Key parameters for warm compaction of high density materials

The mechanical properties of high performance P/M parts depend heavily on material composition and the density reached during processing. Warm compaction technique, which consists of pressing preheated powder in a heated die [1], is known to favour densification of parts, leading to an improvement of sintered properties [2,3]. Warm compaction requires powder mixes with specific physical characteristics to be adequately processed in the temperature range involved in warm pressing. In particular, powder mixes should provide good flowability and good lubrication of the die walls in order to reduce ejection forces. Behaviour and properties of binder and non-binder treated materials processed by the warm compaction technique were investigated both on lab and industrial scales. Specific test programs on a production press were conducted to identify and quantify the effects of different key production parameters such as the compaction pressure, the powder and die temperatures, the production rate and the part size on green and sintered characteristics and ejection forces of parts. The warm compaction process capability is discussed in terms of stability of powder flow rate and apparent density, compacting pressure and temperature as well as weight and density of pressed parts. INTRODUCTION For numerous P/M applications, high sintered densities, typically over 7.3 g/cm are needed in order to achieve high mechanical strength and toughness. Such high densities are difficult to reach using standard compaction and sintering conditions. The warm compaction technique, which consists in pressing preheated powder mix in a heated die [1], is known to increase green density and, in turn, sintered density [2,3]. Final sintered density ranging typically between 7.25 and 7.45 g/cm can be achieved by warm compaction with a single compaction/single sintering treatment by using a compacting pressure and a sintering temperature of 50 tsi (690 MPa) and 1120 °C respectively [3,4]. Still higher densities could even be achieved by using high temperature sintering [5]. Powder mixes used for warm pressing should be able to withstand the temperature involved during the compaction cycle. In particular, the lubricant/binder system should provide good flowability and stability (robustness) at the compacting temperature and should enable a target density on a production scale. The density reached on a production press with more complex and heavier parts could be quite different from