Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

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     

Production of bronze powders by water jet cooled rotating disc atomisation

Abstract

New type of water jet cooled rotating disc atomisation unit was designed and constructed. The raw material was melted in graphite crucible with high frequency induction heating, and atomisation was performed in high purity argon gas atmosphere. Cu–10Sn alloy was atomised to investigate the effect of production parameters, such as disc speed, disc surface condition, liquid metal flowrate, disc fin number and superheat of liquid metal with respect to mean particle size and powder yield rate. The produced powders appeared spherical, rounded, ligamentous, irregular and flaky, depending on particle size. The mean particle size of produced powders was in the range of 100–250 μm with 65–85% powder yield rate depending of atomisation parameters. The ZrO₂ material coated disc with four fins gave the finer mean particle size and higher powder yield rate in comparison with uncoated disc with two fins.     

Guidelines for modelling cold compaction behaviour of various powders

Abstract

Through this study, the authors aim to anticipate powder behaviour by account of their intrinsic characteristics and thus giving guidelines for modelling industrial cold die compaction. Investigations are based on complete experimental testing of various industrial powders. Because of the very distinct material properties (metal or ceramic, presence of binder element or lubricant), as well as the morphology, these powders offer a wide range of compaction behaviours that are analysed in order to establish objective considerations for suitable modelling. A global overview of powder behaviour is then proposed, based on two behaviour subtypes regarding powder hardness. By analysing their main features (a new concept is also detailed) it is then possible to simplify the characterisation and modelling of any powder behaviour.     

Experimental investigation of yield behaviour of metal powder compacts

Abstract

In this paper the compaction and yield response of two steel and two copper powders are examined. These were chosen to determine how the material response depends on the type of material and the morphology of the powder particles. Experiments were conducted in a computer controlled triaxial cell. Here, concentration is on the response during simulated, frictionless closed die compaction, whereby the radial stress is controlled so as to keep the radius of the sample constant. The compaction process was stopped at regular intervals and a series of probing paths were followed in stress space to construct the yield surface for the compact.

The experimentally determined yield surfaces are compared with yield surfaces predicted by empirical models and micromechanical models of the Fleck type, which assume that the compact consists of monosized spherical particles. During the early stages of compaction the form of the yield surfaces for spherical powders are consistent with Fleck's micromechanical model, but the surfaces become less elongated in the direction of loading at high densities. The yield surfaces for irregular shaped powders are significantly different from the predictions of the Fleck micromechanical model. A modified anisotropic Cam-Clay model is proposed, which is able to predict yield surfaces for the four powders at all densification levels.     

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.     

Magnetic and structural properties of hot pressed Cu–SmCo5 composites obtained by mechanical alloying

Abstract

Cu–8 wt-%SmCo₅ alloys were obtained through mechanical milling for novel industrial applications. Copper and SmCo₅ powder mixtures were mechanically alloyed in a planetary ball mill to disperse SmCo₅ fine particles in the copper matrix with the aim to modify the structural, mechanical, electrical and magnetic properties. The resulting alloyed powders were characterised as a function of milling time. Under the magnetic field, SmCo₅ particles achieved M_s to improve the soft magnetic properties of copper–8 wt-%SmCo₅ to be used in dielectromagnetic components. The magnetic properties of Cu–8 wt-%SmCo₅ powders reached their optimum values after milling time ranging from 10 to 15 h. The consolidation of milled alloy powders was performed by uniaxial hot pressing at 923 K for 2 h under argon atmosphere to obtain dense compacts. The consolidation process resulted in good dense metal matrix composite materials with adequate properties of compression strength >900 MPa, 95 HRB in hardness, electrical conductivity up to 43% of that of the International Annealed Copper Standard (IACS) and magnetic properties such as coercive field, saturation and remanent magnetisation obtained at 218 Oe, 70·23 emu g^?1 and 6·09 emu g^?1 respectively at 300 K. The existence of a coercive field and a little magnetic memory of the consolidated system is a typical behaviour of magnetically soft materials. The variation of electric and magnetic properties and its dependence on structure strength change with milling time were discussed.     

