Microstructural characteristics of shock consolidated 2124 Al alloy compacts

2124 alloy powders have been compacted using explosive compaction. The effect of process parameters like explosive pad thickness and impact energy imparted to the powders on the microstructure and hardness across the cross-section of the compact have been investigated. When the thickness of the explosive pad was increased to 9.5 mm, three distinct microstructures with different hardness values were found across the cross-section of the compact. The size and shape of the θ phase precipitates were different in the fine grained structure when compared with that of the original particles and triple point junctions. Central porosity and pipe formation were observed when the thickness of the explosive pad was increased beyond 14.5 mm. Variation in the microstructure of the compact across the cross-section disappeared when the diameter of the compact was increased from 11 to 25 mm. This revised version was published online in November 2006 with corrections to the Cover Date.     

Explosive Compaction of powders,principle and prospects

The explosive compaction method consisting of a cylindrical container surrounded by a proper type and amount of explosive is an inexpensive method to achieve high densities close to theoretical density. The explosive's parameters have to be adjusted to the type of the powder to be compacted. The required explosive's pressure is linearly related to the Vickers hardness of the metal powder particles. If higher pressures are applied, an “explosive liquid phase sinter” - process can be achieved, allowing the welding of individual particles. The residual properties of the material are characterized by a high defect structure and by dislocation densities and subgrain sizes comparable to those after heavy plastic deformation. The same is true of compacted ceramic powders. Enhanced sinter reactivity, chemical and catalytic reactivity may be the result of high values of stored energy observed in the ceramic materials, shock wave treated under conditions of explosive compaction. The properties of materials produced by shock-wave synthesis or by shock-wave transformation are also determined by a high-density defect structure.     

Energy and deformation in implosive compression of axisymmetrical metal cylinders

Implosive powder consolidation of axisymmetrical components is based on the utilization of the available impact energy in both deforming plastically the cylindrical powder container and affecting the compaction of its contents. The level of the energy available for actual compaction depends not only on the mass and radial velocity of the container, but also, for the given overall impact energy delivered, on its material and geometry. In the course of extensive investigations of the process of explosive consolidation of polymer powders and their mixtures, the relationships between the reduction in the outer diameter and the explosive/tube mass and, further, between the generalized strain and impact have been established experimentally. This information in conjunction with the calculated energy data, enables an assessment of the deformation and compaction components of the impact energy to be made. It also provides an insight into the response of a cylinder to implosive shock loading.     

Various microstructures suggesting possible shock compaction mechanisms

In order to reveal mechanisms of the shock compaction process for metallic powders, a shock recovery experiment for equiatomic NiTi alloy powder was performed using the flyer plate impact technique, and the microstructure at the central region of the sample after the shock treatment was observed under a metallurgical microscope. It is clearly confirmed from the microstructures that the powder is compacted by three mechanisms: (1) generation of a molten metal jet and its trapping, (2) dynamic friction between powder particles, and (3) plastic deformation around a void. The microstructure of the shock-compacted powder depends on the range of particle dimensions, which implies that the compaction process based on the above mechanisms is influenced by the powder particle size and its distribution. At the periphery of the sample, two structural features are observed. One, which is characterized by the large melt pool and the marked deformation and breakage of the powder particle, is located near the sample region to which a plane shock wave propagates through the sample capsule. The other shows the long melt band along the bottom and the side of the sample. These features are attributable to a shock wave with a nonuniform shock front and a radial shock wave, respectively.     

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。      

Modelling pressure-assisted densification by power-law creep

Densification of ceramic powders by power-law creep during pressure-assisted compaction is analysed. The proposed densification model is based on two existing power-law creep densification models: one for a relative density up to 0.9 (stage I) and the other for densities above 0.9 (stage II). Using these two models independently in their respective density ranges for predicting hot pressing of homogeneous alumina powder results in a discontinuity in the densification rate time history curves as well as in the radial and hoop stress time histories in the compact. To eliminate these discontinuities a novel method of combining the two models into a single unified model is presented. Blending of the models is based on the assumption that porosity changes gradually from being completely open at the beginning of compaction to completely closed at full density. Experimental data generated by hot pressing homogeneous alumina cylindrical compacts at two different temperatures of 1400 and 1450°C at different pressures were used to obtain the material creep constants that were employed in the unified model. This revised version was published online in November 2006 with corrections to the Cover Date.     

