The sintering process occurs after a mass of atomized powder has been compacted. Further back, the powder metal has been mixed and blended with a liquid phase. Oils and bonding additives have spread uniformly throughout the mix. A closed die now imparts the amorphous mass its dimensional profile. Now, leaving those profile-forming stages behind, we move towards the sintering stages. In here, it’s time for some necking action.
What’s Sintering Necking?
This is when the discrete metal particles first begin to coalesce. The grains are in contact. They’ve been compacted into a desired shape, so the compaction operation is complete. A finely rendered part looks almost ready for its application. But, as just mentioned, the particles are not fully bonded. Applying sintering heat, which requires a temperature that does not exceed the metal’s melting temperature, the grains become plastic. They diffuse and fuse. The grain boundaries are dissolving, and those melting boundaries are developing fused connections between one another. These are the necks, the initial grain diffusion points.
Charting the Intermediate Stage
The neck growths swell as the temperature climbs. Intelligently regulated, the pressing lubricants vapourise and the necks meld into a single mass. The spheroid particles, once loaded with surface curvature, can be seen under a microscope at this point. With the process frozen mid-stage, those curves are transforming into densified crevices. The porous structure is taking shape. Now, with the densification action almost finished, the pore equilibrium stabilizes. Tiny capillaries have taken form, and it’s in this pore network that the powder metal’s lubricating agent takes up residence. Importantly, the pressing lubricant is gone and the interfacial tensions are also reaching a state of equilibrium.
Completing the Densification Process
Dimensionally stable and fully particle diffused, the initial necks are gone and the porous substructure has become properly established. Material to pore quotients are observed and waxy or oily lubricants are in place. A controlled atmosphere has been monitored during the process, so no metal oxides have compromised what should be an entirely pure material base. Happily, the required number of pores are still connected, while the rest are completely closed and isolated. Their job is simply to reinforce the metal’s stress equalizing structure so that the sintered part displays strength and durability.
All three major sintering stages are controlled by sophisticated equipment mechanisms. Otherwise, the pore quotients and/or the product’s inherent strength would be undermined. To facilitate the process monitoring phase, complex densification models determine optimal processing vectors, including the required sintering temperatures and pressures. For example, solidification stages require careful management if the end-line product is to retain its intended dimensional outlines.