Fatigue properties of Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy produced by direct laser deposition

The fatigue properties of the Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy produced by direct laser deposition (DLD) have been investigated. The fatigue limit of the alloy perpendicular to and parallel to the deposition direction at specified life of 107 and stress ration of 0.1 is 337 MPa and 365 MPa, respectively. The porosities of specimens are among 0.014-0.028%, averaged porosity is 0.02%. The fatigue cracks initiate at the pores near the surface, which are located mostly at the column grain boundary during solidification. The size of pore and its distance from the surface are two important factors to control the fatigue life of specimen, i.e., bigger the pore or nearer to the surface, shorter the fatigue life. The fatigue fracture consists of fatigue source, radial slow crack growth and rough fast fracture region. The columnar grains boundaries of Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy by epitaxial growth from the underlying template grains layer-by-layer during direct laser deposition are the weak joints under the cyclic stress and crack formed propagates preferentially along the grain boundaries, which should be a key factor leading to the difference of fracture morphology in the region of fatigue source and final fast fracture, and also the lower fatigue limit of specimen perpendicular to the deposition direction than that parallel to the deposition direction.