本刊第5卷1—4期总目录

研究与试验粉末多孔体的塑性泊松比与致密化方程部分扩散铁基合金粉末的压缩性及烧结 均匀化粉末冶金高强度烧结钢为疲劳特性Fe一Ti合金的液相烧结机理及性能的研究黄培云双对数粉体压制方程的应用热喷涂镍铝放热反应钡钨阴极钨粉某些物理特性的研究稀土氧化物对钻的马氏体相变及机械性能 的影响 工艺与设备高压水雾化生产低含氧量金属粉末装置 及其工艺特征硬质合金姻旋立铣刀刀片的热挤压成形粉末热锻模具设计以机械合金化新工艺制造弥散强化铝一镁 合金采用湿磨工艺提高铁基摩擦材料的性能表面滚压强化对粉末冶金烧结钢疲劳强 度的影响 …     

喷射沉积AlFeVSi合金模压致密化工艺与模锻制品组织性能研究

采用喷射沉积工艺制备了快速凝固AlFeVSi合金圆锭坯.从锭坯上截取试样,采用模压工艺进行了高温(450和500℃)致密化.通过金相显微组织观察、透射电镜分析、扫描电镜分析、力学性能检测等手段,研究了在高温模压过程中的应力应变状态和金属流变规律对喷射沉积AlFeVSi合金坯的致密化过程与组织性能的影响,并采用铝包套高温(450℃)模锻工艺经将喷射沉积Al-9.20Fe-1.37V-2.30Si合金压实坯加工成完全致密化的模锻件.结果表明,高温模压可以明显提高沉积坯的致密度,但在有限的高向加载应力条件下难以使沉积坯达到理想的结合状态,与喷射沉积坯相比,虽然模压制品强度和塑性有明显提高,但仍处于低水平.喷射沉积坯经热压后,采用铝包套模锻成形,可以实现锻坯有效致密化和改善粉末间结合状态,明显提高材料屈服强度、抗拉强度和延伸率.喷射沉积Al-9.20Fe-1.37V-2.30Si合金直径=200 mm铝包套模锻件拉伸力学性能(σ0.2、σb和δ)可达如下指标:室温(25℃),411MPa、463MPa、9.7%,高温(350℃),180MPa、190MPa、8.9%.     

挤压表面致密化和滚压表面致密化工艺对比

选择表面致密化工艺主要包括挤压致密化和滚压致密化,本文基于表面致密化零件的生产,从零件性能检测方面对比分析挤压致密化和滚压致密化工艺的差异性。试验结果表明：两种工艺制备出的零件在齿廓处均能形成一层均匀的表面致密层,零件的表面强度和表面硬度均得到较大提高;挤压致密化工艺会导致烧结件端面翻边,容易造成后续烧结件外观质量问题,而滚压致密化工艺则不会产生类似外观质量问题;挤压致密化工艺中零件挤压进给量取决于模具尺寸,挤压进给量无法随意调整,并且随着模具径向尺寸的磨损,挤压进给量会变化且不可控,而滚压致密化工艺通过调节两副滚轮模具中心轴的距离,可以快速调整零件滚压进给量,制备出符合要求的表面致密层。因此,滚压致密化工艺优于挤压致密化工艺。     

大高径比粉末冶金零件热压致密化工艺

为制备大高径比粉末冶金零件,研究了提高压坯致密度的径向热压工艺,从理论上分析了与该工艺相关的压坯密度、致密化和应力公式。采用该工艺成功地制备了高径比为15.65,密度为3.79g/cm~3的TiAl基合金排气阀。     

基于连续体介质力学的粉末烧结理论

本文综述了基于连续介质力学的粉末烧结理论的基本结构和发展概况。介绍了采用粘性模型、粘弹性模型、粘塑性模型和塑性模型研究粉末烧结体致密化行为的基本思路;同时指出,采用能量耗散方程是研究复杂粉末烧结过程的有效手段。最后,简述了在这些模型基础上粉末烧结理论的实用化进展。     

