Influence of Pulsed Exposure Strategies on Overhang Structures in Powder Bed Fusion of Ti6Al4V Using Laser Beam

Jonas Grünewald,Pirmin Clarkson,Georg Fey,Katrin Wudy,Ryan Salveson

doi:10.3390/met11071125

Jonas Grünewald, Pirmin Clarkson + Show 3 more

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https://doi.org/10.3390/met11071125

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Journal: Metals	Publication Date: Jul 15, 2021
Citations: 9	License type: CC BY 4.0

Affiliation: Technical University of Munich, Starnberg Hospital

Abstract

Manufacturing structures with low overhang angles without support structures is a major challenge in powder bed fusion of metals using laser beam (PBF-LB/M). In the present work, various test specimens and parameter sets with continuous wave (cw) and pulsed exposure are used to investigate whether a reduction of downskin roughness and overhang angle can be achieved in PBF-LB/M of Ti6Al4V. Starting from cw exposure, the limits of overhang angle and surface roughness at the downskin surface are investigated as a reference. Subsequently, the influence of laser power, scanning speed, and hatch distance with fixed pulse duration (τpulse = 25 µs) and repetition rate (υrep = 20 kHz) on surface roughness Ra is investigated. Pulsed exposure strategies enable the manufacturing of flatter overhang angles (≤20° instead of ≥25°). Furthermore, a correlation between the introduced volume energy density and the downskin roughness can be observed for pulsed exposure. As the reduction in volume energy density causes an increase in porosity, the combination of pulsed downskin exposure and commercial cw infill exposure is investigated. The larger the gap in volume energy density between the infill area and downskin area, the more challenging it is combining the two parameter sets. By combining cw infill and pulsed downskin exposure, flatter overhang structures cannot be manufactured, and a reduction in roughness can be achieved.

Highlights

IntroductionIn laser-based powder bed fusion of metals (PBF-LB/M), continuous wave (cw) and vector-based exposure strategies are used as a standard exposure strategy nowadays
Various test specimens and parameter sets with continuous wave and pulsed exposure are used to investigate whether a reduction of downskin roughness and overhang angle can be achieved in PBF-LB/M of Ti6Al4V
Starting from cw exposure, the limits of overhang angle and surface roughness at the downskin surface are investigated as a reference

Summary

Introduction

In laser-based powder bed fusion of metals (PBF-LB/M), continuous wave (cw) and vector-based exposure strategies are used as a standard exposure strategy nowadays This cw exposure with high-line intensities implies high temperature gradients as well as cooling rates in and around the melt pool, which determines the process dynamic and resulting component microstructure. Different beneficial aspects were addressed using pulsed exposure in PBF-LB/M: Minimizing thermal distortion [1]; Increasing spatial resolution for thin structures [2,3]; Tailoring microstructure [1,3]. These advantageous changes result primarily from the more controlled energy input. To sum up these investigations: with high repetition rates, increased overlap, and reduced scan speed, the top surface roughness is reduced, but at the same time, balling effects occur and side surface roughness is increased [4,5]

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