Fatigue of Ti6Al4V manufactured by PBF-LB: A comparison of failure mechanisms between net-shape and electro-chemically milled surface conditions

In the recent years, metal additive manufacturing (AM) has acquired large interest for many industrial applications, principally due to the capability to produce parts with complex geometry. The critical aspect of AM parts is the sensitivity to surface anomalies due to net-shape surfaces, i.e surface microcracks and protrusions, localized stresses caused by coarse surface roughness, or sub-surface features placed below the outer skin in the contour region. To reduce the surface roughness and increase the fatigue properties, proper post-process treatments can be applied. This work investigates the improvement in surface quality and fatigue properties due to the electro-chemical milling process of Hirtisation® compared with net-shape condition, on samples manufactured in Ti6Al4V by laser-powder bed fusion (PBF-LB). Post-processing led to a reduction of surface roughness due to the removal of the peaks and sharp valleys that act as crack initiation sites during fatigue tests, but it exposed the sub-skin contouring defects to the free surface. These were the crack initiation sites resulting in a limited improvement of the potential benefits produced by Hirtisation®. This was confirmed by fatigue life predictions based on propagation of surface features and contouring anomalies.