Assessment by X-ray CT of the effects of geometry and build direction on defects in titanium ALM parts

Additive layer manufacturing (ALM) has the potential to allow engineers almost complete freedom of design, with reduced material wastage and tooling costs, as well as shorter lead times on new designs. Unfortunately, most ALM processes invariably lead to porosity in the material deposited. The ALM process investigated here, selective electron beam melting (SEBM) of a powder bed, is no exception. Although it is known that defects do arise, with this process their dependence on the part geometry and the adopted build strategy has not been resolved. This is of key importance, because experiments have shown porosity can make an order of magnitude difference to the fatigue life of ALM components. X-ray computed tomography (CT) is an ideal tool for fully characterising in 3D the defects seen within ALM parts and has been applied here to study the effect of geometry and build direction on defects in titanium components. The most industrially, realistic worst case scenario was employed using an Arcam machine with a small raster pattern misalignment in order to study all the characteristic flow types possible in the SEBM process. For most of the test samples studied, XCT revealed large elongated pores (> 100 μm) to be present, which grew at a distance of around 1 mm from an edge, following the build direction. The position of this defect type correlated with the misaligned overlap region between the slow contour passes of the electron beam around the periphery of the sample outline and the hatching in fill region of a section. Smaller voids caused by gas entrapment were also resolved by XCT but they appeared to be mostly randomly distributed.