Michael P. Sealy
Date of this Version
R. Karunakaran, 2020. Mechanical and Cellular Behavior from Interlayer Milling Ti-6Al-4V during Directed Energy Deposition, Masters Thesis.
The goal of this research is to enable better performance of biomedical implants through hybrid manufacturing technology. In pursuit of this goal, the research objective of this thesis is to understand how interlayer machining during additive manufacturing of Ti-6Al-4V affects mechanical and cellular behavior. Ti-6Al-4V is bioinert and extensively used in the biomedical industry due to excellent mechanical properties. However, the stiffness is one order of magnitude greater than cortical bone, which results in stress shielding. The implant shields bone from applied loading. The reduced load causes more bone porosity that increases the risk of re-fracture. Additive manufacturing has shown to minimize stress shielding through topology optimization by equating the effective moduli between implant and bone. Conventional manufacturing methods, such as milling, turning, grinding, and casting, are unable to achieve many complex topologically optimized structures. Topologically optimized structures enable promotion of cell growth for better implant fixation and improved healing rates. The challenge is that printed titanium alloys are less ductile, highly anisotropic, and require further processing due to poor tolerances. An alternative approach is hybrid additive manufacturing that couples interlayer machining during printing to alter global mechanical properties. Intermittent milling helps locally adjust mechanical properties, and thereby, cause a global (or bulk) change in mechanical behavior. In this study, the mechanical and cellular behavior from hybrid AM using directed energy deposition (DED) and milling was investigated to determine how strength, ductility, and cell adhesion were affected. The ductility of AM Ti-6Al-4V doubled due to interlayer milling and was comparable to annealed Ti-6Al-4V. The tensile strength was lower than annealed parts. Cell growth was not affected due to interlayer milling. In conclusion, interlayer milling during DED is beneficial in improving the mechanical behavior of Ti‑6Al‑4V without affecting cell growth.
Advisor: Michael P. Sealy