Shock-wave compaction of the granular medium initiated by magnetically pulsed accelerated striker

The experimental and theoretical investigation of the granular medium shock-wave compaction process is made. The striker acceleration in the experiments is realized by pulsed magnetic field force action. The granular medium is alumina nanopowder. The mathematical model of the process describes the energy dissipation, which is stipulated by the work made to compact the powder, considering the loss for the non-ideal reflection of the shock wave from the discontinuity surfaces on the borders of the body being compacted. There is a satisfactory fit of the theoretical calculations and the final compact’s density experimental data. In terms of the low-energy shock front approximation the problem of the time dynamics calculation of the granular medium state is narrowed down to the system of ordinary non-linear differential equations. The limitation the striker to low energies linearizes the system, allowing to take its rigorous solution. The influence of such parameters as starting voltage of the capacitor, powder and striker masses and wave resistances of contacting materials on the compaction process is analyzed.     

Microstructural characteristics of 2124 Al – 40 vol.% SiCp metal matrix composites produced by room temperature shock consolidation and hot shock consolidation

2124 Al – 40 vol.% SiCp metal matrix composites have been consolidated by room temperature shock consolidation using axisymmetric assembly, and by one-dimensional hot shock consolidation using underwater shock wave assembly. Trimonite powder explosive was used in room temperature consolidation and SEP explosive was used in hot shock consolidation. The thickness of the explosive layer used in room temperature consolidation was 27 mm. The thickness of the water layer employed in the hot consolidation experiments was 15 mm. The hot shock consolidation was carried out at 200°C, 300°C and 400°C. The microstructural variations across the cross section of the room temperature shock consolidated compact and, the effect of the temperature on microstructure and hardness of the hot consolidated composites have been investigated. Microstructural comparison was made between the composites produced by both room temperature consolidation and hot consolidation.     

硼粉型含能微囊乳化炸药的制备及其爆轰性能研究

针对传统乳化炸药高能敏感问题,采用悬浮聚合法设计了一种利用聚甲基丙烯酸甲酯（PMMA）包覆硼粉的含能微囊。利用该硼粉型含能微囊作为添加剂制备了乳化炸药。通过激光粒度分析仪和扫描电镜,对硼粉型含能微囊的微观结构进行了表征;利用同步热分析仪、爆热弹和空中爆炸测试系统,研究了硼粉型含能微囊对乳化炸药热稳定性、爆热以及冲击波参数的影响。实验结果表明:硼粉能够提高乳化炸药的冲击波特征参数和爆热,其中,冲击波峰值压力和爆热分别提高了29%和42%以上;而微囊包覆技术可以增加含硼乳化炸药的初始分解温度和活化能,改善其热稳定性。利用微囊包覆含能添加剂的方法,可以在不影响乳化炸药安全性的前提下提高其做功能力,改善工程爆破和爆炸加工效果,为研制安全高能乳化炸药提供了新的思路。     

快淬粉爆炸压制Nd-Fe-B永磁体的性能与结构特征

利用爆炸压制方法制备的快淬永磁体,其磁性能、压缩强度和密度都比相同粉料制备的粘结磁体有明显提高,其中最大磁能积（BH）max,提高了30％,压缩强度σbc增加了40%。扫描电镜观察显示,爆炸压制磁体的粉体颗粒表面出现了局部的熔融区域与大量的微裂纹。借助于场发射扫描电镜进一步观察发现,爆炸样品粉体颗粒表面出现纳米数量级的蜂窝状组织。磁力显微镜观察表明,爆炸压制不仅保持了原始粉末细小的晶粒尺寸,还保持了原始快淬粉细小的磁畴结构,磁体粉体颗粒中存在大量的微裂纹,但是微裂纹对快淬粉爆炸压制磁体的磁性能几乎没有影响。     

