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

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.