Understanding How Aluminum Moves Around a Friction Stir Welding Tool in order to Prevent Welding Defects

该奖项将支持基础科学研究，这些研究将调查铝如何在搅拌摩擦焊接工具周围移动，以及这种移动如何影响亚表面缺陷(空洞)的形成。搅拌摩擦焊由于具有其他焊接工艺所不具备的诸多优点，在众多制造业中迅速得到普及。然而，某些限制减缓了它的大规模实施。这项研究旨在解决人们对搅拌摩擦焊接的两个局限性缺乏了解的问题：(1)在高可靠性应用中(例如：国防和航空航天工业)需要昂贵的焊后亚表面缺陷检查；(2)需要提高处理速度以实现大批量生产(例如：汽车工业)。较快的焊接速度增加了缺陷形成的可能性。目前对搅拌摩擦工具周围材料流动的精确机制及其在亚表面缺陷形成中的作用缺乏基本的了解。这项研究将产生必要的知识，帮助美国制造商预测缺陷形成，从而使他们能够设计出一种能够生产无缺陷焊缝的搅拌摩擦焊接工艺。解决上述限制将大大加快这种加入方法的大规模实施，这将对美国制造业、美国经济和国家安全产生积极影响。该奖项还将促进未来劳动力的培训，因为所有级别的学生都将接触到先进的加入技术并获得经验。从这个项目中产生的知识将通过会议演讲、期刊文章、行业参观和开放参观工程演示来公开传播。这项研究的目标是对金属围绕摩擦搅拌工具的间歇运动(类似挤压的过程)背后的物理原理有一个基本的了解，每次工具旋转一次。通过力测量和完成焊接后对焊缝横截面的研究，这种现象在制造工程界已被广泛报道。然而，对于为什么会发生这种间歇性的材料移动和直接的过程中观察，还不存在理解。一种主要的假设是，在推进工具之后，空腔打开，随后在工具的每次旋转过程中，材料被挤出到这个空腔中。在良好的焊接条件下，这个空洞完全填满了。然而，当材料流动出现故障时，空腔未被完全填充，仍存在缺陷(空洞)。这一假设将通过一种铝合金搅拌摩擦焊接过程的新型过程中质子射线照相来验证。洛斯阿拉莫斯中子科学中心的质子射线照相术提供了产生一系列焊接区的射线图像(如x射线)的能力，这些图像将显示空洞是否正在形成和填充。通过物理实验获得的知识，结合在麦迪逊-威斯康星大学进行的其他实验，将推动间歇流现象的数值模拟。这一现象的模拟在已出版的文献中基本上是没有的。在这项工作中，将使用先进的基于拉格朗日的数值技术来模拟和模拟复杂的材料流动和空洞的形成和修复过程。最终，这将提供预测缺陷形成的手段，并设计在生产过程中避免缺陷的措施。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award will support fundamental scientific research that will investigate how aluminum moves around a friction stir welding tool and how this movement influences the formation of sub-surface defects (voids). Due to its many advantages over other welding processes, friction stir welding has rapidly gained popularity in numerous manufacturing industries. However, certain limitations have slowed its mass implementation. This research seeks to address a lack of knowledge about two limitations in friction stir welding: (1) the need for costly post weld inspection of sub-surface defects in high-reliability applications (for example: defense and aerospace industries), and (2) the desire to increase processing speed to allow for high-volume production (for example: automotive industry). Faster welding speeds increase the likelihood of defect formation. A fundamental understanding of the precise mechanisms of material flow around the friction stir tool and its role in sub-surface defect formation is currently lacking. This research will generate the knowledge necessary to help U.S. manufacturers predict defect formation, hence allow them to design a friction stir welding process that produces defect-free welds. Addressing the aforementioned limitations will greatly expedite the mass implementation of this joining method, which will have a positive impact on U.S. manufacturing, the U.S. economy, and National Security. The award will also facilitate training of the future workforce as students across all levels will gain exposure to and experience in advanced joining technologies. Knowledge generated from this project will be distributed publicly through conference presentations, journal articles, industry tours, and open house engineering expositions.The goal of this research is to generate a fundamental understanding of the physics behind the intermittent movement (an extrusion-like process) of metal around the friction stir tool once per tool revolution. This phenomenon has been widely reported in the manufacturing engineering community by means of force measurements and by studying weld cross-sections after completing the weld. However, an understanding of why this intermittent material movement occurs and direct in-process observations do not exist. One leading hypothesis is that a cavity opens up in the wake of the advancing tool and that material is subsequently extruded into this cavity during each rotation of the tool. In a good welding condition, this cavity is completely filled. However, when there is a breakdown in the material flow, the cavity is not filled completely and a defect (void) remains. This hypothesis will be tested by means of novel, in-process proton radiography of the friction stir welding process of an aluminum alloy. Proton radiography at the Los Alamos Neutron Science Center provides the capability of producing a series of radiographic images (like x-rays) of the weld zone that will show whether or not a cavity is forming and filling. The knowledge gained through this physical experimentation, combined with other experiments conducted at the University of Madison-Wisconsin, will drive the numerical simulation of the intermittent flow phenomenon. Simulation of this phenomenon is largely absent from the published literature. In this work, the complex material flow and cavity formation and filing processes will be modeled and simulated using advanced Lagrangian based numerical techniques. Ultimately, this will provide the means to predict defect formation, and design measures to avoid it during production.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.