Wire-Feeding Based Additive Manufacturing of the Ti–6Al–4V Alloy. Part I. Microstructure

In recent years, metal additive manufacturing (AM), also known as 3D printing, is grown massively in the industry. The ability of AM to build parts directly from the digital representation makes it an excellent alternative compared to traditional manufacturing technologies, such as milling, welding, casting, rolling, stamping, forging and turning for rapidly making highly customized parts. Currently, a number of different powder- and wire-based AM technologies are developed for 3D printing of metals. A number of potential benefits of AM are noted, including the allowance of design freedom, complex parts’ production, the material waste and part weight reductions, material use minimization; it also saves the time and money of the production cycle times. Due to the feasibility of the economically producing large-scale metal components with relatively high deposition rate, low machinery cost, high material efficiency, and shortened lead time as compared to the powder-based AM, the wire-based AM significantly attracted in the industry and academia due to its ability to produce the large components of the medium geometric complexity. During this AM process, the wire is fed by the controlled rate into the melt pool produced by the electric arc, laser or electron beam as the heat source. In the past few decades, the basic research and development efforts are devoted to the wire-based 3D printing parts made of Ti–6Al–4V alloy, which has been widely investigated and used in different fields such as aerospace, automotive, energy, marine industries and in addition to the prosthetics and the orthopaedic implants. Numerous studies in recent years on the influence of the 3D printing parameters have shown a significant difference in the mechanism and kinetics of the microstructure formation in the Ti–6Al–4V alloy samples compared to traditional technologies. It is well investigated that the mechanical properties of such alloy are dependent on the solidification macro- and microstructure, which is controlled by the thermal conditions during 3D printing. In the present review, the main microstructural characteristics, which determine the mechanical properties of the two-phase Ti–6Al–4V alloy, are analysed for the samples obtained by wire-feed 3D printing with various sources used for the wire melting, namely, the electric arc, the laser, and the electron beam. At first, the review introduces the links between the process parameters, resultant microstructures, especially, the morphology, the size and the quantitative ratio of the α and β grains in the as-printed Ti–6Al–4V alloy samples. However, the metallic products manufactured by a vast majority of the AM processes need to be post-processed by heat treatment and/or hot isostatic pressing, which are also discussed in this review.