This paper presents the results of an experimental study to establish process parameters for repeatable, high quality ablated features in ferrous substrates using a Ti:sapphire femtosecond laser system. Initial trials with stainless steel substrates were conducted in ambient atmospheric conditions. Laser power and exposure parameters were varied, in addition to the angle of the substrate relative to the beam. Ablated holes were sectioned, and examined. Data was reduced according to the Taguchi/ANOVA method. The optimal process parameter set minimized the figures of merit for quality or accuracy of the ablated hole. In trials using pulsed ablations, high accuracy holes were associated with laser power greater than 600 mW, substrate angles of 30-45 degrees, and 1000 pulses. In the dwell experiments, high accuracy holes were achieved with a similar power level, and a 1-second dwell time. In contrast to the pulse results, a shallow substrate angle (30 degrees or less) yielded favorable results. In subsequent trials, kovar substrates were processed in a vacuum at constant fluence with a 1-second dwell time. A localized flow of nitrogen removed ablation products. Results were compared to those of the initial trial, leading to significant observations regarding the use of vacuum and secondary process gas.
Femtosecond laser processing is a promising new technology for the fabrication of micro-scale components from engineering materials, such as metals. In the femtosecond time regime, the ablation process is nearly a solid to vapor transition, thereby providing access to cut smaller features. Sandia National Laboratories has constructed a femtosecond laser microfabrication system to study the ability to produce microscale components in metals and glasses. In this paper, we will report on our initial studies to understand the metal ablation process with respect to manufacturing process parameters. With this understanding, we will show that femtosecond laser processing can fabricate complex components with fine feature detail and clean surfaces. A key finding in this work is the substantial effect of layer decrement on resulting recast material deposition when processing in air.
The laser engineered net shaping (LENSTM) process, currently under development, has demonstrated the capability to produce near-net shape, fully dense metallic parts with reasonably complex geometrical features directly from a CAD solid model. Results to date show that excellent mechanical properties can be achieved in alloys such as 316 stainless steel and Inconel 625. In fact, due to the highly localized nature of the laser heating, a fine grain structure will occur resulting in a significant increase in yield strength at no expense of ductility. The current approach lends itself to produce components with a dimensional accuracy of plus or minus .002 inches in the deposition plane and plus or minus .0.015 inches in the growth direction. These results suggest that this process will provide a viable mens for direct fabrication of metallic hardware directly from the CAD solid model.
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