**7. Heat treatment of AM Ti-6Al-4V and corresponding microstructure and mechanical properties**

#### **7.1. Heat treatment temperature: A collection from AM Ti-6Al-4V and corresponding mechanical property**

Results show that the residual stress in AM Ti-6Al-4V can be higher than 1000 MPa [33]. Considering that the yield strength of Ti-6Al-4V is merely around 880 MPa (see Table 2), residual stress can initiate premature failure especially during dynamic testing such as fatigue. It is a major concern for the AM Ti-6Al-4V. Most as-built AM Ti-6Al-4V therefore need heat treatment to mitigate the impact of the residual stress on the mechanical property performance. A collection of heat treatment temperatures being adopted by current studies is shown in Fig. 15 [29-88]. Temperatures of the martensite phase transformation (at ~610°C), α → β (at ~750°C) and β-transus (at ~980°C) are also marked in the figure using dotted lines. The figure indicates that most of the heat treatments are distributed at temperatures between 610°C and 980°C. This is very likely due to the necessity of eliminating the hard-but-brittle martensite phases via phase transformation from the α' and/or the αm phases to stable (α and/or β) phases for the LMD, EBM and SLM approaches. The microstructure of the SLS Ti-6Al-4V is similar to that of the equilibrium state, and can be free from any post heat treatment.

**6. Mechanical property of AM Ti-6Al-4V (As built)**

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UTS and elongation of as-built AM Ti-6Al-4V are summarised into Figure 14 (a) and (b) [29-88]. ASTM requires that as-prepared Ti-6Al-4V should be no lower than 860 MPa in UTS and no lower than 10% in elongation. These two benchmark values are marked in the two figures using dotted lines. These data suggest that, comparatively speaking, EBM is able to provide a combination of good facture strength and good ductility which satisfy the ASTM specifica‐ tions. Ti-6Al-4V made by SLM tends to show the highest fracture strength among the four AM approaches yet the corresponding ductility is the lowest which is mostly below the corre‐ sponding benchmark value (10%). There are no adequate data for LMD but it is reckoned that it should show a similar tendency to that of the SLM due to the similarity between the two processing techniques. Because of poor density (see Figure 6), SLS Ti-6Al-4V normally relies

on post treatment such as HIP to achieve good fracture strength as well as ductility.

**Figure 14.** (a) UTS and (b) elongation of as-built Ti-6Al-4V prepared by the various AM techniques [29-88]. Dotted

**7. Heat treatment of AM Ti-6Al-4V and corresponding microstructure and**

Results show that the residual stress in AM Ti-6Al-4V can be higher than 1000 MPa [33]. Considering that the yield strength of Ti-6Al-4V is merely around 880 MPa (see Table 2), residual stress can initiate premature failure especially during dynamic testing such as fatigue. It is a major concern for the AM Ti-6Al-4V. Most as-built AM Ti-6Al-4V therefore need heat treatment to mitigate the impact of the residual stress on the mechanical property performance. A collection of heat treatment temperatures being adopted by current studies is shown in Fig. 15 [29-88]. Temperatures of the martensite phase transformation (at ~610°C), α → β (at ~750°C)

**7.1. Heat treatment temperature: A collection from AM Ti-6Al-4V and corresponding**

lines in the two figures represent corresponding ASTM specifications.

**mechanical properties**

**mechanical property**

**Figure 15.** Collection of heat treatment temperatures adopted by various studies of the AM Ti-6Al-4V [29-88]. Dotted lines in the figure represent critical temperatures for Ti-6Al-4V (refer to Table 1).

UTS and elongation of the as-annealed AM Ti-6Al-4V are shown in Fig.16 [29-88]. If comparing with the as-built data (see Fig. 14), the general trend is that the fracture strength will be reduced after heat treatment while ductility can be improved. This is mainly resulted from both the reduced residual stress due to the heat treatment and the partially or even fully transformed martensite phases in the as-built microstructure.

**Figure 16.** (a) UTS and (b) elongation of as-annealed AM Ti-6Al-4V [29-88]. Dotted lines in the two figures represent corresponding ASTM specifications.

