Selective laser melting of near-α titanium alloy Ti-6Al-2Zr-1Mo-1V: Parameter optimization, heat treatment and mechanical performance

doi:10.1016/j.jmst.2020.05.004

Abstract

Abstract:

This paper presents a comprehensive study conducted to optimize the selective laser melting (SLM) parameters and subsequent heat-treatment temperatures for near-α high-temperature titanium alloy Ti-6Al-2Zr-1Mo-1 V (TA15), which is widely used in the aerospace industry. Based on the surface morphology and relative density analysis, the optimized process parameters were: laser power from 230 W to 380 W, scan speed from 675 mm/s to 800 mm/s, scan spacing of 0.12 mm, and layer thickness of 0.03 mm. The effects of the laser power and the layer thickness on the phase constitutions, microstructure features, as well as room-temperature and high-temperature (500 °C) tensile properties, were then studied to obtain an in-depth understanding of SLM-built TA15. Six typical temperatures (650, 750, 850, 950, 1000 and 1100 °C) covering three representative temperature ranges, i.e., martensite partial decomposition temperature range, martensite complete decomposition temperature range and above β transus temperature, were subsequently selected as heat-treatment temperatures. The heat treatment-microstructure-mechanical property relationships of SLM-built TA15 were elucidated in detail. These results provide valuable information on the development of SLM-built TA15 alloy for industrial applications, and these findings are also beneficial to additive manufacturing of other near-α Ti alloys with desirable high-temperature properties.

Key words: Selective laser melting, Near-α titanium, Heat treatment, High-temperature tensile properties

Chao Cai, Xu Wu, Wan Liu, Wei Zhu, Hui Chen, Jasper Chua Dong Qiu, Chen-Nan Sun, Jie Liu, Qingsong Wei, Yusheng Shi. Selective laser melting of near-α titanium alloy Ti-6Al-2Zr-1Mo-1V: Parameter optimization, heat treatment and mechanical performance[J]. J. Mater. Sci. Technol., 2020, 57: 51-64.

Figures/Tables 19

Fig. 1. (a, b) Morphology and size distribution of the as-received TA15 powder, (c) an HRPM-II type SLM machine and the SLM scanning strategy, and (d) printed metallographic and tensile specimens.

Fig. 2. Chart of SLM processing parameters for TA15 alloy. Three typical optical metallographic (OM) images demonstrate over-melted, well-melted, and porous top surfaces of printed samples, which are represented by red, green, and blue, respectively.

Fig. 3. Relative density histogram of the printed samples.

Fig. 4. X-ray diffraction patterns of the as-received powder and the SLM-processed samples with different laser power.

Fig. 5. Low-magnification side-surface morphologies of the printed samples: (a) S17, (b) S18, (c) S19, and (d) S20.

Fig. 6. TEM results of SLM-built sample S17: (a) bright-field image, (b) magnified image of nano-size twins in Area A, (c) HRTEM image of nano-size twins in Area B, (d) magnified image of needle-like structures, and (e), (f), and (g) correspond to the SAD patterns of points I, II, and III in (d), respectively.

Fig. 7. Top-surface EBSD grain orientation and IPF maps of TA15 samples: (a) S17, (b) S18, (c) S19, and (d) S20.

Table 1 Room-temperature and high-temperature tensile properties for the printed TA15 alloy samples under varied SLM processing parameters and heat treatments.

Samples	Room temperature		500 ℃
	UTS (MPa)	EI (%)	UTS (MPa)	EI (%)
S17	1422.1 ± 50.2	9.5 ± 1.5	990.2 ± 2.2	15.6 ± 1.9
S18	1362.6 ± 46.2	9.3 ± 0.4	949.6 ± 19.6	13.6 ± 3.2
S19	1349.8 ± 26.7	5.6 ± 0.1	909.4 ± 40.1	12.5 ± 0.4
S20	1063.6 ± 136.9	7.5 ± 0.5	939.5 ± 20.7	15.3 ± 1.7
S26	1234.2 ± 53.1	7.3 ± 0.7	907.5 ± 4.9	11.8 ± 0.4
S26 + HT1	1207.3 ± 3.8	8.9 ± 0.1	853.2 ± 10.7	12.5 ± 1.1
S26 + HT2	1123.6 ± 14.0	11.3 ± 0.7	757.1 ± 7.0	13.4 ± 1.1
S26 + HT3	1024.9 ± 19.5	9.0 ± 0.4	715.3 ± 4.3	13.0 ± 0.9
S26 + HT4	936.5 ± 4.9	8.0 ± 1.0	639.8 ± 6.7	14.3 ± 0.9
S26 + HT5	906.9 ± 5.5	5.8 ± 0.2	602.0 ± 5.6	15.9 ± 5.0
S26 + HT6	780.6 ± 7.2	3.8 ± 0.4	596.3 ± 5.6	15.6 ± 2.5
Wrought TA15 [24]	965	18.8	680	24.8

Fig. 8. Fracture surfaces of the four samples: (a) S17, (b) S18, (c) S19, and (d) S20 after tensile testing at room temperature.

Fig. 9. Fracture surfaces of the four samples: (a) S17, (b) S18, (c) S19, and (d) S20 after tensile testing at 500 °C.

Fig. 10. Side-surface OM microstructures of samples under various layer thicknesses: (a) S17 with 30 μm layer thickness and (b) S26 with 60 μm layer thickness.

Fig. 11. X-ray patterns of untreated S26 sample and S26 samples with heat treatments HT1 (650 °C), HT2 (750 °C), HT3 (850 °C), HT4 (950 °C), HT5 (1000 °C), and HT6 (1100 °C).

Fig. 12. Microstructures of untreated S26 sample and heat-treated samples in the partial martensitic decomposition temperature range: (a, b) untreated S26, (c, d) HT1 (650 °C), and (e, f) HT2 (750 °C). White and yellow arrows indicate needle-like structure and lath-like structures, respectively.

Fig. 13. Microstructures of heat-treated samples in the complete martensitic decomposition temperature range: (a, b) HT3 (850 °C) and (c, d) HT4 (950 °C).

Fig. 14. Microstructures of heat-treated samples above the βtr temperature: (a, b) HT5 (1000 °C) and (c, d) HT6 (1100 °C). Yellow arrows indicate the lamellar structure.

Fig. 15. Top-surface EBSD grain orientation and grain size distribution maps: (a, b) untreated S26, (c, d) HT2, (e, f) HT4, and (g, h) HT5 samples.

Table 2 Microstructure type and constituent phase of the S26 sample and heat-treatment samples in this study.

Sample	Microstructure type and constituent phase
S26	Full needle-like α′ martensitic phase
HT1	Fine lath-like (α + β) with needle-like α′ martensitic phase
HT2	Fine lath-like (α + β) with minority needle-like α′ martensitic phase
HT3	Full lath-like (α + β)
HT4	Full lath-like (α + β)
HT5	Lath-like (α + β) with minority coarse lamellar (α + β)
HT6	Coarse lamellar (α + β) with minority lath-like (α + β)

Fig. 16. Comparison diagram of room-temperature and high-temperature tensile properties for the S26 samples without and with heat treatments. The blue frames show the corresponding tensile testing temperatures. The five-pointed stars and squares represent tensile strength and elongation, respectively.

Fig. 17. High-magnification morphologies of the printed samples: (a) S17, (b) S18, (c) S19, and (d) S20. The yellow and white arrows indicate coarse and fine α′ martensite, respectively.

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