Powder-bed based Additive Manufacturing (AM), also known as 3D printing, is an emerging technology to produce high quality end-parts at a cost and time effective manner as compared to the traditional subtractive manufacturing processes. The study of metal powder-bed processes usually involves two critical steps which includes the spreading of the powder and the fusion of the layers. Spreadability of powders is an essential characteristic in determining the total build time and quality of the final product. The spreadability is thought to be linked to the powder characteristics, mostly the flow behaviour, which is influenced by the individual particle properties and environmental conditions.
This thesis endeavours to investigate the spreading behaviour of two samples of titanium alloy (Ti6Al4V) powders, produced by Gas Atomisation (GA) and Hydrogenation-dehydrogenation (HDH) methods. The spreading behaviour of each sample has been investigated using an in-house spreading rig set up at University of Leeds, with a set of parameters, including the gap size between the blade and build plate (ranging between 191 µm to 508 µm) and spreading velocity (ranging between 50mm/s to 200mm/s) as the variables, while the mass of the fed powder as a constant. The bulk layer density and mass per area are the two measures of spreadability introduced in this study.
It is found that the quality of the spread layer is significantly influenced not only by the powder properties, but also the process parameters. GA powder exhibited spherical shapes, which in turn created homogeneous layers due to improved packing behaviour compared to HDH powder which were characterised by irregular shapes impeding powder flow. Additionally, the bulk layer density of GA powder decreased when the spreading velocity was increased, but this correlation was not established for HDH powder. This owes to the fact that HDH powders are of irregular morphologies, hence they consist of particular particle arrangements under the spreader at certain velocities. An increase in the spreading velocity also resulted in a gradual reduction in the values of the mass per area, suggesting that higher velocities jeopardise the quality of the spread layer. Due to the limitations of spreader rig, only a specific amount of powder to cover the build plate was utilised, however, in reality an infinite amount of powder is fed for spreading the layers.
It is concluded that GA powders outperform HDH powders in terms of reliability and creating higher bulk layer densities, which is essential in ensuring quality layers and, consequently, defect-free end parts. The quality of the layer is determined by calculating the average bulk layer density. In AM, a higher bulk layer density is desirable as it demonstrates better packing quality , homogenous layers of low porosity, which consequently results into end parts of high density and increased mechanical strength.