#### **7.2. Stress relief annealing and corresponding microstructure of as-annealed AM Ti-6Al-4V**

Heat treatment at temperatures below the temperature of the martensite phase transformation may be not able to change the microstructure. Figure 17 provides such an example via an EBM Ti-6Al-4V annealed at 600°C for four hours [69]. The martensite phases are still observable after the annealing. Such heat treatment may only offer stress relief to reduce the residual stress in the as-built AM Ti-6Al-4V.

**Figure 17.** Microstructure of an EBM Ti-6Al-4V annealed at 600°C for four hours. Rectangular (acicular) shaped mar‐ tensite phases remained after the heat treatment [69].

#### **7.3. Heat treatment to achieve equilibrium microstructure of AM Ti-6Al-4V**

#### *7.3.1. Heat treatment at temperature above the martensite phase transformation*

Contrary to the low temperature annealing, heat treatment at temperatures higher than the martensite phase transformation may assist in eliminating the brittle, metastable martensite phases via phase transformation [29-88]. Increasing the heat treatment temperature to higher than the α → β (at ~750°C) or even the β-transus (at ~980°C) will facilitate the transformation from α to β. Figure 18 provides an example of this where heat treatment was conducted up to 1200°C to form equilibrium lamellar (α+β) microstructure [69]. Martensite phases were not observable after such high-temperature heat treatment.

**7.2. Stress relief annealing and corresponding microstructure of as-annealed AM Ti-6Al-4V**

Heat treatment at temperatures below the temperature of the martensite phase transformation may be not able to change the microstructure. Figure 17 provides such an example via an EBM Ti-6Al-4V annealed at 600°C for four hours [69]. The martensite phases are still observable after the annealing. Such heat treatment may only offer stress relief to reduce the residual stress

**Figure 17.** Microstructure of an EBM Ti-6Al-4V annealed at 600°C for four hours. Rectangular (acicular) shaped mar‐

Contrary to the low temperature annealing, heat treatment at temperatures higher than the martensite phase transformation may assist in eliminating the brittle, metastable martensite phases via phase transformation [29-88]. Increasing the heat treatment temperature to higher

**7.3. Heat treatment to achieve equilibrium microstructure of AM Ti-6Al-4V**

*7.3.1. Heat treatment at temperature above the martensite phase transformation*

in the as-built AM Ti-6Al-4V.

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tensite phases remained after the heat treatment [69].

**Figure 18.** Microstructure of an LMD Ti-6Al-4V annealed at 1200°C for two hours. Colony α-phase is observable along with prior β grains and grain boundary α phases [69].

### *7.3.2. Dependence of α colony size on heat treatment and microstructural dependency of mechanical property*

The model developed by Tiley et al. [95,96] suggests that the mechanical performance of the (α+β) dual phase Ti-6Al-4V can be estimated based on a variety of microstructural parameters including the size of the α colony. The schematic graph in Figure 19 suggests that increasing size of the α colony may lead to reduced ductility (*ε*) and yield strength (*σ*0.2) but in the meantime it may also contribute to better resistance to macro-cracks [95,96]. It is noted that heat treatment can coarsen the α colony of the AM Ti-6Al-4V, see Fig. 20 [54]. From this perspective, heat treatment of AM Ti-6Al-4V will have to be selected according to specific requirement and serve the application purpose of the material.

**Figure 19.** Dependency of mechanical property on the α colony size of the Ti-6Al-4V alloy [95,96].

**Figure 20.** The size of the α colony in an SLS Ti-6Al-4V. HIP at (626°C-700°C) increases the α colony size evidently. HIP at higher temperatures can slightly increase the α colony size than those of lower temperatures [54].

### **8. Potential oxygen issue for the AM Ti-6Al-4V and counter measurements**

#### **8.1. Oxygen equivalent and oxygen level in AM Ti-6Al-4V**

**Figure 19.** Dependency of mechanical property on the α colony size of the Ti-6Al-4V alloy [95,96].

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**Figure 20.** The size of the α colony in an SLS Ti-6Al-4V. HIP at (626°C-700°C) increases the α colony size evidently. HIP

at higher temperatures can slightly increase the α colony size than those of lower temperatures [54].

