Electron beam powder bed fusion additive manufacturing of Ti6Al4V alloy lattice structures: orientation-dependent fatigue strength and crack growth behaviour under compressive cyclic loading

Sufficiently high fatigue strength is required for lattices made using electron beam powder bed fusion (EBPBF) for hip implants and understanding the anisotropic fatigue behaviour of EBPBF lattices is necessary for implant design. In this work, the combined effects of loading direction (LD) and cell orientation of EBPBF-Ti6Al4V lattices on the fatigue strength of the structures under cyclic compressive loading have been studied. Simple cubic (SC) ([001]//LD, [011]//LD and [111]//LD) lattices with a relative density of 0.36 were EBPBF made, tested and examined. The fatigue strength of [001]//LD lattices has been determined to be ∼190 MPa at 5 × 10⁶ cycles, ∼8 times higher than that of [011]//LD or [111]//LD lattices. The low fatigue strength of the non-[001]//LD lattices resulted from crack initiation readily occurring in the high tension locations, which are the top and bottom locations of each unit cell. Sideway growth of cracks leading to fracturing along (001) will be shown. This failure mechanism is absent in [001]//LD lattices and thus their fatigue strength is high. Examining the data in the literature has shown that fatigue strength values of all non-SC lattice structures are low, likely due to the same failure mechanism identified for non-[001]//LD SC lattices in this study.