Introduction

BLENDING OF POWDERS is defined as the thorough intermingling of powders of the same nominal composition. The implication with blending is that the constituents in the vessel are virtually identical except for some minor physical characteristics. For example, one may be interested in blending several lots of powder that have slight variations in moisture or color. Premixing is the preparation of a uniform mixture of two or more components. The implication with mixing is that the constituents in the vessel are different. They may differ in chemical composition or may be physically dissimilar. Many properties of powders and sintered parts--such as powder flow, apparent density, ejection stress, delubrication behavior, dimensional change, and mechanical strength--are quite sensitive to even small changes in particle size distribution and to fluctuations in the concentrations of components within a powder mixture.

In many cases, the P/M parts producer is interested in relatively large, uniformly blended powder lots, because costly and time-consuming process adjustments, often of an empirical nature, must be made. Adjustments are often made from lot to lot, even when the various lots are within given specifications.

Lubricated, single-component metal powders comprise the most important two-component systems in powder metallurgy. Important three- and four-component systems include iron-carbon-lubricant, copper-tin-lubricant, iron-copper-carbon-lubricant, and copper-tin-carbon-lubricant. The widespread use of powder mixtures rather than prealloyed powders is attributable to both economic and technical factors: powder mixtures often are less expensive, have better compacting properties, and sometimes permit shorter sintering times.

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