Functionally Graded Materials Are Key to Fast, Effective Multi-Material 3D Printing

A buzzword in the 3D printing industry lately has been “multi-material.” Desktop 3D printers have been released that are capable of printing with more than one material in one build, but industrial 3D printing also has a lot of multi-material development happening. This includes composite materials, such as fiber-reinforced polymers. But what Kevin Eckes, Ph.D, R&D Engineer at Aerosint, describes as the “holy grail” of composite materials are functionally-graded materials (FGMs).

Eckes, who introduced us to Aerosint’s unique approach to multi-material powder bed 3D printing last fall, has been sharing deeper perspectives in thought pieces shared on Medium. He recently shared with us his latest piece focusing on FGMs.

“Unlike traditional composites, in which a reinforcing material is distributed throughout a bulk matrix material, FGMs are composites in which two materials are joined with a graded interface to avoid a distinct boundary between the two bulk materials,” Eckes explains.”The purpose of creating this gradation is to distribute over a larger volume the thermal and mechanical stresses that would otherwise be concentrated at a distinct material boundary and cause part cracking or breakage.”

FGMs, because of their resistance to failure, are useful in extreme environments with high thermal, chemical or mechanical stresses that would cause a single-material part to fail. An example of an FGM part is a metal-to-ceramic plate, in which the two materials cancel out each other’s weaknesses and enhance each other’s strengths. Ceramic by itself is hard and chemically and thermally resistant, but it’s also brittle with low impact toughness. Metal, on the other hand, is strong with high impact toughness, but it’s easily corroded by strong acids and bases and its mechanical performance suffers when heated. Put the two materials together, though, and they become virtually indestructible.

Metal-ceramic FGMs can withstand high heat gradients without cracking or plastic deformation (graphic adapted from: A. Gupta, M. Talha, Progress in Aerospace Sciences. 79, 1–14 (2015)

“A metal-to-ceramic FGM is capable of withstanding blazing temperatures and harsh chemical environments at the ceramic face while maintaining overall strength and resistance to brittle fracture thanks to the metal reinforcement,” says Eckes.

Researchers at Japan’s National Aerospace Laboratories of Science and Technology first began using metal-ceramic FGMs to try to create effective, durable thermal barriers for reusable space plane vehicles. They discovered that a plate composed of compositionally graded layers of nickel superalloy and a stabilized zirconium oxide on a nickel superalloy base could withstand heat gradients of over 1000°C without cracking. In the same conditions, a 100% nickel superalloy coated with 100% zirconium oxide and a chromium alloy as a relaxation layer in between cracked from thermal fatigue.

An FGM combines the best thermal, mechanical, and/or chemical properties (top) of two materials (in this case, steel and copper) into a single part with a graded interface that distributes stresses within a greater volume (below). The goal is to avoid part failure that would likely occur as a result of stress concentration at a material boundary (middle). Key: k = thermal conductivity, ? = stress, ? = strain, ? = coefficient of thermal expansion, T = temperature, V = volume

Metal-to-metal FGMs are useful as well. Steel-copper FGMs, for example, combine the low cost and high strength of steel with the high thermal and electrical conductivity of copper. In a recent example, researchers created an injection molding tool with a steel molding surface and a bulk copper body, joined with a proprietary buffer layer material. Thanks to the copper’s high electrical conductivity, the mold cooling time was reduced by 10 seconds, reducing the overall mold cycle time by 26%. The company estimated that that increased productivity could save them about $60,000 per mold per year.

There are also polymer-polymer FGMs, in which rigid and flexible polymers can be combined for things like rigid arms with soft joints in robotics, for example, or wear-resistant polymers could be joined to tough polymers for non-metallic bearings or motion components.

“Most of the methods used to create FGMs that are described in the research literature are only capable of creating 1D gradients, which are useful only in a limited number of applications like thermal barrier tiles,” continues Eckes. “To create multi-material parts and FGMs with complete 3D freedom, we need voxel-level spatial control over material placement. Several current AM techniques on the market are capable of this degree of compositional control, but they are primarily geared toward prototyping or one-off part fabrication.”

Aerosint specializes in multi-powder deposition, in which powder voxels are selectively transferred from a rotating drum to the build surface. In theory, as many materials as the number of drums can be patterned, though so far Aerosint has demonstrated two-powder deposition. The material can be metal, polymer, or ceramic, as long as the powder flowability and particle size distribution are compatible with the process.

“The most fundamental challenge in our process is bi-material consolidation or sintering,” says Eckes. “There are clear physical limitations in co-sintering two materials with vastly different melting or sintering temperatures. Even so, many researchers have demonstrated successful metal-metal and metal-ceramic co-sintering through prudent selection of materials with compatible properties. The combinations aren’t limitless, but we think they are plentiful enough to create a range of useful FGMs.”

Aerosint’s technology selectively deposits multiple powder materials on a machine build surface in a line-by-line manner. We have successfully co-patterned alumina and titanium (top right), 316L stainless steel and copper (bottom left), and TPU and PA12 (white and black, respectively, bottom right). Scale bar = 1 cm.

The combination of multi-powder deposition and bi-material co-sintering is one of the most effective ways of enabling material-efficient, fast, and productive multi-material 3D printing, according to Eckes. Aerosint is in the process of creating partnerships with research institutions and academics with expertise in material co-sintering and metal and ceramic 3D printing.

Read Eckes’ latest perspective, “How to make cheap, scalable multi-material 3D printing a reality,” in full here.

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