Doubly shrink fitted dies: optimisation by analytical and FEM calculations

Abstract

In some powder metallurgy part forming, when high compacting loads would require a too large single shrink fitted die, or the inner die diameter is very large but the compaction area is modest, twice shrink fitted dies, which are relatively expensive and complex to build, should be used. In both cases, at any given compacting load, the design should minimise dimensions and stresses. The design targets are minimum die size, optimum intermediate diameter and optimum interference, with maximum stresses on each die item lower than those allowable for the selected materials. In dual mode, for fixed overall dimension, intermediate diameters and shrink fitting interference should enable to maximise the compacting load without exceeding the allowable maximum stresses. In the first part of this work, for cylindrical shapes (circular profile dies) and axisymmetric plane stress problems, dimensions, interferences and stress–strain states have been analytically calculated to optimise doubly shrink fitted dies in the abovementioned sense. In the second part, the same evaluations for axisymmetric generic problems have been carried out by finite element method (FEM) approach integrated in an optimisation procedure. The minimum overall dimensions of the die are presented as a function of the ratio between applied load and maximum allowable stress. Corresponding interferences and optimum diameters have been calculated. Some approximate equations for a quick and handy optimisation are presented. The differences between analytical and numerical results obviously depend on the consideration of the actually stressed length, or compact height, and total die height. The comparison indicates that FEM analyses are needed since analytical calculations generally give low approximations, especially for some die geometries.     

Short dies and thin walled inserts for room temperature or warm compaction - numerical determination of design features

Abstract

If a certain physical phenomenon is so complex that simplifying hypotheses are inadequate to represent it, the determination of design parameters through analytical procedures is unsuitable. In these cases, a numerical algorithm may be the alternative solution, to simulate the phenomenon in a manner that is more faithful to the actual physical situation. The design parameters of compaction dies have been determined by FEM. To evaluate the effects of higher temperatures, as in warm compaction, the available literature data have been used to include the variations of characteristic parameters of the involved materials. A wide range of die dimensions has been covered, focusing the interest on progressively shorter dies or thin inserts. A die can be coded as short when its axial extension is small in comparison to the radial size, which can vary significantly. Different insert materials have been considered, from HSS to PM steels and Co-WC. The deviations from classical Lame's formulae depend on the geometry of the die elements. The limitation on limit stresses requires a minimum wall thickness of the insert, which is higher than that considered dangerous for the instability of elastic equilibrium.     

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.     

AN ASSESSMENT OF GLASS-CERAMIC AS AN INSERT MATERIAL FOR DIE COMPACTION OF METAL POWDERS

Abstract

Glass-ceramic inserts have been made and shrink-fitted into steel bolsters to assess the feasibility of glass-ceramic as a die material for the die-compaction of lubricated and unlubricated iron powder. Measurements of compacting pressures and ejection stresses were lower for the glass-ceramic die compared with those for a standard tool-steel die in lubricated conditions, while in unlubricated conditions ejection stresses were appreciably higher, with scoring and brittle fracture of the insert. The results indicate that possibilities may lie in the further development of ceramics as inexpensive die materials for powder compaction in which die-wall friction could be significantly reduced.     

Constitutive data and friction measurements of powders using instrumented die

Abstract

An instrumented die has been developed to measure friction and constitutive data on powders during compaction. Such data is useful for quality assurance and as input data for computer models of die compaction. The measurement system consists of a die with radial stress sensors, punch force measurement, and a displacement transducer to measure punch displacement. The outputs of these sensors enable simultaneous measurement of density, die wall friction coefficient, and the triaxial stresses acting on the powder during the course of compaction.

The die system has been tested at three industrial sites on automated and manual presses measuring ferrous, ceramic, and tungsten carbide powders at applied stresses of up to 650 MPa and speeds of up to 26 mm s^-1. Sensor outputs were sufficiently noise free to permit the recording of useful data down to stresses less than 1 MPa. Typically the run to run reproducibility of friction coefficients was better than ± 0·005, depending on the type of powder and the applied stress. Variations in constitutive data were usually better than ± 4%, again depending on material and the stress. Die wall friction coefficients are found generally to decrease with increasing density and stress.

It has been possible to discriminate between different grades and batches of the same material using the frictional and constitutive data. Constitutive data for all types of powder can be accurately represented by an analytical relationship involving four adjustable parameters. This parametric form of data is suitable as input to finite element mathematical models.     

