Micromechanical behaviour of TA15 alloy cylindrical parts processed by multi-pass flow forming

Analysis of the micromechanical behaviour of titanium alloys is of fundamental importance to microstructure control and property tailoring of titanium components. In this study, the dependence of deformation history on the micromechanical properties of TA15 titanium spun cylindrical parts processed by the multi-pass flow forming were analysed together with the microstructural features. The results indicate that the indentation hardness of primary α grains (α_p) and the transformed β matrix (β_t) both increase with the increase of the flow forming strain. The indentation hardness of α_p is higher than that of β_t, which partly results from the obvious indentation size effect (ISE) of α_p. With the increase of the flow forming strain, numerous substructures are generated in this process, which increases the proportion of statistically stored dislocations during indentation tests and thus weakens the influence of the ISE on the indentation hardness. The Young's modulus of α_p increases rapidly with deformation, whereas it decreases with the increase of the indenter load. This decrease is attributed to the damage accumulating at higher indentation loads. For β_t, there is a minor fluctuation of the Young's modulus with strain variation. After the ISE of α_p is eliminated, effect significance of each microstructural feature on the integrated hardness decreases in the order of α_p, β_t, and the grain interfaces of α_p/β_t. Meanwhile, with the increase of the flow forming strain, the hardness contribution of α_p to the integrated hardness decreases, while those of β_t and α_p/β_t interfaces increase. These findings provide a basis for tailoring desirable properties of TA15 cylindrical parts in the multi-pass flow forming process.