Effect of repeated application of pressure on the nonuniformity of density distribution in compacts

Conclusions A study was made of the effect of repeated load application to powder in a die in the case of single-ended pressing with an unattached bottom. With increasing number of repeated loading cycles the difference in density between the compact ends decreased as a result of the spontaneous development of a pressing process akin to double-ended compaction. Use of a die with a rigidly fixed (blind) bottom with repeated load application increased the nonuniformity of density distribution. In both cases the mean compact density grew.Translated from Poroshkovaya Metallurgiya, No. 12(240), pp. 14–16, December, 1982.     

用温压制造高密度材料的关键参数

温压技术是由在加热的阴模中压制预热的粉末组成[1],已知温压有助于零件密实,从而改进烧结件的性能[2,3]。温压需要在适合温压的温度范围内进行。特别是,粉末混合粉应具有好的流动性,同时对阴模模壁有良好润滑性,以减小脱模力。在试验室和工业生产中都研究了用粘结剂处理的和未经粘剂处理的用温压技术制造的材料的性状与性能。为了确定和定量各种关键生产参数,诸如压制压力,粉末温度与阴模温度,生产速率及零件大小对生坯和烧结件特性和零件脱模力的影响,进行了专门的试验研究。依照粉末流动性与松装密度的稳定性,压制压力与温度以及压制零件的重量与密度讨论了温压的工艺性。     

THE DISPLACEMENT OF GAS FROM POWDERS DURING COMPACTION

Abstract

A soap-bubble method was used to observe in detail the flow of gas out of powder masses during and immediately after compaction. The effects of powder material and size distribution, die size, pressing speed, and degree of compaction were investigated. The amount of gas trapped in the compact at completion of pressing varied from 12 to 83% of the initial amount of gas present in the spaces between the powder particles, over the range of compaction conditions studied. As expected, the amount trapped increased with increase in die size, with increase in pressing speed, and with decrease in particle size. Very little gas escaped during the later stages of compaction, even at slow pressing speeds. The effect of a small punch/die clearance was examined in typical cases, and shown to be minor, except with a coarse powder pressed at a high speed.

That trapped gas can produce cracks was demonstrated by making compacts containing varying amounts of trapped gas, other conditions remaining constant. With the particular powder used, once the amount of trapped gas had passed a certain level, the compacts tended to be fairly badly cracked. It appears, however, that cracks due to gas pressure alone tend to occur only when powders are rapidly compacted to very high densities.     

Study of warm compaction of Alumix 123 L

Abstract

Although powder metallurgy (PM) material is dominated by ferrous alloys, there is a growing interest in Al PM. The usage of Al PM in automotive applications depends on the development of higher density and improved dynamic properties. Several approaches have been proposed to increase density of sintered parts. Warm compaction process of Al powder was used to achieve high density. In this study the authors focused on the effect of warm compaction on Alumix 123 L (ECKA Granules) powder blend. It has been found that warm compaction at 110°C led to a reduction in the ejection force by 27·9%, increased green density to 94% of theoretical density and increased sintered strength to 315 MPa as compared to those pressed at room temperature.     

New technological approach to fabrication of high density PM parts by cold pressing sintering

Abstract

A new technological approach to the fabrication of high density powder metallurgy (PM) parts via single pressing sintering, allowing cold compaction to be performed without admixed lubricants, has been studied. The influence of in pore gas on the compacts' green density and their sintered properties were evaluated. A mathematical expression relating in pore gas pressure in the compacts to the green density was developed. The expression showed that in order to reduce the negative influence of gases trapped in the pores it is necessary to ensure effective air drainage from the compaction zone. In order to ensure sufficient air evacuation during cold compaction, a new design of porous die was developed. The behaviour of powder mixes with different lubricants during cold compaction in porous die was investigated. All the test conditions were evaluated in terms of green and sintered properties, including the ejection force, green and sintered densities, tensile strength and surface hardness. In the context of the experimental work, compaction in porous die promoted the improved combination of green and sintered properties compared with compaction in conventional dies.     

