It is no surprise that the complexity is free axiom is so commonly used when discussing the benefits of additive manufacturing (AM). Indeed, the very nature of this new branch of manufacturing allows designers to make use of unprecedented geometrical freedom, in turn making it possible to tweak the properties of end products. This new approach to component fabrication has already had far-reaching consequences in lightweight design, building in functional compliancy, and optimizing thermal behaviors of parts like injection mold tools. The ability to seamlessly change materials in a single part would drive this design freedom to the next level.
Currently, product designers already have some tools to combine various properties in a single item. One route is to join different materials in a single part by gluing, welding, brazing, fastening or combining during the production process itself. Typical examples are elastomers over-molded on stiff polymer construction to increase grip and feel, hard metal plaques brazed on cutting tools, and Teflon or brass bushings on slide surfaces inserted at friction points, among others. Besides the required extra steps, a disadvantage of this approach is that often the joint is the weak point susceptible to breakdown and eventual loss of functionality.
Another route is to locally modify the chemistry or microstructure of a material in a spatially graded manner. Think of case hardening of steel, selective heat treatments, and ion bombardment of functional polymers. Despite their seemingly more complex nature, these practices are widespread in industry and some have been applied for centuries.
The localized gradual change in material chemistry is the target of research towards functionally graded materials (FGM’s), which are a prime goal for multi-material AM methods. In this branch of materials science, a vast number of potentially useful material combinations have already been identified. What is seemingly overlooked is that a not-insignificant number of metal combinations should be low hanging fruit for a multi-material capable AM technology.
Those working on material and parameter development in the AM world understand that materials in the same class can be processed with very similar or equal parameters without a real loss of quality. Even alloys with an apparently completely different composition can often be processed with the same laser parameters and scan strategy if they have similar thermal properties. Typical examples are stainless steels and cobalt chrome alloys, or tungsten and tantalum — hinting at the fact that these materials might be readily fused together in a multi-material powder bed process without the need to change the laser power parameters on the fly within each layer, or even without interrupting the scan vectors. Nonetheless, the properties of the final materials are sufficiently different to allow for functionality optimization.
Below is a non-exhaustive list of potentially ‘easy’ metal/alloy combinations to be used in multi-material laser powder bed fusion.
With selective powder deposition applied to binder jetting AM, this list could be extended. There are materials which are currently not compatible with laser processing due to microstructural issues and cracking. If multi-material binder jetting could be used to shape green parts containing two materials that sinter at similar temperatures and at a similar rate, complete de-binding and fusing during the thermal cycle is likely possible. Furthermore, a diffusion zone will be generated in a natural fashion, increasing the strength of the bond. To compensate for differences in sintering temperature, sintering rates can be tuned by varying the particle size distribution, as smaller particles will tend to sinter more quickly thanks to their larger surface area-to-volume ratio than larger particles.
Whether combined with LPBF or binder jetting, selective powder deposition can help metal AM mature even further by enabling multi-material processing. Plenty of research must be done to determine which metal combinations can be successfully co-processed, but with such a huge variety of alloys available, it’s likely that at least a handful will prove feasible and practical.
by Bram Neirinck, Ph.D. Senior R&D / Applications Engineer @ Aerosint