国内Cu-Al2O3复合材料致密化加工的研究进展

Cu-Al2O3复合材料具有很多优异的性能,其致密化加工方法主要有热挤压、锻造和冷加工等.本文综述了国内弥散强化铜塑性变形的研究,详细介绍了弥散强化铜致密化加工研究的现状.热挤压是生产弥散强化铜的主要加工方法,不同挤压方式和挤压工艺对材料性能有很大影响.锻造是生产大断面尺寸弥散强化铜的重要手段,处于三向压应力状态锻造后弥散强化铜的性能可优于挤压态弥散强化铜.冷加工也是弥散强化铜生产中的关键步骤,主要作用为材料成形和提高力学性能.本文还对该领域的未来发展进行了展望,将来应进一步从理论上加强对弥散强化铜致密化加工的研究,并开拓新的致密化加工技术.     

高使用性能粉末冶金零件的表面致密化

为保证机构的可靠性,历史上一直在用锻钢合金批量生产高转矩变速器零件,诸如变速器齿轮、高性能链轮、单向离合器座圈及滚珠轴承座圈。虽然用粉末冶金工艺生产的零件成本低廉,但常规粉末冶金钢的静态、疲劳及耐磨性能的局限性,使粉末冶金零件不能应用于上述领域。为解决这些缺欠,近年来,一直在研发用选择性表面致密化(SSD)改进粉末冶金钢性能的各种生产工艺。特别是,PMG集团开发出了一种拥有专利权的表面致密化工艺——DensiForm。利用这种工艺可使零件表面关键部分形成深度达1mm的完全致密化表面层,而零件心部仍然是多孔性的。这种结合与粉末冶金材料合金化的灵活性,使生产的烧结钢零件的使用性能可与在高应力下使用的锻钢性能相比拟。本文将介绍2个例子,即变速器中应用的表面致密化螺旋齿轮和直齿链条的驱动链轮。本文说明了螺旋齿轮和驱动链轮的生产工艺与显微组织,介绍了轿车变速器用螺旋齿轮的性能(其中包括在三轴总成的成对试验台架上得到的滚动接触疲劳的数据)和表面致密化链轮的材料性能与使用性能。     

中间高温均匀化对7175—T736铝合金锻件组织和性能的影响

本文系统地研究了挤压锻坯中间高温均匀化对７１７５－Ｔ７３６铝合金锻件组织性能的影响。结果表明，中间高温均匀化能显著提高锻件（Ｔ７３６状态）的塑性（δ）电导率（ｒ）和应力腐蚀寿命（ＳＣＬ），并能明显改善拉伸断口和应力腐蚀断口的结构，在中间高温均匀化过程中析出的Ａｌ－Ｃｒ化合物的弥散质点，对显微组织和组织参数具有有利影响，因而使Ｔ７３６锻件的塑性和应力腐蚀寿命得到改善。     

粉末烧结体镦粗致密与成形的有限元分析

粉末锻造因同时具有粉末冶金和锻造工艺的共同优点已被证实为一种提高烧结制品密度和强度的有效方法。本文选择典型的镦粗工艺对烧结的致密与成形行为和规律进行研究，提出了多孔材料的致密模型并简要介绍了多孔材料的塑性理论。然后，采用有限元法模拟了不同镦粗压下量的烧结坯的致密与成形过程，密度与等效应变分布的一致性结果表明，烧结坯的致密强烈依赖于材料的塑性变形。此外，塑性变形过程中的平均应力也对烧结坯的致密产生一定程度的影响。通过对冶炼材料的镦粗变形比较发现，烧结坯较易满足屈服条件而发生塑性变形且成形力较低，但因存在内何加工硬化现象而使成形力增加较快。     

过共晶Al-Si20合金粉末包套-等径角挤压致密化行为

在500℃下采用包套-等径角挤压(PITS-ECAP)工艺将Al-Si合金粉末颗粒固结成高致密、块体细晶材料。结果表明,PITS-ECAP工艺对粉末材料具有强烈的致密化效果。1道次PITS-ECAP后,试样整体相对密度比初始预装粉致密度提高了50%;2道次PITS-ECAP工艺变形后,合金粉末初晶硅明显细化、整体致密化强烈,C路径能高效破碎组织中粗大板片状初晶硅,使晶粒尺寸变小,圆整度提高,分布均匀,粉末材料相对密度达95%,显微硬度值增加到220 HV25,初晶硅形状系数为0.64,致密化及初晶硅细晶化效果最好;A路径虽致密效果不好但有利于等温处理后的初晶硅细晶化,Bc路径产生的初晶硅细晶化、致密化效果较差。在PITS-ECAP工艺变形过程中,剧烈塑性剪切变形、较高静水压力和有效应变积累是合金粉末致密化、初晶硅细晶化的主要驱动因素。     

