During the sintering process, as applied to powdered metals, a densely packed, cavity-shaped cluster of machine-pulverized particles plasticizes and diffuses. They do not melt. That’s a common misconception. Now, assuming an optimal sintering temperature is employed, the compacted grains experience the material diffusion cycle. They solidify. However, process temperatures can go awry. On knowing this possibility exists, we need to know about the consequences of such a thermally elicited gaffe.
Diffusion Mechanics: Distorted By Thermal Energy
Whatever the reason, the amount of thermal energy required to densify a P/M (Powder Metallurgical) component has been miscalculated. That’s something of a disturbing error, especially when we consider the relative leanness of a particular alloy’s sintering sweet spot. That leanness is attributed to the complex nature of the alloy powder, plus the binders and other process additives caught up in the blended mix. Anyway, back to the subject at hand, the smallest of thermal changes can result in a proportionally larger diffusion error. If the temperature drops lower than the desired sintering threshold, the process suffers. Likewise, a rise above the upper-temperature boundary can produce a poorly realized product structure.
The Upper and Lower Temperature Repercussions
If the miscalculated energy quotient falls too low, the porous structure grows in volume. That might not seem like much of a problem at first, but this increase of lubricant-carrying space comes at a cost. The densification effect takes a hit. The alloy structure is weakened by the larger pores. That same effect can place a hit on other part parameters. For example, a component that’s not as dense as initially intended won’t conduct electricity as well as a correctly densified part. If electrical conductivity is an issue, then this current attenuating effect cannot be allowed to take place. Happily, things improve when the error hits the high end of the temperature threshold. The part becomes denser, harder when too much thermal energy is applied. However, there comes a point when this effect also becomes unwanted. Denser, harder metals are brittle. They’re also less ductile, so they’re more fatigue-susceptible.
Incidentally, too much thermal energy will also reduce the number of pores, so the part’s capillaries won’t develop properly. Without capillary action, a bearing’s lubricant cannot take up residence within the product’s metal rings. That means an overly heated, sintered bearing won’t produce enough self-lubricating influence. It’ll run dry, then it’ll fail when a drive system starts or switches direction, and it will eventually age itself out of commission. Critically, sintering temperature miscalculations must be addressed and corrected at once so that the grains plasticise and “neck” in a process-predicted manner.