Fiber Metal Laminates Made by the VARTM Process

Langley Research Center, Hampton, Virginia

Fiber metal laminates (FMLs) are multicomponent materials utilizing metals, fibers, and matrix resins. Tailoring their properties is readily achievable by varying one or more of these components. Two new processes for manufacturing FMLs using vacuum assisted resin transfer molding (VARTM) have been developed.

Typical FMLs are prepared by stacking alternating layers of metal foils and fiber/matrix resin prepregs, followed by consolidation in a press or autoclave. FMLs consisting of aluminum sheets and aramid fiber/epoxy prepregs were first developed in the 1980s and then, in 1991, FMLs with glass fibers instead of aramid fibers were introduced.

Current commercially available FMLs can be expensive to produce, and part size is limited due to the required prepreg and use of an autoclave or press in consolidation. However, infusing liquid resin into dry fabric layers solely by vacuum pressure to produce high-quality materials has proven to be a more cost-effective process for preparing composites. VARTM utilizes a flow distribution media to allow the resin to proceed rapidly on the surface over the length of the part, followed by the slower through-the- thickness infusion through the part, thereby decreasing infusion times.

A VARTM process was developed for FMLs that utilizes small flow pathways in the metal layers to allow for through-the-thickness resin infusion. The materials produced by this process are referred to as VARTMFML. A second VARTM process developed for FMLs utilizes porous metal-coated fabrics to allow through-the-thickness infusion. These laminates are referred to as VARTMPCL. For each process, many combinations of metal, fiber, and resin can be selected to tailor the material for specific applications.

The processing of VARTMFML involves stacking alternate layers of the metal foils containing resin flow pathways and reinforcement fabric. The metal foil/fabric preform is then infused with a resin via a VARTM process. The VARTMFML provides good mechanical properties that can be optimized by proper selection of metal foil, fiber, resin, and size and distribution of the pathways.

The processing of VARTMPCL first requires the production of the metal-coated fabrics. The fabric is coated with a metal layer using an inductively-coupled plasma deposition process. The metal layer produced is porous so that stacked metal-coated fabric layers can be infused to produce novel FMLs. Metal powders are axially injected into inert argon/helium plasma at low pressures. As the particles pass through the plasma, they become partially molten droplets. These metal droplets impinge onto the as-received fabric at low velocity, rapidly cooling and forming a metal-to-fabric bond. The advantage of the VARTMPCL is most likely an improvement in functional properties, including electrical conductivity (e.g., lightning strike protection) or thermal conductivity (e.g., heating for deicing) relative to the parent polymer matrix composite.

This work was done by B. J. Jensen, R. J. Cano, S. J. Hales, B. W. Grimsley, and E. S. Weiser of Langley Research Center; and J. A. Alexa of Lockheed Martin Engineering and Sciences. LAR-17165-1/485-1/-2

This article first appeared in the August, 2015 issue of NASA Tech Briefs Magazine.

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