Constitutive model for powders with characterisation of low pressure region of yield surface

Abstract

Numerical simulations of manufacturing processes rely on material characterisation. This highlights the need for a close combination of experimental and numerical work to model cold die powder compaction. Based on a previous experimental work by the authors, this document shows how the data collected can be used to formulate a constitutive model reflecting the material behaviour in a broad range of conditions. A plasticity model based on pressure and deviatoric stress was chosen. Particular attention was given to the definition of the yield properties under low pressure levels. This choice was motivated by the previous review of experimental techniques for powder characterisation and, in contrast with popular models which focus on the behaviour of the powder in conditions close to idealised frictionless uniaxial compaction, the updated model achieves a very good agreement with out of die tests for two different materials while retaining a single equation formulation for the yield surface.     

氧乙炔燃流氧化处理制备氧化物包覆ZrB_2/SiC核壳结构粉末特征与机理的研究

ZrB_2/SiC复合粉体熔点高,在等离子喷涂过程中难以获得良好的熔融状态,变形颗粒间存在大量缺陷,导致涂层抗氧化性能急剧下降。为此,本文设计了氧化物包覆ZrB_2/SiC复合粉末,并探索采用氧乙炔火焰热处理方法进行核壳结构粉末的制备。采用扫描电镜对不同试验阶段粉体表面及截面进行观察,发现氧化热处理后的粉体可以得到明显核壳结构的团聚粉,粉体的致密性也得到了明显的提高。采用EDS对三种粉体的成分分布进行了观察,探究了原位氧化法制备得到的核壳结构的粉体中Zr、Si、O主要元素的分布情况,结合XRD进行的物相分析,进一步证实了扫描电镜观察的结果,并对结果形成原因进行了分析。     

Cold Compaction of Copper Powders Under Mechanical Vibration and Uniaxial Compression

Physical experiments were carried out to study the cold compaction of copper powders under uniaxial compression using our self-designed equipment. Two kinds of copper powders with different particle sizes and distributions were considered. One-dimensional vibrations were utilized before compaction to systematically study the effect of parameters such as vibration frequency ω, amplitude A, and vibration intensity Г on the initial packing density. The macro-property and corresponding microstructures of compacts obtained from initial packings with and without vibrations were compared and analyzed. The results show that higher packing density can be obtained in the compaction of coarse powders with broad size distribution when other experimental conditions are fixed. For each powder, the evolution of packing density vs pressure takes on exponential correlation with high R ² value. Much denser and more uniform compacts can be realized with the aid of vibration which can improve the particle rearrangement and result in the filling of macro pores formed in initial packing, and the characterization on the microstructure identifies that the particles inside the compact become polyhedrons with regular shape and uniformly distributed.     

Analysis of compaction behaviour of wet granulated aluminium alloy powder

Abstract

The compaction behaviours of wet granulated aluminium powder were examined by uniaxial die compaction, and their effect on rearrangement and plastic deformation was analysed by using the Cooper–Eaton equation. Based on the calculation results and structure/morphology of the granulated powder, a new compaction model for granulated powder, which consists of three compaction mechanisms (macrorearrangement, microrearrangement, and plastic deformation), and a modified equation has been proposed in this study. A macrorearrangement indicates it to be a dominant factor on the compaction behaviour of granulated powder and the modified equation is sufficient to analyse the compaction behaviour.     

PM compaction equations applied for the modelling of titanium hydride powders compressibility data

ABSTRACT

The suitability of compaction equations commonly used in PM was investigated for the modelling of TiH₂ powder compaction. Compressibility data of TiH₂ powder fractions (<45, <150 and <355?μm) were obtained up to 800?MPa and fitted to the Heckel equation, as well as to the models of Gerdemann and Jablonski and Cooper and Eaton. Although a partial correlation was observed for the Heckel equation, the model provided a consistent approximation of TiH₂ powder yield strength. An accurate fit was observed for the Gerdemann and Jablonski equation; however, considering the brittleness of TiH₂, a more realistic depiction of the physical process was verified from the Cooper and Eaton model. By the addition of an exponential term to the original equation an excellent fit was attained, and compaction of TiH₂ powders could be appropriately described according to the mechanisms of initial density, particle rearrangement, fragmentation and plastic deformation.     

