Functionally-graded materials (FGMs) differ from traditional composites in that two materials are joined with a graded interface in order to avoid a boundary between them. As FGMs are resistant to failure, they’re perfect for use in extreme environments with high chemical, mechanical, or thermal stresses that would cause a single-material part to fail.
FGMs based in metals used to be more theoretical, but thanks to 3D printing, they can now be made by interweaving two, or even more, independent metals through thickness with a specific gradient – resulting in a product that combines the best properties of all the materials, from strength and corrosion resistance to toughness and thermal conductivity.
However, very few metal FGMs have moved beyond conception to actual production. But that’s changing now, thanks to Fabrisonic and its hybrid metal 3D printing process. Ultrasonic Additive Manufacturing (UAM) is a patented method of using high frequency ultrasonic vibrations to merge layers of metal foil together in a solid-state. The technology makes it possible to 3D print thin metal foils, without the use of melting, in order to build solid metal parts at temperatures that are close to ambient.
“The bond permits joining of dissimilar metals without the undesirable metallurgical interactions seen in other additive processes that melt,” Fabrisonic’s president and CEO Mark Norfolk wrote about UAM in a company post.
By using UAM technology to make FGMs, it’s possible to include a wide range of material combinations in one part, which enables the production of engineered materials and property gradients. Since the process only reaches about 250°F, UAM technology can also be used to embed sensors and electronics in a solid metal structure without damaging it, as well as create objects with complex internal geometry.
“Fabrisonic builds to near-net shape, then comes in with a CNC to finish the part shape to exact specifications. Combining the subtractive with the additive is where we get our hybrid process,” Norfolk explained.
There are many examples of UAM technology being used to leverage the possibility of FGMs, such as the company’s work with the US Army Research Lab (ARL). The technology had been used in the past to build of compositions of titanium and aluminum, and ARL experts were able to build upon this work and simulate ballistic behavior to make a gradient “formula” which offers the optimal combination of ductility and strength through the thickness. Then, each ‘recipe’ was 3D printed using UAM technology, and the resulting large panels successfully passed rigorous ballistic requirements.
Several thermal applications use custom FGMs made with dissimilar metals, like cooling systems for electronics packages. Fabrisonic frequently uses copper and aluminum FGMs to 3D print high performing heat exchangers – it’s possible to wick heat from a critical location, without needing to build the whole structure out of heavy, costly copper, just by printing the material in a few strategic locations.
Electronic shielding is also an application for 3D printed FGMs in the aerospace field, as delicate electronics can be shielded from radiation by weaving in layers of tungsten or tantalum on an aluminum panel.
Norfolk wrote, “Both thermal and rad hardening FGM’s can be printed in the same panel at the same time, further optimizing performance.”
Most industry cold plates are made of aluminum or copper, both of which have drastically different CTEs (coefficient of thermal expansion) than electronic packages, which are mostly made of silicon. But by interweaving Invar and molybdenum materials through the thickness, Fabrisonic can use its UAM technology to create FGMs that can achieve optimized thermal conductivity, “without inducing high thermal stresses across the interface.”
While FGMs can be completely made of metal, UAM technology can also be used to integrate other materials, like ceramic.
“Recent work has focused on printing ceramic fibers into a metal matrix creating a metal matrix composite (MMC),” Norfolk explained. T”his is akin to adding rebar to concrete as the ceramics can have 5-10 times the strength of the monolithic aluminum.”
Entire parts can be made more lightweight by using UAM to 3D print ceramic fibers in certain high-loaded regions. In addition, fatigue life has been shown to improve by several orders of magnitude thanks to embedded ceramics.
By taking advantage of Fabrisonic’s innovative UAM 3D printing process, it’s possible to engineer dissimilar metals into unique FGM solutions.
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