Ultrasonic Additive Manufacturing: Weld Optimization for Aluminum 6061, Development of Scarf Joints for Aluminum Sheet Metal, and Joining of High Strength Metals

Ultrasonic additive manufacturing (UAM) is a low temperature, solid-state manufacturing process that enables the creation of layered, solid metal structures with designed anisotropies and embedded materials. As a low temperature process, UAM enables the creation of active composites containing smart materials, components with embedded sensors, thermal management devices, and many others. The focus of this work is on the improvement and characterization of UAM aluminum structures, advancing the capabilities of ultrasonic joining into sheet geometries, and examination of dissimilar material joints using the technology.;Optimized process parameters for Al 6061 were identified via a design of experiments study indicating a weld amplitude of 32.8 synum and a weld speed of 200 in/min as optimal. Weld force and temperature were not significant within the levels studied. A methodology of creating large scale builds is proposed, including a prescribed random stacking sequence and overlap of 0.0035 in. (0.0889 mm) for foils to minimize voids and maximize mechanical strength. Utilization of heat treatments is shown to significantly increase mechanical properties of UAM builds, within 90% of bulk material.;The applied loads during the UAM process were investigated to determine the stress fields and plastic deformation induced during the process. Modeling of the contact mechanics via Hertzian contact equations shows that significant stress is applied via sonotrode contact in the process. Contact modeling using finite element analysis (FEA), including plasticity, indicates that 5000 N normal loads result in plastic deformation in bulk aluminum foil, while at 3000 N no plastic deformation occurs. FEA studies on the applied loads during the process, specifically a 3000 N normal force and 2000 N shear force, show that high stresses and plastic deformation occur at the edges of a welded foil, and base of the UAM build. Microstructural investigations of heat treated foils confirms that plastic deformation occurs in the bulk of the foil, while previous studies have only identified microstructural changes to the bond interface region.;A methodology for joining aluminum 6061 sheet material 0.076 in. (1.93 mm) thick is proposed based on iterative design studies which identified a scarf joint configuration as viable. Design of experiments studies indicate optimal properties can be achieved using a scarf joint angle of 10SO. Room temperature and elevated temperature tensile, and room temperature fatigue testing exhibit joint mechanical properties similar to solid, homogeneous material.;Successful joints were achieved for Al/Ti, aluminum to steel, steel to aluminum, and steel to steel combinations. Mechanical characterization studies of Al/Ti combinations indicate that post-process heat treatments can significantly increase mechanical properties. Microstructural evaluations including electron back scatter diffraction show significant deformation within the softer aluminum layers. Investigations of Al/steel combinations indicate that mostly voidless interfaces occur and that plastic deformation is present in the steel layers only. Steel to steel combinations, while proven possible, require further work to enhance the consistency of the joints and improve the ability to build larger structures.