Oxygen is detrimental to ductility of Ti-6Al-4V especially when it exceeds certain level [97-99]. Miura et al. [26] suggests the critical oxygen level for PM Ti-6Al-4V is 0.33 wt.%, above which ductility drops rapidly and can be much lower than the corresponding ASTM specification (see Fig. 1b). It needs to be noted that C, N and Fe may affect the mechanical performance of Ti-6Al-4V in a similar way to that of the oxygen, as suggested by the following Eq. (3) [100] and Eq.(4) [101].

$$\text{[O]}\_{\text{EQ}} = \begin{bmatrix} \text{O} \end{bmatrix} + 2\begin{bmatrix} \text{N} \end{bmatrix} + 2\langle \text{N} \| \begin{bmatrix} \text{C} \end{bmatrix} \begin{pmatrix} \text{wt.}\,\% \end{pmatrix} \tag{3}$$

$$
\begin{bmatrix} \mathbf{O} \end{bmatrix}\_{\text{EQ}} = \begin{bmatrix} \mathbf{O} \end{bmatrix} + 2.77 \begin{bmatrix} \mathbf{N} \end{bmatrix} + 0.1 \begin{bmatrix} \mathbf{Fe} \end{bmatrix} \begin{pmatrix} \text{wt.} \% \end{pmatrix} \tag{4}
$$

Generally speaking, AM processing only mildly increases oxygen level and the oxygen level in most of the as-built AM Ti-6Al-4V remain low [29-88]. This is mostly contributed to the extra-low interstitial (ELI) raw powders used, good vacuum condition and/or inert gas protection achievable during AM processing. Only occasional report can be found where oxygen level is higher than the 0.2 wt.%O benchmark value, see Figure 21 [29-88].

**Figure 21.** Variation of oxygen content before and after AM [29-88]. Dotted line in the figure represents corresponding ASTM specification (0.2 wt.%O).

#### **8.2. Price issue of raw AM powders**

Figure 21 indicates that the oxygen issue may have not been serious to the AM Ti-6Al-4V so far. This is mainly thanks to the use of ELI powders that are prepared by gas-atomisation or rotating electrode. The high cost of the ELI raw powders, however, can be an issue to AM Ti-6Al-4V from cost perspective. Figure 22 compares the price of a variety of powders, showing that the cost on the AM powders is extraordinary [4-13]. They are mostly higher than \$400 per kg while hydrogenation-dehydrogenation (HDH) Ti powders that are widely used in con‐ ventional PM Ti are merely around \$30 per kg. The high cost can be the bottle-neck issue for AM Ti-6Al-4V to be fully embraced by industries. For this reason, low-cost, high-interstitial (i.e. oxygen), fine powders are more desired for developing cost-effective AM Ti alloys. Sun et al. [102] recently developed an approach to manipulate irregularly-shaped HDH powders to make round-shaped, fine particles that are suitable for AM. The cost of their powders is higher than the HDH Ti powder but still markedly lower than the current AM powders, making it be more reliable for developing AM Ti and Ti alloys.

**Figure 22.** Comparison on the cost of various Ti materials, including raw material (TiO2), intermediate material (TiCl4) and Ti powders produced via the Kroll, Hunter and HDH approaches (price of Ti-6Al-4V is similar to these Ti pow‐ ders). Price of fine powders of Ti-6Al-4V for the 3D printing is extraordinary [4-13].

#### **8.3. Counter measurements**

Counter measurements need to be consulted when the high impurity (oxygen) powders are to be used for developing AM Ti-6Al-4V. Research has shown that the RE elements are capable to scavenge oxygen and their oxygen scavenging capability follows this sequence: Y>Er>Dy>Tb>Gd (see Figure 23a) [103, 104]. The high potency of yttrium in scavenging oxygen from AM titanium alloys is demonstrated in Figure 23 (b), where the uniformly distributed Y2O3 dispersoids are resulted from an addition of 0.1 wt.%Y to an EBM titanium alloy [105]. RE hydrides such as YH2 have been further demonstrated to be able to scavenge, aside from oxygen, Cl, another important impurity element to Ti materials, Fig. 24 [106,107].