用温压制造汽车变速器输出轴毂

汽车中应用的铁基粉末冶金零件在不断增加,这主要是由于密度的增高和动态性能的改进。粉末冶金零件的新应用还必须通过不断采用新工艺的最终形成形能力及减少制造工序,降低生产成本。用于制造某些新零件的生产工艺,还必须能生产出几何形状复杂的薄壁零件。采用温压工艺(ANCORDENSETM),用一次压制可达到较高密度水平。这种工艺还能增高生坯强度与减小脱模力。汽车变速器输出轴毂在20世纪90年代以前,是用二次压制/二次烧结工艺大量生产的一个重要粉末冶金零件。1995年前后,用温压工艺进行了制造这种零件的试验,并和用二次压制/二次烧结工艺生产的零件进行了对比。     

基于离散元的不同粒径配比粉末压制相对密度与力链分析

利用PFC三维数值模拟软件,通过改变粉末颗粒粒径分布建立各组冷压模型,得到压制过程中相对密度变化规律与力链分布情况。在特定粉末粒径配比下,能够得到相对密度最高的压坯。结果表明:在大、中、小粒径颗粒质量比为60:15:25的粒径配比下,压坯相对密度最高,压坯相对密度并不会随着细粉比例不断增加而一直提高;在压制过程中,随着附加细粉占比上升,压制方向上能产生更大应变。侧压系数与泊松比受粉末粒径分布影响较小,且在压制后期,在压坯已获得较高相对密度的情况下,会因缺乏足够的驱动力与位移空间发生下降;混合粒径粉末试样的力链数量远大于单一粒径粉末试样,在强力链数目充足的前提下,结合大量的弱力链能获得更高的压坯相对密度。     

Investigation of warm compaction and sintering behaviour of aluminium alloys

Abstract

The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejection force during the powder shaping process. This paper presents the significant advantages and drawbacks of using the warm compaction process for commercial PM aluminium alloys.     

Two-Dimensional Multiparticle Finite Element Modeling on the Cold Isostatic Pressing of Al Powder

In this article, the cold isostatic pressing (CIP) of Al powders with different initial packing structures was numerically simulated in two dimensions using the multiparticle finite element method from particulate scale. The effects of external pressure and initial packing structures on the packing densification and performance of the compacts were systematically studied. Various macro/micro properties (such as relative density and distribution, local stress and distribution, and particle deformation behavior) and the densification dynamics/mechanisms were characterized and analyzed. Corresponding modeling on die compaction was also conducted for comparison. The results show that the relative displacement of particles is much less and the particle shape is more regular for CIP of ordered initial packing structure than those for CIP of random initial packing structure, and the distributions of relative density and local stress in the compact for the former case are more homogeneous. Compared with die compaction, the formability of CIP is much better; also, the compact has higher relative density and more uniform density and stress distributions. The normal stresses are transmitted in all directions in the compact during CIP because the external load is applied uniformly to the outer surface of the rubber mold from all directions in the entire process, while the normal stresses are directional along the loading path during die compaction.

     

Numerical simulation of metal powder die compaction with special consideration of cracking

Abstract

Powder die compaction is modelled using the finite element method and a phenomenological material model. The Drucker–Prager cap model is modified with the goal to describe the formation of cracks during powder transfer, compaction, unloading, and ejection of the parts from the die. This is achieved by considering the cohesive strength and the cohesion slope, which characterise the current strength of the powder compact in the Drucker–Prager model, as state dependent variables. Evolution equations are formulated for these variables, so that the strength increases by densification and decreases by forced shear deformation. Some of the parameters appearing in the evolution equations are determined from measured green strength values. An iron based powder (Distaloy AE) is used for the experiments. Examples are shown to demonstrate that the density distribution can be calculated accurately as compared with an experiment, that cracking can be modelled at least qualitatively correctly, and that the compaction of complex 3D parts can be simulated.     

