Effect of Post-Deposition Thermal Treatments on Tensile Properties of Cold Sprayed Ti6Al4V
Abstract
:1. Introduction
2. Materials and Methods
2.1. Substrate Material and Feedstock Powder
2.2. Cold Spray System and Process Conditions
2.3. Post-Deposition Thermal Treatments
- (i)
- Solution treatment and ageing (STA): STA is an established technique that can improve the strength of Ti6Al4V alloy while maintaining its ductility. A wide range of microstructure and mechanical properties can be achieved in α-β alloys after STA treatment, which originates in the instability of high-temperature β phase at lower temperatures. Heating α-β alloy to the solution-treating temperature (generally 25–85 °C below β-transus) increases the ratio of β to α phase. This partitioning of phases is maintained following quenching. Subsequent ageing treatment (425–650 °C) promotes the decomposition of unstable β phase and any martensite, producing a finely divided mixture of α and β phase, resulting in increased strength [26,27]. In this study, solution treatment was performed under vacuum at 940 °C for 1 h followed by argon fast cooling and aged at 480 °C for 8 h followed by furnace cooling to room temperature.
- (ii)
- Hot isostatic pressing (HIP): HIP is a widely accepted thermo-mechanical method for closing internal defects (except surface connected pores) in titanium castings or additively manufactured parts. In the HIP process, chemically cleaned parts were loaded inside a HIP chamber which later on was filled with high-purity argon gas to create high pressure (usually in the range of 70–205 MPa) and temperature (900–955 °C for titanium) for some fixed dwell time, e.g., 2–4 h. As the yield strength of the material drops at high temperatures, the application of pressure results in high diffusion rates and the plastic flow of the material, which helps to close any voids or microcracks [27,28]. In this study, HIP treatment was performed at 140 MPa and 930 °C for a duration of 4 h.
- (iii)
- HIP followed by STA (HIP + STA): Several studies reported that HIP temperatures can coarsen α platelets, causing a reduction in tensile strength of the material. Therefore, to recover the strength of the material, STA treatment is generally performed after HIP [27,28]. Parameters used for HIP + STA treatment was the same as mentioned earlier for STA and HIP.
2.4. Microstructural Characterisation
2.5. Evaluation of Mechanical Properties
3. Results
3.1. Microstructure and Porosity
3.2. Mechanical Properties
3.2.1. Hardness
3.2.2. Tensile Properties
4. Discussion
4.1. Overview
Process Gas | Temperature (°C)/Pressure (MPa) | Testing Condition | Porosity (%) | UTS (MPa) | Elongation (%) | E (GPa) | Measurement Method | References (et al.) |
---|---|---|---|---|---|---|---|---|
He | 450–530/2.4 | As-deposited | 18–26 | 52–109 | 0.16–0.48 | 28 | ASTM E8 (sub-size round) | Blose [13] |
Annealing a1 | 18–26 | 236–244 | 0.64–0.68 | 61 | ||||
Encapsulated HIP b1 | ~0 | 890–1024 | 12.3–14.0 | 115 | ||||
350/4 | As-deposited | ~0.3 | 445 ± 145 | 3.8 ± 0.8 * | - | MFT e1 | Vo [3] | |
Annealing a2 | ~0.3 | 764 ± 189 | 6.3 ± 0.5 * | - | ||||
950/2 | As-deposited | ~1.2 | 373 ± 10 | ~0.46 | 75 | ASTM E8 (sub-size flat) | Chen [14] | |
HIP b2 | ~0.04 | 962 ± 31 | ~1.76 | 80 | ||||
N2 | 800/4 | As-deposited | ~6.7 ~6.7 | ~160 | - | - | MFT e1 | Wong [8] |
Annealing a3 | ~160 | - | - | |||||
800/4 | As-deposited | 5–12 | 154 ± 81 | 2.3 ± 0.9 * | - | MFT e1 | Vo [3] | |
Annealing a4 | 5–12 | 251–273 | 3.2–4.7 * | - | ||||
Annealing a5 | 5–12 | 144–219 | 2.2–3.5 * | - | ||||
Annealing a6 | 5–12 | 180–462 | 3.0–5.8 * | - | ||||
800–1000/4–5 | As-deposited | 4.6–10.4 | 182–263 | - | - | TCT e2 | List [9] | |
157–295 | - | - | MFT e1 | |||||
950/5 | As-deposited | ~2.4 | 85 ± 3 | ~0.27 | 31 | ASTM E8 (sub-size flat) | Chen [14] | |
HIP b2 | ~1.5 | 664 ± 21 | ~1.40 | 55 | ||||
