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 andα/βphase inter-face,columnar prior-βgrain 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 lamellarαp phases and colonies,and the decrease of the number ofαlaths with an angle(ϕ)relative to the applied stress direction ranging from 75°to 90°.In the Paris regime,the fatigue cracking path can be deflected at colony boundaries or columnar prior-βgrain boundaries.The larger the deflection angle,the more tortuous the cracking path and the lower the fatigue crack growth rate.The angle(γ)of the columnar prior-βgrain growth direction relative to the build direction affects not onlyϕof differentαvariants,but also the fatigue cracking path deflection angle(θij)at columnar prior-βgrain boundaries.An optimal combination ofγ=0°-15°-0°-15°for several adjacent columnar prior-βgrains is derived from the theoretical analysis,and that can effectively avoidϕbeing in the range from 75°to 90°and makeθij 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.