Selecting a Process

Selecting the proper process for consolidating powder to produce a part requires making many decisions. Using the performance requirements as the accept-reject criteria, the wide range of possible processes can be narrowed down, but usually the designer/manufacturer still finds that several methods are possible for producing a particular part. The final selection is then usually based on the availability of equipment, the experience with particular processes, and the marketplace requirements of cost, delivery, and quantity.

Some guidelines and corollaries to these guidelines are presented here as an example of process selection decision making. These are not hard-and-fast rules for several reasons. First, the craft or skill aspect of parts making will alter these guidelines for each and every company. Second, new methods, new powders, and new equipment constantly require that any such set of guidelines be adjusted. Third, the marketplace itself dictates changes and additions to such guidelines. The reader is encouraged to take these guidelines and corollaries as a starting point, then modify and add new statements with quantified details wherever possible so that the decision making process within each organization is captured.

Guideline 1: Control of porosity is the most important aspect of a consolidation process. The size, shape, distribution, and volume fraction of pores is the singlemost important property of a powder metallurgy part. The performance of the part is directly dependent on these aspects of porosity. Therefore, the partsmaker must control porosity in order to achieve a usable part, and the consolidation process is critical to controlling part porosity.

Corollary: Dynamic properties such as toughness and fatigue resistance improve dramatically as porosity is eliminated. Higher part stresses during application require higher densities. If a powder metallurgy part must compete with a wrought part, full density must be achieved in all critically stressed regions. This is especially true for applications that require high toughness or where cyclic loading is a life limiting condition.

Guideline 2a: Control of dimensions is the second most important aspect of the consolidation process. The successful implementation of parts production using powder metallurgy techniques is due to the minimization or elimination of machining operations. Therefore, the consolidation process must offer substantial control of the final part dimensions so that secondary operations are minimal.

Corollary: The use of rigid tooling provides superior control of dimensions in comparison to flexible tooling. Therefore, die pressing, injection molding, and powder forging are preferred over cold and hot isostatic pressing in terms of dimensional control. The shape of specific sections of isostatically pressed components can be better controlled by the selective use of rigid fixtures such as mandrels or pressing plates.

Corollary: The effect of friction on porosity distribution must be taken into account in order to maintain control of dimensions. Porosity gradients due to friction can cause nonuniform dimensional change during sintering. This is especially critical in die compaction where friction along vertical die walls and punch faces must be minimized through the use of admixed lubricants or die wall lubricants.

Corollary: Uniform powder loading is required to achieve a uniform dimensional change during sintering. Because the sintering rate is density dependent, a nonuniform distribution of powder will produce a nonuniform dimensional change during sintering. For injection molding, this means that the volume fraction of binder should be uniform throughout the green part. For multilevel die pressed compacts, this means that multiple pressing motions should be used so that the proper powder distribution is achieved before consolidation starts. In die pressing there is negligible transfer of powder between cavity sections while pressure is applied.

Corollary: Prior to sintering but after debinding, injection molded parts must be supported because particle bonding at this point is simple cohesion. After ejection from the mold, an injection molded shape is held together by the binder. Prior to sintering, the binder is removed, leaving a time period when the only force holding the powder mass together is particle cohesion plus residual binder. A supporting bed of an unreactive matter such as alumina spheres may be necessary.

Guideline 2b: Ignore Guideline 2a if powder metallurgy is the only method that can be used to produce a particular material. In some cases powder metallurgy offers a way to process a unique material. In these cases, the uniqueness is the material often outweighs the economics of dimensional control so that substantial machining or secondary processing is acceptable.

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