Residual oxygen content and powder recycling: effects on microstructure and mechanical properties of additively manufactured Ti-6Al-4V parts

Abstract

The laser-based powder bed fusion of metals (PBF-LB/M) offers a variety of advantages over conventional processing techniques and the possibility to recycle and reuse powder increases its sustainability. However, the process and resulting part properties are influenced by a variety of factors including powder recycling grade and residual oxygen content of the process atmosphere. Especially in terms of reactive materials like Ti-6Al-4V, oxidation during processing and recycling determines process stability and reproducibility. This work therefore focusses on the influence of the conventionally varied processing parameters as well as atmosphere residual oxygen content process and powder recycling on the microstructure and mechanical properties. For this purpose, the design of experiments approach is used and by evaluation of regression models, effect sizes and interactions are given. Additionally, two different etching techniques were employed to reveal different aspects of the microstructure. While no significant influence of powder recycling and residual oxygen on the microstructure could be observed, they both significantly influence the mechanical properties. A maximum hardness of 470 HV0.1, a maximum ultimate tensile strength of 1252.3 MPa, and a maximum elongation at break of 17.8 % were obtained. The results demonstrate the importance of the processing atmosphere’s residual oxygen content and of taking into account the changing powder characteristics during recycling as well as its effect on the part properties.

The microstructure, mechanical and electrochemical properties of 3D printed alloys with reusing powders

“…Larger agglomerates (satellite particles) removed during sieving might be the main reason for the narrowing of the PSD, and, together with the reduction of the humidity of the powder, also results in improved flowability and consequently the lower porosity 18 . It is assumed that reusing the powder would have an influence on its porosity 22 . The conclusions of previous studies are, however, contradictory in this regard.…”

“…The efficiency of recycling also has a large impact on the properties of a 3D printed object. Some influences of exposing the powder to heat and gases in the printing atmosphere, such as oxygen uptake, and changes to the chemical composition and microstructure, cannot be restrained by recycling 15 , 22 , 28 , 29 . In order to achieve the target properties, it is therefore of great interest to understand the impacts of these factors on the properties of the final object.…”

Characterization, preparation, and reuse of metallic powders for laser powder bed fusion: a review

“…In terms of the microstructure of the powders, it is found that both the virgin and reused Ti6Al4V powders consist of a dark brown with an irregular lighter region after etching with ammonium bifluoride (figures 13(b-i) and (b-ii)) [247]. However, the lamellar structure can be observed in the lighter area for reused powders (figure 13(b-ii)) Yet, the mechanisms behind the variation in the microstructure or phase composition were not given in the study.…”

“…Finally, it is reported that the density of the samples fabricated using virgin and reused CP-Ti powders for 10 times [209] is the same. However, Soltani-Tehrani et al [183] observed that the porosity of the parts tended to decrease first and then increase as the [247], with permission from Springer Nature. (c) EBSD map and phase distribution of virgin (i and ii) and reused (iii and iv) samples.…”

“…(b) Cross-section of virgin (i) and reused (ii) powder after etching. Reproduced from[247], with permission from Springer Nature. (c) EBSD map and phase distribution of virgin (i and ii) and reused (iii and iv) samples.…”