Metal powder compacting dies: optimised design by analytical or numerical methods

Abstract

The initial data needed to design metal powder compaction die are: compact shape and density, powder mix composition, compaction and radial pressure, part number and tool materials. The design targets are: diameters of insert and ring, sometimes number of rings and interference or interferences. The constraints include: no tensile stresses on the insert, no risk of relative motion at part ejection, no unwanted alteration of material microstructures and maximum stresses always below the allowable limits. Usually the design is based on engineering experience, company knowhow, and approximated analytical calculations and cost considerations.

This study is focused on the use of numerical methods to determine the design parameters of dies for powder compaction. Both room temperature and warm compaction have been investigated. Numerical algorithms, implemented into FEM calculation codes, enable one to optimise the common diameter of insert and ring, corresponding to the lowest stresses on both items, or to find the minimum value of the outer diameter. A wide range of compaction pressures, die materials and geometries, interferences and allowable stresses have been explored. To compare the results, based either on analytical or numerical methods, circular dies have been investigated. The differences among the results depend on the consideration of the actual stressed length, or compact height, and total die length. The calculations by analytical methods overestimate the stresses. The paper presents some suitable nomograms for the comparison of results of calculations performed either by Laméformulas or by sophisticated numerical methods.     

超细粉末的高压软模成形方法

以近纳米粉体WC—Ni—Fe和W-Cu为原料，采用软模成形的方法，重点研究软模成形工艺参数对压坯密度和压坯强度的影响；对钢模压制和软模压制两种成形方法进行比较。结果表明：高压软模成形工艺能有效提高分层压力，使压坯相对密度大幅度提高，由33％提高到60％；压坯横向断裂强度超过17N／mm^2；所得压坯能够进行机械加工。     

The Effect of Density Distribution on the Radial/Axial Shrinkage Ratio of Iron Powder Compacts

Abstract

The effect of compacting pressure on the radial/axial (R/A) shrinkage ratio for iron powder compacts was studied. It was observed that R/A and compacting pressure have a linear relationship. The effect of density distribution on R/A ratio inside the green compact was determined. The observed changes in R/A are attributed to the change in density distribution in the green compact.     

Effect of tool coatings on ejection characteristics of iron powder compacts

Abstract

Powder metallurgy (PM) part makers heavily rely on part density as a mean of controlling part performance. Higher compaction pressures may be used to obtain higher densities and better properties. However,ejection stresses usually increases with compacting pressure. Those stresses may affect significantly part quality (surface finish, formation of cracks and lamination) and tool wear.

Different methods may be used to minimise ejection stresses, such as the use of admixed lubricant, die wall lubrication and the modification of tool surfaces. This paper presents an approach to evaluate the effect of tool coatings on the ejection of ferrous compacts. The method consists of evaluating the ejection characteristics of core rods with different coatings. The results obtained show that ejection characteristics are sensitive to tool coatings. Coating the surface of the core rods yields important variations of the stripping pressure (2×) and ejection energy (1·6×). No clear correlations between the ejection characteristics and the part surface finish were observed.     

用温压法制备铁氧体

为了改进铁氧体磁性材料的成形性，拓宽其应用领域，研究了一种介于干压成形与热压铸成形之间的新的铁氧体成形工艺——温压成形。该成形方法将干压成形与热压铸成形有机结合，汲取各自的优点，克服了干压成形中压力不均匀、压坯内应力过大，热压铸成形中粘结剂过多、收缩率大、磁性能低的缺点。在温压成形过程中，只掺入少量粘结剂与添加剂，使其既能保证粉料良好的成形性与取向性，又能保证脱脂与烧结过程中不会产生过大的收缩与变形。结果表明，用该成形方法压制得到的压坯烧结性较好，可以得到90％以上的烧结相对密度。磁性能比热压铸磁体高很多，与干压磁场成形方法相当。该工艺能够适用于形状较为复杂并对磁性能有一定要求的中小型产品。     

Effects of Aging and Steam Oxidation on Sintered Irons and Steels Containing Phosphorus

