Characterization of femtosecond laser micromachined specimens extracted from PBF-LB/M microstruts: Analyzing surfaces fabricated via internally linked machined kerfs

Abstract

The as-built surface morphology on micro-scale parts produced by laser powder bed fusion (PBF-LB/M) considerably influences their mechanical behavior. Femtosecond pulsed laser was utilized to selectively ablate surface regions of stainless steel microstruts produced by PBF-LB/M, allowing the fabrication of micro-scale specimens for tensile and surface characterization of its intrinsic or "roughness-free" mechanical behavior. The study fabricates 280μmx400μm cross-sectioned specimens from vertically built PBF-LB/M microstruts of 500μm nominal diameter. The new micromachining strategy utilized in this work was designed to ensure similar roughness at the edges and faces of the specimens, the average Ra are 1.2±0.4μm and 0.9±0.2μm, respectively, as well as to impart low curvature at edges, i.e. ρ=3±1μm. In addition, the micromachined surfaces revealed an increased average hardness from 276±11HV to 340±14HV at near-surface depths <3.6μm and similar bidirectional compressive residual stresses in laser incidence and scan directions, i.e. −371±65MPa and −429±41MPa, respectively. The effect of surface morphology and hardened surface layer on tensile elongation are negligible, as the average yield strength, ultimate tensile strength, and uniform tensile strain obtained for the micromachined specimens are comparable to those obtained from electropolished counterparts.