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，研究铝如何在搅拌摩擦焊工具周围移动，以及这种移动如何影响表面下缺陷（空隙）的形成。搅拌摩擦焊由于其相对于其他焊接工艺的许多优点，在许多制造业中迅速普及。然而，某些限制减缓了其大规模实施。本研究旨在解决对搅拌摩擦焊中两个局限性缺乏了解的问题：（1）在高可靠性应用（例如：国防和航空航天工业）中需要对亚表面缺陷进行昂贵的焊后检查，以及（2）希望提高处理速度以允许大批量生产（例如：汽车工业）。更快的焊接速度会增加形成缺陷的可能性。目前缺乏对摩擦搅拌工具周围材料流动的精确机制及其在次表面缺陷形成中的作用的基本理解。这项研究将产生必要的知识，以帮助美国制造商预测缺陷的形成，从而使他们能够设计一个搅拌摩擦焊接工艺，生产无缺陷的焊缝。解决上述限制将大大加快这种连接方法的大规模实施，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，因为各级学生将获得先进连接技术的接触和经验。从这个项目中产生的知识将通过会议演讲，期刊文章，行业图尔斯，和开放的房子工程expositions.The研究的目标是产生一个基本的理解背后的间歇运动（挤压过程）的金属在摩擦搅拌工具工具每一次工具革命的物理。这种现象已经在制造工程界通过力测量和通过在完成焊接后研究焊接横截面而被广泛报道。然而，对于为什么会发生这种间歇性材料移动以及直接的过程中观察结果的理解并不存在。一个主要的假设是，腔体在前进的工具之后打开，并且材料随后在工具的每次旋转期间被挤出到该腔体中。在良好的焊接条件下，该空腔被完全填充。然而，当材料流中出现故障时，空腔没有完全填充，并且仍然存在缺陷（空隙）。这一假设将通过铝合金搅拌摩擦焊过程的新型过程中质子射线照相术进行测试。Los Alamos中子科学中心的质子射线照相提供了产生焊接区的一系列射线照相图像（如X射线）的能力，这些图像将显示空腔是否正在形成和填充。通过这个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，将驱动间歇流现象的数值模拟。这种现象的模拟在很大程度上是缺席出版的文献。在这项工作中，复杂的材料流和空腔的形成和填充过程将建模和模拟使用先进的拉格朗日为基础的数值技术。最终，这将提供预测缺陷形成的方法，以及在生产过程中避免缺陷的设计措施。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。

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.