## 3 LMD of Ti6 Al4V
### Heat Treatment and Mechanical Properties  
The heat treatment that includes solution treatment and aging is a prefered method to get favorable properties, balanced strength and ductility. The temperature for solution treatment is most significant for getting the expected microstructures and properties, and is, therefore, set 40-100 \({}^{\circ}\)C lower than \(\beta\) transus temperature, in order to get a certain proportion between primary \(\alpha\) and primary \(\beta\) phases through a following rapid cooling (air cooling) [54]. Oversaturated, metastable \(\beta_{m}\) phases can also be generated, which enable the growth of primary \(\alpha\) phases and the participation of secondary \(\alpha\) phases during aging.  
Stress-relief annealing is another typical treatment for enhancing the ductility. The stress-relief annealing weakens the \(\beta\)-\(\alpha\) phase transformation and facilitates, thereby, the precipitation and growth of \(\alpha\) phases, leaving the type of the microstructure unchanged. Primary \(\alpha\) become coarse after annealing [7].  
In the heat treatment, the cooling rate is another important factor which has an impact on the microstructure and the mechanical properties besides the temperature. The relationship between the cooling rate and the microstructure is shown in Figure 3.  
The cooling method used after the higher-temperature annealing can affect tensile properties [19]. The thermal profile of heat treatment influences the final microstructure [60]. As shown in Figure 3, after the fast cooling, such as water cooling with a rate around 1000 \({}^{\circ}\)C/s, the microstructure can be fully transformed into \(\alpha^{\prime}\) martensite. The air cooling rate is as estimated at around 20 \({}^{\circ}\)C/s from 1100 \({}^{\circ}\)C to 500 \({}^{\circ}\)C [59]. In the case of cooling with an intermediate rate of about 3.5 \({}^{\circ}\)C/s, the martensitic transformation is accompanied by diffusional transformation, and hence, the volume fraction of martensitic phases decreases to the benefit of the stabilization of the \(\alpha\) and \(\beta\) phases [55]. The cooling rate of a furnace cooling condition is around 2 \({}^{\circ}\)C/s [61]. With an extremely slow cooling rate, a fully lamellar or a duplex microstructure could even be achieved [62].  
Figure 3: Continuous Cooling Transformation (CCT) diagram for Ti6 Al4V. The three colorful curves represent the thermal profiles of water, air and furnace cooling(adapted from [59]).  
Based on the literature review, the mechanical properties (yield strength, tensile strength and elongation at break) of Ti6 Al4V fabricated by LMD with and without heat treatment have been summarized in Table 4.  
As shown in Table 4, the tensile strength and the yield strength are strongly influenced by the parameters and the experimental systems. The strengths and observed elongation in the work of [PERSON] et al. meet the specification of annealed material, while the results from others are of higher strengths and lower ductilities. Some studies are focused on the anisotropic tensile behavior: the ductility along the build direction is significantly higher than that along the scanning direction. After heat treatment, the elongations can be strongly increased with lightly decreased strengths.