Bending Of Metallic Thin Foil Via High Energy Pulsed Laser Peening
AuthorYocom, Christopher James
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Laser Peen Forming (LPF), a novel method used to form thin metallic engineering foils,is a manufacturing process utilizing strong shockwaves induced by high pressuresintroduced to the surface of a target by high energy, pulsed laser beams. It is a non-thermal,non-contact approach that improves the hardness and fatigue life of components byimparting beneficial residual stresses on the surface of the material. First, a review of theparameters of LPF processing, the forming mechanisms, process modeling techniques, andalternative LPF methods is discussed, to understand the procedures of industrialmanufacturing standards. Next, a simplification of metallic thin foils is proposed to developa novel low-cost and simplified LPF process design.The first segment pursues the general principles of LPF by reviewing its recentprogress, and that of comparable techniques. It discusses the process design, mechanismsattributed to forming, laser-material interactions, simulation methods, alternativeapproaches, and limitations thereof. The effect of laser intensity, material thickness,overlapping ratio, and number of scan tracks on bend angle, is discussed. Mechanisms ofLPF are reviewed by elucidating the mechanics of convex and concave curvature.Particularly, the Stress Gradient Mechanism (SGM) and the Stress Bending Mechanism(SBM), are modeled. In addition, the effect that drag bending has on the net bending of thesample piece is considered. The Fabbro model is followed for laser-material interactions.Advances in femtosecond and heat assisted LPF are considered with a focus on theadvantages and disadvantages over nanosecond methods.In the second segment, a streamlined LPF process design that eliminates ablativecoating and confining media is developed and explored to determine the feasibility ofbending of thin metallic foils. Experiments were conducted in a fashion similar totraditional LPF processing, with additional equipment unique to this new method, todetermine the final bend angle. Evidence has shown that laser intensity, sample thickness,and material strength are primary parameters in determining bending direction, amplitudeand process efficiency. In this part of the study, the effect of the number of passes and laserintensity on bending angle was examined. The results show that with increasing number ofpasses, there is an increase in bending angle, and, likewise, with an increase in laserintensity, there is an increase in bending angle. Then, the surface profile of the sample wasinspected, and the laser-processed area was characterized with respect to surfaceroughness. It was found that, relative to the original sample, the surface roughness of thelaser-processed area was not significantly impacted. Finally, the bend angle of LPF withand without an ablative coating, using equivalent experimental parameters, was compared.This portion of the study indicates that using an ablative coating under this new LPFprocess design is counterproductive.