Toward developing Ti alloys with high fatigue crack growth resistance by additive manufacturing

Fatigue crack growth behaviors were investigated by three-point bending tests for TA19 alloy fabricated by laser metal deposition and four kinds of heat-treated samples. The crack growth resistance of the TA19 samples in the near-threshold regime and Paris regime was evaluated through the experimental characterization and theoretical analysis of the interaction between fatigue crack and alpha/beta phase interface, columnar prior-beta grain boundary and colony boundary. The results show that in the near-threshold regime, the fatigue crack propagation threshold and resistance increase with the increase of widths of lamellar alpha(p) phases and colonies, and the decrease of the number of alpha laths with an angle (phi) relative to the applied stress direction ranging from 75 degrees to 90 degrees. In the Paris regime, the fatigue cracking path can be deflected at colony boundaries or columnar prior-beta grain boundaries. The larger the deflection angle, the more tortuous the cracking path and the lower the fatigue crack growth rate. The angle (gamma) of the columnar prior-beta grain growth direction relative to the build direction affects not only phi of different a variants, but also the fatigue cracking path deflection angle (theta(ij)) at columnar prior-beta grain boundaries. An optimal combination of gamma = 0 degrees-15 degrees-0 degrees-15 degrees for several adjacent columnar prior-beta grains is derived from the theoretical analysis, and that can effectively avoid phi being in the range from 75 degrees to 90 degrees and make theta(ij) as large as possible. Such findings provide a guide for the selection of scanning strategies and process parameters to additively manufacture Ti alloys with high fatigue damage tolerance. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.