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.     

EPMA drives collaborative development projects

Abstract

An analysis has been performed on the selective removal of particles from the edges and thin lands on green iron powder compacts. The voids created as a result of this ‘picking’ are believed to be due to large (+ 125 μm (+ 120 mesh)) particles in the iron powder mix. Microscopic examinations indicated that in all cases picking occurred within isolated regions of large pore and particle size. Two possible independent causes for this problem have been advanced: segregation of large particles within the feed shoe during die filling; and an uneven load distribution across the face of a part due to the presence of tooling impressions on the upper punch. A series of follow up tests showed no change in the relative position of the large pore size region with upper punch orientation. Therefore, by deduction, particle segregation is advanced as the main cause of the observed picking. Suggestions are presented to alleviate this problem. PM/0495     

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     

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.     

Bearing Materials by Powder Metallurgy

Abstract

The dependence of green strength on green density and on compacting pressure was investigated for the bidirectional die pressed and isostatically pressed Cu powder compacts. The breaking strength of the pressed Cu compact was found to increase with green density and also with compacting pressure. The green strength seemed to be directly proportional to the contact area between powder particles. A theoretical equation for the relationship between green density and contact area was derived from a geometrical consideration, and agreed well with experimental findings. PM/0272     

PM processing and characterisation of Ti–7Fe low cost titanium alloys

Abstract

This work studies a set of low cost beta alloys with the composition Ti–7Fe, processed by conventional powder metallurgy (PM). The materials were prepared by conventional blending of elemental Ti hydride–dehydride powder with three different Fe powder additions: water atomised Fe, Fe carbonyl and master alloy Fe–25Ti. The optimal sintering behaviour and the best mechanical properties were attained with the use of Fe carbonyl powder, which reached a sintered density of up to 93% of the theoretical density, with UTS values of 800 MPa in the ‘as sintered’ condition. Coarse water atomised powder particles promoted reactive sintering, and coarse porosity was found due to the coalescence of Kirkendall porosity and by the pores generated during the exothermic reaction between Ti and Fe. The addition of Fe–25Ti produced brittle materials, as its low purity (91·5%) was found to be unsuitable for formulating Ti alloys.     

Measurement of friction for powder compaction modelling – comparison between laboratories

Abstract

A ‘round robin’ assessment of methods for measuring friction for die compaction of powders has been undertaken as part of the European PM Modnet programme. Measurements have been made on a Distaloy AE with 0·5% graphite and 1·0% wax using five different shear based techniques and six techniques using instrumented dies. The potential accuracy of friction measurement required as input to models of the compaction process has been assessed using results from modelling sensitivity studies where friction was varied, together with specifications from industry of the accuracy of model output required. These studies suggest that an accuracy of measurement of friction coefficient of ±0·01 is preferred, but that an accuracy of ±0·02 will still give predictions of acceptable levels of accuracy. Friction data have been compared by separating the measurements into three regimes, respectively relevant to: ? surfaces perpendicular to punch movement during compaction (Type 1), ? surfaces parallel to punch movement during compaction (Type 2), ? surfaces parallel to punch movement during ejection (Type 3). The majority of data relate to the second of these cases. Type 2 friction data from shear techniques fall within a band of ±0·02 around a linear relation with normal pressure and density. Mean friction falls from 0·14 to 0·07 as density increases from 4 to 7·5 g cm^-3 and from 0·11 to 0·06 as normal pressure rises from 0 to 600 MPa. Type 2 friction data from dies show a wider variation than data from shear techniques. A number of reasons for this has been suggested. The use of a varying ratio of radial to axial pressure at die surfaces during compaction has been demonstrated to bring closer agreement between data from die and shear techniques, especially at pressures above 200 MPa.     

PILOT STUDY OF PREFORM DESIGN FOR SINTER FORGING

Abstract

An important difference between sinter forging and conventional forging is that of consolidation of the PM preform. It therefore follows that sinter forging would seem to be a hot-compaction process with some bulk metal plastic flow. The permitted amount of plastic flow is considerably less for a PM preform than for a preform produced from wrought material of equivalent composition, since PM preforms exhibit poor ductility and are therefore more prone to cracking. The purpose of this study is to produce some guidelines to assist in designing preforms for sinter forging and to attempt to minimize the ‘trial and error’ approach. This has been undertaken by the production and study of a component suitable for PM forging, which at present is produced by conventional forging.

The lateral flow of metal was observed and investigations undertaken as to how density variations across the preform might assist the consolidation process. An iron powder containing 1% graphite and 1% zinc stearate was used in the production of components.     

Densification of Powders by Particle Deformation

Abstract

Based on a previous experimental study of particle deformation during powder compaction, a model is developed for describing the densification behaviour of an irregular packing of spherical particles. Using the radial density function of a ‘random dense packing’, the increase in both the average size and the number of contact faces are calculated. A simple criterion for local yielding allows the compaction pressure to be determined for relative densities up to 90%. In the final stage of compaction, particle deformation, now constrained by neighbouring contacts, is modelled by extrusion into the remaining pore space. A compaction equation encompassing both stages is presented; its application to non-spherical powders elucidates the role of particle shape during powder densification. PM/0150     

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.     

