Loose-powder sintering

Porous parts and bearings are made by both the press and sinter techniques, whereas filters are made by loose powder sintering. The metals most commonly used for P/M porous products are bron2e, stainless steel (type 316), nickel-base alloys (Monel, Inconel, nickel), titanium, and aluminum. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

Figure 2.31 Development of ceramic microstructure during sintering (a) Loose powder particles (b) initial stage (c) intermediate stage and (d) final stage. From W. E. Lee and W. M. Rainforth, Ceramic Microstructures, p. 37. Copyright 1994 by William E. Lee and W. Mark Rainforth, with kind permission of Kluwer Academic Publishers.

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

The dry-powder processes generally employ wire screen precut to the so-called master plaque dimension. The screens are placed in molds with loose powder on each side. They are then typically sintered in a belt furnace in a reducing atmosphere at 800 to lOOO C. 

Local consolidation of the material takes place, so in this way three-dimensional parts of any shape can be produced. No supporting structures are needed to produce the parts, because the unsintered powder remains in the bed and supports the part. At the end of the production process, the loose powder has to be removed from the parts produced in this way. The sintering process is pressureless, so density is less than that attainable in the processes in which the material is melted completely. Once the parts have been cleaned, the surface can be treated by sanding or by shot-blasting. 

Sintering is a thermal process through which a loose mass of particles is transformed to a coherent body. It usually takes place at a temperature equal to two-thirds the melting point, or ca 800—1000°C for nickel. The sintered nickel stmcture without active material is called a plaque and it can be prepared by either dry or wet processes (see Metallurgy, powder). 

Shimizu et al. has developed a low-cost MgO porous particle separator, which possessed both the high porosity ( 85%) of the BN felt and the high mechanical resistance of powder separators. The separator was prepared by loosely sintering fine MgO powder with Mg(NOs)2 as the binder. The particles thus formed showed excellent performance as separators in Li—Al/FeS r cells. 

The sintering process utilizes fusion as a means of size-enlargement. This process, used mainly for ores and minerals and some powdered metals, employs heated air that is passed through a loose bed of finely ground material. The particles partially fuse together without the assistance of a binder. Sintering frequently is accompanied by the volatilization of impurities and the removal of undesired moisture. 

Figure 7.32 Selective laser sintering process. A laser is bounced off a mirror, which is controlled by a scanner to selectively trace the outline of a cross section of a part on a powder bed. When one layer has been traced, the part is lowered and fresh powder is rolled into place. After finishing, the part is removed from the loose, unfused powder.

Porous structures in green agglomerates can be achieved if the feed particles are preagglomerated (Fig. 5.49) or consist of hi- or multi-modal particle size fractions. Another method is the production of loose packings by sedimentation. Then, to retain the open porosity, the sintering process must be carried out at a low temperature and for a short time or, to obtain more open pores, pore forming additives are mixed with the powder. 

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