Previously, we discussed how selective material deposition in an existing powder compaction die could be used to make high-value multi-material metal and ceramic components. This idea can be taken yet a step further — what if, instead of depositing it into a pre-formed die, the part material and “die” material are co-deposited layer by layer? Allow me to explain.
A common trick during sintering of green powder compacts to prevent warping is to bury parts in coarse powder (e.g. sand or ceramic) with a much higher sintering temperature than the part material. At the temperature needed to densify the green parts, the surrounding powder remains unsintered — acting as a space filler that mechanically supports the powder compacts during the sintering process. Done this way, the green compacts don’t deform under their own weight at high temperatures.
With a selective powder deposition system like Aerosint’s we could ditch the part-specific die, and well, almost the entire powder compaction machinery. How? We directly co-deposit the space-filling, non-sintering material (sand or ceramic) alongside powdered part material (metal or ceramic) into a crucible of a generic shape. Because the powder deposition contours can be changed on the fly and each layer can be composed of two materials, the non-sintering material can be patterned to effectively fill all the space around the part material contour for each layer. The non-sintering material thus acts as a sort of shape-maintaining “die” deposited layer-by-layer with the sinterable part material. What is created by the end of the process is a bed of two spatially arranged powders that can be transferred to a furnace for selective sintering of the part material.
We demonstrated this basic methodology in house using glass powder as part material and aluminum oxide powder as the non-sintering support. The startup company Iro3D has also demonstrated this concept very impressively with metal part materials using their pipette-based point-by-point selective powder deposition system.
To compensate for part shrinkage leading to the settling of support material, one can sinter until necking and complete the full densification in a pressure-less cycle. Infiltration with a low melting temperature alloy is another option that was used widely in the early days of metal rapid prototyping and is currently used by Iro3D.
In the case of pressure-assisted sintering, it is even feasible to employ a support material that can be compacted and comply with the part during sintering. Graphite or boron nitride are good options, depending on their chemical compatibility with the part material.
The use of compactible support powder also means that green parts can be produced “offline” by co-depositing non-sintering support and compactible part material in a die, followed by uniaxial pressing, thus forming a pre-compacted green part inside a powder bed. This complex powder can be sintered as a whole, relying on the non-sintering character of the support to remove it afterward. We found it is even possible to form a freestanding green compact of the sinterable powder if the support powder does not cake after compaction. Using this route, we’ve produced sintered aluminum oxide parts.
All in all, by using selective powder deposition in this manner we can bridge the gap between layer-wise additive manufacturing and traditional press-and-sinter manufacturing by producing pre-shaped parts through “on-the-fly” die shaping. Since this approach doesn’t require expensive part-specific die tooling, it allows for fast, cheap part design prototyping and iteration. Multiple part forms can also be constructed at once in the same build, allowing for the same “mass customization” as laser powder bed fusion AM processes but at a potentially higher productivity and much lower cost.
by Bram Neirinck, Ph.D., Senior R&D / Applications Engineer @ Aerosint