FACTORS CONTROLLING THE SHAPE REGULARITY OF SINTERED U02 PELLETS

Abstract

As explained mathematically, the magnitude of the ‘diabolo’ effect in sintered nuClear-fuel pellets is a function of the slope and position of the sinterability curve and the density gradient existing in the green pellets. If sinterability is representedby a straight line passing through the origin, the diabolo effect is negligible; this condition can be fulfilled when using a deactivated powder.

In any other case, the density gradient must be kept as low as possible to produce sintered pellets of regular shape. This is discussed as a function of powder roughness, lubricant quality, lubrication technique, and compaction characteristics.

When the sinterability curve cannot be adapted and powder quality and lubrication conditions are fixed, the possibility still exists of minimizing irregularities in sintered diameter by using a single-action press and compacting the powder in an adapted conical die with a moving lower punch.     

注重流动性、充填性的粉末冶金用预混合铁粉的开发

压制生产率是粉末冶金生产过程中的一个重要课题。控制压制生产率的生产作业有：（1）粉末从料仓中的排出;（2）将粉末充填于模具型腔中。为了评定粉末的排出特性,设计了简易的粉末排出装置,用来测定铁基预混合粉的临界流出径。已证明临界流出径和料仓中粉末的流动型式密切相关。在将粉末充填于模具型腔内的作业中,发现将空气从模具型腔内排出是一个重要现象。与常规装粉靴相比,可将空气加速从模具型腔中排出的“搅拌式装粉靴”,能够提高装粉速率和减小粉末压坯重量的波动。     

Powder metallurgy and high-entropy alloys: update on new opportunities

ABSTRACT

This article addresses the development of high-entropy alloys (HEAs) fabricated via Powder Metallurgy (PM) techniques. There are potential opportunities for PM techniques to produce ‘different’ HEAs and offer alternative routes to obtain special compositions. The potential for PM is vast, and the obtained properties are highly competitive with those from HEAs produced by ingot casting. Considerable work must still be done to provide the market with reasons to use PM instead of other processing routes. Since article [Torralba JM, Alvaredo P, García-Junceda A. High-entropy alloys fabricated via powder metallurgy. A critical review. Powder Metall. 2019;62(2), doi:10.1080/00325899.2019.1584454] was published in 2019, more than 100 research works have been issued on this topic. The objective of this paper is to provide an update on some of the new opportunities that have been proposed in the last months.     

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.     

改善还原铁粉流动性能的试验

流动性是粉末冶金重要的工艺参数之一,其优劣直接影响生产效率,同时对于形状复杂件、细长件、大高度件等,流动性差将导致装粉时模具充填不佳甚至出现搭桥从而影响制件的成形。实际生产中还原粉的流动性能控制相对较难,本文提供了几个改善流动性的试验,主要包括:调整粉末粒度组成、配入雾化粉、调整原料松装密度并延长合批时间、调整还原温度。以上方法均可以改善还原铁粉的流动性,其中第三个最可行。     

Selected Problems of Die‐Forging of PM Steel Products

The paper discusses the selected technological and material problems of closed‐die forging of PM steel preforms. The first part presents technological variants of hot forging routs of PM materials. In the second part the influence of hot forging parameters on the density, mechanical properties and structure are discussed. The influence of the size of powder particles, powder manufacturing method, chemical composition and heat treatment conditions on mechanical properties after forging is shown. The research on thermomechanical forging of PM preforms has resulted in potential possibilities of shortening of the manufacturing route to obtain products with high density and good mechanical properties from one heating before forming.     

Densification behaviour of titanium alloy powder compacts at high temperature

Abstract

Densification behaviours of titanium alloy (Ti–6Al–4V) powder compacts were investigated under uniaxial compression at high temperature. The yield function and a strain hardening law proposed by Kim were implemented into a finite element program (Abaqus) to compare with experimental data of porous titanium alloy powder preforms under PM forging. Experimental data were compared with finite element calculations for the variations of relative density with the applied pressure. Deformed geometry and density distributions of titanium alloy powder compacts under PM forging were also compared with finite element calculations.     

DETERMINATION OF HOOP STRESSES INDUCED IN A CYLINDRICAL STEEL DIE BY COMPACTING METAL POWDERS

Abstract

Because of the lack of stress data, steel dies for compacting metal powders were designed in the past on the assumption of hydraulic pressure transmission by powders.

True die stresses existing in a hardened steel die during the compacting of various lubricated and non-lubricated powders have been measured by mounting strain-gauges on the periphery of the die at numerous points along its length.

Values of the hoop stress varied from approximately 0·05 to 0·40 of the calculated and measured hydraulic fluid stress. For a particular metal powder, the stress was a function of the average particle size, the powder height, and the amount of lubrication.     

Dienet: conclusions and achievements

Abstract

The European Union funded thematic networks PM Modnet and PM Dienet have encouraged development of modelling technology to improve the production of complex ferrous parts and other materials. The networks sought to demonstrate via case studies the potential of the technology to meet industry requirements. The technical results obtained in three case studies (on iron, ceramics, hardmetals and soft magnets) are summarised and requirements for effective dissemination of simulation into industry are proposed. Further progress will involve complementary research on die filling and crack simulation and characterisation.     

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.