Modeling of hot isostatic pressing of metal powder with temperature–dependent cap plasticity model

In this paper, the coupled thermo–mechanical simulation of hot isostatic pressing (HIPing) is presented for metal powders during densification process. The densification of powder is assumed to occur due to plastic hardening of metal particles. The constitutive model developed is used to describe the nonlinear behavior of metal powder. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, powder plasticity behavior, and frictional contact algorithm. A Lagrangian finite element formulation is employed for the large powder deformations. A modified cap plasticity model considering temperature effects is used in numerical simulation of nonlinear powder behavior. The influence of powder-tool friction is simulated by the use of penalty approach in which a plasticity theory of friction is incorporated to model sliding resistance at the powder-tool interface. Finally, numerical examples are analyzed to demonstrate the feasibility of the proposed thermo–mechanical simulation using the modified cap plasticity model in the hot isostatic forming process of powder compaction.     

温压技术中的致密化机制

根据聚合物的流动特性和粉末成形特点 ,试验研究了温压条件下金属粉末的塑性变形 ,理论分析了温压技术中的聚合物膜的成形和致密化机制。结果表明 :在压制初期 ,聚合物良好的流动性和较低的摩擦因数改善了金属粉末的充填行为。在压制末期 ,聚合物在金属表面形成一层微米或亚微米级薄膜。这层薄膜与基体结合牢固 ,将金属粉末与粉末隔离开来 ,促进粉末的塑性变形。     

An Analytical Model for Explosive Compaction of Powder to Cylindrical Billets through Axial Detonation

An analytical model, describing an explosive compaction process performed axially on a powder assembly of cylindrical geometry, is discussed. The powder is encapsulated in a cylindrical metal container surrounded by an explosive pad, which is detonated parallel to the major axis of the compact. The pressure generated in the powder is a function of the nature and the thickness of the explosive material as well as the powder characteristics. The model is based on the principle of shock propagation in powder aggregate and, the detonation as well as the refraction wave characteristics of the explosives. For the purpose of validation and illustration, this investigation considers the explosive compaction of aluminium powder particles for different explosive pad thicknesses. The model brings-out a closed-form solution for densification of powders. The density of the final powder compact depends on the pad thickness. Inadequate pad thickness leads to under compacted core, while higher pad thickness leads to melting at the core leading to over all low density. The optimum pad thickness of the explosive to produce the highest densification is thus determined using the model. The densification depends on the size of the powder particles also, since; the heat generated by the high pressure shock wave melts the surface of the powder particles depending on the specific heat, thermal conductivity and the latent heat of the powder material. The study essentially covers the effect of the explosive pad thickness and the particle size of the powder on densification. The analytical results are compared with a few experimental data and the comparison is found to be satisfactory.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Optimization of aluminum oxynitride compaction process using a Gray-coded genetic algorithm

The densification process for the ceramic spinel aluminum oxinitride (ALON) has been analyzed using biologically inspired genetic algorithms. A micro-mechanical model for powder compaction is coupled with the available experimental data and the optimized parameter set for the densification process is obtained through genetic algorithms and the mechanism of compaction highlighted.     

粉末高速压制成形密度分布的数值模拟及影响因素分析

将研究不连续体力学行为的离散单元法应用于粉末高速压制致密化过程的研究,将粉末视为黏弹性的离散颗粒,建立粉末高速压制过程颗粒接触模型及每个颗粒的基本运动方程,推导了力与位移表达的粉末高速压制黏弹性本构关系。基于PFC软件实现了铁粉高速压制过程中粉末颗粒二维流动情况及压坯密度分布的数值模拟,模拟结果的密度分布规律与实际压制的密度分布规律较为一致;利用数值模拟结果对影响压坯密度分布的摩擦因数、高径比、双向压制因素进行了具体分析。     

Cu-Cr粉体致密化颗粒变形及粒子流动三维数值模拟

基于MSC.Marc有限元软件对Cu-Cr粉体颗粒的单、双向致密化过程进行了细观数值模拟分析。研究了不同压制方式及摩檫系数对Cu-Cr粉体颗粒致密度及形貌变化的影响。结果表明:随着摩擦系数的增大,单向压制Cu-Cr粉体颗粒的致密化程度越高,摩擦系数为0.5时,单向压制的Cu-Cr粉体颗粒最高致密度为96.4040%;随着摩擦系数的减小,双向压制Cu-Cr粉体颗粒的致密化程度越高,在无摩擦理想条件下,双向压制Cu-Cr粉体颗粒致密度最高为89.1630%。在相同条件（摩擦系数、压制力）下,单向比双向压制Cu-Cr粉体颗粒有较高的流动性和致密度,Cu颗粒的应变量差值为1.3385,但双向致密化Cu-Cr粉体颗粒比单向压制的粒度均匀性好。模拟结果与实验结果相符合,验证了模型的准确性。      

THE ELECTROIMPACT COMPACTION OIF POWDERS: MECHANICS, STRUCTURE AND PROPERTIES

An electroimpact compaction method recently developed for powder consolidation is described In terms of the basic principles of electric discharge and dynamic compaction processes. The influence of processing parameters, microstructural characteristics and mechanical properties of preforms obtained are discussed. Mathematical models for the mechanics of compaction, electrical resistance, discharge current variations and comparisons with experimental results are presented. The best set of properties are obtained when electrical discharge is applied to cause interparticle fusion at the instant when dynamic compaction pressure attains its peak level.     

