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

相加制造(AM)是一种革命性的直接成形结构部件的方法。它正在改变制造业，然而，安全关键部件的质量保证测试正在造成技术进步的瓶颈。金属零件的凝固过程涉及到复杂的物理过程，导致了多个长度尺度上的不均匀。许多不均匀，如空洞和缺乏融合，可以在离线、基于X射线的计算机体层摄影(CT)扫描中检测到，但其他发生在微观尺度上的不均匀仍然很难量化。目前，还没有一种无损的方法能够在实际制造过程中估计材料的强度、性能和制造质量。通过材料的弹性波传播，这表明材料的弹性性质，将被用来解决这一缺点。本文研究了如何有效地利用激光诱导的热应力在材料表面层产生弹性波，以及如何从测量的波传播中提取材料的特性。通过使用非接触式激光光源，该技术可以集成到制造环境中，以在逐层形成的基础上对材料进行表征。这些信息将被用来提供质量保证测试和反馈到闭环过程控制中，并消除将AM用于安全关键部件商业化的瓶颈。作为该项目的一部分，这一创新、对学生的培训以及对教师的推广将帮助美国保持在制造业中的领导地位。目前没有一种技术能够在添加剂制造(AM)过程中询问材料的微观结构。然而，非线性超声波有能力非破坏性地表征决定强度的材料微结构的特征。虽然非线性超声常用的接触式换能器不适用于AM环境，但激光激励和超声接收是容易处理的，因此非线性激光超声被建议作为解决方案。该项目的研究目标是在AM环境中将弹性波与材料微结构的相互作用关联起来。为了实现这一目标，脉冲激光产生的瑞利表面波在层层沉积过程中在金属部件中传播的假设将根据叠加原理混合在一起的假设将不得不被证伪。将研究一种双缝掩模/透镜组件，目的是在两个主要频率上激励瑞利波。这些瑞利波的非线性混频将产生和差谐波。通过证伪假设，弹性表面波的相互作用将被证明是AM过程中非破坏性材料表征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的非线性，并提供和/差组合谐波与引起它们的与强度相关的材料非线性之间的关联。

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.