By Ilya Mingareev, Tobias Bonhoff, Ashraf F. El-Sherif, Wilhelm Meiners, Ingomar Kelbassa, Tim Biermann and Martin Richardson
Laser Additive Manufacturing (LAM) is a rapidly developing field of advanced fabrication technologies that will benefit many industries by enabling near-net shape manufacturing of high-value components from metals, ceramics and compound materials. However, the geometry and the surface quality of parts produced by LAM can be significantly affected by heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the laser tool motion. The dimensional accuracy is insufficient for many application areas, thus requiring a certain amount of post-processing such as CNC milling and polishing. While efficient for solid and bulk components, conventional post-processing techniques cannot be applied to parts made of brittle, heat-sensitive materials, many multi-layer material systems and components with engineered porosity.
In this study, post-processing of metal parts fabricated by LAM was performed by utilizing high-repetition rate, high-pulse energy femtosecond and picosecond laser radiation. Ultrafast laser machining is an established micro-processing technique that takes advantage of the ultrafast deposition of the optical energy into the material leading to a significant reduction of the heat-affected zone compared to longer pulses. Utilizing this technique, different multi-layer laser scanning strategies were adopted to remove excessive material, reduce surface roughness and improve the geometrical quality of complex micro-scale topology features. Curved surfaces were processed in multiple overlapping tracks resulting in layers that followed the final design shape at a constant vertical offset until the final design shape was revealed.
For this work, 3D-shaped parts were manufactured from nickel-base and titanium alloys using Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Ultrafast laser irradiation of surfaces in 3-7 layers resulted in the reduction of the average surface roughness from initial several tens to less than 2 micrometers. The proper choice of the starting focus position relative to the non-processed surface is crucial for an optimal application of the high-intensity region of the laser beam to surface irregularities, e.g., individual powder particles, and thus for the resulting surface quality. In addition, a well-defined laser intensity profile must be maintained in the laser-matter interaction area, which may be challenging for complex part geometries.
Given the scalability of the presented post-processing approach, and using state-of-the-art scanning system technology and multi-100W laser sources with sub-picosecond pulse duration, the productivity of this method can be estimated to approximately 2 square inches per second. Furthermore, this hybrid post-processing capability can be integrated into existing LAM machines in order to ensure proper positioning and dimensional accuracy of parts according to the CAD data.
Ilya Mingareev, Tobias Bonhoff, Ashraf F. El-Sherif and Martin Richardson are with Townes Laser Institute, CREOL, University of Central Florida. Wilhelm Meiners and Ingomar Kelbassa are with Fraunhofer Institute for Laser Technology. Tim Biermann is with Joining Technologies Research Center.