Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes

Morales, Rosa Edith

Graduate Thesis Or Dissertation

Laser Ultrasonic Monitoring of Laser-Induced Thermal Processes Public Deposited

Analytics

Download PDF

Citations

Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/pg15bg401

Abstract

Intra- and inter-layer integrity of components fabricated with advanced manufacturing techniques, such as laser powder bed fusion, is dependent upon rapid heating, melting, and solidification processes. There is a need for new techniques to provide in situ feedback of these processes. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. Furthermore, laser-based ultrasound experimental results which monitor the transient change of surface wave travel time associated with high power laser surface heating are provided. A pulsed laser is used to generate surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. Qualitative agreement is observed between theory and experiment with both showing a rapid reduction in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. It is demonstrated that changes in the surface wave velocity can be used to track local heating and detect the onset of surface melting in real time.

Additionally, we develop three-dimensional finite element acoustic models to study rapid, depth-varying laser heating. We fabricated a fast acquisition experimental setup built to probe transient depth-dependent temperature fields and melt pool depths. Agreement between theory and experiment is observed with both showing significant surface acoustic wave dispersion resulting from the rapid laser-heating. These dispersive effects are more pronounced when the heating laser power is sufficient to cause melting as the high frequency SAW components are further delayed by the presence of the melt.

The work presented here demonstrates the efficacy of using laser-based ultrasonics for in-situ monitoring of transient laser-induced heating and melting processes. This technique may ultimately find application in the mapping of transient laser-induced thermal fields and melt zones, providing critical information for real-time feedback and process control in advanced manufacturing systems.

Creator