Exploring the influence of powder properties and handling on the selective laser melting process

Metal additive manufacturing (AM) processes are promising techniques that offer several benefits such as weight reduction, complex shapes and reduction of inventories. Besides the weight reduction achieved by selectively melting the part geometry in the selective laser melting (SLM) process, it is also possible to reuse the raw material that does not form the final part. In this case, at the end of the process, the powder deposited in layers on the build platform is collected in a ‘buffer’ container and ‘handled’ to be used again. The ‘handling’ consists of sieving the powder, mixing it with virgin fresh material – the so-called ‘rejuvenating’ treatment – and then loading again into the machine for another build job or a continuation of a lengthy one. During the time the powder is out of the machine, either for being stored or handled, it can be in contact with contaminants such as moisture, while the effect of moisture on SLM metal powders is not yet well understood.

In this thesis, special attention is paid to the influence of the general handling procedures on the atomized powder and final SLM part properties. General understanding of the
handling steps is provided and a procedure to compensate its effects are proposed in this manuscript. In addition, to mitigate the effect of humidity on the repeatability of SLM
process, pre-drying treatments are proposed and investigated.

Since reusing the metal powders in SLM is key to ensure a degree of sustainability and costefficiency in the process, this has promoted some investigations on typically printed materials such as Ti6Al4V and Inconel 718. However, other alloy systems such as aluminium alloys are not yet well understood in terms of reusability. In addition, literature and preliminary studies of the powders have shown the high affinity of these alloys to contaminants. For this reason, also Scalmalloy and AlSi10Mg are important focus materials in this study.

The first part of this thesis focuses on understanding the influence of reuse, humidity and pre-drying treatments on the metal powders’ properties and flowability/spreadability
behaviour, which is important for thin layer application. A powder characterization methodology is designed to characterize four materials typically used in SLM: Inconel 718, Ti6Al4V, AlSi10Mg and Scalmalloy. In the second part the materials and mechanical properties of AlSi10Mg and Scalmalloy are studied. Their processability and feasibility to be used in aerospace applications are assessed, also by measuring their anisotropic behaviour. In addition, a detailed assessment of Scalmalloy build properties from virgin and reused powder is discussed.

There are a number of interesting findings from this experimental study. AlSi10Mg showed sensitivity to reuse and the highest affinity to humidity of all studied materials. The other three materials appear to tolerate more variability in the storing conditions. The mechanical tests performed to the aluminium alloys revealed that Scalmalloy is a more suitable candidate for aerospace applications: with similar density as AlSi10Mg, the mechanical properties are outstanding and the processability is much better. Finally, reusing the Scalmalloy powder appears to be safe once the handling is correctly carried out by applying strict process control during the sieving process, rejuvenation and climate protection.