Optimisation of mechanical milling process for production of AA 7075/(SiC or TiB2) composite powders

Abstract

The present work concerns the processing of composite powders based on 7075 aluminium alloy by mechanical milling. A premixed powder (Alumix 431D, Ecka Granules, Germany) was used as the matrix material, and two different ceramic reinforcements (SiC and TiB₂) were chosen as reinforcements. The main objective was to evaluate the effect of the content and addition method of the process control agent as well as the content and type of reinforcement on the microstructural and morphological evolutions of the powder particles during milling process and the as milled properties of the processed materials. Results showed that regardless of the starting composition, alloying took place through three stages, in which deformation, cold welding and fracturing of powder particles were the main mechanisms involved respectively. The mechanically milled composite powders showed a fine and homogenous distribution of reinforcement particles. A higher content of reinforcement resulted in a lower crystalline size for the milled powders (～18 nm for composite powders containing 15 vol.-% ceramic particles).     

FACTORS AFFECTING THE COMPACTION OF TUNGSTEN POWDERS

Abstract

It is difficult to form tungsten powders into compacts by pressure-forming methods. The brittleness of the powder particles causes them to fracture under pressure instead of producing the typical “point welds” exhibited by more ductile particles. Because of this, the powder characteristics such as particle size, size distribution, and particle shape play a most important role in the compacting of tungsten powders.

Both regular- and irregular-shaped particles of tungsten powder are discussed as regards the formation of strong and dense compacts from these powders. Powders composed of irregular-shaped particles gave stronger, but less dense compacts. The effects of particle size and particle-size distribution are also considered. Each of these factors has individual as well as combined effects. It was found that certain critical particle-size distributions produced the densest compacts.

It is concluded that interlocking of particles, which is brought about by surface irregularities, and interfit, which is determined by correct particle-size distribution, are the determining factors in the compaction of tungsten powders.     

Predicting segregation of metal powders

Abstract

Close tolerances in powder quality are necessary for efficient production and control of these tolerances can be achieved only by detailed analysis of materials and processes. The quantification and identification of segregation effects has been a challenge, especially with the powders typically used in PM. An attempt to quantify segregation as a first step towards the predicting segregation behaviour of general powders is reported. Experiments have been undertaken using a lubricated iron alloy powder to analyse rolling segregation, and using am unlubricated powder to determine the effect of height of discharge on segregation. Bulk particulates characterised by small particle sizes, narrow size distributions, free flow, high particle densities and high inter-particle forces require careful experimental procedures to obtain repeatable results.     

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.     

Microstructural investigation of as cast and PREP atomised Ti–6Al–4V alloy

Abstract

A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V powders from an alloy electrode. The process parameters and their impact on the material properties are described. The effects of electrode rotation speed on the particle size distribution, particle shape and crystal structure are investigated in detail. Optical microscopy and scanning electron microscopy are used for microstructural characterisation. The analysis shows that cast and extruded Ti–6Al–4V alloys have equiaxial α and α+β phase structures, while plasma rotating electrode processed powder from the same alloy compositions has an acicular or martensitic (α) structure. The microstructure scale depends on the particle size. Microhardness measurements are used to assess mechanical property dependence on the microstructure of this alloy. The rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.     

THE COMPACTION OF METAL POWDERS BY ROLLING. II.–AN EXAMINATION OF THE COMPACTION PROCESS

Abstract

The compaction process is examined in detail. It is shown that, where particle deformation is concerned, compaction by rolling is similar to compaction by static pressing, with the addition of elongation of the particles in the rolling direction when the rolling pressure is sufficiently high. A method for determining the average roll pressure is described. A comparison of the rolling of a metal powder with the rolling of a solid bar, and the determination of the effect of particle shape and mean size, indicates that not only roll/powder friction but also the slip between particles plays an important role in the compaction process. This leads to an examination of the flow properties of powders, which are measured in terms of a “powder-viscosity factor” that indicates whether and at what order of rolling speed a powder can be coherently compacted. Finally, a mechanism of compaction is proposed on the basis of the present findings and on the authors’ earlier work.     

Densification rate of metal powders during hot uniaxial compaction

Abstract

A model to describe the densification rate of a metal powder aggregate undergoing constant uniaxial pressure and temperature conditions is proposed. The model is based on the power law creep equation, and it is obtained by using the equivalent simple cubic system, a theoretical tool proposed by the authors in previous work. This theoretical tool assumes that it is possible to predict the evolution of the densification under the pressure of an actual powder system via the study of the same problem in a system of deforming spheres packed into a simple cubic lattice. The proposed model is validated with the help of experimental data obtained from uniaxial hot compaction experiments carried out with aluminium, tin and lead powders. The agreement obtained between theoretical curves and experimental data is reasonably good.