Metal powder industry
Powder metallurgy is a field of technology covering a set of methods for manufacturing metal powders and metal-like compounds, semi-finished products and products from them (or mixtures thereof with non-metallic powders) without melting the main component.
Powder metallurgy technology includes the following operations:
- Production of initial metal powders and preparation of charge (mixture) from them with specified chemical composition and technological characteristics;
- Shaping of powders or their mixtures into preforms with predetermined shape and dimensions (mainly by pressing);
- Agglomeration, ie heat treatment of blanks at a temperature below the melting point of the entire metal or its main part.
After sintering, the products usually have some porosity (from a few percent to 30-40%, and in some cases up to 60%). In order to reduce porosity (or even completely eliminate it), improve mechanical properties and fine-tune to exact dimensions, additional pressure treatment (cold or hot) of sintered products is used; sometimes additional thermal, thermochemical or thermomechanical treatment is also used.
In some variations of powder technology, the molding operation is eliminated: powders are agglomerated, poured into the appropriate molds.
Stages of powder metallurgy technology
- Mechanical grinding of metals in vortex, vibration and ball mills (obtaining large (100 or more microns) powders of irregular shape);
- Atomization of liquid metals into air or water: its advantages are the possibility of effective purification of the melt from many impurities, high productivity;
- Obtaining powders of iron, copper, tungsten, molybdenum by high-temperature metal reduction (usually from oxides) with carbon or hydrogen;
- Electrolytic deposition of metals;
- Thermal dissociation of volatile metal carbonyls (carbonyl method).
Advantages - obtaining finely dispersed (0-20 microns) iron powder of the correct form, with certain radio engineering properties.
The main method for molding metal powders is pressing in molds made of hardened steel under a pressure of 200–1000 MN/m2 on high-speed automatic presses. Pressings have a shape, dimensions and density, given the change in these characteristics during sintering and subsequent operations.
The importance of such new cold forming methods as isostatic pressing of powders under uniform pressure, rolling and MIM technology is growing.
Sintering is carried out in a protective environment (hydrogen; an atmosphere containing carbon compounds; vacuum; protective fillings) at a temperature of about 70-85% of the absolute melting point, and for multicomponent alloys - slightly higher than the melting point of the most fusible component. The protective environment must ensure the reduction of oxides, prevent the formation of undesirable product contamination, prevent the burnout of individual components (for example, carbon in hard alloys), and ensure the safety of the sintering process.
The design of sintering furnaces should provide for not only heating, but also cooling of products in a protective environment. The purpose of sintering is to obtain finished products with a given density, dimensions and properties, or semi-finished products with the characteristics necessary for subsequent processing. The use of hot pressing (sintering under pressure), in particular isostatic, is expanding.
Advantages of Powder Metallurgy
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The possibility of obtaining materials that are difficult or impossible to obtain by other methods. These include:
- Some refractory metals (tungsten, tantalum);
- Alloys and compositions based on refractory compounds (hard alloys based on tungsten carbides, titanium, etc.): compositions and so-called pseudo-alloys of metals that do not mix in molten form, especially with a significant difference in melting temperatures (for example, tungsten - copper) ;
- Compositions of metals and non-metals (copper - graphite, iron - plastic, aluminum - aluminum oxide, etc.);
- Porous materials (for bearings, filters, seals, heat exchangers), etc.
- The possibility of obtaining some materials and products with higher technical and economic performance. Powder metallurgy allows you to save metal and significantly reduce the cost of production (for example, in the manufacture of parts by casting and cutting, sometimes up to 60-80% of the metal is lost in sprues, goes into shavings, etc.).
- By using pure initial powders (such as carbonyl method), it is possible to obtain sintered materials with a lower content of impurities and with a more accurate match to the desired composition than conventional cast alloys.
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With the same composition and density, sintered materials, due to the peculiarity of their structure, in some cases have higher properties than fused ones, in particular, the adverse effect of the preferred orientation (texture) that occurs in a number of cast metals (for example, beryllium) is less affected due to specific conditions of melt solidification.
A big drawback of some cast alloys (for example, high-speed steels and some heat-resistant steels) is a sharp discontinuity of the local composition caused by segregation (the process of separating an initially homogeneous melt when the temperature is lowered into two immiscible liquids of different composition) during solidification.
- The dimensions and shape of the structural elements of sintered materials are easier to control, and which is the most important, it is possible to obtain such types of mutual arrangement and shape of grains that are unattainable for molten metal. Due to these structural features, sintered metals are more heat-resistant, better tolerate the effects of cyclic temperature fluctuations and stresses, as well as nuclear irradiation, which is very important for materials of new technology.
Disadvantages of Powder Metallurgy
- Comparatively high cost of metal powders;
- The need for sintering in a protective atmosphere, which also increases the cost of powder metallurgy products;
- The difficulty of manufacturing in some cases products and blanks of large sizes;
- Difficulty in obtaining metals and alloys in a compact, non-porous state;
- The need to use pure initial powders to obtain pure metals.
The disadvantages of powder metallurgy and some of its advantages cannot be considered as permanent factors: they largely depend on the state and development of both the powder metallurgy itself and other industries.
As technology develops, powder metallurgy can be forced out of some areas and, conversely, conquer others.