The Warm Compaction Process.Basics, Advantages and Limitations 980334
The origins of the new compacting method are surveyed. The densification enhancement derives from yield strength drop versus temperature and from a most effective lubrication mechanism. For any mix the pore free density figures out the limit density after compaction. The allowances corresponding to volume increase on ejection are evaluated. The curves of porosity versus compaction pressure show that graphite additions can contribute to densification. For part shapes with upper profiles replicating the upper punch faces, the porosity distribution attained by warm compaction is better than that attained by room temperature compaction. The changes with respect to room temperature compaction bring about a higher radial pressure at densification end but a lower residual radial pressure at ejection start. These are the reasons for a relatively high spring-back but low ejection strength. The low porosity of compacts obliges to reexamine the thermal profile on sintering, because permeability to reducing gases strongly decreases. To avoid embedding of oxide flakes the refining stage of sintering should be adequately long. High thickness P/M parts can be undersintered at the core, if the reducing gases cannot reach their most internal sites. The new concept of reduction depth is introduced. The bonds linked to new (high Mo) fully prealloyed powders, which can allow a completely ferritic sintering, are examined. The analysis of powder properties shows the importance of a reduced inclusion content in case of warm compaction and very law sintered porosity. Finally, the tools operating conditions are reviewed and the differences with respect to room temperature compaction are underlined. The needs for an innovative and sophisticated design calculation, and for a suitable EDM procedure for dies as well, are discussed and summarized.