A comprehensive review of aspects of columnar to equiaxed transition, phase stability, and properties in novel Ti-alloys developed by additive manufacturing

Abstract

Development of novel Ti-alloys via additive manufacturing (AM) by exploring newer β Ti alloy space is gaining significant importance. Although the AM process addresses the segregation issue associated with the Ti-alloys with higher β stabilizer content, the formation of columnar prior β-grains leading to microstructural and property anisotropy becomes challenging. Thermodynamics-guided alloying strategy can be adopted for facilitating the columnar to equiaxed transition (CET) of AM built Ti alloys by minimizing the steep thermal gradients. The co-relation between the governing thermodynamic parameters (which are functions of composition) and CET, for various types (Isomorphous and Eutectoid) of β stabilizers has been studied with the help of CALPHAD simulated phase diagrams and demonstrated via comparative plots to understand the correct alloying strategy. The effect of different β-alloying on the resulting mechanical properties are discussed via comparative studies that will help in selecting a suitable β-Ti alloy (both high-strength metastable β alloys and stabilized β alloys) for specific application. The β-isomorphous alloys find their primary applicability in bio-medical industries, whereas the β-eutectoid high-strength Ti-alloys are applicable for structural applications in aerospace and automobile industries. This study also highlights the additional challenges (unmelted powders, porosity, brittle phase formation and reduced ductility) with various types of β alloying and strategies to overcome those issues. Datapoints for new Ti-alloys showing CET and improved properties are mapped in the molecular orbital ($\overline{\mathit{Bo}}-\overline{\mathit{Md}}$) space, and new vectors have been introduced for designing newer alloys based on Ti64.