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，在铝合金的连接中得到了越来越广泛的应用，而传统的熔焊工艺很难实现这种连接。将直接受益于更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行业。然而，大多数搅拌摩擦焊接的开发都是通过实验反复试验来完成的，这限制了其影响，并减缓了其引入潜在的减轻重量应用的速度。搅拌摩擦焊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)和机械连接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，因为报告的摩擦和换热系数模型输入值存在数量级的变化。这项研究的目的是利用热波技术来更准确地测量换热系数。更好地了解和测量热传递，从而改进建模预测，将加快搅拌摩擦焊接的发展，使更轻的车辆结构、更安全的压力容器和更耐用的核废料罐等得以生产。如果成功，这项技术也可以应用于传统的加工过程，在这些过程中，以前使用热电偶进行的热测量工作充其量是间接的和近似的。除了技术方面，该项目还将使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，从而加强他们的劳动力准备。外展活动针对的是该大学的工程女性小组、当地一所技术学院的焊接项目以及当地的高中农业和技术教师。这项工作的研究目标是确定在动态工艺条件下利用热波测量热传递系数的可行性。热波是材料中的温度变化，通过调制入射激光热源的强度产生，并作为工具(或底板)抛光表面的光学反射比的调制变化进行测量。在该系统中，波从刀具内部进入工件，两部分之间的热阻大小改变了所测得的热波的幅度和相位。换热系数值将通过将所得到的相与多层热四极模型进行拟合来获得。为了验证这一新的计量技术，将设计一个双光纤探头，并将其放置在工具和基板内，以在焊接过程中产生和检测热波。H13钢和铝合金圆盘之间的静态压缩试验将验证探头准确测量换热系数的能力。测量值还将与成熟的分析模型进行比较，这些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接触热阻。然后，测量的参数将被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提高对一系列条件和工具设计的载荷、温度和材料流动的预测。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。

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.