Q. Describe the various characteristics of metal powders to be considered before their selection for any process.
Ans. Metal Powder Characteristics
Both the consolidation and the properties of the final product depend upon certain characteristics of metal powders which are discussed below :
1. Surface Area : This is the area per unit mass of the powder. It indicates the area available for bonding and also the area on which adsorbed films or contaminants may be present.
2. Density : The degree of porosity in a sintered or unsintered powder metallurgy component is determined indirectly by making density measurements. The density of a metal in three different conditions is commonly dealt with in the science of powder metallurgy. These are given below :
(i) Theoretical density : Theoretical density or True density is the mass per unit volume of the solid material. It is the density of a non-porous metal in the solid state, that is, when it has been manufactured by non-powder metallurgy methods such as casting followed by hot and cold working. Any and all the pores get completely closed and the material becomes fully homogeneous. The theoretical density of a material is the highest possible density (100%) it can have. This density is used as a standard basis of comparison with other types of density. For iron, its value is 7.87 gm/cm3.
(ii) Apparent density : The 'Apparent density' in gm/cm3 defines the actual volume filled out by the losse powder. It is a very important property of the powder and depends upon the particle shape, size and size distribution. The apparent density irregularly shaped particles will be lower than that of the spherical particles and so also of the coarser particles as compared to finer particles of the same shape. It is often expressed as a percentage of the fully dense material, that is, as a percentage of true density.
It is measured by subtracting the weight of an empty container from the weight of that container and the powder filling it and then dividing by the inside volume of the container. For iron powder, its value ranges from 2.5 to 3.5 gm/cm3.
(iii)Tap density : This density is obtained by tapping or vibrating the
container and signifies the compaction achievable without the application of pressure.
(iv) Green density : It is the mass of compacted powder in an unsintered pressed briquette per unit volume of the briquette. The green density of iron powder usually rangers from 6.7 to 7.5 gm/cm3.
3. Compressibility and Compression Ratio : The term compressibility denotes the change in green density at some specified pressure (450 to 500 MPa). Compression ratio is defined as the ratio of the unpressed volume and pressed volume of the powder for a given mass. Both these properties depend upon the shape, size and size distribution of the powder particles. The compression ratio usually varies between 1.5 to 3. Of course, for very fine powders it may be as high as 10. However a lower, compression ratio is preferred to reduce the depth of die and the plunger movement in the die and thereby the wear and tear of the tools. 'Compacting ratio' is defined as the ratio of the green density of the part to the apparent density and it means the same thing as the compression ratio.
Other related terms used in powder Metallurgy practice are :-
(a) A compact : It is the briquette made by compacting powder. This compact is called "Green compact". After this compact is sintered, it is called "Sintered compact".
(b) Fill or Powder fill : It is the amount of powder needed to completely fill the die or container cavity, prior to the compacting operation. The cavity should be filled in the shortest possible time to avoid delay of production. The amount of powder representing this fill is usually expressed as the height of the powder in the die-cavity needed to result in a compact of the desired dimensions and density. Thus,
Fill = Median height of compact desired x Compression Ratio
4. Flow Rate : It is defined as the time required for a measured quantity of powder to flow out of a standard orifice. It is an important property of the powder because the time needed to fill the die should be small to obtain higher production rates and economy. Flow rate or flowability depends upon the shape, size and size distribution of the powder particles. Smaller and spherically shaped particles have the maximum flow rate. Very fine particles (of the order of 1 to 10 microns) will flow just like a liquid. When such a powder is pressed in die, it will flow into complex die cavities. Flowability can also be increased by the addition of stearate to the mix.
5. Particle Shape, Size and Size Distribution : As discussed above, these three properties greatly influence the apparent density, compressibility and flow rate of the powder.
The particle shape depends largely on the method of powder manufacture. It may be spheroidal, nodular, irregular, angular, lamellar, acicular and dendritic. Spheroidal particles have excellent sintering properties. However, irregularly shaped particles are superior for practical moulding and achieving good green strength of the part because they will interlock on compacting.
Particle size or fineness is expressed by the diameter of the spheriodal particles and by the average diameter of the other particles. It is determined by passing the powder through standard sieves or by microscopic measurement. Particle size should neither be too large nor too small. Too large particles may not display the required structure for the P/M route and may not result I high densities. On the other hand, it is difficult to handle fine particles and they may large quantities of undesirable absorbed substances and on metals, also oxides. However, finer powders have excellent sintering qualities and are particularly important for 'Slip Casting'. The usual size of powders used in P/M varies from 4 to 200 microns, with 100 microns being the most used size.
Particle size distribution refers to the quantity of each standard particle size in the mix (powder). It influences apparent density, compressibility flowability, final porosity and the strength of the part. For a given material, the strength of the part will be proportional to its final density after pressing. Theoretically, a powder containing varying particle sizes will result in greater density, because smaller particles will fill up the interspaces between the large particles. However, when mixing, the finer particles have the tendency to separate and segregate. Due to this reason, many users prefer uniform size particles and rely on the compacting pressure to get the final density required.