The Effect of Microstructural Defects on High-Cycle Fatigue of Ti Grade 2 Manufactured by PBF-LB and Hydrostatic Extrusion

Abstract

The aim of this study was to show the effect of manufacturing defects in a commercially pure Ti Grade 2 produced by a laser beam powder bed fusion (PBF-LB) process on its high-cycle fatigue life. For this purpose, the high-cycle fatigue performance of PBF-LB Ti Grade 2 was compared to its ultrafine-grained (UFG) counterpart processed by hydrostatic extrusion exhibiting a similar mechanical properties under static tensile. The yield strength of the PBF-LB and UFG Ti Grade 2 was 740 and 783 MPa, respectively. The PBF-LB Ti Grade 2 consisted of a typical columnar of prior β grains with an acicular martensite α’ microstructure, while UFG Ti Grade 2 was mainly composed of fine, equiaxed α phase grains/subgrains with a size of 50–150 nm. A residual porosity of 0.21% was observed in the PBF-LB Ti Grade 2 by X-ray computed tomography, and, despite similar yield strength, a significantly higher endurance fatigue limit was noticed for UFG Ti Grade 2 (420 MPa) compared to PBF-LB Ti Grade 2 (330 MPa). Fatigue striation analysis showed that the fatigue crack propagation rate was not affected by manufacturing technology. In turn, the high-cycle fatigue life was drastically reduced as the size of manufacturing defects (such as pores or lack of fusion zones) increased.

Mechanical Properties of Ti Grade 2 Manufactured Using Laser Beam Powder Bed Fusion (PBF-LB) with Checkerboard Laser Scanning and In Situ Oxygen Strengthening

“…The PBF-LB process for HCF samples fabricated in Set III.1 was kept in 0.1-0.3 vol.% of oxygen in the building chamber, so the resulting part had 0.13 wt.% of oxygen measured in the high-temperature extraction method. Furthermore, a study on manufacturing defects in PBF-LB fabricated titanium grade 1, as reported by Majchrowicz et al [49], indicated that a significantly lower endurance fatigue limit was observed while the yield strength was similar to the commercial material. Therefore, the fabrication method should be improved to eliminate the presence of porosity.…”