2.5. Powder metallurgy

There are three types of metals and alloys (according to technological features):

- cast, that are castings and ingots;

- wrought alloys, i.e. alloys after metal forming;

- sintered alloys, that are alloys produced by methods of powder metallurgy.

Powder metallurgy uses metal and non-metal elements and their chemical compounds for manufacture of products. The powder metallurgy techniques comprises following stages:

- powders production;

- preparation of mixture of powders and technological additions;

- forming of an article by pressing process (compressing a briquette or green compact);

- sintering that renders the article proper strength.

This technique is more complex and expensive than casting or plastic working methods. But the powder metallurgy techniques attracts more and more attention since they offer ample scope for production of materials and parts with high heat and wear resistance, which display stable magnetic properties or specific physicochemical properties. Main advantage of these techniques is that mentioned properties are impossible to be obtained by casting or plastic working methods.

Powders are produced by mechanical and physicochemical methods. Mechanical methods do not change the chemical composition of material and prepare powder by two ways:

- grinding solids in ball mills, vortex chambers and vibratory mills;

- granulating the melt, that is spraying the liquid metal.

Mechanical methods are applicable only for hard and brittle materials, which are the base material of all cermets. These are the powdered carbides of such metals as tungsten titanium and tantalum the hardness of which is close to that of diamond.

Physicochemical methods enable to reduce crushed oxides (ores) or carbides to metal powders. Sizes of metal powders range from 0.005 to 0.5 mm.

Ball mills or vibratory mixers may be used to blend ingredients in required proportions.

Moulding the blend into various shapes is the process of single-action or double-action compaction (pressing) in dies by mechanical or hydraulic presses at a pressure of 150 to 800 MPa (Fig. 2.18).

Fig. 2.18. Single-action (a) and double-action (b) pressing of powders: 1-bottom; 2-powder;

3-container; 4-plunger

Another method of powder moulding is compaction of metal powder into strip, including bimetallic strip.

Sintering is the process of heating green compacts in vacuum furnace or in furnace with shielding gases (argon, nitrogen, hydrogen) at a temperature from 60 to 80 % of the melting temperature of the base metal and holding time from 1 to 2 hours.

To impart the parts the final shapes and desired properties, the sintered articles can be put through additional processing: heat treatment, diffusion heat treatment, and coining or sizing.

Powder metallurgy techniques are used for manufacture of:

- filters, because of porosity of articles (porosity ranges from 10 % to 50 % of volume; it is determined by moulding pressure);

- frictional materials, produced by additions of asbestos, oxides and carbides to metallic powders;

- antifrictional materials, produced by additions of graphite and plastics to metallic powders;

- cutting tools by sintering carbides of tungsten, titanium and tantalum with cobalt powder;

- fireproof materials by sintering oxides and carbides, which have high melting point;

- pseudoalloys, components of which can't form solution in liquid state (e.g. iron and lead).