Influence of powder reuse on metallurgical and mechanical properties of Ti-6Al-4V EB-PBF manufactured components

The laser-based powder bed fusion (LB-PBF) and electron beam powder bed fusion (EB-PBF) processes, both of which fall under the additive manufacturing (AM) category known as Powder Bed Fusion (PBF), are two of the most used techniques in the fabrication of metal components. These procedures allow for the manufacturing of extremely detailed forms in a single production step, removing the need for complex post-processing operations prevalent in traditional manufacturing processes (such as turning, drilling, etc.). They also reduce the necessity for external joints, such as gluing or riveting. It is, thus, a group of innovative processes that start with a raw material represented by powder. The powder used in PBF processes is obtained through complex and expensive atomization processes (water atomization, gas atomization, plasma atomization, and so on), and the more the powder has characteristics such as high sphericity, few deformations, and few internal porosities, the higher the cost of the powder. However, it is worth mentioning that in these types of manufacturing processes, the powder that does not participate in the creation of the desired component (the powder that is not melted) is frequently in large proportion to the initial powder. This has the consequence of generating far too much waste at the end of each manufacturing cycle to consider the process sustainable and efficient, resulting in massive cost increases. As a result of the previous evidence, the importance of reusing unconsumed powder that had not been utilized in the production of the planned component becomes clear. This approach plays an important role for lowering production costs while increasing overall efficiency. Depending on the number of reuses and the quality of the original virgin powder, the process of reuse waste powder can result in surface degradation of theindividual powder particles, accumulation of by-products known as spatters, and variation in microstructure. Based on these considerations, it becomes essential to understand the degradation mechanisms involved in the powders and how this deterioration impacts the final components produced. This dissertation aimed to evaluate and study both the metallurgical properties of powders and the metallurgical and mechanical characteristics of produced components to fully understand how powder reuse causes a deterioration in final quality. Specifically, in the EB-PBF process, powder degradation for Ti-6Al-4V powder particles was analysed and studied, beginning with a distinction and differentiation of one batch of virgin powder, one reused five times with the addition of ELI (low-oxygen) powder at each reuse, and one batch of highly reused powder, more than 100 times, with only four refills performed. Starting with many satellites in the virgin powder due to the atomization process and a moderate amount of internal porosity, it has been found that these defects decrease with increasing reuses. This reduction can be ascribed to both thermal and mechanical factors. The fabricated parts examined showed no apparent change in microstructure since all three kinds of components were found to be of the basketweave type. Also, in terms of internal and external defects, there was no difference, as all three samples contained macro-porosities, micro-porosities, Lack of Fusion (LOF) defects, and surface roughness. From a mechanical aspect, since from the literature reviewed, most of the mechanical tests performed from the researchers were tensile tests, the current dissertation wanted to focus more on fatigue tests and crack growth propagation tests to provide different findings. However, whereas earlier research indicated a decrease in tensile strength with repeated powder reuse, the present study found just an insignificant and irrelevant difference on fatigue and crack growth propagation tests. This observed result is probably caused by the non-optimization of process parameters, which overshadows the impact of powder reuse. As a result, all three kinds of samples exhibited similar porosity and mechanical strength.