Characteristic of Metal Powders
Characteristic of Metal Powders: The performance of powder metallurgical parts is totally dependent upon the characteristics of metal powders. Most important characteristics of metal powders are powder particle size, size distribution, particle shape, purity, chemical composition, flow characteristics and particle microstructure. Some of the important properties are discussed as under.
Powder particle size and size distribution: Particle size of metal powder is expressed by the diameter for spherical shaped particles and by the average diameter for non-spherical particle as determined by sieving method ormicroscopic examination. Metal powders used in powder metallurgy usually vary in size from 20 to 200 microns. Particle size influences density/porosity of the compact, mold strength, permeability, flow and mixing characteristics, dimensional stability, etc. Particle size distribution is specified in terms of a sieve analysis i.e. the amount of powder passing through 20 or 40 mesh sieves.
Particle shape: There are various shapes of metal powders namely spherical, sub-rounded, rounded, angular, sub-angular, flakes etc. Particles shape influences the packing and flow characteristics of the powders.
Chemical composition: Chemical composition of metallic powder implies the type and percentage of alloying elements and impurities. It usually determines the particle hardness and compressibility. The chemical composition of a powder can be determined by chemical analysis methods.
Particle microstructure: Particle microstructure reveals various phases, inclusions and internal porosity.
Apparent density: Apparent density is defined as the weight, of a loosely heated quantity of powder necessary to fill a given die cavity completely.
Flow characteristics: Flow-ability of metal powders is most important in cases where moulds have to be filled quickly. Metal powders with good flow characteristics fill a mould cavity uniformly.
Mixing or Blending of Metallic Powders: After the formation of metallic powders, proper mixing or blending of powders is the first step in the forming of powder metal parts. The mixing is being carried out either wet or dry using an efficient mixer to produce a homogeneous mixture.
Compacting of Powder: Compacting is the technique of converting loose powder in to compact accurately defined shape and size. This is carried out at room temperature in a die on press machine. The press used for compacting may be either mechanically or hydraulically operated. The die consists of a cavity of the shape of the desired part. Metal powder is poured in the die cavity and pressure is applied using punches, which usually work from the top and bottom of the die as shown in Fig 1. Dies are usually made of high grade steel, but sometimes carbide dies are used for long production runs. In compacting process, the pressure applied should be uniform and applied simultaneously from above and below. The pressure applied should be high enough to produce cold welding of the powder. Cold welding imparts a green strength, which holds the parts together and allows them to be handled. The metal parts obtained after compacting are not strong and dense. To improve these properties, the parts should be sintered.
Sintering: Sintering is the process of heating of compacted products in a furnace to below the melting point of at least one of the major constituents under a controlled atmosphere. The sintering temperature and time vary with the following factors-
- Type of metal powder
- Compressive load used, and
- Strength requirements of the finished parts.
Fig. 1 Powder metallurgy die setup
In the sintering furnace, the metal parts are gradually heated and soaked at the required temperature. During this gradually heating process, powders bond themselves into coherent bodies. Sintering results in strengthening of fragile green compacts produced by the pressing operation. It also increases electrical conductivity, density and ductility of the powder metal parts.