Mechanical properties of green compacts. II. Effect of powder relative bulk density on the strength of compacts with different forming temperature conditions

Mechanisms of strength for green compacts made from powders of iron, nickel and its alloys, copper, tin, and zinc are analyzed. The strength of green compacts prepared from metal powders of medium fineness with a relative bulk density (RBD) from 0.119 to 0.568 by two-way compaction in rigid dies with homologous temperatures from 0.15 to 0.59 (pressure from 200 to 800 MPa, powder deformation rate 10^?2–10^?3 m/sec) is studied. Compact strength is determined by diametric compression of cylindrical compacts. The dependence of strength on compact porosity is studied by the Bal’shin equation. The possibility is demonstrated of using this relationship in order to describe hot compaction and formally describe cold compaction of powders with RBD up to 0.40. The effect of homologous temperature and powder RBD on compact strength is determined. The homologous temperature for transition from warm to hot compaction and the effect of compact density (degree of deformation) on this temperature is studied. It is shown that linear approximation is possible for the dependence of compact strength on powder RBD according to the equation σ _f.c = 87–217?RBD.     

A photoelastic technique for examining contact stresses in powder compaction

Conclusions A photoelastic technique has been employed for investigating the contact stresses generated in powder compaction. The technique permits both qualitative and quantitative estimation of the stresses arising under operating conditions. In simplicity and directness, it is superior to other methods for the experimental investigation of stress distribution. The results obtained have confirmed the existing views on nonuniform stress distribution on the die wall and the upper and lower punches. In addition, they have revealed certain new phenomena, such as a nonuniform decrease of the transverse pressure and friction forces on the die walls in single action compaction, pressure transmission via particle chains, etc.Translated from Poroshkovaya Metallurgiya, No. 11(71), pp. 13–20, November, 1968.     

粉末冶金齿毂模具裂纹分析与解决方法

从压制阴模模具材料、热处理制度、金相组织等方面研究了粉末冶金压制阴模键子处裂纹形成原因,讨论了粉末冶金齿毂模结构及压制模具设计结构的合理性,通过分析数据得到了开裂的原因并加以改正。结果表明,压制阴模本身的材质选择和热处理制度没有问题,导致裂纹产生的主要原因是整体模具结构设计不合理,导致压制阴模键子损坏。改进原有模具结构可以提高压制模具寿命,降低模具使用成本,保证生产顺利进行。     

The process of compaction of molybdenum disilicide

Summary A study was made in this investigation of the influence exerted by the time of holding of powder under load, repeated compaction, percentage moisture content, and plasticizer on the compressibility of molybdenum disilicide. In addition, the distribution of forces during compaction and the elastic recovery of compacts were examined.The density of a molybdenum disilicide compact is independent of the time of holding under load, and, consequently the compaction process may be carried out practically instantaneously. It is desirable to employ double or treble compaction with intermediate rubbing through a sieve, the first compaction being performed at a pressure of 35–40 kN/cm² and the second at a pressure of 15–25 kN/cm².It was established that the compaction of molybdenum disilicide is described by semilogarithmic functions of compaction pressure and relative volume. The constants of these equations were determined.The optimum moisture content for the compaction of MoSi₂ powder is 1–3%, and the optimum amount of bentonite addition 3–4%.The use of punch face and die wall lubrication reduces friction between the powder and the die walls, thereby increasing the density of the compact.The minimum elastic recovery (0.4%) is observed at a compaction pressure of 20 kN/cm².The author thanks G. V. Samsonov, Corresponding Member, Acad. Sci. UkrSSR, for his assistance in a number of problems which arose during this investigation and evaluation of the results obtained.     

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.

     

超声振动对纯铁粉压坯密度的影响

为提高粉末冶金制品的密度,提出超声粉末压制的新方法,以纯铁粉为原料进行压制实验,研究超声振动对粉末冶金制品密度的影响。研究结果表明:超声振动能促进粉末颗粒内部的运动与重排,降低粉末颗粒与模壁之间的摩擦,同时也有利于降低粉末塑性变形阶段的冷焊与加工硬化程度;与常规压制相比,超声压制的压坯密度提高0.1～0.3 g/cm3,轴向最大密度差降低0.16 g/cm3,孔隙率降低4.3%;随着压制力提高与压制时间增加,超声振动对提高压坯密度的效果更明显,粉末压坯密度进一步提高。     

Effect of temperature on the behaviour of lubricants during powder compaction

Abstract

The study of the influence of temperature on the performance of admixed lubricants is important since higher densities are desired while keeping the ejection force at a reasonable level. Therefore, three lubricants admixed with iron powder were evaluated during compaction at 25, 65, and 110°C. An instrumented die permitting the measurement of the applied and transmitted pressures through the compact lead to the evaluation of the slide coefficient. This empirical parameter is related to the stress ratio and to the friction coefficient characterising the friction of the compact on the die wall. The evolution of the slide coefficient revealed a different behaviour at the beginning of compaction, where a higher shear resistance is desirable, compared with the end of compaction, which was more influenced by the amount of lubricant at the interface between the compact and the die wall. A too low shear resistance at that stage could however lead to stick–slip phenomenon.     

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.     

