Role of structural hierarchy on mechanics and electrochemistry of wire arc additive manufactured (WAAM) single phase titanium

Abstract

The absence of understanding of correlations between structure, mechanics, and electrochemical behavior in single-phase additive-manufactured titanium components impedes the advancement of large-scale processing. The present study addresses this scientific knowledge gap by investigating the evolution of structural hierarchy and its impact on the hardness and corrosion resistance of wire arc additive manufactured (WAAM) commercially pure titanium (cp-Ti) along three mutually orthogonal deposition directions. Integrated X-ray diffraction and optical microscopy revealed that a single-phase α-Ti monolithic structure is devoid of observable deposition tracks and interfaces with traces of titanium oxide. Homogeneous nucleation in the single phase at low undercooling and slow solidification rates during WAAM resulted in coarse grains of size 1 mm. The grains are randomly oriented with slight preferential texture along the hexagonal close-packed basal {0001} planes due to their parallel orientation with the close-packed body-centered cubic {011} plane during the β – α phase transition. The single-phase structure results in a uniform distribution of microhardness along layer-by-layer buildup direction and the bulk with an average of 1.6 GPa, which is about 80 % of conventionally cast cp-Ti. An increased dislocation density at the surface and from thermal strain during WAAM enhances nanohardness to 3.0 GPa. The absence of galvanic potential difference between two adjacent grains and the coarse size imparts high corrosion resistance with a rate of 80 × 10−5 mm per year, less than 50 % of other additive-manufactured titanium alloys. Overall, this study advances the state-of-the-art in homogenous, single-phase WAAM processing as a potential large-sale additive manufacturing alternative to conventionally processed Ti components.