950/4.5 | As-deposited | 4–5 | 337 ± 17 | 4.4–5.3 * | - | MFT e1 | Tan [10] | |
600/5 | As-deposited | 7.5 | 68 ± 5 | 0.54 | 99 | ASTM E8 (sub-size flat) | Petrovskiy [15] | |
Encapsulated HIP b3 | 0.2 | 956 ± 5 | 13.5 ± 0.5 | 99 | ||||
800/4 | As-deposited | ~6.5 | 105 | 0.65 | - | ASTM E8 (sub-size flat) | Petrovskiy [16] | |
HIP b4 | 4.9–5.2 | 635 | 0.97 | - | ||||
- | - | As-deposited | - | 147 | 0.24 | 61 | MFT specimen with DIC e3 | Ligda [11] |
HIP b5 | - | 798 | 2.5 | 86 | ||||
HSPT c1 | - | 859 | 3.1 | 91 | ||||
N2 | 1100/5 | As-deposited | 2.25 | 287 ± 7 | 0.44 ± 0.03 | 77 ± 2 | ASTM E8 (sub-size flat) | This study |
STA d1 | 1.74 | 868 ± 44 | 1.23 ± 0.27 | 109 ± 1 | ||||
HIP b6 | 1.78 | 747 ± 42 | 0.76 ± 0.08 | 110 ± 3 | ||||
HIP b6 + STA d1 | 4.40 | 521 ± 14 | 0.61 ± 0.08 | 95 ± 10 |
4.2. Effect of Thermal Treatments on Ultimate Tensile Strength
4.3. Effect of Thermal Treatments on Elongation
4.4. Effect of Thermal Treatments on Elastic Modulus
4.5. Key Takeaways
5. Conclusions
- Post-deposition thermal treatments led to complete disappearance of the microstructural features found in the as-deposited condition, resulting in coarsened microstructure having equiaxed α grains with intergranular β precipitates. Porosity (in terms of area fraction) was reduced from 2.3% in the as-deposited condition to 1.7% after STA, and/or HIP treatment.
- STA and HIP treatments lowered hardness by 15–17% due to microstructural softening in the CS deposits caused by static recovery mechanisms during high-temperature thermal treatments.
- Post-deposition thermal treatments resulted in significant improvement in the tensile strength, which is mainly attributed to increase in cohesion strength between deposited particles due to improved metallurgical inter-particle diffusion bonding, homogenisation of microstructure, and in some cases reduction in porosity. For the fully CS material, the ultimate tensile strength (UTS) was improved the most by around 200%, from 287 MPa in AD to 868 MPa after STA.
- Elongation was much lower for all investigated cases with the highest measured value (averaged) of 1.23% after STA.
- For CS repair specimens (repair ratio 2:5), both STA and HIP treatments improved the UTS by 209%, from 308 to 951 MPa, elongation from 0.35% to 3.01% after STA, and to 4.32% after HIP. Better performance in repair specimens (compared to fully CS specimens) could be due to the contribution of the substrate material (mil annealed Ti6Al4V) towards the overall performance.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A. Tensile Strength for Specimens with Different Repair Ratios
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CS system setup | Gun | CS system | Impact 5/11 |
Nozzle | T24-SiC | ||
Pre-chamber | Long (128.6 mm) | ||
Process gas | Nitrogen (N2) | ||
Gas pressure (MPa) | 5 | ||
Gas temperature (°C) | 1100 | ||
Powder feeder | Dosing disk rotation speed (rpm) | 3 | |
Powder feed rate (g/min) | 24.67 | ||
Carrier gas flow rate (m3/h) | 3 | ||
Nozzle cooling medium | Water | ||
Robot and toolpath setup | Gun traverse or scanning speed (mm/s) | 500 | |
Track spacing (mm) | 2 | ||
Spray angle (°) | 90 | ||
Standoff distance (mm) | 30 | ||
Toolpath pattern | Cross-hatch |
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Boruah, D.; Zhang, X.; McNutt, P.; Khan, R.; Begg, H. Effect of Post-Deposition Thermal Treatments on Tensile Properties of Cold Sprayed Ti6Al4V. Metals 2022, 12, 1908. https://doi.org/10.3390/met12111908
Boruah D, Zhang X, McNutt P, Khan R, Begg H. Effect of Post-Deposition Thermal Treatments on Tensile Properties of Cold Sprayed Ti6Al4V. Metals. 2022; 12(11):1908. https://doi.org/10.3390/met12111908
Chicago/Turabian StyleBoruah, Dibakor, Xiang Zhang, Philip McNutt, Raja Khan, and Henry Begg. 2022. "Effect of Post-Deposition Thermal Treatments on Tensile Properties of Cold Sprayed Ti6Al4V" Metals 12, no. 11: 1908. https://doi.org/10.3390/met12111908