Analysing the powder properties and spreadability of a Ti-Ta blend in a powder bed fusion system
Creators: name: De Silva, Sarukkalige Tirunika
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401401
subject: Additive manufacturing

description: Powder bed fusion systems in additive manufacturing rely on the application of a thin layer of powder over a build platform which is termed spreadability. PBF systems generally utilise spherical powders due to their exceptional flowability and spreadability, resulting in uniformly packed powder bed layers. This leads to the creation of parts with excellent mechanical properties, such as high strength.

However, the production of spherical powders relies on expensive processes such as gas atomisation, plasma rotating electrode and plasma spheroidization. Considering this, the cost of producing parts in additive manufacturing can be greatly reduced if more economical non-spherical powders can be employed. Current traditional methods such as the Hall and Carney flowmeter can quantify the flowability of powders. However, these methods are performed external to the PBF systems and only give an idea of how well a powder flows within the test devices. These tests are inadequate to determine whether powders, especially fine and irregularly shaped powders, can flow and spread sufficiently well in a PBF system. Consequently, this led to the development of the Universal Powder Bed by CSIRO which can more accurately analyse powder behaviour by simulating the powder raking process similar to an ARCAM A1 electron beam melting PBF system.

The objective of this project was to analyse the powder and powder bed properties of an 80 wt.% spherical titanium (45-106 μm) and a 20 wt.% irregularly shaped tantalum (10-30 μm) powder blend (Ti-20Ta) which has potential applicability in the biomedical industry. The aim was to address two main challenges – the expensive nature of spherical powders and the lack of experimental studies addressing spreadability in electron beam melting systems. By doing so, it helps to bridge the knowledge gap in this research field. Three relative characterisation measurements were used to assess the thin layer of powder: area fraction of build surface covered with powder, area surface roughness, and powder bed composition. These measures were evaluated against increasing rake speeds of 30% to 100% of the nominal rake speed used in the ARCAM and powder layer thicknesses of 100 μm to 200 μm.

The characterisation techniques in this study revealed valuable insights. Firstly, it demonstrated that the flowability properties of the Ti-20Ta blend were slightly inferior to 100% Ti powder but significantly superior to irregularly shaped Ta powders. Secondly, it established that increasing layer thicknesses and slower rake speeds led to more efficiently packed powder layers. Furthermore, two types of segregation were observed in this study. The first type was segregation between opposite sides of the powder bed, and this was effectively reduced by increasing the layer thickness and reducing the rake speed. Interestingly, most of the Ta particles accumulated predominantly towards the end of the bed, in contrast to other studies where finer particles settled at the start of the build plate. This observation was attributed to the cohesiveness experienced by the fine, irregular Ta particles and the use of a double-staggered comb in the electron beam melting machine. The second type of segregation observed was vertical segregation of Ta, which remained noticeable at larger layer thicknesses and slower rake speeds. However, it was found to be less prominent and more likely to be offset by the effect of raking between both sides of the plate. In summary, the study concluded that refining the spreading parameters could lead to achieving acceptable levels of spreadability.