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

目前市售的金属骨科植入物是永久性的，经常导致并发症，需要昂贵的二次移除或修复手术。一种潜在的解决方案是可溶解的、可生物降解的镁合金植入物，它可以保持足够长的机械完整性，使受影响的骨骼能够愈合，然后溶解，从而消除了进一步手术干预的需要。实现这一目标的根本挑战是减缓和控制镁合金在体内的溶解速度。该学院早期职业发展（Career）项目奖旨在通过应用激光强化来解决这一问题，激光强化是一种局部变形过程，可以产生有利的压应力并降低腐蚀速率，而不仅仅是传统的表面处理。这将通过在增材制造的中间阶段实施激光强化来实现，增材制造是一种逐层构建零件的过程。如果成功，该项目将产生新的制造知识，可扩展到镁合金以外的金属材料的印刷定义机械性能，从而可以为美国制造基地提供新的能力，以加强其竞争力。该奖项的教育组成部分将提高未来劳动力的准备，包括：(1)鼓励美国学生追求制造业的高级学位，特别是来自STEM中代表性不足的群体；(2)通过工程创业竞赛和参与NSF I-Corps计划，促进基础增材制造研究的创业培训和商业化；(3)通过增材制造的学术学徒为有资格的学生提供就业途径。此外，将该项目与大学与纳瓦霍技术大学建立的外展计划结合起来，将使Dine的学生更好地从事研究型职业，并在部落土地上创业。本项目的技术目标是分离由分散的喷孔层（即阻隔层）引起的强化机制和残余应力如何抑制位错运动并影响应力腐蚀行为。该方法在打印过程中对多层进行激光喷丸处理，从而在整个建筑体量中形成一个整体的表面完整性，而不是传统的外部表面修改。该目标的核心是了解热输入（来自沉积下一层材料）和机械输入（来自后期喷丸阶段）如何影响早期喷丸步骤引起的有利压应力分布。这将通过1)量化加工硬化、晶粒细化和残余应力导致的位错密度，2)验证不同空间处理频率导致的镁合金强度和延展性，以及3)验证增加的应力腐蚀抗性来实现。这将产生一个新的数学模型来预测镁靶的异步打印和喷丸的频率，以减轻先前喷丸步骤的热和机械抵消。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.