Character of porosity of compact materials obtained from metalorganics

The possibility of obtaining highly porous (60–80% pores) materials by combining techniques of powder metallurgy with chemical-metallurgical processes of formation of nanodimensional activators of sintering during the thermal destruction of metalorganics is shown. It is noted that, irrespective of the composition of starting charges, the surface porosity of such materials is represented by pores <30 μm, the fraction of which does not exceed 1%. The basis (86–89%) is represented by pores <4 μm. The character of porosity inside the sintered articles is determined by the sizes and shape of the powderlike skeleton material introduced into the starting charge to prevent the outflow of metalorganics during thermal treatment. When using powders with highly developed particle surfaces as such, the character of the internal porosity is similar to the surface porosity. The combination of scale particles up to 100 μm in size and highly dispersed (<10 μm) particles promotes the formation of long porous channels (100–350 μm) 10–50 μm in width.     

Method of manufacture of high-speed steel from swarf and its properties

Conclusions The feasibility is demonstrated of direct processing of fine high-speed steel swarf by a powder metallurgy technique including prepressing, extrusion, and heat treatment operations The process developed enables sintered steel to be produced possessing properties similar to those of standard steel. The true mechanical properties of standard (cast) and sintered (extruded) steels were determined, with the aid of the theory of plasticity of porous materials, by plotting their deformation diagrams. It is shown that the above theory is applicable to materials possessing coarse structural porosity. Quantitative discrepancy was found between compression and tensile curves, resulting from the presence in such materials of isolated nonmetallic inclusions and from the discontinuity of their microstructure, which may be regarded as initial porosity.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 35–40, April, 1980.     

FATIGUE BEHAVIOUR OF SINTERED METALS AND ALLOYS

Abstract

Published information on the fatigue behaviour of sintered materials is reviewed.

Porous sintered materials exhibit similar fatigue characteristics to cast and wrought materials, including fatigue limits in ferrous materials. Their endurance ratios are slightly lower than those of similar wrought materials and they may depend on porosity content. In some cases fatigue data for sintered materials show less scatter than those for similar wrought materials. The total porosity content, which is mainly determined by compacting conditions, is the most important factor influencing fatigue behaviour. Endurance limit decreases as the porosity content increases. In the copper- and iron-base materials investigated, fatigue behaviour is influenced only slightly by powder characteristics, sintering temperature, atmosphere, and time, and by post-sintering treatments. Environmental and surface conditions seem to influence the fatigue behaviour in the same manner as pore-free materials. However, notches have a less severe effect than on pore-free materials. Fatigue fracture appears to occur in the same manner as in pore-free materials. Fatigue cracks tend to start at the free surface of the specimen in preference to the internal surfaces of pores, in agreement with theoretical predictions. Sintered low-alloy steels can be heat-treated to give a wide range of fatigue strengths, and they are less notch-sensitive than pore-free steels. The fatigue properties of sintered and pore-free materials are compared and sintered materials are shown to possess fatigue strengths in the same range as cast and wrought materials.     

Tribological Characteristics of Materials Based on Bronze for Small Friction Assemblies

The effect of the fineness of atomized nonspherical bronze powder and compaction pressure on the porosity and pore size of sintered materials is studied. The optimum sintering temperature for achieving the maximum strength and porosity of bronze carcasses is determined. The effect of bronze original porosity and powder particle size on the wear resistance and friction coefficient of sintered carcasses impregnated with polymer and oil under dry friction conditions and a self-lubricating regime is established.     

制备铁基高温合金多孔材料的新方法

向Fe-Cr-W-Ti-Y合金粉末中添加原子比为3∶1的Fe/Al混合粉末,利用Kirkendall效应和Fe、Al反应造孔制备铁基高温合金多孔材料.研究Fe/Al混合粉末的添加量对Fe-Cr-W-Ti-Y高温多孔材料开孔隙率及孔结构的影响.结果表明,加入Fe/Al混合粉末能显著提高烧结坯的开孔隙率,但并非所加Fe/Al混合粉末含量越多越好,Fe/Al混合粉末质量分数为15％时,烧结坯开孔隙率达到最大值33％.Fe/Al混合粉添加量在15％～45％范围内,随Fe/Al混合粉末含量增加烧结坯的开孔隙率下降.     

Damping properties of sintered high-porosity materials based on powders and discrete copper fibers

Experimental relationships are established between the vibration decrement and the level of maximum cyclical deformations during the bending of sintered high-porosity materials based on powders and discrete copper fibers. The quality of interparticle contacts is evaluated in the sintered materials. Comparison of curves of the vibration decrement versus porosity for fixed maximum cyclical deformations indicated that the vibration decrement of all materials decreases with increasing porosity (within the range from 40 to 77%); however, it is appreciably greater vibration decrement of the matrix material (compact copper). Sintered high-porosity materials based on discrete copper fibers have the highest indicators of energy dispersion, especially at relatively high levels of maximum cyclical deformations as compared with sintered materials based on copper powders.     

