**4. Densification of AM Ti-6Al-4V**

#### **4.1. As-built density of AM Ti-6Al-4V**

Figure 6 summarises reports on as-built density of the AM Ti-6Al-4V using LMD, SLM, EBM, and SLS [29-88]. The figure shows that the as-built density of AM Ti-6Al-4V is mostly higher than 99% of theoretical density and the material can be fully dense if processing parameters are appropriately selected. SLS Ti-6Al-4V is exceptional to this conclusion, which as-built densities are around 95% and similar to the as-sintered density of the conventional PM Ti-6Al-4V [4-13].

**Figure 6.** Density of as-built AM Ti-6Al-4V [29-88].

### **4.2. Densification of SLS Ti-6Al-4V: Solid state sintering and liquid phase sintering**

**Parameter**

**Scanning speed** Up to 7000 mm/s (mostly 50 mm/s - 100 mm/s)

**Feed type** Mostly 37 μm - 74 μm (should be similar to SLM) **Build temperature** - (can use preheated powder at e.g. 600°C)

Figure 6 summarises reports on as-built density of the AM Ti-6Al-4V using LMD, SLM, EBM, and SLS [29-88]. The figure shows that the as-built density of AM Ti-6Al-4V is mostly higher than 99% of theoretical density and the material can be fully dense if processing parameters are appropriately selected. SLS Ti-6Al-4V is exceptional to this conclusion, which as-built densities are around 95% and similar to the as-sintered density of the conventional PM

**Substrate** Normally Ti, can be heated to e.g. 230 °C

**Typical equipment** EOSINT M270

84 Sintering Techniques of Materials

**Layer thickness** 20 μm - 100 μm

**Build density** Mostly below 99%

**Table 6.** Typical technical parameters of the SLS [77-88].

**4. Densification of AM Ti-6Al-4V**

**4.1. As-built density of AM Ti-6Al-4V**

**Figure 6.** Density of as-built AM Ti-6Al-4V [29-88].

Ti-6Al-4V [4-13].

**Build volume** 250 mm \* 250 mm \* 215 mm **Laser type** Yb-fiber laser, 200 W **Powder supply** 5.5 kW max. **Laser beam size** 100 μm - 500 μm

The fact that the as-built density of the SLS Ti-6Al-4V is similar to that of the conventional PM Ti-6Al-4V (see Figure 6) implies the two processing approaches should have analogous densification mechanism. This is understandable by referring to Figure 5 which shows that the SLS processing uses laser as the heating source while for the conventional PM Ti-6Al-4V, heating is mainly through conductive and/or radiation heat. Other than this the two processing pathways are essentially same. This further suggests that the driving force and activation energy for densification of the conventional PM Ti-6Al-4V should be applicable to the SLS Ti-6Al-4V. During SLS of Ti-6Al-4V, however, there is possibility that liquid phase sintering (LPS) occurs due to local overheating to temperatures higher than the liquidus temperature of Ti-6Al-4V (~1660°C). LPS provides extra driving force for sintering for it has extra surface energies, i.e. resulted from liquid surface and from liquid-solid interface [89,90]. It involves different sintering mechanisms, e.g. pore-filling, from the solid state sintering. These normally can contribute to an as-sintered density of up to 99.5% high [89,90]. Since the as-sintered density of the SLS Ti-6Al-4V is lower than this value, the overwhelming densification mechanism for the SLS Ti-6Al-4V should still remain as that of the solid state sintering.

#### **4.3. Densification of EBM, SLM, and LMD Ti-6Al-4V: Solidification from liquid**

Full or nearly full denseness is achievable in EBM, SLM and LMD Ti-6Al-4V (see Figure 6), implying that the densification mechanism for these AM approaches is different from that of

**Figure 7.** Simulated T-T-T curve of Ti-6Al-4V, which can be used to forecast the phase selection during solidification of the alloy [91]. Ms in the figure denotes the martensite phase transformation. Open squares in the figure are for 0.05wt. %O, circles for 0.1 wt.% and stars for 0.2 wt.%O. The solid line is drafted based on experimental results. Oxygen (O) is found to be able to lower the temperature for the martensite phase transformation.

the SLS. Indeed, for these three laser-based processing approaches, the densification process is more of solidification from liquid rather than the normal sense of sintering. The so-called time-temperature-transformation (T-T-T) of Ti-6Al-4V regulates phase selection and phase constitution of the solidified microstructure. Figure 7 provides the T-T-T curve of the Ti-6Al-4V alloy containing different levels of oxygen [91]. Under equilibrium conditions (i.e. low cooling rates), the resultant microstructure will be a mixture of thermodynamically stable α and β phases, while high cooling rates can enable formation of martensite phases.
