Date of Award

5-2022

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Chair/Advisor

Xin Zhao

Committee Member

Garrett Pataky

Committee Member

Rodrigo Martinez-Duarte

Abstract

This study demonstrates the viability of femtosecond laser peen forming for bidirectional sheet metal bending. Ultrafast laser forming is a valuable method thanks to its flexibility, negation of springback, no need for a confining or protective layer, and minimalization of thermal degradation in comparison to other forming processes. However, femtosecond lasers have traditionally been believed to be unsuitable for laser forming in comparison to nanosecond lasers, particularly bidirectional bending, because the induced shock waves were expected to be weak. Conversely, in this study it is elucidated that the femtosecond laser-induced shock waves are extremely strong (several hundred GPa) but the shock wave penetration depth is less than 100 µm, limiting the bendable metal sheet thickness. It is found that below thicknesses of 254 µm, angles can be made ranging 90 degrees towards the laser (concave) and 210 away from the laser (convex). The underlying mechanisms of the femtosecond laser peen forming process are studied through the variables of laser fluence, scan speed, scan pitch, total scan area, material thickness, and a single line bending test. The effective laser fluence is defined and proposed as the deterministic factor, which is supported by the experimental tests. As the effective laser fluence increases, the shock wave intensity and penetration depth rise, leading to the transition from stress gradient mechanism to shock bending mechanism, and hence convex to concave bending. Although, high fluence yields high ablation, which can lead to a loss of stiffness and elastic properties compared to non-laser bending.

Available for download on Monday, April 17, 2023

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