Assessment of porosity and structure of powder materials based on mechanically activated Ni-Fe-NaCl charges

Electron-probe microanalysis methods were used to study the formation of structure and the distribution of various components in high-porosity powder materials during heat treatment of compacts based on Ni-Fe-NaCl charges mechanically activated in fluid media. We confirmed the hypothesis that agglomerates are formed during mechanical activation of Ni-and Fe-based composite-particle charges in fluid media. A causal relationship was established between formation of structure in compacts and structure of the high-porosity powder material after heat treatment. A technique was proposed for determining the local porosity of the sintered high-porosity powder material. The structure of the high-porosity powder material was found to consist of large pores surrounded by powder material containing pores of smaller diameter.     

Microstructural modelling of electrical conductivity and mechanical properties of sintered ferrous materials

Abstract

The role of microstructure on mechanical properties of sintered ferrous materials was studied using a method based on electrical conductivity measurement. The method was accompanied by quantitative fractography to evaluate the dewaxing and sintering process in iron compacts. The effects of manufacturing parameters, such as compacting pressure in the range of 150–800 MPa, sintering temperature from 400 to 1300°C, sintering time up to 8 h, and lubrication mode were investigated. Several mathematical models were checked to obtain the best one for prediction of electrical conductivity changes as a function of manufacturing parameters. The mechanical properties of the sintered compacts were also evaluated to establish a relationship between conductivity, total porosity, pore morphology, and mechanical behaviour. The results show that the electrical conductivity/resistivity of sintered materials is closely related to its microstructure, so that measuring these properties can replace destructive test methods for prediction of mechanical strength of sintered materials with homogeneous matrix microstructure. The application of the method is shown for sintered Fe, Fe–0·8%C, and Fe–1·5%Mo–0·7%C compacts.     

NiO掺杂对菱镁矿烧结性能的影响

以天然菱镁矿为原料,添加不同质量分数的NiO粉末,采用高温煅烧法制备了镁砂。利用显气孔体密测定仪、X射线衍射仪、扫描电子显微镜分析NiO粉末掺杂对镁砂熟料烧结性能、物相组成、微观结构的影响。结果表明,在菱镁矿中添加NiO粉末有利于提高镁砂烧结致密性,促进MgO晶粒长大。当不添加NiO粉末时,试样在1600℃烧结前后线变化率为8.72%,体积密度为2.96 g·cm-3,显气孔率为19.2%,MgO晶粒尺寸为1.36 μm;当加入质量百分数为0.75% NiO粉末时,试样烧结前后线变化率为18.06%,制备镁砂体积密度为3.29 g·cm-3,显气孔率为11.5%,MgO晶粒尺寸为3.81 μm。在1600℃对添加NiO粉末的菱镁矿进行烧结,NiO可以与MgO完全固溶,引起MgO晶格畸变,降低晶体活化能,提高MgO烧结致密性能。但添加过多NiO,在短时间内无法与MgO完全固溶,妨碍烧结相的直接接触,影响传质过程,导致样品烧结性能降低。     

粉末冶金高强度烧结钢的疲劳特性

通过测定N-110烧结钢旋转弯曲疲劳下的S-N曲线、疲劳裂纹扩展速率da/dN以及疲劳门坎△K_th,对烧结材料的疲劳特性进行了探讨,初步阐明了孔隙的存在对各种疲劳性能的影响规律以及热处理状态与疲劳性能之间的关系。     

Mechanical properties and strain-strengthening of sintered iron

Conclusions The particle size of the starting powder, like the grain size of sintered specimens, does not have a pronounced effect on the mechanical properties of sintered iron because the effective length or a slip plane is determined by the size of the smaller fragments bounded by the pores in the particles and interfaces. The strength and ductility characteristics of sintered iron in a wide range of porosity variation are satisfactorily described by the expression obtained in [3]. The strain-strengthening of sintered porous iron, like that of polycrystalline nonporous iron, has a parabolic character, with the exponent of strainstrengthening being independent of porosity and the coefficient of strain-strengthening decreasing with, increasing porosity according to the same law as the limit of proportionality.Translated from Poroshkovaya Metallurgiya, No. 7(259), pp. 68–72, July, 1984.     

Correcting for container temperature inhomogeneity in sintering diamond-metal composites

A model is used for thermal conduction in a homogenized medium to calculate the temperature field in a container enclosing diamond-metal composites. The temperature pattern and porosity of the sintered materials have been determined for various values of the heat-transfer coefficient of the oven (0.01–0.1). The results obtained for β=0.03 are compared with experiment. The calculated porosity agrees well with experiment. Superhard Materials Institute, Ukraine National Academy of Sciences Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 86–92, January–February, 1999.     

Influence of the structure on the elastic modulus of highly porous copper-based materials

The effect of the structure on the elastic modulus of two groups of highly porous sintered copper-based materials (foam and fiber metal) with 40 to 90% porosity is investigated. The elastic modulus of the materials is described within the sphere-rod model developed for foam plastic with an open porosity; the model uses an equation containing a structure parameter. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 79–83, March–April, 2000.     

