As a wad of powdered metal is pressed into its final form by a compaction mechanism, hydrostatic pressures trigger the start of the densification process. Particles of metal slide past one another, they lock tightly together, and then a series of intricately embedded capillary systems take shape. On one side of a datasheet, pressure parameters get printed out in blocks of text. On the other side, there are graphs and curves to control the process.
Studying Powdered Metal Densification Kinetics
The first two compaction determining factors are found on those graphs. On the X-axis, the applied pressure is labelled in GN/m² (Giga Newtons per Square Metre). On the Y-axis, there’s the Relative Density, a parameter that curves subtly upwards in response to more compaction pressure. The third major process initiator is sintering temperature, which triggers the powder coalescing phase, but we’re getting ahead of ourselves. The metal grains won’t neck until they enter the sintering chamber.
Adding Hold Times to the Mix
Time is the final component. Like any other process, a product can’t be created instantaneously. After the pressure is applied, the powdered grains are held in place within the compaction mechanism for a set amount of time. The hardened die is subjected to what’s known a predetermined “Hold Time.” This grain stabilizing factor stretches from the point the pressure is applied to the point the sintered end-product leaves the compaction stage. However, this value cannot be exceeded, for extended hold intervals can produce coarsened product microstructures. To sidestep such process deviations, it’s better to ramp up the sintering temperature.
Employing a Densification Roadmap
At this point, the application of thermal energy is imminent. The Green compact is about to exit its die and head straight into a sintering chamber. There could even be heat treatment work incorporated into this phase of the process. Before that can happen, dimensional constraints are enforced. The yield factor, how the grain boundaries contract, is calculated so that a product’s desired shape and size are accommodated. From the top of the roadmap, the plasticity of a chosen alloy and its additive-to-metal mix ratio determine compaction pressure. From here, though, there are numerous stresses to offset.
So alloy malleability and hardness still count when a metal grain is almost too small to see with the human eye. Mixed between those powdered metal grains, process additives expand the mix. These mixing factors are accompanied by several densification-dependent elements as well. There’s the Hold Timing, the yield profile, frictional conditions and grain boundary tensions to plot out as well. By carefully regulating these elements, creep flow and plasticity threshold stresses are rapidly overcome so that the Green compact assumes its final shape and size.