*3.5. Effect of Annealing Temperature on Mechanical Properties Micromachines* **2022**, *13*, 331 12 of 16

Table 4 and Figure 11 show the tensile properties of the SLM-printed samples at different annealing temperatures. It can be seen that the tensile strength decreased gradually with the increase in the annealing temperature. When the annealing temperature of 750 ◦C was applied, the tensile strength of the sample decreased to 1094 MPa, which is 9% lower than that of the printed sample. The elongation of 7% was similar to that of the printed sample. The maximum elongation of 14% could be obtained at 950 ◦C, which is 79% higher than that of the as-printed sample. Table 4 and Figure 11 show the tensile properties of the SLM-printed samples at different annealing temperatures. It can be seen that the tensile strength decreased gradually with the increase in the annealing temperature. When the annealing temperature of 750 °C was applied, the tensile strength of the sample decreased to 1094 MPa, which is 9% lower than that of the printed sample. The elongation of 7% was similar to that of the printed sample. The maximum elongation of 14% could be obtained at 950 °C, which is


**Table 4.** Mechanical properties of the SLM-printed Ti–6Al–4V samples. **Table 4.** Mechanical properties of the SLM-printed Ti–6Al–4V samples.

79% higher than that of the as-printed sample.

**Figure 11.** Effect of annealing temperature on the mechanical properties of the SLM-printed Ti–6Al– 4V samples (laser power of 170 W, scanning speed of 1300 mm/s, and hatch space of 0.07 mm). **Figure 11.** Effect of annealing temperature on the mechanical properties of the SLM-printed Ti–6Al–4V samples (laser power of 170 W, scanning speed of 1300 mm/s, and hatch space of 0.07 mm).

After annealing at 750 °C, some brittle and hard α' martensites decomposed into the α + β phase with relatively high ductility. However, the partial decomposition suppressed the change in the elongation of the sample but reduced its tensile strength. When the annealing temperature exceeded 800 °C, the acicular α' martensite completely decomposed into the α and β phases, which decreased the tensile strength and gradually increased the elongation. When the annealing temperature exceeded the β phase transition temperature, the formed coarse grains and the lath-shaped α phases inside them could hinder the slip of dislocations, causing stress concentration at the interface of the α and β phases and After annealing at 750 ◦C, some brittle and hard α 0 martensites decomposed into the α + β phase with relatively high ductility. However, the partial decomposition suppressed the change in the elongation of the sample but reduced its tensile strength. When the annealing temperature exceeded 800 ◦C, the acicular α 0 martensite completely decomposed into the α and β phases, which decreased the tensile strength and gradually increased the elongation. When the annealing temperature exceeded the β phase transition temperature, the formed coarse grains and the lath-shaped α phases inside them could hinder the slip of dislocations, causing stress concentration at the interface of the α and β phases and eventually reducing the ductility.

eventually reducing the ductility. Figure 12 shows the fracture morphology of the samples at different annealing temperatures. It can be observed that the fracture morphology of the samples annealed at 750 °C and 850 °C was similar to that of the as-printed samples, including cleavage facets and Figure 12 shows the fracture morphology of the samples at different annealing temperatures. It can be observed that the fracture morphology of the samples annealed at 750 ◦C and 850 ◦C was similar to that of the as-printed samples, including cleavage facets

dimples. However, when the annealing temperature increased to 950 °C, the cleavage fac-

and dimples. However, when the annealing temperature increased to 950 ◦C, the cleavage facets disappeared and dense dimples with large sizes were formed, indicating a ductile fracture. When the local stress at the phase interface exceeded the interfacial bonding force in the tensile process, micropores occurred and consumed a large amount of strain energy. Micropores lead to dimples in the aggregation process, and denser dimples indicate the better ductility. After annealing at 1050 ◦C, the dimple size increased, and a small number of tear ridges appeared, which reduced the elongation of the sample. ets disappeared and dense dimples with large sizes were formed, indicating a ductile fracture. When the local stress at the phase interface exceeded the interfacial bonding force in the tensile process, micropores occurred and consumed a large amount of strain energy. Micropores lead to dimples in the aggregation process, and denser dimples indicate the better ductility. After annealing at 1050 °C, the dimple size increased, and a small number of tear ridges appeared, which reduced the elongation of the sample.

