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，并且经常导致需要昂贵的二次移除或修复手术的并发症。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，其保持足够长的机械完整性以使分解的骨愈合，然后溶解，从而消除任何进一步的外科干预的需要。 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率。这个教师早期职业发展（CAREER）计划奖旨在通过应用激光喷丸来解决这个问题，激光喷丸是一种局部变形过程，可以诱导有利的压缩应力并降低腐蚀速率，整个材料体积，而不仅仅是传统的表面。这将通过在增材制造期间的中间阶段实施激光喷丸来实现，增材制造是一种以逐层方式构建零件的过程。如果成功，该项目将产生新的制造知识，在金属材料中打印定义的机械性能，可扩展到镁合金之外，从而为美国制造基地提供新的能力，以加强其竞争力。 该奖项的教育部分将增加未来的劳动力准备包括：（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，促进创业培训和基础增材制造研究的商业化;以及（3）通过增材制造的学术学徒制为有资格的学生提供就业途径。此外，利用该项目与大学的既定推广计划与纳瓦霍技术大学将更好地定位Dine学生追求以研究为导向的职业生涯，并开始业务，留在部落的土地上。该项目的技术目标是隔离如何加强机制和残余应力所造成的穿插喷丸层（即，阻挡层）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个层进行激光喷丸处理，以在整个构建体积中形成聚合物表面完整性，而不是传统的外表面改性。这一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缩应力分布。这将通过以下方式实现：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）验证增加的耐应力腐蚀性。 这将产生一个新的数学模型来预测异步打印和喷丸镁靶的频率，以减轻先前喷丸步骤的热和机械取消。该奖项反映了NSF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.