Densification of porous materials by power-law creep

The densification of a porous intermetallic alloy (Ti-14wt%Al-21wt%Nb) during the final stage of densification has been investigated under various states of stress and compared with the predictions of current models for densification by power-law creep. The experiments generally confirm the model predictions that the densification rate is a sensitive function of the stress state. Experimentally, unconstrained uniaxial compression resulted in the largest densification rates, while constrained uniaxial compression resulted in the lowest. Hydrostatic loading resulted in a densification rate similar (but slightly higher) than that of constrained compression. This ordering of the densification rates agreed well with the model predictions. However, the magnitudes of the measured densification rates are found not to be accurately predicted. A number of factors, including pore shape, pore spatial distribution and matrix microstructure have been observed to affect the densification rate, and the significance of each of these factors to predictive modelling of creep consolidation processes is assessed.     

Workability Studies in Forming of Sintered Fe-0.35C Powder Metallurgy Preform During Cold Upsetting

 An experimental investigation on the workability behaviour of sintered Fe-035C steel preforms under cold upsetting, have been studied in order to understand the influence of aspect ratio and lubrication condition on the workability process. The above mentioned powder metallurgy sintered preform with constant initial theoretical density of 84% of different aspect ratios, namely, 04 and 06 respectively were prepared using a suitable die-set assembly on a 1 MN capacity hydraulic press and sintered for 90 min at 1200 ℃. Each sintered preform was cold upset under nil/no and graphite frictional constraint, respectively. Under the condition of triaxial stress densification state, axial stress, hoop stress, hydrostatic stress, effective stress and formability stress index against axial strain relationship was established and presented in this work. Further more, attained density was considered to establish formability stress index and various stress ratio parameters behaviour.     

Influence of Carbon Content on Strain Hardening Behaviour of Sintered Plain Carbon Steel Preforms

 Complete experimental investigation on the instantaneous strain hardening behaviour of powder metallurgy (P/M) preforms of pure iron, Fe-0.35%C, Fe-0.75%C and Fe-1.1%C was carried out. The strain hardening behaviour of the above-mentioned P/M sintered steel preforms with aspect ratio of 0.4 under triaxial stress state condition was determined by cold upsetting under nil/no and graphite lubricant conditions. The instantaneous strain hardening value (ni), strength coefficient (Ki), and the stress as a function of strain and densification were obtained and analyzed. Furthermore, a relation was obtained from a semi-log plot of stress against relative density and analyzed to study the hardening behaviour owing to densification as stress was a function of induced strain as well as densification in the P/M materials.     

Anisotropic shrinkage during sintering of glass-powder compacts under uniaxial stresses: Qualitative assessment of experimental evidence

The shrinkage anisotropy that occurs during sintering of glass-powder compacts under uniaxial compressive stresses is considered. Available experimental data from the literature are used to calculate a shrinkage anisotropy factor (k) and to study its dependence on the applied stress and progress of densification. The factor k is defined by the ratio between axial and radial strains. The experimental data are interpreted in terms of qualitative trends proposed for the variation of k with the progress of sintering for a constant applied load. It is shown that when sintering densification predominates over viscous deformation induced by an external load at the onset of the process, k increases with the sintering time. The existence of a stage at which the shrinkage anisotropy factor becomes negative is confirmed by the analysis of available experimental data. The trends for the variation of k are also given for the case of the externally induced deformation predominating over sintering densification. However, no sufficient experimental data were found to validate the proposed trends in this case. The prediction of the theoretical model of Scherer for viscous sintering under uniaxial stresses is shown to agree qualitatively with the experimental trends. In connection with the use of dilatometry in sintering kinetic studies, the effect of small stresses on shrinkage anisotropy is highlighted.     

