Thermal and Mechanical Redistribution of Residual Stress in Hybrid Additive Manufacturing

The long-term goal of this research is to develop advanced manufacturing technologies that enable fabrication of dissolvable load-bearing devices with customized degradation rates. In pursuit of this goal, the overarching research objective was to understand how interlayer coldworking treatments like laser and ultrasonic peening performed improved mechanical and chemical behavior of printed parts by locally altering stress fields and microstructure. The challenge in such a hybrid additive manufacturing approach involving interlayer coldworking was that the corrosion behavior of printed structures was largely unexplored. Furthermore, there was limited scientific advancement in optimizing the selection of appropriate layer interfaces for coldworking to attain the required degradation rates from printed devices. This dissertation was aimed at addressing the challenges in hybrid AM by investigating the corrosion kinetics of a magnesium alloy printed by coupling powder bed fusion with interlayer ultrasonic peening. This exploratory investigation was the first to demonstrate the ability of interlayer coldworked interfaces to delay the corrosion kinetics of powder bed fusion printed magnesium. A finite element framework was developed to investigate residual stress formation in parts subjected to cyclic thermal and mechanical loading from printing and cold working. An analytical model was established to accelerate residual stress simulations and develop design principles for directing stress concentration within parts that customize mechanical and chemical performance. This research forms the basis for fabricating time-resolved loadbearing orthopedic implants and dissolvable hydraulic fracking plugs.