*Micromachines* **2022**, *13*, 331 13 of 16

**Figure 12.** Fracture morphology of the printed samples after annealing at different temperatures: (**a**) 750 °C; (**b**) 850 °C; (**c**) 950 °C; (**d**) 1050 °C. **Figure 12.** Fracture morphology of the printed samples after annealing at different temperatures: (**a**) 750 ◦C; (**b**) 850 ◦C; (**c**) 950 ◦C; (**d**) 1050 ◦C.

Table 5 shows the mechanical properties of Ti–6Al–4V samples after different posttreatments as compared to those reported in previous studies. It can be seen that the sample after 950 °C heat treatment exhibited a superior tensile strength and reasonable elongation. Heat treatment temperatures below 950 °C reduced the β phase [23], resulting in a higher tensile strength of the printed alloy. The samples with higher elongations were treated by HIP [26,27], which was more conducive to tailoring the microstructure and reducing the pore defects in the sample. Table 5 shows the mechanical properties of Ti–6Al–4V samples after different posttreatments as compared to those reported in previous studies. It can be seen that the sample after 950 ◦C heat treatment exhibited a superior tensile strength and reasonable elongation. Heat treatment temperatures below 950 ◦C reduced the β phase [23], resulting in a higher tensile strength of the printed alloy. The samples with higher elongations were treated by HIP [26,27], which was more conducive to tailoring the microstructure and reducing the pore defects in the sample.


**Table 5.** Mechanical properties of the SLM-printed Ti–6Al–4V samples after different post treatments.

#### **4. Conclusions**

This work investigated the microstructure and mechanical properties of the SLMprinted Ti–6Al–4V alloy post-treated by annealing. The effects of process parameters on the relative density, microstructure, and mechanical properties of SLM-printed samples were studied. The effects of annealing temperature on microstructure and mechanical properties of the printed samples were further studied. The main findings are presented below.

The relative density of the SLM-printed sample was significantly affected by the scanning speed. In particular, the SLM-printed sample could obtain the highest density of 99.51% with a laser power of 170 W, a scanning speed of 1300 mm/s, a layer thickness of 0.03 mm, and a hatch space of 0.07 mm.

The microstructure of the printed sample was composed of β columnar crystals, which contained a large number of acicular α 0 martensite, resulting in higher strength and lower plasticity of the sample. The width of the β columnar crystals decreased with the increase in the scanning speed, as determined by the decrease in the energy density. The maximum tensile strength of 1265 MPa was achieved at a scanning speed of 900 mm/s, while its elongation could reach the highest value of 7.8% at a scanning speed of 1300 mm/s.

The annealing temperature had a significant effect on the microstructure of the sample. After annealing, the acicular α 0 martensite was decomposed into the α + β dual phase. With the increase in the annealing temperature, the tensile strength gradually decreased, while the elongation increased first and then decreased. Annealing at 950 ◦C could result in the highest elongation of 14%, which is 79% higher than that of the as-printed sample, without a significant reduction in the tensile strength.

**Author Contributions:** Conceptualization, D.W. and C.H.; funding acquisition, D.W.; investigation, H.W.; methodology, X.C.; project administration, D.W.; resources, H.W. and X.C.; supervision, Y.L. and C.H.; validation, X.C.; visualization, H.W. and C.H.; writing—original draft, D.W. and H.W.; writing—review and editing, Y.L., D.L., X.L. and C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported in part by the Guangdong Provincial Basic and Applied Basic Research Fund under Grant 2019B1515120094, 2021A1515110527, and in part by the open project founded by the State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization under Grant 2021P4FZG11A.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

