Q.2. Explain the different properties of metal powder suitable for powder metallurgy process. Also discuss applications of powder metallurgy. (AKTU. 2011 - 12)
Ans. Main characteristic of powder for powder metallurgy: -
1. Morphology - The morphology of powders can be critical in determining both flow characteristics as well as playing an important role in heat and momentum transfer to the powder during spraying. Generally, the morphology of powders can be described as irregular, blocky or spherical. Irregular powders are characterised by the presence of a wide range of shapes from cubic like structures through needles. Blocky powders tend to have shapes where the largest and smallest dimensions of the powder particles are quite close. Spherical powders are broadly spherical in shape.
2. Powder size range and distribution - In general, powders are designated in terms of a simple size range. Historically these are often quoted in terms of the mest size used in traditional sieving classification. The mesh number actually defines the number of wires used per inch in a “standard” sieve. The symbol + and - are also used to denote, respectively, powder which is either retained by or passes through a particular sieve.
While sieves may still be used for classification of powders, measurement of size is frequently carried out by instrument using laser light scattering. These plot the size of the powder particles in terms of an “equivalent” diameter of a projected sphere. For spherical powder this is quite accurate but for irregular shapes (i.e., most material) the sizes given will always be larger than the “actual” sizes. Since in most thermal spray processes spray parameters are optimised so that the majority of powder particles reach the appropriate state it is clear that any significant shift in size distribution will have important consequences. Finer than usual powders will become overheated and vapourise and oxidise while coarser material will be insufficiently melted or accelerated. As a result, more and more end users are paying particular attention to the distribution of powder on a lot by lot basis.
The particle size distribution is important to the end user in the following ways:
* Direct impact on finished product quality
* Ease and efficiency of filling a die
* Wide size distributions allow voids between larger particles to be filled with smaller particles
* Porosity-fine particles leave smaller pores easily closed during the sintering process
* An excess of fine has negative effects on flow properties
* Caking, environmental contamination, pyrophoricity
3. Powder Flowability - The flowability of powders is very important in all thermal spray processes. Poor flow leads at the very least to inconsistancies in the powder feed rate and thus coating build-up rates. Generally, Hall flow is used to characterize powders. The Hall flow is measured by timing the flow of a sample of powder through a standard orifice. Hall flow is affected by both powder morphology and size range. Generally the more irregular shaped and finer the powder the worse is the flow.
4. Chemical composition - Obviously although the general chemical specification can be set, there can be wide variations in chemistry from lot to lot of powder. These variations can often be blended out by mixing different lots together. Howevere, there can also be problems in that while the average chemistry is correct e.g. less than 0.05% Fe, this impurity can be formed by clumps of iron particles present due to a mechanical treatment during sizing.
5. Sprayability - Regardless of how and what is specified in the resultant powder product, sprayability of the powder is often the only sure way of ensuring that the product is consistent. There is strong evidence that for sensitive material, the only way to ensure that the powder is suitable to carry out a spray trial prior to use. Here, coating characteristics are used to check the performance of each lot prior to release.
Application - Powder metallurgy is used in manufacture of parts for:
1. Automobile industry: motros, gears assemblies, brake pads
2. Abrasive: polishing and grinding wheels
3. Manufacturing: Cutting and drilling tools (using hard metals)
4. Electric and magnetic devices: magnets, soft magnetic cores, batteries
5. Medical and dental: prostheses, amalgams
6. Aerospace: motors, heat shields, structural parts
7. Welding : solder, electrodes
8. Energy: electrodes, fuel cells
9. Other: porous filters, bearings, sporting goods etc.
