Powder Properties

Metal powder characteristics directly determine the parameters of the CIP process. In mold filling, particle size distribution and particle shape are controlling factors in determining tap density of the filled mold. For billet molds, where final shape is determined by deformation processing operations, exact fill is not as important as in near-net-shape parts. Nevertheless, reproducible particle size and shape factors would be required to ensure repeatable post-CIP processing. In the case of net-shape parts, the mold must be filled to a uniform and repeatable tap density at all locations in the mold to yield predictable green part dimensions. This requires exact control and reproducibility of the above-indicated factors. German (Ref 1) has presented the existing knowledge base covering particle packing characteristics. In the case of blends of powders with disparate characteristics, that is, tungsten plus copper, for example, particle characteristics including size, shape, and density control the extent of separation during handling and mold filling (Ref 1).

Powder blending represents the most adaptable method for creating alloy compositions using CIP. Elemental powders can be blended with master alloy powders to create ranges of compositions. Separation of the blend must be avoided by selecting appropriate size and shape factors for the constituents (Ref 1).

In CIP compaction, a series of events takes place including particle rearrangement, localized deformation, homogeneous deformation, and elastic bulk compression. These phases of compaction depend on yield strength, strain hardening rate, and elastic properties, all of which are generally inferred from bulk properties of the related metal or alloy. Hardness is a shorthand indicator of compaction response because it is a property derived from the plastic flow properties of the bulk material. The softer the bulk metal is, the lower will be the required compaction pressure. With blends of powders with dissimilar properties, that is, hard and soft, compaction response is governed by the constituent with the greatest connectivity, that is, that phase of the blend for which a continuous path between like particles can be traced throughout the structure without intersection in the alternate phase.

Although produced regularly in tonnage quantities in a variety of compositions, powders atomized from the melt have a generally spherical particle shape and have been found difficult to use in CIP processing. This behavior is attributed to absence of shear and interlocking mechanisms during compaction, processes that occur in chemically produced metal powders with irregular shapes. Atomized powders can be altered in particle shape by mechanical processing such as milling or attrition.

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