Mixing Elastic Waves to Nondestructively Characterize Microstructure During Additive Manufacturing of Metals

增材制造（AM）是直接形成结构部件的革命性方法。它正在改变制造业，但安全关键部件的质量保证测试正在成为技术进步的瓶颈。金属部件凝固过程中涉及的复杂物理过程导致多个长度尺度上的不均匀性。许多非均匀性，如空洞和缺乏融合，是可检测的离线，基于X射线，计算机断层扫描（CT）扫描，但其他的，发生在微观尺度，仍然难以量化。 目前还没有能够在实际制造过程中估计材料强度特性和构建质量的非破坏性方法。通过材料的弹性波传播，这是材料的弹性性能的指示，将被用来解决这个缺点。本文研究了如何有效地利用激光诱导热应力在材料表层产生弹性波，以及如何从测量的波传播中提取材料特性。通过使用非接触式激光源，该技术可以集成到制造环境中，以在逐层形成时表征材料。这些信息将被用于提供质量保证测试和反馈到闭环过程控制中，并清除AM商业化的瓶颈。作为该项目的一部分，这种创新、学生培训和教师推广将有助于美国在制造业中保持领导地位。目前没有技术能够在增材制造（AM）期间询问材料的微观结构。然而，非线性超声具有非破坏性地表征决定强度的材料微观结构特征的能力。虽然非线性超声常用的接触式换能器不适用于AM环境，但超声的激光激励和接收是易于处理的，因此建议将非线性激光超声作为解决方案。本计画的研究目标是在AM环境中，将弹性波与材料微结构的相互作用关联起来。为了实现这一目标，假设瑞利表面波产生的脉冲激光和传播的金属零件在逐层沉积混合在一起，根据叠加原理将不得不被证伪。双缝掩模/透镜组件将被研究，其目的是在两个主频率处激励瑞利波。这些瑞利波的非线性混合将产生和差谐波。通过证伪这一假设，弹性表面波的相互作用将被证明是一种可行的方法，无损材料表征AM。这项研究将导致了解如何描述材料的非线性瑞利波的相互作用，并提供和/差组合谐波和强度相关的材料非线性引起它们之间的相关性。

Additive manufacturing (AM) is a revolutionary way to directly form structural components. It is transforming the manufacturing industry, however quality assurance testing of safety critical components is creating a bottleneck in advancement of the state of the art. The complex physics involved in the solidification of metal parts results in nonuniformities across multiple length scales. Many non-uniformities, such as voids and lack of fusion, are detectable in off-line, x-ray based, computed tomography (CT) scans, but others, occurring at the micro-scale, are still difficult to quantify. Currently there is no nondestructive method capable of estimating material strength properties and build quality during the actual manufacturing process. Elastic wave propagation through the material, which is indicative of the material's elastic properties, will be used to address this short coming. This work researches how to effectively use laser induced thermal stresses to generate the elastic waves in the material surface layers, and how to extract material properties from the measured wave propagation. By using a noncontact laser source, the technique can be integrated into the manufacturing environment to characterize material as it is formed on a layer-by-layer basis. This information will be leveraged to provide quality assurance testing and feedback into closed loop process control and to clear the bottleneck in commercializing AM for safety critical components. This innovation, training of students, and outreach to teachers as part of this project will help the United States maintain a leadership role in manufacturing. No current technology is capable of interrogating the microstructure of material during additive manufacturing (AM). However, nonlinear ultrasonics has the capability to nondestructively characterize the features of material microstructure that dictate strength. While the contact transducers customary for nonlinear ultrasonics are not appropriate for the AM environment, laser actuation and reception of ultrasound is tractable, thus nonlinear laser ultrasonics is suggested as the solution. The research objective of this project is to correlate the interaction of elastic waves with material microstructure within an AM environment. To achieve this objective, the hypothesis that Rayleigh surface waves generated by a pulsed laser and propagating in a metal part during layer-by-layer deposition mix together according to the principle of superposition will have to be falsified. A dual slit mask/lens assembly will be researched with the aim of actuating Rayleigh waves at two dominant frequencies. Nonlinear mixing of these Rayleigh waves will generate sum and difference harmonics. By falsifying the hypothesis, the interaction of elastic surface waves will be proven to be a viable method for nondestructive material characterization during AM. The research will lead to understanding how to describe material nonlinearity in terms of the mutual interaction of Rayleigh waves and provide a correlation between sum/difference combinational harmonics and the strength-related material nonlinearity causing them.