Propagation of long fatigue cracks in Ti6Al4V alloy produced by direct metal laser sintering

Abstract

The growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering was experimentally determined on CT specimens. The crack propagation was investigated in three different orientations with regard to the building direction. The as-built CT specimens were heat treated at temperatures 380, 740 and 900 °C. The crack growth curves with the threshold values of the stress intensity factor ΔKth were determined. The measurement was carried out in laboratory air at room temperature and stress ratios R = 0.1 and 0.8.It was found that the crack growth rate in the Paris region is identical for all three heat treatments for the same R ratio. For the particular stress intensity factor range ∆K the crack growth rates at R = 0.1 and 0.8 differs by factor of two. Contrary to the Paris region there is a clear effect of the heat treatment in the near threshold region. For the material heat treated at 380 °C and loaded at R = 0.1 the threshold is 3.7 MPam1/2, for 740 °C 2.7 MPam1/2 and for 900 °C 3.5 MPam1/2. The explanation of the observed dependence of the threshold on the post processing heat treatment is discussed in terms of microstructure and residual stresses.