Warm compaction of metal powders: why it works, why it requires a sophisticated engineering approach

Abstract

The science of materials indicates that the compressibility of metal powders decreases as yield strength increases. In heated powder mixes the yield strength drops and an effective lubrication mechanism can occur. For any mix the pore free density determines the limit density after compaction. The allowances corresponding to volume increase on ejection have been evaluated, and the experimental results show that with warm compaction a porosity lower than 2% can be attained, and that graphite contributes to densification. Tests on compacts with upper profiles replicating upper punch faces show that after warm compaction the porosity distribution is better than that attained by room temperature compaction. The analysis of pressure ratios shows that in warm compaction the radial pressure at the end of densification is higher, whereas the residual radial pressure at the ejection start is lower. Consequently, the springback is higher and the ejection strength lower. Compacts with very low porosity require a suitable thermal profile on sintering, because their permeability to reducing gases strongly decreases. To avoid embedding oxide flakes, the refining stage before sintering should be adequately long. Thick walled PM parts can be undersintered at the core if the reducing gases cannot reach their most internal sites. A new concept is introduced in the present paper using the definition of reduction depth, and some possible experimental techniques for its determination are proposed. The analysis of powder properties in the case of warm compaction shows the importance of a reduced inclusion content and of a very low porosity after sintering, since the fatigue properties of high density materials can be adversely affected by even a few coarse inclusions. The operating conditions of the tools are reviewed and the differences with respect to room temperature compaction are underlined. The need for an innovative and sophisticated design calculation, and for a suitable electrodischarge machining procedure for dies, is discussed and summarised.