**2. Preliminary knowledge: Fundamental properties of Ti-6Al-4V and conventional PM Ti-6Al-4V**

#### **2.1. Densification of PM Ti-6Al-4V and typical microstructure and mechanical property**


An Overview of Densification, Microstructure and Mechanical Property of Additively Manufactured Ti-6Al-4V… http://dx.doi.org/10.5772/59275 79

**Figure 1.** (a) Scanning electron microscopy (SEM) image to show the typical microstructure of PM Ti-6Al-4V [25] and (b) ductility of PM Ti-6Al-4V as a function of oxygen [26].

#### **2.2. Ti-6Al-4V: Basic physical and mechanical properties**

**2. Preliminary knowledge: Fundamental properties of Ti-6Al-4V and**

**2.1. Densification of PM Ti-6Al-4V and typical microstructure and mechanical property**

**• Sintering mechanism:** Sintering/densification of conventional PM Ti-6Al-4V is mostly through solid state sintering. Sintering temperatures are normally selected in the β phase region [10-13], at a temperature (e.g. at 1300ºC) well below the liquidus temperature (~1660 ºC for Ti-6Al-4V, Table 1) [14,15]. For sintering under pressure such as via hot pressing or spark plasma sintering, the sintering temperature can be lower, for instance, around 900°C [16]. Driving force for densification during solid state sintering is reduction of surface area and surface free energy by eliminating solid-vapour interfaces [17-19]. Sintering procedure mainly involves diffusional flow of composing elements, including surface diffusion, grain

**• Sintering activation energy:** Using the master sintering curve approach, Crosby [20] estimated that sintering activation energy, the *Q* value, of Ti-6Al-4V is about 130 kJ/mole. This is consistent with the reported *Q* value of the self-diffusion of titanium (92.5 kJ/mole-158 kJ/mole) over the β-Ti range from 900 to 1250 ºC, affirming that the densification of Ti-6Al-4V

**• Sintered density:** Most of the as-sintered Ti-6Al-4V materials show 95%-99% of theoret‐ ical density [4-13]. The as-sintered density depends on a few factors such as compac‐ tion pressure and powder size. Using TiH2 as the starting powder may assist in improving the as-sintered density [21,22], but to achieve a pore-free alloy (no less than 99.8% of theoretical density) post treatment such as via extrusion or hot isostatic pressing (HIP)

**• Microstructure of PM Ti-6Al-4V**: PM Ti-6Al-4V is expected to show the following microstructural characteristics [24]: (a) The overall microstructure is close to the equilibri‐ um state due to low cooling rate adopted for most sintering practices. (b) There could be some annealing and/or aging effect resulting from the slow cooling process from the isothermal sintering temperature to room temperature. Aging-induced phases such as isothermal ω may form during cooling. (c) Pores are part of the as-sintered microstruc‐ ture due to the difficulty to achieve a pore-free microstructure in most cases. Fig. 1(a) provides a typical SEM image of the as-sintered PM Ti-6Al-4V, consisting of grain boundary α, α lath and β phases [25]; the overall volume fraction of the α phase is more

**• Mechanical property of PM Ti-6Al-4V:** Mechanical properties of the PM Ti-6Al-4V are highly dependent upon oxygen level. Typical cases are shown in Figure 1(b) to demonstrate ductility of PM Ti-6Al-4V as a function of oxygen [26]. The fracture strength of PM Ti-6Al-4V

is comparable or even higher than the wrought material (ASTM B348) [9].

**conventional PM Ti-6Al-4V**

78 Sintering Techniques of Materials

is necessary [23].

than 85% [10-13].

boundary diffusion and lattice diffusion [17-19].

is mainly controlled by self-diffusion of titanium [8].

Table 1 and Table 2 list the fundamental physical and mechanical properties of the Ti-6Al-4V, respectively [14,15,27,28]. They serve as a point of reference and provide benchmark values for the following discussion on the AM Ti-6Al-4V.


**Table 1.** Fundamental physical properties of Ti-6Al-4V [14,15,27,28]


**Table 2.** Mechanical properties of Ti-6Al-4V achievable via forged- then-annealed [14,15,27,28]
