Powder Compaction
Introduction
The main purpose of the compacting process is the forming of metal powders into compacts or desired shape with sufficient strength to withstand ejection from the tools and subsequent handling up to the completion of sintering, without breakage or damage it is the subsequent sintering operation which imparts the final strength.
Although there are various methods of powder compaction, only few of them are widely .employed commercially. It is convenient to group the various .,methods into the following classification :
(i) Pressure-less shaping , technique
(ii) Cold pressures shaping technique
(iii) Hot pressures shaping technique
Pressure less Shaping Technique
1) Loose sintering (loose shaping, gravity sintering, or pressure less Slater log).
In this process, metal powder is poured or vibrated mechanically into the mold which is a negative impression of the article and heated to the sintering temperature1. This is the simplest method involving low cost of equipment. It has been mainly employed to a large extension a commercial basis for producing product with porosities varying from 30% up to 90% by volume However this process is not used for the production of complex-shaped parts :
(a) the difficulty of part-removal from the mold after sintering
(b) flow characterieristics of the powder and
(c) considerable amount of shrinkage during sintering.
The die employed in this method must have adequate strength to contain the powder during sintering and usually carbide, graphite, stainless steel, or cast iron is used as the die material1.By selecting suitable powder, sintering temperature and time, it is possible to produce high density part of most metals and alloys, but in most of the cases, poor dimensional accuracy is obtained. This method may be employed to produce very large ingots ( up to 2 tons) from carbonyl iron and nickel powder which may be further treated by rolling or forging Highly porous filter materials such as bronze stainless steel, monel, tungsten, ets, are easily formed by this technique. Extremely porous nickel sheets used as electrodes and fuel cell and beryllium bars are another application of this process.
1) Slip Casting
Slip casting is commonly used for the production of ceramic articles, although it may be used to a limited extent for metals. The slip for casting is prepared from metal or ceramic powder finer than 5μ a liquid to serve as a suspending medium and a small amount of suspension agent and defloecatant (which prevents the settling and aggregation of powder particles and maintains the desirable viscosity of the slip) and binders, The process consists in filling the highly absorbent plaster of Paris mold ( made by the casting around a wooden or metal pattern) with this slip partial drying in the mold, removal of the " slip casting " from the mold, drying in an oven and sintering of the compact at the proper temperature and time in order to achieve the desired properties of the of the Parties of the sintered materials. Fig. 21 illustrates the stages of this method.
(a) (b) (c)
Fig. 21. Principles of slip casting : (a) Assembled mould: (b) filling the mould: (c) absorbing water from the slip : (d)
finished plece, removed from the mould and irimmed.
Thus, a casting slip ii a, suspension of particles usually than 5μ and these are prevented from aggregation or formation particle build-ups and controlled by the, adjustment of the factor1. A suspending medium to fulfill the following basic requirements is selected : proper viscosity, chemical stability, fluidity, and value to allow proper mold filling. Mostly water is use as the liquid vehicle or suspending medium, but absolute alcohol and other organic liquids may also be employed. The most useful additives employed in the preparation of cast able slip are ammonium and sodium salts of alginic acids such as Marcx and Keltex because they, serve the three fold functions: act as deflocculant, suspension agent, and binding agent to improve the green strength of the compact.
The suspension is made either manually or mechanically. it is advisable to shake or vibrate the mold while pouring in order to obtain a high quality product with less final porosity. Various, lubricants may be employed to prevent the product adhering to the mold walls ; for example; Spraying a 0.2% solution of sodium alginate onto the internal mold wall surface ensures easy removal of the product without affecting the moisture absorption.
It is possible to prepare both low and density articles by this technique. It has been employed for the production of.. tubes boats, crucibles, cones, hollows, turbine blades, rocket guidance fins, and other parts made from Mo, W, Ta mid CT powder. Paris fabricated by this method usually possess a lower density than of loose sintered components.
The main drawback of. this process is that it is a relatively slower and costlier process and therefore not suited, for . mass production of P/M parts.
3) Slurry Casting
This is a similar to slip casting except that the slurry or cream of the metal powders with suitable liquids, various additives, and binders is poured into a mold and lowed to dry before extraction. the solvent present in It may be removed either by absorption into the mold, often made of plaster of Paris or, by slow evaporation. Alternatively, solidifying liquid resins or self-solidifying liquids may lie used. Another modification of the process consists in using n non-absorbent mold, vibrating the cream or slurries in a vacuum, injecting the slurries along with vibration into the mold and freezing the castings.
The main drawback of this process is the difficulty in obtaining slurries of sufficiently high density, which can be poured and which will leave no residue sintering. This is also a slower process and produces produces with poor dimensional accuracy.
The advantages of slip and slurry cresting are. that the parts can be produced in various shapes and . sizes which are not possible by the conventional compaction technique and the expensive dies and equipment's .are eliminated here.
It is applied to produce turbo super charger bucfet5, very high porosity sheet (up to 90 volume % porosity) for use as electrodes in fuel cells and Ni-Cd rechargeable batteries, etc3.