Effect of die temperature on mechanical properties of hot pressed P/M

Abstract

In conventional warm compaction, both powder and die are heated to a certain temperature during<br><br>compaction. This is a technique for producing P/M compacts with higher green and sintered strength as<br><br>compared to room temperature pressing. However, there is a certain limit to powder temperature due to flow<br><br>problems at higher temperatures. Heating the die above this practical limit can further improve properties. In<br><br>this work, the effect of die temperature on green and sintered properties of Astaloy CrM powder has been<br><br>investigated. Here, the powder at 135 oC was fed to the die at different temperatures. Density and strength<br><br>for samples in green and sintered conditions were evaluated for two compaction pressures of 500 and<br><br>650MPa and temperatures ranging from ranging from 135 to 165 oC. Comparison of samples compressed at<br><br>room temperature showed marked improvement in density and strength properties. A 22% increase in<br><br>density, as well as 40% increase in green strength was observed. Tensile and impact strengths were<br><br>improved by about 10% and 20% respectively. SEM micrographs showed more rounded pores and hence<br><br>reduced stress raising sites. The improvement in properties can be mainly attributed to changes in powder<br><br>morphology and die wall lubrication due to migration of hot lubricant from interparticle space to die walls. The<br><br>latter will reduce particle spacing and bring about more intimate metal-metal contacts as well as better<br><br>lubrication on die walls.