Correlating the fatigue strength with surface finish imperfections in LPBF Ti-6Al-4V alloys

To establish the basics for defect-tolerant design of laser powder bed-fused (LPBF) components, it is essential to correlate their fatigue resistance not only with the size of internal defects, such as gas-filled pores or lack-of-fusion defects, but also with surface finish imperfections.

To this end, Ti64 axial and prismatic four-point bending fatigue specimens were fabricated by LPBF, with the bending specimens built with orientations varying from 45 to 135^o. The specimens were heat-treated and divided into two groups: a) as-built and b) surface-finished. The surface-finished specimens were subjected to one of three finishing operations: machining, chemical milling and tumbling. All specimens were inspected to quantify internal and surface process-induced imperfections and subsequently subjected to fatigue testing. All printed specimens manifested similar chemical compositions and duplex (ɑ+β) microstructures and contained a limited number of gas-filled pores.

The static tensile properties were comparable for all specimens (Sy≈900 MPa), except for the elongation to failure which was lower for the as-built samples than for their machined counterparts (10 vs 18%). The runout stress amplitude (10^7 cycles) ranged from 200 to 500 MPa depending on the size of LPBF-induced imperfections measured by computed tomography and validated by fractography and optical profilometry. These data were used to construct a Kitagawa-Takahashi type diagram correlating internal defects and surface finish imperfections with the fatigue resistance of printed components. The proposed approach provides a framework for supporting defect-tolerant design methodologies for additively manufactured Ti64 components.