Ultrasonic Additive Manufacturing of NiTi - Aluminum Matrix Composites

Purpose: To understand and improve the interfacial shear strength (ISS) of metal matrix composites fabricated via very high power (VHP) ultrasonic additive manufacturing (UAM).   UAM combines the solid-state ultrasonic welding of metallic foils, an automated foil feeder to build-up material in an additive fashion, and a CNC machining center for periodic machining of components during and post welding.  The result is an additive manufacturing process where complex and relatively large metallic components can be fabricated at or near room temperature.  VHP-UAM is a key improvement in the original UAM process where the ultrasonic weld power has increased nearly an order of magnitude.  This increase in weld power remedies poor interfacial properties observed in early UAM processes.  As a consequence of VHP-UAM’s low formation temperature, recent efforts have been devoted to the fabrication of aluminum matrix composites with embedded NiTi shape memory alloy fibers for thermal expansion control applications.  VHP-UAM NiTi-Al composites have shown a dramatic decrease in thermal expansion in the aluminum matrix, yet thermal blocking stresses developed during thermal cycling have been found to degrade and eventually cause interface failure.  As a result, to improve understanding of the interface and guide the development of stronger NiTi-Al composites, ISS was investigated through the use of single fiber characterization methods developed within the fiber composite industry. In addition to estimating ISS, the influence of various fiber surface treatments on interface properties was studied in order to potentially improve ISS.  Bond characteristics were studied with the use of various microstructure characterization tools.