Temporal and Spatial Beam Shaping in LPBF for Fine and Porous Ti-Alloy Structures for Regenerative Fuel Cell Applications

Abstract

AbstractLaser Powder Bed Fusion (LPBF) presents itself as a potential method to produce thin porous structures, which have numerous applications in the medical and energy industries, due to its in-process pore formation capabilities. Particularly, regenerative fuel cells, which are capable of both producing and storing energy through the use of hydrogen-based electrochemical fuel cell and electrolysers, respectively, can benefit from the LPBF-induced porosity for it porous layer components in the electrode. Numerous studies have reported that process parameters, such as laser power, scan speed and hatch spacing, are key factors affecting the formation of pores in LPBF material due to their control over the energy density and melt pool formation during the build. Contemporary fibre lasers offer novel temporal and spatial beam shaping capabilities. Temporal laser control means that the laser can use pulsed wave (PW) or single point exposure (SPE), and spatial beam shaping refers to variations in the intensity distribution of the laser, which can be modulated from Gaussian to ring shape via the use of multi-core fibers. These have seldom been studied in combination with LPBF. Therefore, the aim of this study was to utilise temporal and spatial beam shaping in LPBF to produce thin porous structures. To do this, PW and SPE laser temporal strategies were utilised and the duty cycle (which relates the on and off time of the laser) was varied between 50% and 100%. Beam shape indexes 0 (Gaussian), 3 and 6 (ring) were also investigated alongside more standard LPBF process parameters such as laser power and scan speed to manufacture thin porous walls, as well as fine struts. The thinnest wall obtained was 130 μm thick, while the smallest strut had a diameter of 168 μm. The duty cycle had a clear effect on the porosity of thin walls, where a duty cycle of 50% produced the highest number of porous walls and had the highest porosity due to its ability to control the intensity of the energy density during the LPBF process. The different beam shape indexes corresponded to different spatial distribution of the power density, and hence, modifying the temperature distribution in the meltpool during the laser material interaction. Beam shape index 6 (corresponding to a ring mode with lower peak irradiance) created more porous specimens and smaller meltpool sizes, with respect to its beam size. Overall, this study showed that temporal and spatial control of the beam (through duty cycle and beam shape index) are powerful tools which can control the distribution and intensity of the energy density during the LPBF process to produce thin porous structures for energy applications.