A two-decade odyssey in fusion-based additive manufacturing of titanium alloys and composites

In the past two decades, the advent of metal additive manufacturing (AM) or three-dimensional (3D) printing technology has led to exciting breakthroughs in the metallurgy of titanium alloys, including improved properties, tailored microstructures, and multi-component compositions. However, unlike conventional manufacturing methods, additive manufacturing of titanium alloys (AM Ti alloys) poses challenges owing to the complex microstructural evolution and phase transformation kinetics. Consequently, various strategies have been devised based on a substantial body of knowledge gained to render Ti alloys more amenable to printing. This review critically examines the research progress made over the past two decades, with a focus on how the processes affect the microstructural evolution, phase transformations, mechanical properties, and corrosion behavior. The review includes a statistical analysis of research outputs over the past two decades to gain quantifiable knowledge of the field. Despites the extensive work that has been done on Ti alloys, our statistical analysis show that research on AM Ti alloys/composites is still growing exponentially, with the number of publications increasing at a rate of 35 % per annum, largely driven by applications of Ti in the aerospace and biomedical sectors. This review outlines the evolution of 3D printing technologies and classifications of various AM processes for Ti alloys. The review focuses on fusion-based AM and discusses the mechanisms controlling the microstructural evolution and fundamental phenomena associated with the kinetics of α− and β-transformations in a rapidly solidifying AM Ti alloys. In addition, the review summarizes and compares the mechanical properties of AM Ti alloys and composites. Regardless of the AM process, it was observed that the major benchmarks for the room temperature ultimate tensile strength and elongation to failure of current AM Ti alloys and composites are within the range of 800–1200 MPa and 2.5–6.0 %, respectively. The areas of opportunities and major research gaps identified in this assessment can provide guidance for future studies.