In one previous article in this series we proposed using selective powder deposition to be able to shape metal or ceramic preforms without machined dies typically used in press-and-sinter manufacturing. In this application note we describe a related idea for producing high density, minimally porous parts.
The goal of most powder-based manufacturing processes, e.g. powder metallurgy (PM), is to produce dense parts — often aiming to reach less than 1% porosity — without melting the starting material. The loose powder one starts with typically has a rather low stacking density. The theoretical maximum density of randomly stacked perfectly spherical mono-modal particles is only 64%, after all. With proper powder particle size distributions or deformable powders, packing densities of over 90% can be obtained. But this requires applying a significant force. Applying pressure, such as in die pressing processes, can hence aid significantly in obtaining satisfactory results. However, these uniaxial pressing processes are limited in application as they tend to lead to differences in density in the pressing direction for thick parts. For bulk parts, isostatic pressing is often used instead.
In Cold Isostatic Pressing (CIP), a low-density green part or even loose powder is place in a sealed flexible container. This container is submerged in liquid within a pressure vessel, and a pressure of several thousand bar is applied, compressing the green compact as close to its maximum packing density as possible. This higher initial density will significantly speed up consolidation to final density in the thermal cycle.
Pressure can also be applied during the thermal cycle. In uniaxial pressure-assisted sintering, typically only plate-like products can be produced. For more complex parts, Hot Isostatic Pressing (HIP) is used. HIP is carried out in a gaseous atmosphere, so the major requirement is that the outer surface of the treated parts be gas tight to start with. As such, this process can be used to produce components by vacuum sealing part powder in a mild steel shell. The uniform application of pressure ensures the shape is retained as the part and shell shrinks. Powders processed using a HIP strategy are typically not compatible with melting processes, either requiring excessive temperatures or yielding unfavorable microstructures. Nonetheless, the extra effort that is made to process these materials indicates their use in demanding applications where even a low level of porosity is not acceptable.
Both the CIP and HIP process use multi-material constructs: polymer shells with ceramic powders in CIP, and welded mild steel sheet material casing with high-end powder metallurgy alloys in HIP. Both methods thus present an opportunity for single step pre-processing of constructs via multi-material additive manufacturing. This approach could be particularly beneficial for producing small series or prototypes, and might even decrease manufacturing costs by allowing for increased part complexity with fewer processing steps and time.
Using Aerosint’s selective deposition technology one can simultaneously deposit different materials. When a flexible thermoplastic polyurethane (TPU) is deposited with a compressible, CIP-compatible ceramic, the TPU can be selectively sintered by laser or infrared lamp energy while the ceramic remains in powder form. In this way, we could generate a shaped, liquid-tight polymer shell surrounding uncompacted ceramic powder in the shape of the final part. Since the loose powder is completely enclosed by the polymer shell, the entire construct than be transported easily and placed inside a CIP machine to yield a green compact that can be removed from the polymer shell and sintered in a furnace to nearly full density.
For parts that must be processed using HIP, a similar approach could be used. A mild tool steel and a PM specific alloy could be co-deposited using a selective powder deposition system, and only the tool steel exterior would need to be consolidated. As the PM alloy does not need to be melted it is easy to imagine producing a prefilled casing with piping that would enable weld sealing this casing under vacuum, just as in HIP-produced parts produced from welded sheet metal. The rest of the HIP processing can then follow the traditional route.
In short, selective powder deposition can create a useful bridge between new additive manufacturing (AM) technology and widely accepted traditional manufacturing of high-end components. As the pressure treatment and thermal cycling processes are well understood and available in industry, this approach could help ease anxiety about items produced via AM. After all, these parts would only be shaped using a new AM method but consolidated using long-established technology.
by Bram Neirinck, Ph.D. Senior R&D / Applications Engineer @ Aerosint;
editing help and graphics by Kevin Eckes
