Various Methods to Assess the Degree of Sintering: a Review

摘要

The end users of PM parts, to ensure the fulfilment of most common application requirements, relyrnon standards issued by different bodies. These standards specify the material’s propertiesrncurrently achieved by usual industrial processes of powder metallurgy. In some cases, however,rneven if the properties of PM parts supplied by different manufacturers fulfil the specifications, andrnthe raw materials are the same, operating failures occur only for some parts. Therefore, somernuncontrolled factor, or property, not yet included in the standards, should justify the observedrndifferences. This “unspecified” factor is the degree of sintering, as characterised not only byrnprocess conditions and corresponding typical mechanical properties. The intensity of the process,rnand the consequent "mesostructure", namely the combined pattern of porosity feature, microconstituentsrntype and distribution, and diffusion level of alloy additions, may be the reason of thernbehaviour's difference, for instance in case of Hertzian contact pressures. On this ground, after anrnapproach to sintering based on the well-known Larson-Miller criterion, other methods, presumablyrnmore suited to differentiate the effect of sintering on material's mesostructure, and performance asrnwell, are reviewed. A survey of existing literature enables to compare the points of strength andrnweakness of different methods proposed to rate the effect of sintering on the whole of materialrnproperties. This approach completes and integrates the Larson-Miller criterion, which appearsrnmore suitable to characterise the process, than the effects of the process, on performances ofrnsintered materials subject to different and heavily demanding stress pattern.