Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

C. Pauzon,S. Murugesan,G. Bruno,S. Dubiez-Le Goff,E. Hryha,S. Evsevleev,T. Mishurova

doi:10.1016/j.addma.2021.102340

Abstract

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas’s high thermal conductivity, heat capacity, and thermal diffusivity.

Highlights

Metal additive manufacturing (AM) and, in particular, laser powder bed fusion (L-PBF) have disrupted traditional manufacturing routes and created almost unlimited possibilities for designers to revise compo nents’ geometries and assemblies to improve component functionality and maximize benefits from this technology
Two sets of laser parameters were employed to produce both geometries with the three gases (12 sample types): the standard laser parameters developed by the machine manufacturer under the license Ti64_PerformanceM291 1.10 and a custom set with increased scanning speed
Regardless of the process atmosphere, not all the nitrogen from the feedstock powder was trans ferred to the built material during L-PBF, which should be investigated in future studies

Summary

Introduction

Metal additive manufacturing (AM) and, in particular, laser powder bed fusion (L-PBF) have disrupted traditional manufacturing routes and created almost unlimited possibilities for designers to revise compo nents’ geometries and assemblies to improve component functionality and maximize benefits from this technology. L-PBF uses the en ergy from a laser to locally melt and solidify a bed of metallic powder in a layer-wise fashion. This approach allows the production of near-netshape complex parts with thin features and integrated functionality. With the development of L-PBF hardware and L-PBF processing, the most used alloys, such as 316L stainless steel, Alloy 718, and Ti-6Al-4V can be produced to full density with strength comparable to or higher than conventionally produced materials [1]. Process, and quality control are key aspects of such challenges

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