脉冲电流烧结机理的研究进展

脉冲电流烧结（Ｐｕｌｓｅ ｅｌｅｃｔｒｉｃ ｃｕｒｒｅｎｔ ｓｉｎｔｅｒｉｎｇ,ＰＥＣＳ）是材料科学领域开发出的一种新型快速烧结技术,已广泛应用于金属与合金、结构陶瓷、氧化物超导体、复合材料、热电材料、高分子材料以及功能梯度材料的制备．本文简介脉冲电流烧结特征,结合ＰＥＣＳ烧结条件对铜粉末和氧化铝粉体致密化及显微结构影响的实验证据,就脉冲电流烧结过程和机理进行探讨．     

磁场在金属材料制备成形中的应用

讨论了稳恒磁场、变化磁场对金属凝固组织、基体中合金元素的固溶度以及Al-Cu合金热裂的影响,介绍了动磁压制技术的原理、特点及应用情况,磁场在提高NdFeB永磁体的磁性能和在粉末固相、液相烧结致密化中的应用,并展望了磁场在金属凝固和粉末冶金中未来的研究和应用发展前景.     

Powder material parameters establishment through warm forming route

This paper presents the establishment of the properties of powder materials through experimentation at elevated temperature ranging from room temperature (30 °C) to 150 °C. Uniaxial die compaction experiments were conducted to establish the powder properties such as densification, Young’s modulus, spring-back, plastic index, elastic index, and plastic hardening coefficient. Shearing experiments were conducted to establish the temperature dependent friction coefficient. Iron powder ASC 100.29, manufactured by Höganäs AB Company was used during the experiment. The results showed that the deformation properties of powder materials especially iron powder are temperature dependent. The values of some properties are increased as the compaction temperature increased and vice versa.     

Preparation of Porous Ceramics from Nanocrystalline Zirconia and Their Microstructure

Data are presented on the compaction behavior of nanocrystalline yttria partially stabilized zirconia powder and the effects of compaction pressure, sintering temperature, and sintering time on the microstructure of the resultant ceramics. It is shown that even relatively low (50 MPa) compaction pressures lead to the disintegration of powder particles and aggregates. The compaction behavior of the powder points to changes in the densification mechanism: from quasi-liquid sliding of powder particles at the beginning of the process to the breakdown of large microstructural constituents at the end. In the initial stages of sintering, a robust skeleton forms, which plays a key role in determining the pore structure of the ceramic.     

Densification mechanisms in spark plasma sintering of nanocrystalline ceramics

Effect of the particle size on the possible electric discharge during the SPS was examined. Nanoparticle compacts enable accumulation of high electric charge, and discharge under conventional voltages used for the SPS. The critical particle size for the electric discharge is both morphological and material dependent. The early stages of densification of the nanocrystalline powder compact proceed either by the plastic deformation or grain-rotation coalescence and sliding, aided by softening of the particle surfaces. The active densification mechanism depends on the changes both in the mechanical and electrical properties with temperature. Densification of 11 nm nc-MgO particles with low yield stress proceeds by plastic deformation already at 700 °C. However, densification of 34 nm nc-YAG particles with high yield stress proceeds by nano-grain rotation aided by particle surface softening. Densification at the final stages of SPS is associated with diffusional processes, where curvature driven grain growth predominates.     

Effect of liquid phase on densification in electric-discharge compaction

The effect of liquid phase on densification in electric-discharge compaction (EDC) was explored in the present work. The temperature at contact area of particles in EDC was estimated from random packing model incorporated with electric current distributions. Consolidation of cemented carbide and tungsten heavy alloys was conducted under varying current densities. WC-11Co/Fe/WC-11Co sandwich powder compacts were designed to investigate the effect of liquid phase flow. It is found that the densification occurred only when liquid phase formed, and relative density increased with the increasing of liquid phase volume. In the case of WC-11Co powders, the faceted grain evolution occurred but the significant grain growth was hardly observed, which meant the densification was mainly induced by particle rearrangement. The depth of liquid penetration of Fe in WC-11Co/Fe/WC-11Co sandwich compact also agreed with that caused by particle rearrangement processing. The possible effects of electric current on densification were also discussed.     

