Functionally Graded Metal-Metal Composite Structures

NASA's plasma-deposition process provides the ability to tailor various properties while designing functional parts by selecting specific materials and processing parameters to meet the end goal. Specifically, the plasma process deposits metal particles that are heated as they travel axially at low velocity through an inert gas plasma. The accelerated powder particles become molten, strike the substrate fabric (uniaxial, biaxial, and multiaxial) and rapidly solidify, imparting very little heat to the substrate while forming a metal-to-fiber bond, as well as a metal-to-metal bond. The resulting metal-coated fabric is porous, so the polymer matrix can pass through the product precursor during the infusion process. The amount of metal deposited can be controlled, as can the number of plies of fabric that are ultimately stacked to produce the preform for the polymer matrix infusion process. A variety of infusion processes can be utilized to prepare the FML, including resin transfer molding (RTM), resin film infusion (RFI), and vacuum-assisted resin transfer molding (VARTM). The tailorable aspect of the process allows for specific product design. By varying the combination of metal particle, fiber, fabric type, metal layer thickness, fabric direction, number of layers, polymer matrix resin, infusion process, and cure conditions, the characteristics of the final part can meet the needs of various applications.

The FMLs resulting from the NASA process have similar properties to traditionally produced metal/composite hybrid laminates including, as compared to either the composite or metal only structures, improved load carrying capability, lighter weight, improved stiffness, improved impact resistance and damage tolerance, and improved permeation resistance. The NASA process can be applied to various FML types, including GLARE (glass, aluminum, epoxy), and TIGR (titanium, graphite). Typical manufacturing processes are costly and complex shapes are hard to produce, whereby the NASA process enables use of these kinds of laminates without an autoclave or press, thus increasing the size that can be produced and decreasing the cost. The resin pathways in the foils enable connection between the plies that can improve the interlaminar strength of the final part. Functionally the NASA process creates resin columns in the transverse direction of the plies. NASA is working to optimize the final properties by varying the size and distribution of the pathways.

Heat treatment of the deformed welds is desirable in order to restore the properties of the alloy negatively affected in the weld region. In these alloys, abnormal grain growth frequently occurs in friction stir welds during solution heat treatment, and is known to degrade key materials properties, such as strength, ductility and toughness. The innovation of inserting an intermediate annealing step covered here reduces abnormal grain growth during post-welding heat treatment, thereby allowing optimum mechanical properties. This is important where Al-Li alloys (and other heat treatable alloys) are friction stir welded followed by deformation processing and high performance, high reliability structural components are required for aerospace vehicles.

In-situ alloyed, additively manufactured GRCop components are produced by mixing elemental copper, chromium, and niobium powders and then selectively laser melting the powder mixture using powder bed fusion additive manufacturing. During laser melting, the chromium and niobium powders react to form Cr2Nb precipitates in the molten state, which are then dispersed in the pure copper matrix upon solidification. This dispersed precipitate phase is encapsulated within the pure copper matrix upon solidification, resulting in the characteristic microstructure of GRCop alloys with high thermal conductivity from the copper combined with superior mechanical properties from the precipitates. Optimization of process parameters like laser power has enabled up to 89% conversion of the constituent elements into Cr2Nb precipitates. The achieved rapid cooling rates of ~10^4-10^6 K/s enable formation of fine Cr2Nb precipitates around 0.1-1 micrometers in size. The technology is currently at a TRL 4, and NASA has developed and tested coupons of material using the new process. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.

EBAM technology is capable of making full-density, functional metallic components for numerous engineering applications; the technology is particularly advantageous in the aerospace, automotive, and biomedical industries where high-value, low-volume, custom-design productions are required. A key challenge in EBAM is overcoming deformation of overhangs that are the result of severe thermal gradients generated by the poor thermal conductivity of metallic powders used in the fabrication process. Conventional support structures (Figure 1a) address the deformation challenge; however, they are bonded to the component and need to be removed in post- processing using a mechanical tool. This process is laborious, time consuming, and degrades the surface quality of the product. The invented support design (Figure 1b) fabricates a support underneath an overhang by building the support up from the build plate and placing a support surface underneath an overhang with a certain gap (no contact with overhang). The technology deposits one or more layers of un-melted metallic powder in an elongate gap between an upper horizontal surface of the support structure and a lower surface of the overhang geometry. The support structure acts as a heat sink to enhance heat transfer and reduce the temperature and thermal gradients. Because the support structure is not connected to the part, the support structure can be removed freely without any post-processing step. Future work will compare experimental data with simulation results in order to validate process models as well as to study process parameter effects on the thermal characteristics of the EBAM process.