powder resulted in an enhanced densification rate compared with that of atomized powder. The pressure-loading mode had a significant effect on the mechanical properties of the samples consolidated by SPS. The consolidated compact revealed differences in mechanical response when tested along the SPS loading axis and radial directions. Higher sintering pressures improved both the strength and ductility of the samples. The influence of grain size on diffusion was considered on the basis of available diffusion equations, and the results show that densification was attributed primarily to a plastic flow mechanism during the loading pressure period. Once the final pressure was applied, power law creep became the dominant densification mechanism. Higher sintering temperature improved the ductility of the consolidated compact at the expense of strength, whereas samples sintered at lower temperature exhibited brittle behavior. Finally, densification rate was found to be inversely proportional to the particle size.     

An examination of the interparticle contact area during sintering of W-0.3 Wt Pct Co

As a powder compact sinters, its microstructure evolves. One way to quantify the scale of the microstructure is to consider the interparticle contact area. This study examines two known models for calculating the interparticle contact area: the classic two-sphere model and the Voronoi cell model. Both models have particular assumptions about the microstructure that make them not applicable for treating densification to near full density with concurrent grain growth. The classic two-sphere model assumes a regular packing of particles and a perfectly spherical particle geometry and neglects an increasing particle coordination number with sintering. The Voronoi cell model assumes that the scale of the microstructure remains constant; i.e., as long as the compact is densifying, grain growth does not occur. We propose a modified Voronoi cell that accounts for an increasing grain size, making it applicable to a general case where grain growth occurs during sintering. The three models are compared to the interparticle contact area data, obtained by stereology techniques, for W-0.3 wt pct Co sintered from green state to near full density. The original Voronoi cell model fits the data only at low temperatures, before the onset of grain growth. Below approximately 90 pct relative density, the two-sphere model with an assumed coordination number of six (coordination number in a green compact) and the modified Voronoi cell model provide a good fit to the data. At higher densities, both models overestimate the interparticle contact area.     

Microstructural change during liquid phase sintering of W−Ni−Fe alloy

The changes of bulk density and microstructures during heating and liquid phase sintering of 98W-1Ni-1Fe compacts prepared from 1 and 5 μm W powders have been observed in order to characterize the densification behavior. The compact prepared from a fine (1 μm) W powder begins to densify rapidly at about 1200°C in the solid state during heating, attaining about 95 pct density upon reaching the liquid phase sintering temperature of 1460°C. The compact prepared from a coarse (5 μm) W powder begins to densify rapidly at about 1400°C in the solid state, attaining about 87 pct density upon reaching the liquid phase sintering temperature. Thus, the skeleton of grains is already formed prior to liquid formation. During the isothermal liquid phase sintering, substantial grain growth occurs, and the liquid flows into both open and closed pores, filling them sequentially from the regions with small cross-sections. The grains subsequently grow, into, the liquid pockets which have been formed at the pore sites. The sequential pore filling by first liquid thus is shown to be the dominant densification process during the liquid phase sintering of this alloy, as has been demonstrated earlier with spherical model pores and as predicted theoretically.     

钨粉粒径对W-15Cu导热性能的影响

采用不同粒径的钨粉，用熔渗法制取钨铜两相假合金，分别测量其热导率，并在扫描电镜下观察熔渗后铜网络的分布，分析了钨粉粒径的大小对W-15Cu烧结行为的影响及其导热性能的影响。     

Effects of nickel on the sintering behavior of Fe-Ni compacts made from composite and elemental powders

Injection-molded Fe-Ni parts made from composite and elemental powders were prepared, and the effect of nickel on the sintering of iron compacts was investigated. Dilatometry analyses showed that the alpha-gamma phase transformation temperature of the Fe-Ni compact changed from a fixed 912 °C for pure iron to a temperature range between 700 °C and 912 °C where two phases coexisted. The microstructure indicated that nickel impeded surface diffusion and slowed down the neck growth rate of iron powder in the early sintering stage. The dual phase and the small neck size at low temperatures suppressed the exaggerated grain growth, which usually occurs on carbonyl iron powders at 912 °C. It was also observed that nickel impeded the grain growth of iron at high temperatures. Thus, by reducing the exaggerated grain growth during phase transformation, impeding the grain growth at high temperatures, and with high diffusion rates of iron in Ni-rich areas, enhanced densification was obtained for Fe-Ni systems, particularly for those systems made from composite powders. However, when coarse nickel powder was added, expansion was observed due to the presence of large pores around nickel powders. These pores were formed because of the particle rearrangement which was caused by the Kirkendall effect.