Ultrasonic consolidation (UC), also referred to as ultrasonic additive manufacturing (UAM), is a recent solid-state, low temperature fabrication process that can be used for layered creation of solid metal structures. The process uses ultrasonic energy to induce plastic deformation and nascent surface formation at the interface between layers of metal foil causing bonding between the surfaces. UAM is an inherently stochastic process with a number of unknown facets that can affect the bond quality between layers. In order to take advantage of the unique benefits of UAM, it is necessary to understand the relationship between manufacturing parameters (machine settings) and bond quality by quantifying the mechanical strength of UAM builds. The goal of this thesis is to identify the optimum combination of processing parameters for manufacture of metallic composites.
The research presented consists of three parts; the first part deals with characterizing the mechanical strength of titanium-aluminum builds by statistically analyzing the effects of four main process parameters on the USS and UTTS of the builds. A multi-factorial experiment was designed to study the effects of the manufacturing parameters normal force, oscillation amplitude, weld speed, and number of bilayers on the outcome measures Ultimate Shear Strength (USS) and Ultimate Transverse Tensile Strength (UTTS). For a given factor, the operating levels were selected to cover the full range of machine capabilities. A generalized linear model was formulated to study the statistical significance of each factor. Transverse shear and transverse tensile experiments were conducted to evaluate the bond strength of the builds. Optimum levels of each parameter were established based upon the statistical contrast trend analyses. The results from trend analyses indicate that high mechanical strength can be achieved with a process window bounded by a 1500 N normal force, 30 μm oscillation amplitude, about 42 mm/s weld speed, and two bilayers. The effects of each process parameter on bond strength are discussed and explained. Microstructural analysis was performed on samples in order to evaluate interfacial bonding and indicated that the composites strength were mainly due to mechanical interlocking between the soft aluminum and hard titanium layers.
The second part investigates the effect of the process parameters tack force, weld
force, oscillation amplitude, and weld rate on the USS and UTTS of 3003-H18 aluminum UAM built samples. A multi-factorial experiment was designed and an analysis of variance was performed. Additional statistical tools were employed in order to develop a complete understanding of the effects and interactions of the process parameters. The results of the statistical analyses indicate that a relatively high mechanical strength can be achieved with a process window bounded by a 350 N tack force, 1000 N weld force, 26 μm oscillation amplitude, and about 42 mm/s weld rate. The effects of each process parameter on bond strength are discussed and explained. A correlation and proposed hypothesis between the process parameters, the mechanical strength of the specimens, and their resulting microstructure was developed based upon analyses using optical microscopy. Bond characterization analyses indicate that all the specimens had close to the same amount of percent bonded area and that a high linear weld density (LWD) did not directly correlate to a high mechanical strength. The final part of this thesis explores the fatigue life of 3003 aluminum UAM built specimens loaded axially using fully reversible bending.