FRICTION COEFFICIENTS BETWEEN IRON POWDER COMPACTS AND DIE WALL DURING EJECTION USING VARIOUS ADMIXED ZINC STEARATE LUBRICANTS

Abstract

Laboratory compaction and ejection studies have been made using a reduced iron powder mixed with a number of zinc stearates having median particle sizes between 4 and 22μm. Comparable experiments were carried out on a fully instrumented production press, which was operated at compacting pressures between 300 and 500 MN/m² to produce compacts with true densities ranging from 5·90 to 6·70 g/cm³. Determination of ejection forces by the two methods enabled calculations of the coefficients of friction between compact and die wall to be made for mixtures containing 0·5–2·0 wt.% zinc stearate. These showed that the behaviour during compaction and ejection was comparable on both laboratory and production scales and gave very similar results. An interpretation of the results is given and values of coefficients of friction are presented which show that these are dependent on the type of zinc stearate used.     

Behaviour of metal powders during cold and warm compaction

Abstract

An instrumented die was used to investigate the behaviour of metal powders during cold (ambienttemperature) and warm (up to 140^°C) compaction. This instrument enables simultaneousmeasurement of density, die wall friction coefficient, the triaxial stresses acting on the powderduring the course of compaction and ejection pressure. Commercial iron, titanium, aluminium,316L stainless steel (SS) and aluminium–silicon powders were employed for investigation. Theresults demonstrated the advantages of powder preheating on the compaction behaviour of metalpowders concerning green density, dimensional changes, frictional behaviour, ejectioncharacteristics and compactibility. However, the outlines also determined that the response ofthe non-ferrous powders to powder preheating is somehow different from those of the ferrouspowders. In this context, the behaviour of prealloy aluminium–silicon powders during compactionwas found of particular interest, as their compactibility is strongly affected by powder preheating,whereas the dimensional changes after ejection decrease considerably. This article presents theeffect of cold and warm compaction on the consolidation and ejection characteristics of ferrousand non-ferrous metal powders. The influence of compaction condition (pressure andtemperature) with considering of the powder characteristics and densification mechanisms areunderlined.     

数值仿真技术在粉末冶金零件制造中的应用及研究进展

数值仿真技术可用于粉末零件制造的工艺设计与优化,为缺陷预测与预防、零部件性能改善提供有效的理论支持和技术指导。基于粉末零件压制和烧结机理的本构模型是进行准确可靠的数值计算的必要条件。本文主要介绍了粉末压制模拟中常用的本构建模方法,如粉末烧结体塑性力学方法、广义塑性力学方法及微观力学方法,和粉末烧结模拟中的微观结构模拟方法,如分子动力学法、相场法、蒙地卡罗法及元胞自动机法等,并对以上各种建模方法的计算原理、适用范围及近些年的应用进展做了简要介绍,最后对数值仿真技术在粉末零件制造中的发展方向进行了展望。     

几种润滑剂对温压工艺的影响

温压工艺的技术关键之一是润滑剂 ,其主要作用是减小压制过程中粉末颗粒与模壁之间及粉末颗粒之间的摩擦 ,增大有效压力 ,从而使压坯密度相对于传统压制工艺明显提高。不同润滑剂的润滑效果不同 ,最佳压制温度也有差异。本文选用 3种不同的润滑剂 ,在不同温度、压力条件下采用温压技术制备铁基粉末冶金材料 ,研究了压坯密度的变化规律和它的力学性能 ,探讨了润滑剂对温压工艺的影响     

粘结剂处理的铁基混合粉的温压特性

研究的目标是结合粉末粘结技术和温压技术以制备高质量、高性能的粉末冶金材料。预粘结粉末的一大优点是能有效地降低成分偏析，因此在混粉过程中应尽量避免偏聚。本文采用两种混粉工艺制备预粘结铁基混合粉，通过不同的压制压力、不同的粘结剂含量以常温压制和温压压制制备出铁基粉末冶金材料，测量其压坯密度并进行比较分析。研究结果表明，不同预粘结工艺、压制温度和粘结剂含量对压坯密度影响很大。在一定的粘结剂含量范围内，生坯密度随粘结剂含量增加而呈线性减少。不同的预粘结工艺对生坯密度的影响随着粘结剂含量的增加而增大。     

粉末冶金温压的致密化机理

通过对铁粉的动态压制曲线、脱模力曲线、 X射线衍射、摩擦和润滑的研究, 揭示了温压的致密化机理： 温压成形过程可分为3个阶段, 在初期阶段, 粉末的颗粒重排过程占主导地位, 颗粒重排份额与冷压相比提高15%～31%; 而在后期, 温压致密化以塑性变形为主, 铁粉塑性变形程度的改善又为粉末颗粒的二次重排起了协调作用, 使铁粉获得最大程度的颗粒填充密度; 其间, 温压润滑剂对致密化起了重要作用, 它降低了摩擦因数, 改善了粉末和模壁、粉末和粉末之间的润滑条件, 有效地降低了粉末成形的摩擦阻力, 有利于粉末致密化的顺利进行.     

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.