Metal powders flowability

Plow chaiacterlstic.s. Angle of repose and flowability are measurable charac teristics for which standard tests are available (e.g., ASTM Test B213-48, Flow Rate of Metal Powders, etc.). A steeper angle of repose would indicate less flowability. The term Hubricity has sometimes been used for solid particles to correspond roughly to viscosity of a fluid. 

There is, of course, a direct way of testing flowability by measuring the time for a standard amount of powder to be discharged from a specified funnel or hopper. This is sometimes referred to as free flow time , as for example specified by an ASTM standard method40 for testing metal powders the funnel used is the same as in the International Standard ISO 3923, Part 1. The ASTM method was used in measuring the flow time of sands in a study of particle morphology (see Ref. 10, Chapter 17). 

Powders are finely divided solids, smaller than 1000 in its maximum dimension. A particle is defined as the smallest unit of a powder. The particles of powder may assume various forms and sizes, whereas the powders, as an association of such particles, exhibit, more or less, the same characteristics as if they were formed under identical conditions and if the manipulation of the deposits after removal from the electrode was the same [1,2]. The size of particles of many metal powders can vary in a quite wide range from a few nanometers to several hundreds of micrometers. The most important properties of a metal powder are the specific surface, the apparent density, the flowability, and the particle grain size distribution. These properties, called decisive properties, characterize the behavior of a metal powder. 

The more irregular the particle shape is, the less efficient is the powder flow. Resistance to flow is the main feature of friction, which decreases as the particles approach a smooth spherical shape. The effect of particle size distribution on the powder flowability is also important. If the powder consists of monosized particles, which are more or less in point contact with one another, making the contact surface as low as possible, powdered deposits can flow. If the powder consists of different particles, the interstitial voids of the larger particles can be filled by the smaller ones, the contact surface area increases, and the flow of the powder is less efficient [1]. Hence, the best conditions for the free flow of the powder are fulfilled if the powder consists of monosized particles of spherical shape with a surface structure approaching to the stmcture of a smooth metal surface. This happens when the surface parts of the particles corresponding to the metal segments are larger than, or equal to, the pores between them [45], as illustrated by Fig. 6.12, and can be discussed quantitatively as follows. 

As stated in Introduction, some properties, called the decisive properties, characterize the behavior of metal powder. The most important of them are the specific surface, the apparent density, the flowability, the particle grain size, and the particle size distribution [1]. These properties were analyzed by Popov et al. [58-73] which showed that some of them can be mutually related, as well as that the specific surface of copper powder can be related to the overpotential of electrodeposition [58, 59]. 

Bulk characteristics, such as temperature, bulk density, and flowability, may be adjusted in a preparatory step to improve size enlargement. Prior to briquetting into ration-sized agglomerates, vegetables, food pulps, and fruit juices maybe frozen. Metal dusts and powders, as well as certain minerals, are often heated and briquetted hot to make use of their increased malleability at elevated temperatures. 

It is well known that flowability of copper powders [3, 64] mainly depends on the apparent density of a copper powder. It was shown by Peissker [64] that free flow of a copper powder can be only expected if the apparent density of the powder is larger than 2.2-2.3 g cm , while poor flow is possible at lower densities also. This can be explained in the following way. It was shown [44] that copper powder can be treated as a continuous medium, the density of which is equal to the apparent density of the powder pad- The density of compact metal is p . As shown in Fig. 6.13, the continuous medium can be divided into equal cubes with edge height a. It is obvious that spheres in the radius of which is aJ2 occupy effectively the same volume and that a powder consisting of spherical particles equal to each other will be characterized by ftee flow. Hence, the representative particle of a copper... 

For the analysis of the flowabUity of the powder, due to the existence of nonsieved powders which can flow, the shape and the structure of the powder particles are more important than particle size distribution. Flowability of nonsieved powders occurs when the surface parts of the particles corresponding to the metal segments are larger than or equal to the pores between them [65]. 

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