In the field of additive manufacturing, new approaches are constantly being developed to avoid the localized heating effects during the build process. An emerging development is that of methods that build the desired geometry in which the underlying powder particles are first weakly bonded together to form a green part with a binder or “glue,” followed by a thermal cycle to debind and sinter the particles together. Examples of such methods include binder jetting and multi-material fused deposition modeling. With these methods, the issue of localized heating effects during the build process is now reframed as an issue of overall heating effects during the sintering process. Generally, simulation users are interested in predicting the shrinkage and gravitational deformation during the sintering process to check end dimensional tolerances.
Background
Metal additive manufacturing methods such as powder bed fusion (PBF) and directed energy deposition (DED) share a commonality in that a localized heat source is used to consolidate a material to form a part, analogous to how a sketch is formed in a “stroke-by-stroke” manner by pencil, allowing complex geometries to be incrementally or additively built. However, the high temperatures and thermal gradients introduced during the build process lead to localized warpage and buildup of residual stresses within the part, which may further complicate removal of the part from the baseplate. Additionally, the localized heating and cooling cycles evolve the underlying microstructure, subsequently affecting the mechanical properties of the part.
Methods such as binder jetting (BJ) eliminate the localized thermal effects by introducing a binder into the print nozzle to consolidate the powder together in a weaker manner to build the part. Once the part is built, it then undergoes two stages of heat treatments like those in metal injection molding (MIM), namely:
Debinding – Relatively weak thermal load (furnace) is applied to “evaporate” the binder, and
Sintering – Relatively stronger thermal load (furnace) is applied so that the powder now undergoes densification to achieve the required engineering requirements.
An advantage of this global approach to sintering is that microstructures—and consequently, mechanical properties—competitive with those seen in casting manufacturing can be produced with low spatial variability in the part, as has already been established in near net-shape manufacturing methods like powder metallurgy (PM) and MIM. Methods like binder jetting are a re-visit to these near net-shape manufacturing methods, but also allow for the incorporation of more complexity in the design of the part (such as overhangs and cavities). However, during the design stage, one should account for the shrinkage and gravitation bending of the part due to creep during sintering of the part. The increased ability to design complex parts also means that it is no longer sufficient to solely design the part around shrinkage of the part as has been practiced in PM and MIM.
How Simulation Can Assist with Sintering Challenges
Being able to simulate the shrinkage and gravitational warpage of complex parts will not only help reduce trial-and-error required for the design of parts to meet tolerance requirements, but it will also expand the design exploration space.
Additionally, once a good calibration has been established for a particular material system with good repeatability of results, compensation algorithms can be applied to achieve the compensated shape that will result in the desired dimensional specification.