Ultrasonic Characterization of Iron Powder Metallurgy Compacts during and after Compaction

Ultrasonic measurements in powder metallurgy (PM) compacts at various stages of production are presented both as a practical means of improving PM production and as a method of providing a fuller understanding of PM materials. Ultrasonic monitoring during powder compaction, a novel process instrumentation technique to follow powder densification, is reviewed. Measurements taken during the compaction of simple PM disk demonstrate that the ultrasonic velocity can be used as a measure of the in situ density. This connection arises due to the acoustic equivalence between powder during compaction and PM compacts after sintering. Ultrasonic monitoring during compaction of a two-level PM part is demonstrated to be fully capable of independently following the density in each level. The results also provide evidence of different regimes of powder flow behaviour during compaction. Ultrasonic velocity mapping of the two-level compact after sintering provides confirmation of the monitoring results. Subsequently, measurements of the ultrasonic velocity in green PM compacts are shown to be consistent with a dependence on the quality of inter-particle bonding. Finally, laser ultrasonic measurements in PM compacts are used to determine the ultrasonic attenuation. Attenuation values in a sintered compact are shown to follow a simple Rayleigh scattering dependence on frequency which yields a powder particle size consistent with the known value.     

Cold compaction of an aluminium/short fibre alumina powder composite

Al-20Si-3 Cu-1 Mg-0.3 Fe (wt%) rapidly solidified powder homogeneously mixed with 0, 5,10 and 20 vol % Saffil alumina fibres, was subjected to cold uni-axial, single-action die-compression with progressively increasing pressures up to 400 MPa and intermediate relaxation. The powders with 0 and 20 vol % fibres were also compacted to this target pressure in single steps and the resulting densification curves were compared to the progressive series curves. The densification behaviour during tapping (jolting) of the different powders was studied. Densification data were compared with two general and widely quoted compaction equations, being those due to Konopicky-Shapiro and to Kawakita. Above critical pressures, good agreement of experimental data with these relations was found. The tapping process of densification of the materials could be described by the Kawakita relation. The effect of both tapping and rigid-die compaction on fibre size was evaluated. Mean fibre size was not significantly affected by the tapping treatment. The extent of breakage during die compaction depended on the applied pressure.     

热复合磁脉冲粉末压坯致密度的试验研究

对加热Cu粉末进行磁脉冲致密试验研究,探索提高压坯致密度且不使粉末颗粒产生明显长大的致密方法.通过压坯平均致密度和微观金相形貌分析,揭示了加热温度、放电参数和粉末体高径比等工艺参数对Cu粉末热复合磁脉冲致密压坯致密度的影响规律.研究表明:压坯致密度不随温度升高而线性增加,200℃时的致密度最高;在给定放电能量和200℃下,压坯的致密度随电压和电容量增加而提高,随粉末质量增加而降低;3次放电显著改善压坯致密效果,致密度达98.75%,再增加次数的影响甚微.     

Fully coupled thermal–electric-sintering simulation of electric field assisted sintering of net-shape compacts

A fully coupled thermal–electric-sintering finite element model was developed and implemented to predict heterogeneous densification in net-shape compacts using electric field assisted sintering techniques (FAST). FAST is a single-step processing operation for producing bulk materials from powders, in which the powder is heated by the application of electric current under pressure. Previous modeling efforts on FAST have mostly considered the thermal–electric aspect of the problem and have largely neglected the sintering aspect of the problem. A new model was developed by integrating a phenomenological sintering model into a previously established thermal–electric finite element framework to predict the densification kinetics of the sample. The model was used to quantify the effect of specimen geometry on the evolution of thermoelectric gradients and resulting heterogeneous sintering kinetics during FAST processing of a conductive powder. It is shown that the new model which considers sintering kinetics and density-dependent properties provides a substantial increase in accuracy compared to thermal–electric only models. It is also shown that small changes in local resistance due to densification can greatly impact the distribution of thermoelectric gradients during the process, which are exacerbated by heterogeneous stress states induced by sample geometry. Experimental characterization of sintered specimens is used to provide qualitative validation of the model predictions.     

Plasma Activated Sintering (PAS) of Tungsten Powders

Plasma Activated Sintering (PAS) is a short time, high temperature densification process based on three main contributions: resistance sintering, pressure application, and plasma generation. PAS was applied to the densification of untreated tungsten powder at temperatures between 2050 and 2685 K for 2 to 8 minutes in air or vacuum. The density values ranged between 80.2 and 91.5 % of theoretical density. The initial micron grain size was retained after consolidation at temperatures of 2350 and 2400 K. Net shape consolidation of a cutting tool was achieved by PAS densification of WC-Co powders.

The clean grain boundary observed by high resolution electron microscopy in some of the PAS consolidated specimens suggests that activation of the powder surfaces may take place to enhance the densification process. This physical surface activation may be responsible for enhanced sintering of tungsten particles with no need for additives. The major benefits from this new non-conventional technique include an unusual short densification time (minutes as compared to hours for conventional densification), retention of unique initial microstructures and properties, as well as sintering in one single step with no requirements for binders, prior cold compaction or controlled atmospheres.