Low-temperature sintering of A1N ceramics with shock wave treated powder

A1N powder was shock wave treated under a pressure of 9.8 GPa. X-ray line broadening effect was observed in the shocked powder, which is attributed to the stored strain in the lattice caused by the shock wave treatment. The strain is appreciable and has a value up to about 3 × 10⁻³ for the (100) plane, which is released during sintering process and provides a driving force for densification. Sintered at 1610 °C for 4 h, the shocked compact doped with additives has a density of about 98% of the theoretical density, while only 80% for the unshocked compact. This suggests that the shock waves have an “active” impact on the A1N powder and make it possible for low-temperature sintering.     

Shock compaction of silicon carbide powder

A series of shock compaction experiments on SiC powder were carried out over a wide range of shock pressures and shock temperatures up to 30 GPa and 3400 K. Large changes in some physical properties and a variety of unique microstructures were observed in the shock-treated samples with changes in impact conditions. For an iron plate impactor, the optimum impact condition, which depends on the initial density, is 2.5 km sec^–1 for 70% initial density and 2.0 km sec^–1 for 50%. The best-sintered compact of SiC with 97% density and micro-Vicker's hardness of 2700 kg mm^–2 was obtained under optimum conditions. Good compacts with high relative density and high strength exhibit the disruptive effects of the shock wave, which are indicated by microstrain increase and crystallite size reduction. The skin model is presented here in order to estimate the heterogeneous shock state which is realized under and after shock loading of the initial powder aggregates.     

Cold compaction of polymeric powders

Using single end compaction in an instrumented cylindrical die, the compaction characteristics of a variety of polymeric powders have been studied. The powders tested range from those which compact very well to those which do not compact at all. Using scanning electron microscopy, the particle size and shape for each powder has been noted and used in conjunction with other physical properties and deformation characteristics to identify the factors which enhance compactability. In general it has been found that a powder with a combination of small particle size, irregular shaped particles and a relatively low bulk modulus is most likely to compact well. As the physical characteristics depart from these desirable conditions the compactability of the powder is reduced.     

粉末冶金高致密化的新途径

针对粉末冶金行业近年来出现的提高制品致密化的新途径，重点综述了温压技术、模壁润滑技术、动态磁力压制技术、爆炸压制技术、高速压制技术、等离子放电烧结技术的原理、特点和应用情况。     

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.     

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.     

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.     

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。     

Densification model for powder compacts

The driving force of densification has traditionally been modeled on the basis of local curvature changes between powder particle pairs. Extension of particle pair analysis to powder compacts involving billions of particles has not been successful because of the geometric difference between the two cases. In this paper, a densification stress model for grain boundary and lattice diffusion controlled densification is developed on the basis of a powder compact's thermodynamics and the internal surface area evolution. For compacts with a constant grain size, the model predicts that the densification stress increases as a function of relative density, which is in agreement with experimental trends. With grain growth, the densification stress becomes relatively constant throughout the intermediate stage of densification, in agreement with experimental data in the literature. Comparison of densification rate data with densification rate model employing the developed densification stress relation also gives good functional agreement. These agreements indicate that modelling densification stress and densification rate on the basis of internal surface area captures the essential physics of powder compact densification.     

Densification of the granular medium by the low amplitude shock waves

The process of the granular medium densification by multiple low amplitude shock-wave treatment is studied. The granular media are alumina based nanopowders. The mathematical model of the process describes the energy dissipation that is governed by the powder densification work being made and also connected with the non-ideality of the shock waves reflection from the discontinuity surfaces on the borders of the body being compacted. The maximum possible densification limit upon the multiple shock treatment on the granular medium at the conditions of fixed starting velocity of the striker is determined analytically. The shock-wave densification of the nanopowders depending on the number of the shock actions is calculated numerically. The influence of such parameters as individual characteristics of the powder, powder and striker masses, striker speed, and wave resistances of the contacting materials on the process is analyzed.