AbstractRecycling of the unused surplus powder after laser beam powder fusion (LB-PBF) and electron beam powder bed fusion (EB-PBF) processes improve process efficiency. However, some of the recycled powder particles may have different physical and chemical characteristics due to degradation during part manufacturing, recovery, powder handling, and reuse that can affect the quality of the build parts. Therefore, it is important to understand the properties of the recycled powder, especially when it is used for manufacturing safety critical parts for aerospace applications. The present work aims to highlight different types and mechanisms of Ti6Al4V powder degradation, as a consequence of powder recycling, and its subsequent impact on the build properties.
The first part of the thesis presents an evaluation of the evolution of powder properties up to 10 build cycles using a simulated powder recycling strategy that enabled minimal virgin powder top-up in EB-PBF process. The results show heterogeneous powder degradation, with the powder particles located near the melt zone suffering maximum degradation. Degradation in the powder physical characteristics caused an increase in the fraction of lack-of-fusion defects in the specimens produced from the recycled powder. However, post-thermal treatment processes, hot isostatic pressing (HIPing) + solution treatment + ageing appears to be promising in eliminating the lack- of-fusion defects in the recycled builds. Due to the reactive nature of the Ti6Al4V alloy, an increase in the oxygen (O) content was unavoidable and therefore, the recycled powder showed an 0.02 wt.% increased O content compared to the virgin powder. However, since the O content in the recycled powder stayed within the maximum limit (0.13 wt.%), only a slight increase in the material yield and tensile strength (by 10 MPa) with negligible changes in ductility were observed.
Normally, in industry, the recycled powder is often blended or topped-up with virgin powder during recycling to maintain the build volume. The recycled powder particles present in the blend that have different physical and chemical properties compared to the virgin powder may cause localised inhomogeneity in the build properties. The change in powder physical characteristics with recycling are marginal and so the subsequent impact on part properties can be restored by performing additional post-thermal treatments.
Therefore, the second part of this thesis presents an investigation on the effects of blending a high and low O content Ti6Al4V powder on the build properties using in situ and ex situ experiments in the LB-PBF process. A small batch of Ti6Al4V powder (virgin powder) with O content of 0.12 wt.% was artificially oxidised to 0.40 wt.% (oxidised powder). The oxidised powder was blended with virgin powder in a suitable ratio such that the blended powder had 0.20 wt.% O. Due to problems during manufacture which resulted in large lack-of-fusion- defects, the mechanical properties were more significantly influenced by the process-related defects than the O content in the powder feedstock. Therefore, to further understand the effects of powder oxidation, in situ X-ray imaging experiments were conducted in a miniature LB-PBF process replicator on powders with two different O levels, 0.12 wt.%, and 0.40 wt.%. The results indicated that the high O content powder particles had a positive effect in reducing the number of melt ejections, surface roughness and defect population in the build parts.
|Date of Award||2022|
|Sponsors||Lloyd’s Register Foundation|
|Supervisor||Michael Fitzpatrick (Supervisor), Bo Chen (Supervisor), Carol Johnston (Supervisor) & Raja H.U. Khan (Supervisor)|