Friction stir processing of AM parts

Use of Additive Manufactured (AM) parts for structural applications is currently limited due to their mechanical behaviour, particularly the fatigue performance. Microstructural inhomogeneity, porosity issues and residual stresses are the main reasons for such low performance. This project aims at improving mechanical properties of Al AM parts through post-processing via Friction Stir Processing (FSP). This technique may be implemented on a milling machine and consists in introducing a rotating tool in the material, that then traverses through the part to process. FSP can thus easily preceed machining finishing operations in a single combined step, unlike other post-processing alternatives, e.g. Hot Isostatic Pressing. FSP produces a thermomechanical treatment effect involving friction and deformation heating, in solid state. It is thus a flexible process that allows treatment of critical mechanically solicitated regions of large parts. The PhD will be aimed at understanding the effect of FSP post-processing on the microstructure (porosity and second phase size and spatial distribution) and analysing the consequences on the mechanical behaviour (residual stresses and fatigue) of AlSi10Mg AM fabricated parts. The project will also feature an analysis of the influence of processing parameters through a chained thermal and microstructural model, aiming at defining adequate FSP patterning strategy. The first obtained results are very promising, showing porosity neutralisation and homogenisation upon FSP post-processing of the AlSi10Mg AM parts.