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microstructural characteristics that can be controlled to tailor the mechanical, thermal, and filtration properties include porosity content, shape, and size distributions, as well as grain size, shape, and distribution. Control of P/M variables in processing has traditionally been based on experience gained by extensive and systematic trials. This leads to a workable but perhaps not an optimized manufacturing process. Some of the important fundamental correlations between processing variables, powder characteristics, and properties are discussed in greater detail elsewhere.1"3 However new materials, increasingly complex geometries, and higher performance specifications require new, more efficient methods for developing and optimizing P/M processes. With the advent of fast computing and improved scientific theories of constitutive behavior, many manufacturers are developing computer-aided design and processing solutions to maintain a robust processing environment. Economics often dictate many of the process-variable choices, including the level of automation to achieve the desired production volume and component quality. In general automated, near-net-shaped fabrication makes P/M competitive with other metalworking processes. This article addresses powdermetallurgy processing variables and their relationship to powder compaction as a basis for understanding the complexity of modeling the P/M process. Powder-metallurgy processing variables will be grouped into two primary areas: (1) variables associated with the powder and how it is prepared, and (2) consolidation parameters such as die design and loading conditions, which affect how powders are densified and bonded to-

gether. A distinct advantage of P/M is the flexibility and control available in both of the areas just listed, which allows a wide spectrum of materials and desirable microstructures and properties to be "tailored" into the final product. Characteristics of the Compaction Process Powders The powder itself may be the single most important variable to consider in powder compaction. As described by Dinger and Funk in this issue, the physical characteristics and properties of a powder determine how it flows into a die, packs to fill space, and compacts under an applied force. The characteristics of a powder are determined by the process used to make it, as well as by how it is subsequently processed. Powders used in compaction processes often are improved by milling, mixing, and/or granulating. Manufacturing Methods. The means by which powders are manufactured principally determines their characteristics. The particle morphology, size and size distribution, chemistry, porosity, surface area, and mechanical properties are all dependent on the powder fabrication route and contribute to the powder-packing density, mechanical and chemical bonding during compaction, densification behavior during compaction and consolidation, and thus the final product properties. There are many methods for manufacturing and preparing powders, including mechanical comminution