Sintering is all about post-compaction heating, about finding a way to control the coalescence of metal powders. Knowing the definition helps, of course, but it doesn’t really tell us what’s taking place within the coalescing mass. That’s a harder system to understand, but its various mechanisms have been examined in some depth. It begins with a pre-heating phase, a process that’s split into three stages.
The component is almost ready. It’s gained its form, and that shape really is geometrically exquisite. Let’s say this workpiece is destined to be a form-fitting bushing. It’s partially porous, and tiny capillaries have formed throughout the alloy part. That having been said, the metal particles are still separate. They’re in contact, pressed up against each other due to the immense compaction energies that were applied in the previous stage, but they haven’t yet coalesced into a single mass. To kick off this phase, the furnace melts the lubricants and softens the base metal. Next, the true sintering work commences.
Entering the Sintering Furnace
Heated until the furnace temperature is almost hot enough to melt the in-process alloy mix, the discrete particles fuse and consolidate, one to the other, each to its neighbour. Instead of a mass of tightly segregated metal grains, the tiny granules merge together into a single capillary-filled body, which retains its shape, as imbued by the compaction mechanism. From here, the third furnace treatment phase is encountered. The hot, fusion packed part rolls into a cooling chamber. More than a temperature equalizing chamber, the atmospherically controlled space is sealed so that the sintered metal can’t oxidize. Here’s a short list of the atmospheric controllers used in this oxidation prevention stage:
• A vacuum, which isn’t actually a control gas
• Nitrogen/Hydrogen gas (disassociated ammonia)
• Endothermic gaseous atmospheres
Critically, the generation of a suitable sintering gas can only be decided after the nature of the workpiece alloy is determined.
The goal of the sintering process is to metallurgically fuse, to coalesce and blend powdered metal particles without causing those granular bits to melt. The grains fuse, they bond to their adjacent neighbours. Under controlled circumstances, the sintering equipment heats a subject component, one that’s just been compacted. Eliminating oxidization influences, diffusing the grain boundaries, the formerly compacted workpiece is transformed into a solid mass. Solid, yes, but the mass still incorporates a measure of porosity, through which a lubricating agent can flow. Finally, using a rapid cooling mechanism and an atmospherically controlled gas, the process can sinter harden the outgoing product and imbue it with martensitic strength.