Compressive Deformation Behavior of Highly Porous AA2014-Cenosphere Closed Cell Hybrid Foam Prepared Using CaH<Subscript>2</Subscript> as Foaming Agent: Comparison with Aluminum Foam and Syntactic Foam

Closed Cell AA2014-cenosphere hybrid foams have been prepared through stir-casting technique using varying amount of CaH₂ powder as foaming agent. The cenospheres in hybrid foams created micro-pores in the cell wall and in the plateau region. It reduced the requirement of CaH₂ for foaming by 30–40% by attaining equivalent level of relative density. These foams have been characterized for of microarchitechtural characteristics and mechanical properties like strength, densification strain and energy absorption. The properties of hybrid foams have been compared with those of conventional AA2014-SiC foam and Al-cenosphere syntactic foam. The closed cell AA2014-cenosphere hybrid foam exhibited comparable plateau stress, densification strain and energy absorption characteristics to those of AA2014-SiC foams with same relative density. Empirical relations to correlate plateau stress, densification strain and energy absorption for entire range of porosities have been established.     

粉末冶金多孔材料新型制备与应用技术的探讨

在归纳分析国内外最近几年金属多孔材料新技术发展的基础上，结合钢铁研究总院采用粉末冶金技术在大尺寸、异型金属多孔元件、等静压制备、多孔金属复合管挤压成型、亚微米不对称复合膜、多孔催化材料和充气模铸等金属多孔材料制备与应用领域的一些进展，围绕能源、石化和煤的洁净加工等应用技术，对粉末冶金多孔材料技术当前发展的问题和特点进行了研究和探讨。     

On the development of constitutive relations for metallic powders

This article describes the development of elastic and plastic constitutive relations as functions of relative density for partially consolidated —100 mesh aluminum powder. First, measurements of yield stress as a function of stress state and relative density are described. Measurements of the plastic strain increments associated with yielding in unconstrained compression tests and elastic properties, both as functions of relative density, are also described. The experimental results are combined with the associated flow rule to show that the yield surface is asymmetric with respect to hydrostatic tension and compression. Second, it is shown that the yield stress results can be represented by a two-part (capped Drucker-Prager) yield surface. The consoli-dation yield surface moves along the hydrostatic stress axis during densification, while the shear yield surface approaches the Mises yield surface. For the Al powder used in the present inves-tigation, superposition of shear stress on a hydrostatic stress state aids the densification process. However, the hydrostatic stress requirement was found to be reduced by only about 20 pct for relative densities below 0. 98. Formerly with the Department of Materials Science Formerly with the Department of Materials Science     

Shear deformation and compaction of nickel aluminide powders at elevated temperatures

Powder compacts of nickel aluminide were compressed under uniaxial load above 1373 K, using a method that allowed the material to move laterally. Lateral and axial displacements were measured by means of three LVDTs. The resulting data fully described the applied stress state and the strain state as a function of time. That allowed us to obtain simultaneous measurements of the time dependent, and density dependent shear and densification behavior of the powder compact. The shear rate was non-linear in stress suggesting a dislocation flow mechanism. A model for densification by power law creep was applied to the data. It greatly overestimated the measured densification rates. Interestingly it was found that it is difficult to densify the powder to a value more than about 0.80 (relative) using uniaxial compression. In further experiments the powder was hot-pressed in a constraint cavity, allowing large hydrostatic pressures to be applied to the specimen. Near theoretical densities were obtained, presumably because the hydrostatic pressure promoted the diffusional transport mechanism of densification. The hot-pressing data were combined with the sinter forging data to obtain the correlation between densification rate and applied pressure. The diffusional mechanism of densification gave a good quantitative explanation for the densification behavior. In a broader context, we think that powder consolidation techniques ought to be optimized with a view to both shear strain as well as hydrostatic pressure. The shear strain can promote microstructure refinement through dynamic recrystallization, while pressure provides a driving force for diffusional densification.     

Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700 °C to 1400 °C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200 °C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb₂O₅ and the vaporization of MoO₃. An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900 °C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.