Why combine the advantages of two materials, such as metal and technical ceramics, in one component, and how is it possible?
So-called hybrid materials represent this combination of two different types of materials and utilize their advantages while compensating for their disadvantages. However, there are hybrid materials of different types, which are presented in the following.
Components with homogeneous mixing ratio
In the industry, hybrid materials are used in a wide variety of areas. In cutting and chipping tools, the hardness can be increased by a ceramic, while the metal exhibits a tougher behavior. The ceramic is held together by the metal.
The combination of cross-material blends and the geometric freedom of 3D printing introduces in a new generation of hybrid components. For example, in addition to the high hardness of a cutting tool, it is also possible to optimize heat transfer through internal cooling channels.
WZR conducted initial trials using a 316L stainless steel as the matrix and aluminum oxide as the hardness enhancing additive. The material was mixed in different proportions and printed through our WASP 2040 Clay material extrusion printer.
Due to the metallic matrix, tensile bars were printed as test specimens and then sintered.
Initial results showed the following microstructure after sintering:
Microstructure of a hybrid material
Individual aluminum oxide particles can be seen in the sintered, metallic matrix. Nevertheless, isolated pores can be seen, which is why further sintering tests must be carried out.
The tensile test showed a more ceramic-typical behavior: The specimens broke in a brittle fracture at the clamping surfaces.
In principle, an altered deformability behavior is not surprising with the combination of ceramic and metal. On the one hand, the aluminum oxide achieves a higher hardness but on the other hand it impacts the elastic deformability and thus favors a brittle fracture. In future tests, we will therefore resort to test methods for brittle materials, such as the 3-point bending test method.
Multi-material with clear border
In addition to the mixed distribution of both components, materials can also be present in clearly defined areas in the component. Such a multi-material structure makes it possible to print electrical conductors (made of MoSi, for example) in an insulator. A practical example of this can be found here (german version).
Multi-material structure with clear delineation
However, multi-material can also support the process: In order to be able to remove support structures more easily, it is possible to print them from a water-soluble material. In this way, they provide support during the printing of complex structures, but can be easily removed from the component afterwards.
Material with function gradient
The process of additive manufacturing also makes it possible to vary the mixing ratio of the components in a component – an option that injection molding or casting and pressing do not offer with the same precision and accuracy. In this way, components can be produced with a so-called “functional gradient”. This means that the distribution of the different material components is not the same throughout the entire component, but that the ratio of one material to the other increases over a distance. This makes it easier, for example, to absorb thermal stress as the temperature rises.
In order to produce a material with a functional gradient in the material extrusion process, a mixing extruder can be used which mixes component A and B. The mixing extruder can react accordingly in areas where a higher proportion of A is required. In these areas, the mixing extruder can react accordingly and thus increase this proportion. In this way, for example, a component can be produced with a higher hardness at the edges but a high toughness on the inside.
Combination of several multi-material concepts
But not only material extrusion can be used for the production of hybrid components. Binder jetting with particle-filled inks opens up further possibilities for multi-material concepts: local modification of the microstructure. For example, appropriate microstructure reinforcement can be applied to areas where reinforcement of the component is required. With multiple print heads, it is even possible to print more than just one particle-filled ink, thus introducing yet another material into a component.
Metal-organic compounds are also conceivable. In this case, the secondary material is not present in particle form but as a metal-organic compound in the ink, which forms metal oxides during the sintering process and is subsequently finely distributed in the structure.
Now, to return to the initial question: As the previously listed possible applications show, there are good reasons for selecting a multi-material structure. However, it must be noted that various physical quantities, such as the thermal expansion of the two materials, must be adjusted to each other. In addition, compromises may have to be made with certain material combinations, as can be seen in the example of MEX tension rods.
