The process atmosphere as a parameter in the Laser-Powder Bed Fusion process
Licentiate thesis, 2019
Laser-Powder Bed Fusion (L-PBF) is an Additive Manufacturing (AM) technique that allows to produce near-net shape components of complex design. Applied to metallic materials, L-PBF allows to consolidate subsequent layers of powder using the heat from laser radiation. The powder particles are typically micron-sized and have a high specific surface area, making them prone to surface oxidation.
As several layers are consolidated, the powder feedstock and solidified material are exposed to repeated thermal cycles, involving high peak temperatures and high heating/cooling rates. This heat input is likely to trigger oxidation by enhancing the reaction of the heated material with the residual oxygen present in the process atmosphere. To limit the extent of oxidation, protective atmospheres are used. These atmospheres are typically established by blanketing the powder bed using an inert gas, which permits the dilution of the present oxygen, and to some extent other impurities such as nitrogen and humidity. The gas will also drag away possible process by-products that are likely to introduce defects within the material upon their re-deposition. Although the gas is necessary for the L-PBF process, its role has mainly been disregarded until now as focus has been placed on other process parameters such as laser related ones (e.g. laser power and scanning speed). As a result, the available gases for L-PBF are limited to the noble argon and the relatively inert nitrogen, leading to the L-PBF machines being designed only for the use of these gases.
The present study aims to raise awareness of the significant role of the gas properties and control for the L-PBF process. The effect of the residual oxygen and processing gas properties are addressed. The results highlight that the residual oxygen guidelines should be proposed based on the sensitivity of the material to the oxygen and nitrogen exposure. While Ti-6Al-4V shows a difference in density at 1000 ppm O2 compared to 100 ppm O2, 316L stainless steel exhibits consistent mechanical properties for any oxygen level below 1000 ppm O2. Upon higher oxygen partial pressure (e.g. 2000 ppm O2), the 316L stainless steel powder particles develop oxide features on the surface. These features are consistent with an increased oxygen pick-up by the material, and the further reduction of its impact toughness, reflecting an increase in oxide inclusions content within the built material.
Furthermore, the work conducted in this thesis gives an insight on the effect of the gas density and thermal properties on the produced Ti-6Al-4V material. Helium gas, which is significantly lighter than argon and nitrogen, was successfully implemented in the L-PBF process of Ti-6Al-4V. Helium and argon-helium gas mixtures enable a reduction in the porosity upon higher build rates. This is attributed to the positive balance of density and thermal properties offered by these mixtures. The obtained results could be used to initiate the development of new gas mixtures aiming at increasing the L-PBF productivity and process robustness.
316L stainless steel
Laser-Powder Bed Fusion