The Decomposition of Ag Oxalate in Ball Drop, Rod Drop, and Ball Milling Experiments: A Tentative Estimation of the Volume of Trapped Powder

The current study provides an experimental estimate of the volume of powder trapped at collision during the mechanical processing by ball milling (BM). The result has been obtained by carrying out ball drop and BM experiments on anhydrous Ag oxalate powders. In both cases, the mechanical stresses operating at collision induce the decomposition of Ag oxalate into metallic Ag and gaseous carbon dioxide. The evolution of carbon dioxide enabled us to measure the amount of Ag oxalate decomposed at each collision by gravimetric analyses. Ball drop and rod drop experiments, respectively, with selected thickness of powder layers and volume of powders, were carried out to relate the amount of decomposed Ag oxalate to the volume of powder trapped at each collision during BM experiments.     

机械镀锌过程中锌粉的受力及运动分析

采用粒径0.1-8.0μm范围的金属锌粉,在室温下以机械镀方式形成锌镀层。对镀桶内物质运动状态、机械碰撞力及锌粉颗粒之间的相互作用力进行了分析计算。结果表明：表面张力、液桥作用力和气泡团聚力是引起锌粉颗粒聚团的主要原因;分子间作用力、双电层作用力和溶剂化膜作用力对锌粉颗粒的聚团作用影响甚微。锌粉的紧实变形和镶嵌成层主要依靠机械碰撞力。     

Towards stiffness prediction of cellular structures made by electron beam melting (EBM)

Abstract

Additive manufacturing is a novel way of processing metallic cellular structures from a powder bed. However, differences in geometry have been observed between the CAD and the produced structures. Struts geometry has been analysed using X-ray microtomography. From the 3D images, a criterion of ‘mechanically efficient volume’ is defined for stiffness prediction. The variation of this criterion with process parameters, strut size and orientation has been studied. The effective stiffness of struts is computed by finite element analysis on the images obtained by X-ray tomography. Comparison between the predicted stiffness and the effective one tends to show that the efficient volume ratio leads to a slight underestimation of the stiffness. Finally, the effective stiffness is used at the scale of a unit cell. This can help define the build orientation and loading direction that lead to the highest stiffness.     

How the nozzle position affects the geometry of the melt pool in directed energy deposition process

ABSTRACT

This work deals with Directed Energy Deposition (DED) that is a process in which the material is delivered directly into the melt pool. Several kinds of nozzle have been developed and used in this process. However, the position of the nozzle, mainly for the lateral one, plays a crucial role in the geometry and porosity content of the melt pool. Therefore, to investigate the correlation between the lateral nozzle position and the geometry of the melt pool, three different sets of single tracks of Ti–6Al–4V were deposited at different nozzle positions. It was found that helpful information regarding the selection of the optimal position of the lateral nozzle can be obtained from the melt pool features. It can be noticed that by increasing the distance between the nozzle position and substrate, the powder capture decreases and accordingly results in a high depth of fusion zone and also keyhole defect formation.     

Cleaning the basis metal of surface films in the spray-deposition of powders

Conclusions In gasothermic spray-deposition the surface of the basis material experiences self-cleaning as a result of the disintegration of surface films by frictional forces set up by the spreading layer of particles and their subsequent removal to the periphery of the contact zone. The intensity of self-cleaning of the surface of a basis material grows with increasing velocity of the particles being deposited. The self-cleaning phenomenon occurring on the basis material in the zone of collision with particles being deposited facilitates the formation of physical contact with the powder particles subsequently interacting with this zone on the surface of the basis material.Translated from Poroshkovaya Metallurgiya, No. 11(251), pp. 41–47, November, 1983.     

Process simulation of powder injection moulding: identification of significant parameters during mould filling phase

Abstract

Simulating and optimising the powder injection process are complex problems since a number of linked material, geometry and process variables have to be considered. In addition, it is very difficult to identify critical parameters for designing binder systems, feedstocks, parts, moulds and processing conditions owing to the fact that multiple objective functions have to be considered. Towards the goal of identifying the level of significance of various material, process and geometry parameters during powder injection moulding, a systematic procedure for sensitivity analysis has been successfully developed for the mould filling phase of the PIM process. In this sensitivity analysis, all input parameters were defined for the mould filling simulation and all output parameters for optimum design of part, mould and processing conditions and dimensionless sensitivity values for all input and output parameters were calculated, which allow parameters with different units to be compared quantitatively. The sensitivity analysis procedure developed will be an invaluable tool for both the design engineer in the PIM industry who has to determine the critical input parameters for given design targets, as well as for the production engineer who has to optimise and monitor the production stage.     

