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

增材制造（AM）是一种直接形成结构部件的革命性方法。它正在改变制造业，然而，对安全关键部件的质量保证测试正在成为技术进步的瓶颈。复杂的物理过程涉及到金属零件的凝固，导致在多个长度尺度上的不均匀性。许多不均匀性，如空洞和缺乏融合，可以在离线、基于x射线的计算机断层扫描（CT）中检测到，但其他发生在微观尺度上的不均匀性仍然难以量化。目前还没有一种无损的方法能够在实际制造过程中评估材料的强度特性和制造质量。弹性波在材料中的传播，表明了材料的弹性特性，将用于解决这一缺点。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，以在逐层形成的基础上表征材料。这些信息将用于为闭环过程控制提供质量保证测试和反馈，并清除安全关键部件增材制造商业化的瓶颈。作为该项目的一部分，这种创新、对学生的培训和对教师的拓展将有助于美国在制造业中保持领导地位。目前没有一种技术能够在增材制造（AM）过程中询问材料的微观结构。然而，非线性超声具有无损表征材料微观结构特征的能力，这些特征决定了材料的强度。由于非线性超声的接触式换能器不适用于AM环境，而激光的驱动和接收是可处理的，因此建议采用非线性激光超声作为解决方案。该项目的研究目标是在AM环境中将弹性波与材料微观结构的相互作用联系起来。为了实现这一目标，必须证明由脉冲激光产生的瑞利表面波在金属零件逐层沉积过程中根据叠加原理混合在一起的假设是错误的。本文将研究一种双狭缝掩模/透镜组件，其目的是在两个主频率下驱动瑞利波。这些瑞利波的非线性混合将产生和谐波和差谐波。通过对这一假设的证伪，弹性表面波的相互作用将被证明是在AM过程中表征无损材料的一种可行方法。这项研究将有助于理解如何用瑞利波的相互作用来描述材料非线性，并提供和/差组合谐波与引起它们的强度相关材料非线性之间的相关性。

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.