Abstract

Dramatic reduction of ejection forces in a metal powder compaction process has been achieved using a balanced die method. The approach uses a toolset consisting of a thin walled die in a fairly rigid metal, with a rubber sleeve capable of transmitting high radial pressure. By maintaining a pressure balance between internal compaction pressure and external ‘bolstering’ pressure across the thin wall die, the ejection forces were reduced to almost zero. With limited equipment available, the compaction process was carried out in incremental stages to produce cylindrical metal billets of 32 mm diameter by 20 mm height. Green densities of 6·49 Mg m^?3 were achieved for NC100 iron powder supplied by Höganäs at much lower compaction pressure than those reported elsewhere. PM/0711     

粉末热锻凸轮的数值模拟

凸轮作为装配式凸轮轴上的关键零部件之一,其制造技术很大程度上影响着凸轮轴及发动机的发展。采用验证实验的方法确认有限元法模拟粉末热锻凸轮的压制及热锻成形过程的合理性,在此基础上利用数值模拟法对凸轮的压制方式进行改进、优化热锻工艺参数。针对粉末压制成形工艺,对不同压制方式下(单向压制、双向压制、浮动压制、摩擦压制)的压坯密度进行模拟分析。而针对热锻工艺则研究分析锻压速度、摩擦因数、坯料加热温度、模具预热温度、坯料初始相对密度5组因素对锻件密度分布的影响,从而获得最佳的工艺方案。     

粉末冷压成形弹性后效及非同步双向压制模拟研究

本文采用有限单元法,基于更新的Lagrange格式和弹塑性大变形理论,模拟分析了金属粉末冷压成形后的弹性后效现象,以及非同步双向压制下的压坯密度分布规律。结果表明:脱模后的节点密度下降量与高径比呈线性递增关系,据此可有效辅助模具设计,提高压坯尺寸精度。距离运动模冲越近,由于积聚的应变能较大,脱模后节点密度下降得越多,弹性后效大。应力的重新分布使距离运动模冲较远处的密度反而略微增加。降低摩擦能整体减小弹性后效程度和密度下降量。非同步双向压制对台阶件等复杂零件生产的适应性要优于同步双向压制。非同步双向压制获得的压坯密度均匀性取决于两模冲的压下量。采取先动作模冲的压下量接近所需的总压下量,之后另一模冲轻微反向压制到最终尺寸的方法,可使密度均匀性大为提高,达到甚至超过同步双向压制的均匀度。     

Densification behavior of dynamically shock compacted AI2O3/ZrO2 powders synthesized through rapid solidification

The rapidly solidified alumina-zirconia eutectic contains high volume fractions of nanocrystallinet- ZrO₂, which makes the material a promising precursor for the manufacturer of fracture-resistant ceramic specimens. Unfortunately, conventional powder processing and sintering techniques are inadequate for the fabrication of dense specimens using this material. We have used dynamic shock compaction to facilitate the achievement of high density specimens which retain the unique microstructure of the precursor material. In an attempt to quantify the dynamics of the microstructural evolution which occurs during the compaction process, we have investigated the effect of various particle size distributions on the densification behavior of the material during the shock compaction and postcompaction sintering cycles. The shock compaction process produced high densities (∼73 to 78 pct of single-crystal theoretical) by inducing a highly efficient packing of the particles. A bimodal powder distribution was also compacted and this specimen exhibited a relative density of 86.2 pct, approximately 10 pct higher that those of the unimodal compositions. In this compact, the small particles efficiently filled the interstices between the larger particles. The high density of the bimodal compact did not translate to a high sintered density, however. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

Computer modelling of warm powder compaction in rigid dies: Effect of nonuniform heating

Computer modelling is used to study warm compaction. Relationships of irreversible porous body flow and the finite element method are used. It is established that in view of nonuniform temperature distribution within a compact there is radial flow of powder. The more heated layer of the material is more intensely compacted than the less heated layer. Nonuniform temperature distribution affects the magnitude of axial and radial pressures. Modelling of compaction of a stepped article of complex shape makes it possible to establish that nonuniform heating may increase radial powder flow that in turn promotes levelling of the density. The results of modelling may be used in order to optimize warm compaction of powder articles by choosing special billet heating schemes. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 7–13, March–April, 2006.