CAREER: Hierarchical Structure Integrity of Magnesium Alloys via Asynchronous Laser and Additive Processing

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Current commercially available metallic orthopedic implants are permanent and often result in complications that require costly secondary removal or repair surgeries. A potential solution is dissolvable, biodegradable magnesium alloy implants that maintain sufficient mechanical integrity long enough for the factored bone to heal, and then dissolve thus eliminating the need for any further surgical intervention. The fundamental challenge in achieving this is to slow down and control the dissolution rate of magnesium alloys within the body. This Faculty Early Career Development (CAREER) Program award seeks to address this by applying laser peening, a local deformation process that induces favorable compressive stresses and reduces corrosion rates, to the entire material volume and not just its surface as conventionally undertaken. This will be done by implementing laser peening at intermediate stages during additive manufacturing, a process that builds parts in a layer by layer manner. If successful this project will result in new manufacturing knowledge on printing defined mechanical properties in metallic materials that is extendable beyond magnesium alloys, and thus can provide the US manufacturing base with new capabilities to strengthen their competitiveness. Educational components of the award that will increase future workforce preparedness include: (1) encouraging U.S. students' pursuit of advanced degrees in manufacturing, especially from underrepresented groups in STEM; (2) promoting entrepreneurship training and commercialization of fundamental additive manufacturing research through an engineering startup competition and participation in the NSF I-Corps Program; and (3) provide a pathway to employment for credentialed students through academic apprenticeships in additive manufacturing. Further, leveraging this project with the University's established outreach program with Navajo Technical University will better position Dine students to pursue research-oriented careers and start businesses that remain on tribal lands.The technical goal of this project is isolate how strengthening mechanisms and residual stress caused by interspersing peened layers (i.e., barrier layers) inhibit dislocation motion and affect stress-corrosion behavior. The approach applies laser peening on multiple layers during printing to form an aggregate surface integrity throughout the entire build volume as opposed to a traditional external surface modification. Central to this goal is to understand how thermal inputs (from depositing the next layer of material), and mechanical inputs (from later peening stages) affect the favorable compressive stress profile induced by earlier peening steps. This will be achieved by 1) quantification of dislocation densities resulting from work hardening, grain refinement, and residual stresses, 2) verification of magnesium alloy strength and ductility resulting from varied spatial treatment frequencies, and 3) verification of increased stress-corrosion resistance. This will result in a new mathematical model to predict how frequently to asynchronously print and peen a magnesium target to mitigate thermal and mechanical cancellation of previous peening steps.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.