Manufacturing porous materials from metalorganic mixtures: Report 2

The morphology of the surface and fracture of highly porous materials obtained during the thermal destruction of metalorganic mixtures (MOM) was investigated. The size and shape of pore channels in the finished product were shown to be determined through the fractional composition and the particle shape in MOMs. The effect of the porosity on the permeability and strength of sintered materials was considered. The permeability was demonstrated to depend not only on the value, but also on the shape of pore channels. Combining the techniques of powder metallurgy with chemical-metallurgical processes allows us to obtain materials with a porosity of 70–80% and a strength of no less than 5–25 MPa. If the pore size was from 10 to 100 μm, the permeability of materials was (in 10⁻¹² m²) 1–3 for Mo, 0.5–6 for Mo-Ni, and 2–6 for Ti-Mo.     

Activation of the sintering process of iron powders by introducing nanodispersed additives

The results of studying of the effect of additions on nanodispersed iron powder (Fe-NP) obtained by the electrical explosion method on the formation and sintering of industrial coarse-grain iron powder are presented. The effect of amount of Fe-NP added to the charge material on compactibility and moldability of compacts, properties, microstructure, and phase composition of sintered material is analyzed. Electrically explosive Fe-NP is shown to be low-technologic. Charges based on the coarse-grained iron powder that contain up to 20% Fe-NP have good compactibility and moldability. It is established that introduction of Fe-NP into the charge causes activation of sintering and is favorable for obtaining the sintered samples with fine-grain crystalline structure and increased physicomechanical characteristics. The effect of amount of the Fe-NP addition to effective activation energy of sintering is evaluated.     

Effect of compaction pressure and powder grade on microstructure and hardness of steam oxidised sintered iron

Abstract

Steam oxidation has proven to be an effective process to improve the properties of sintered iron components. The oxide formed on the surface and in the interconnected porosity strongly influences both the tribological and mechanical properties of these materials, for example through the extent of pore closure and the nature and morphology of the oxide produced. In this paper, the influences of compaction pressure and powder size on the microstructure, oxide content, hardness, and surface topography of steam treated sintered iron are analysed. Specimens prepared from atomised iron powders of different sizes (<65, 65–90, 90–125, and >125 µm) were compacted at four different pressures (300, 400, 500, and 600 MPa), sintered for 30 min at 1120°C and then subjected to a continuous steam treatment at 540°C for 2 h. A clear influence of the processing parameters on porosity was highlighted. Low porosity was always associated with high compaction pressure and greater powder size. Pore size was affected in the same way by compaction pressure, even though the effect of powder size acted in the opposite sense. Changes in compaction pressure and powder size had no significant effect on pore shape. Decreasing powder size always led to high hardness. The effect of compaction pressure on hardness is clear evidence of a compromise between porosity and blockage of the pore network by oxide. Samples produced with smaller powder sizes showed a continuous decrease in hardness as the compaction pressure increased, although for the large powder size there was a slight increase to a constant value of ultimate hardness. For the intermediate powder size a maximum hardness was obtained as the compaction pressure increased. X-ray diffraction showed that the oxide layer is composed of magnetite and haematite.     

Technology and properties of aluminum-based powder porous materials produced by sintering of a loose powder charge in air in the presence of a flux and additives

A method of obtaining aluminum-based porous material is suggested which consists in sintering of a loose powder charge in air using silumin and KAlF₄ as a flux. The effect of the dispersity of the initial powders and of the composition of the initial mixture on the porosity and properties of the sintered powder material has been investigated. It is shown that, in the presence of a small amount of a flux and a low-melting component (silumin), no substantial oxidation of aluminum occurs and that a material with a porosity from 30 to 35% and satisfactory physical properties can be obtained.     

Analysis of the Hydraulic Properties of Permeable Materials with Bimodal Porosity

The possibility of controlling the hydraulic properties of permeable materials with bimodal porosity is analyzed. It is shown that creation of a porous structure (meso- and submesostructure) makes it possible to solve the contradictory problem of obtaining high-quality, highly-porous sintered materials with a high level of hydraulic characteristics.__________Translated from Poroshkovaya Metallurgiya, Nos. 3–4(442), pp. 31–40, March–April, 2005.     

孔隙度对烧结不锈钢纤维多孔材料压缩性能的影响

采用丝径为100μm的不锈钢纤维经松装烧结工艺制备了孔隙度在70%～95%之间的金属纤维多孔板材.在MTS858材料试验机上检测压缩性能,结果表明:孔隙度在82.8%以下的试件的面内应力应变曲线大致分为三个阶段,应变很低情况下的线性弹性区、屈服平台区和应力急剧增大情况下的致密化区,其中屈服平台区较长,说明该材料具有较强的能量吸收能力;随着孔隙度的增加,不锈钢纤维多孔材料的卸载模量、屈服强度减小,其中孔隙度为70%的烧结不锈钢纤维多孔材料的平均卸载模量为5.2GPa,平均屈服强度达到了23MPa;孔隙度大于90.1%的试件屈服强度很低,能量吸收能力很小.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.