## 3 LMD of Ti6 Al4V
### Macrostructure and Microstructure  
In \(\alpha+\beta\) alloy, the room-temperature equilibrium microstructure consists mainly of \(\alpha\) phases (hexagonal close packed) with some retained \(\beta\) phases (body center cubic). The \(\beta\) phases are fully or partly transformed into \(\alpha\) with a martensitic type, which has two different forms; namely, the hexagonal \(\alpha\) (\(\alpha^{\prime}\)) and the orthorhombic \(\alpha\) (\(\alpha^{\prime\prime}\)) [50]. The phase transformation strongly depends on the temperature history and the cooling rate during deposition [51]. \(\alpha^{\prime}\) martensite forms within a specific cooling rate range between 20 and 410 \({}^{\circ}\)C/s [52]. The formation of \(\alpha\) prime (\(\alpha^{\prime}\)) martensite phase is beneficially produced by the rapid cooling through diffusionless transformation, while \(\alpha+\beta\) transformation is a diffusional process [53]. The \(\alpha\) platelets are thicker and shorter when the cooling rate decreases, and even transformed into a global type with very low cooling rates. The four typical microstructures in \(\alpha+\beta\) alloys are shown in Figure 1.  
Figure 1: Typical microstructures of \(\alpha+\beta\) titanium alloys: **(a)** Widmansätten; **(b)** duplex microstructure; **(c)** basket-weave microstructure; **(d)** equiaxed structure (adapted from [54]). There are numerous choices of etchants for revealing Ti6 Al4V's microstructures: [PERSON]'s reagent, oxalic reagent, ammonium bifluoride and so on [17]. Among those, [PERSON]'s reagent is most widely used, as it can easily meet almost all the demands in the laboratory. Chemical etching by [PERSON]'s reagent for Ti6 Al4V is listed in ASTM E407 as etchant number 192. [PERSON]'s Reagent is a solution mixture of 75 mL H2O, 22 mL HNO3 and 3 mL HF. The metallurgical samples are dipped into the solution and stirred gently for 20-30 s. These typical microstructures can be summarized as follows:  
* Widmanstatten: Primary \(\beta\) grains are coarse and complete. Continuous \(\alpha\) particles grow on the grain boundary. The colonies of \(\alpha\) lamellae are thick and parallel. After slow cooling (furnace cooling) from \(\beta\) phase zone, Widmanstatten structure with a low ductility and a high fatigue performance could be seen. * Duplex microstructure: No more than 50% of the discontinuous equiaxed \(\alpha\) particles distribute in transformed \(\beta\) matrix. When alloy is heated or deformed in the upper part of \(\alpha+\beta\) phase zone, a duplex microstructure with a comprehensive performance of mechanical properties can be achieved. * Basket-weave microstructure: primary \(\beta\) grain boundaries are destroyed and the \(\alpha\) lamellae become shorter in multiple orientations. When a large deformation is performed near \(\beta\) transus temperature, basket-weave structure with enhanced ductility is formed. * Equiaxed structure: More than 50% of the primary \(\alpha\) grains and a certain amount of the transformed \(\beta\) grains display an irregular polygon shape. With a higher deformation rate, higher temperature, longer time of temperature holding, the equiaxial level is lifted, with a result of an excellent overall performance. An increase in cooling rate leads to a refinement of the microstructure--both \(\alpha\) colony size and \(\alpha\)-lamellae thickness reduce. New colonies tend to nucleate not only on \(\beta\)-phase boundaries but also on boundaries of other colonies and grow perpendicularly to the existing lamellae [55]. In the case of fabrication of large structures by LMD of titanium alloys, the heat is found mainly dissipated away through the conduction along Z-direction by conduction through the previously deposited material during the deposition process. The typical macrostructure and microstructure of laser metal deposited Ti6 Al4V are illustrated in Figure 2.  
As shown in Figure 2, primary \(\beta\) grains decorated with fine \(\alpha\) phases grow approximately in the build direction and through the deposition layers [57; 58].