**8.2. Price issue of raw AM powders**

96 Sintering Techniques of Materials

making it be more reliable for developing AM Ti and Ti alloys.

ders). Price of fine powders of Ti-6Al-4V for the 3D printing is extraordinary [4-13].

**8.3. Counter measurements**

Figure 21 indicates that the oxygen issue may have not been serious to the AM Ti-6Al-4V so far. This is mainly thanks to the use of ELI powders that are prepared by gas-atomisation or rotating electrode. The high cost of the ELI raw powders, however, can be an issue to AM Ti-6Al-4V from cost perspective. Figure 22 compares the price of a variety of powders, showing that the cost on the AM powders is extraordinary [4-13]. They are mostly higher than \$400 per kg while hydrogenation-dehydrogenation (HDH) Ti powders that are widely used in con‐ ventional PM Ti are merely around \$30 per kg. The high cost can be the bottle-neck issue for AM Ti-6Al-4V to be fully embraced by industries. For this reason, low-cost, high-interstitial (i.e. oxygen), fine powders are more desired for developing cost-effective AM Ti alloys. Sun et al. [102] recently developed an approach to manipulate irregularly-shaped HDH powders to make round-shaped, fine particles that are suitable for AM. The cost of their powders is higher than the HDH Ti powder but still markedly lower than the current AM powders,

**Figure 22.** Comparison on the cost of various Ti materials, including raw material (TiO2), intermediate material (TiCl4) and Ti powders produced via the Kroll, Hunter and HDH approaches (price of Ti-6Al-4V is similar to these Ti pow‐

Counter measurements need to be consulted when the high impurity (oxygen) powders are to be used for developing AM Ti-6Al-4V. Research has shown that the RE elements are capable to scavenge oxygen and their oxygen scavenging capability follows this sequence: Y>Er>Dy>Tb>Gd (see Figure 23a) [103, 104]. The high potency of yttrium in scavenging oxygen from AM titanium alloys is demonstrated in Figure 23 (b), where the uniformly distributed Y2O3 dispersoids are resulted from an addition of 0.1 wt.%Y to an EBM titanium alloy [105].

**Figure 23.** The free Gipps energy (∆*G*) of formation of various oxide materials, showing their thermodynamic stability as well as their affinity for oxygen [104]. Rare earth elements such as Y and some alkaline earth elements such as Ca have higher affinity for oxygen than Ti. (b) Precipitation of fine Y2O3 dispersoids (due to the addition of 0.1 wt.%Y) in the α-Ti matrix of Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si-0.1Y (wt.%) which contained 0.07 wt.% oxygen. The alloy was addi‐ tively manufactured by EBM [105].

**Figure 24.** (a) SEM back-scatter-electron (BSE) image of Y-Cl particles in an YH2-doped, as-sintered Ti alloy, (b) SEM energy dispersive x-ray (EDX) spectrum showing the enrichment of both Y and Cl in the phase and (c) TEM selected area electron diffraction (SAED) analysis of the phase [107].

## **9. Concluding remarks**

3D printing has becoming a focusing topic not only to research community and industry but also to the general public, and AM Ti and Ti alloys is one of the most promising and interested areas to be further developed. For the time being, although a few issues persist such as the microstructural inhomogeneity in the as-built material, some of the AM Ti-6Al-4V have already been able to achieve mechanical properties no lower than the corresponding ASTM specifications. The cost of the ELI AM Ti powders is one of the most challenging issues to limit the scale-up of the AM products. Employing low-cost powders to replace existing expensive powders to reduce the overall cost is a valuable research direction for further developing AM Ti-6Al-4V. Counter measurements to deal with the impurity issue associated with the highinterstitial Ti powders can be one of the key research elements for such development.
