Improvement of Modeling Predictions in Friction Stir Welding by More Accurate Measurement of Heat Transfer Between Tooling and Workpiece

搅拌摩擦焊是一种固态连接方法，越来越多地应用于传统熔焊工艺难以连接的铝合金的连接。将直接受益于高质量焊接组件的工业部门包括航空航天、轻轨、船舶和汽车等美国核心产业。然而，大多数搅拌摩擦焊的发展都是通过实验试错来完成的，这限制了它的影响，并减缓了它在潜在的减重应用中的应用。搅拌摩擦焊接的数值模拟始于大约20年前，在预测关键工艺条件（焊接温度、材料流动）和力学接头性能方面取得了进展。不幸的是，这些模型的预测价值是有限的，因为报告的摩擦和传热系数模型输入值存在数量级变化。本研究旨在利用热波技术比以前更准确地测量传热系数。更好地理解和测量传热，从而改进建模预测，将加速搅拌摩擦焊接的发展，使生产更轻的车辆结构、更安全的压力容器和更耐用的核废料罐等成为可能。如果成功，该技术也可以应用于传统的机械加工过程，其中以前的热电偶热测量工作是间接的，充其量是近似的。除了技术方面，该项目还将吸引研究生和本科生参与研究，并为他们提供与工业用户互动的机会，从而提高他们的劳动力准备。外展活动的目标是该大学的女性工程小组、当地技术学院的焊接课程以及当地高中的农业和技术教师。本工作的研究目的是确定在动态加工条件下利用热波测量换热系数的可行性。热波是通过调制入射激光热源的强度而产生的材料中的温度变化，并作为工具（或底板）抛光表面光学反射率的调制变化来测量。在该系统中，热波从刀具内部穿透到工件中，两部分之间热阻的大小改变了热波的测量幅度和相位。将所得相拟合到多层热四极模型中，得到传热系数值。为了验证这种新的计量技术，将设计一个双光纤探头，并将其放置在工具和底板内，以在焊接过程中产生和检测热波。该探头精确测量传热系数的能力将通过H13钢和铝合金圆盘之间的静态压缩试验来验证。测量值还将与成熟的分析模型进行比较，该模型预测具有已知表面粗糙度值和静态接触压力的材料的热接触电阻。然后，测量的参数将用作搅拌摩擦焊接有限元模型的输入，以演示改进的输入参数值如何在一系列条件和工具设计中推进对载荷、温度和材料流的预测。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。

Friction stir welding is a solid-state joining method that is finding increased application in joining aluminum alloys that are very difficult to join by conventional fusion welding processes. Industrial sectors that will directly benefit from better quality welded assemblies include core American industries such as aerospace, light rail, marine, and automotive. However, most friction stir welding development is done by experimental trial-and-error, limiting its impact, and slowing its introduction into potential weight-saving applications. Numerical simulation of friction stir welding began about 20 years ago, with advances made in predicting key process conditions (welding temperatures, material flow) and mechanical joint properties. Unfortunately, the predictive value of these models is limited because order of magnitude variations exist in reported friction and heat transfer coefficient model input values. This research aims to utilize thermal wave techniques to measure heat transfer coefficients more accurately than previously achievable. A better understanding and measurement of heat transfer, leading to improvements in modeling predictions, will speed development of friction stir welding, enabling the production of lighter vehicle structures, safer pressure vessels, and more durable nuclear waste canisters, among others. If successful, the technique can also be applied to conventional machining processes where prior thermal measurement efforts with thermocouples have been indirect and approximate at best. In addition to the technical aspects, this project will engage graduate and undergraduates in research and will provide them with opportunities to interact with industrial users, thus increasing their workforce preparedness. Outreach activities are aimed at the university’s Women in Engineering group, a local technical college’s welding program, and local high school agriculture and technology teachers.The research objective of this work is to determine the feasibility of utilizing thermal waves to measure heat transfer coefficients under dynamic processing conditions. Thermal waves are temperature variations in a material that are created by modulating the intensity of an incident laser heat source and are measured as a modulated change in the optical reflectance of the polished surface of the tool (or baseplate). In this system, the waves penetrate from inside the tool into the workpiece, and the magnitude of the thermal resistance between the two parts changes the measured amplitude and phase of the thermal wave. Heat transfer coefficient values will be obtained by fitting the resulting phase to a multi-layered thermal quadrupole model. To verify this novel metrology technique, a dual fiber optic probe will be designed and placed inside the tool and baseplate to create and detect thermal waves during the welding process. The ability of the probe to accurately measure heat transfer coefficients will be verified by static compression tests between discs of H13 steel and aluminum alloys. The measured values will also be compared to well-established analytical models that predict thermal contact resistances of materials with known surface roughness values and static contact pressures. The measured parameters will then be used as inputs in friction stir welding finite element models to demonstrate how improved input parameter values can advance the predictions of loads, temperatures, and material flow for a range of conditions and tool designs.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.