Solid-State Infiltration of 6061-T6 Aluminum Alloy Into Carbon Fibers Via Friction Stir Welding

Hybrid welding/joining of lightweight metals to carbon fiber reinforced polymers (CFRPs) typically relies on the adhesive bond created when the molten polymer matrix hardens in contact with the metallic surface. It is hypothesized that these bonds can be improved upon by fully displacing the polymer and infiltrating the carbon fibers with the metallic constituent to create load-bearing fibers that bridge the two materials. Friction stir welding (FSW) holds potential to melt and displace the polymer matrix, plasticize the metal constituent, and force the plasticized metal to flow around the fibers. Preliminary investigations were performed by FSW in AA 6061-T6 plates sandwiched against dry carbon fiber bundles. The FSW process plasticizes the aluminum while applying pressure, forcing the material to flow around the fibers. Cross-sectional images of the samples were used to measure the distance of infiltration of the aluminum into the carbon fiber bed. A fiber infiltration model previously developed to describe the infiltration of carbon fibers with epoxy resins during resin transfer molding was applied to this solid-state infiltration situation, thus modeling the plasticized aluminum as a fluid with an effective viscosity. Promising agreement was seen between the measured distances of infiltration and the predicted distances of infiltration when using effective viscosity values predicted by computational fluid dynamics (CFD) simulations of FSW found in literature. This work indicates that the well-established epoxy infiltration model can form the basis of a model to describe solid-state infiltration of carbon fibers with a plasticized metal.