Model of the nonlinear elastic behavior of a powder composite material. I. Properties of heteroresistant powder materials

A nonlinear elastic isotropic body model is proposed to describe the mechanical behavior of a powder composite material with different properties under tension and compression. The composite is considered as a three-phase body with an elastic matrix, throughout the volume of which pores and rigid inclusions are distributed. The unsymmetrical properties of this material are connected with the nucleation of defects, in particular with decohesion or the formation of discontinuities at the matrix—inclusion interfaces.     

Numerical Modeling of the Compaction of Powder Articles of Complex Shape in Rigid Dies: Effect of Pressing Method on Density Distribution. 1. Mechanical Model of Powder Densification

A theory of plasticity of a porous body was formulated taking into account the specifics of powder behavior under pressing. The proposed model of the material under compression is one-parameter with all functions depending on the current density. In order to determine the parameters of the model, use was made of the equilibrium density attained during the compression of an unbonded powder body, that value of density beyond which further deformation is not accompanied by volume change. Methods for determining the material parameters of the model are described.     

The effect of parameters of dynamic loading on the propagation character of shock waves in a powder

Based on the previously introduced mathematical model of the dynamic compaction of a powder material, the effect of the initial velocity of a tool and the ratio of its weight to the blank weight, which varies in a range of 10²–10⁴, on the shock velocity in the powder is considered using a titanium sponge as an example. According to the model, nonreversible volume changes in the material, which occur under the shock action, are considered on the basis of general conservation equations of mass, linear momentum, and energy, which are written for the discontinuity surface. It is shown that, in the investigated range of parameters of dynamic loading, the relationship between the velocities of the shock wave front and the tool can be approximated by a linear dependence. The pressure at the shock wave front and the shock pulse time are determined.     

Densification behavior of dynamically shock compacted AI2O3/ZrO2 powders synthesized through rapid solidification

The rapidly solidified alumina-zirconia eutectic contains high volume fractions of nanocrystallinet- ZrO₂, which makes the material a promising precursor for the manufacturer of fracture-resistant ceramic specimens. Unfortunately, conventional powder processing and sintering techniques are inadequate for the fabrication of dense specimens using this material. We have used dynamic shock compaction to facilitate the achievement of high density specimens which retain the unique microstructure of the precursor material. In an attempt to quantify the dynamics of the microstructural evolution which occurs during the compaction process, we have investigated the effect of various particle size distributions on the densification behavior of the material during the shock compaction and postcompaction sintering cycles. The shock compaction process produced high densities (∼73 to 78 pct of single-crystal theoretical) by inducing a highly efficient packing of the particles. A bimodal powder distribution was also compacted and this specimen exhibited a relative density of 86.2 pct, approximately 10 pct higher that those of the unimodal compositions. In this compact, the small particles efficiently filled the interstices between the larger particles. The high density of the bimodal compact did not translate to a high sintered density, however. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

A simple but realistic model for laser cladding

A model which takes into account the main phenomena occurring during the laser-cladding process is proposed. For a given laser power, beam radius, powder jet geometry, and clad height, this model evaluates two other processing parameters, namely, the laser-beam velocity and the powder feed rate. It considers the interactions between the powder particles, the laser beam, and the molten pool. The laser power reaching the surface of the workpiece is estimated and, assuming this power is used to remelt the substrate with the clad having been predeposited, the melt-pool shape is computed using a three-dimensional (3-D) analytical model, which produces mmediate results, even on personal computers. The predictions obtained with this numerical model are in good agreement with experimental results. Processing engineers may therefore use this model to choose the correct processing parameters and to establish cladding maps. Formerly Postdoctoral Fellow, Physical Metallurgy Laboratory.     

