Optimisation of electron beam powder bed fusion process for an innovative titanium alloy

Electron Beam Powder Bed Fusion (EB-PBF) is one of the most relevant additive manufacturing technologies for producing Titanium components. Over the years, EB-PBF has had several application in the aerospace and biomedical fields thanks to the possibility of producing components with complex geometry and an excellent quality materials: all these features would be difficult to obtain with other production technologies. The process starts with a pre-heating step: the powder in the start plate is completely pre-heated by a series of passages of the defocused electron beam at low power and high speed. This step is really important by providing the formation of circular necks that connect the particles, increasing the thermal conductivity of the powder bed. This initial step is necessary to avoid the smoke effect which may occurs like an explosion during the interaction between electrons and powder. The reasons why the smoke effects occurs can be due to 1) the generated impact force which is greater than the cohesion force between the particles; 2) the generated repulsion force between the charge of the powder bed and the electrons is higher than the weight of a particle. All the process is performed in a vacuum environment to avoid the deflection of the electrons due to the presence of atoms and impurity in the chamber. Differently from other AM process, EB-PBF is a hot process due to the high temperature reached in the melting chamber; thanks to this, it is possible to process materials with a high melting point and the final parts do not need any stress-relieving treatments. This study presents an explorative investigation onto the EB-PBF processing conditions of an innovative Titanium beta alloy. Particularly, the preheating conditions are investigated and a design of experiment approach is adopted to investigate the optimal melting conditions. Archimedes density measurements and images analysis are performed to analyse the material density and the shape and distribution of the pore. The results shows that no issue are identified for processing the material at different working temperature. However, large deformations are detected when processing the material using certain process parameters. The contour strategy is found to be beneficial in reducing these deformation.