![]() Alternatively, a low laser heating power can lead to an incomplete melting and solidification process and lack of fusion defects 7, 8. If the heating laser power is too high for a given scan speed, it can lead to evaporation of the material from a molten pool, a subsequent recoil force, and collapse of the melt pool, resulting in porosity in the component. Intra- and inter-layer integrity is dependent upon the rapid heating, melting, and solidification processes during which defects and material discontinuities are likely to form 3, 4, 5, 6. For example, in laser powder bed fusion an object is built in consecutive layers by laser melting of powder mechanically distributed over the build surface 3, 4. In these applications, laser energy is absorbed by a material leading to local heating, melting, and vaporization, and in situ control of these laser-induced processes is critical to ensure the integrity of the final product 2. Lasers have seen widespread use in materials processing and manufacturing applications including laser cutting, welding, ablation, drilling, surface texturing, and advanced manufacturing 1. ![]() 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. 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. A pulsed laser is used to generate high frequency surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. 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. 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. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. There is a need for new techniques to provide in situ feedback of these processes. 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.
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