堇青石材料的应用

堇青石因具有体积密度小、热膨胀系数小和结构较疏松等特点而被广泛用作优质耐火材料、电子封装材料、催化剂载体、泡沫陶瓷、生物陶瓷、低温热辐射材料等。可以采用溶胶—凝胶法和水解—沉淀法合成堇青石超微粉 ,其成分均匀 ,优于传统的固相法     

Model of Plastic Deformation of Powder Materials Taking Account of the Proportion of Contact Volume

A structural model is suggested that takes account of the contact nature of plastic deformation of a powder body. It is shown that in averaging local stresses and strain rates the volume of averaging can be presented as a combination of straight cylinders constructed at all contact areas of a particle and having a common nucleus formed by the intersection of cylindrical bodies. On the whole a plastically deformed powder body is an orientated contact-rod system composed of cylinders that are in contact by their bases and that experience uniform tension-compression deformation. The volume fraction of powder body contact volume determined using Bal'shin's and Zhdanovich's equations can be a quantitative measure of the volume fraction of plastically deformed material. The contact-rod model satisfies boundary conditions for the poured state of powder and it is good agreement with experimental data for isostatically compacted metal powders.     

Powder Metallurgy Group Meeting,Edinburgh, 24–26 October 1983: Report of proceedings

Abstract

Porosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of the diameter of the pore, whereas the total volume of material to be damaged between pores is proportional to the cube of the interpore spacing. The present study found that cycles to initiation clearly depended on the volume of highly stressed material adjacent to pores, relative to the volume of void free material between pores. The correlation suggests that porosity effects on fatigue crack initiation are primarily stress concentration effects: pores as crack precursors seem less important. PM/0323     

A multi-phase model for plumes in powder injection refining processes

A one-dimensional steady-state model for momentum transfer in ascending gas-liquid-powder plumes has been developed for conditions relevant to powder injection refining processes. Inter-phase transport of momentum permits the calculation of the volume fraction and velocity of the gas, liquid, and solid phases as they rise in the melt. The effects of plume geometry, initial conditions, density of the phases, head of liquid, bubble size, and powder loading and properties are assessed. Liquid velocities in gas-only injection into water compare favorably with available experimental data. Significantly lower liquid velocities are predicted in liquid metals. At solids loadings used in commercial powder injection stations, the powder is expected to have only a small effect on liquid velocity. For wire injection conditions with little gas release, liquid velocities are considerably smaller. Formerly with McMaster University     

Microstructural characterisation of CuAgZr powder particles produced by argon gas atomisation

CuAgZr alloy is known for its high strength and high conductivity, which are reasons for its wide application. In this work, CuAgZr powder alloys were produced by high-pressure argon gas atomisation. The results indicate that the surface morphology of the powder is a cellular crystal structure with a pentagonal crystal structure, with a ‘rose-like’ lattice and cross-shaped dendrites. A three-dimensional model of the primary dendritic changes in the CuAgZr alloy powder with different powder diameters was established by studying the surface morphology and internal microstructure of the powder. The relationship between the powder diameter and the powder cooling rate was established as the function of the primary dendritic length (PDL), the primary dendritic volume fraction (PDVF), the secondary dendrite arm spacing (SDAS) and the secondary dendritic volume fraction (SDVF). A confidence validation was performed, and the function fitting error is within 10%. The results establish that the function is meaningful.     

Development of the Aluminum Powder Composite Based on the Al–Si–Ni System and Technology of Billet Fabrication of This Composite

Results of an investigation into the development of the composition and fabrication technology of compact billets of the aluminum powder composite material based on the Al–Si–Ni system for space-rocket hardware are presented. The composite production was performed as follows: initially the powder of the matrix alloy is prepared by gas sputtering, and then a mixture of the matrix alloy powder and alloying dispersed additives is subjected to mechanical alloying in high-energy apparatuses. The method of degassing the mechanically alloyed composition in a thin layer (in order to exclude material ejection from a container when degassing a larger volume of powder) and process regimes of composition compaction are developed using the unique equipment available at OAO Kompozit (Korolev, Moscow oblast)—a vacuum press. Using this technology, cylindrical briquettes up to 100 mm in diameter and up to 120 mm in height are fabricated. Newly developed and patented Kompal-301 composite material has substantial advantages over SAS-1-50 power alloy applied for similar purposes. Its thermal linear expansion coefficient is lower by a factor of 1.5, while the precision limit of elasticity is higher by a factor of 2–3 upon similar strength characteristics. The final structure of a compact briquette is a matrix in which dispersed particles of excess silicon are distributed rather uniformly against the background of the aluminum solid solution. Coarser isolated silicon particles are met in separate regions of the structure. Unfortunately, they are the cause of low plasticity of briquettes, which prevents the formation of semifinished products by plastic deformation; however, such low plasticity does not immediately negatively affect the fabrication of the briquettes themselves.