Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Powder Characterization
3.2. As-Built Microstructure and Microhardness
3.3. Phase Stability and Heat Treatment Optimization
3.4. T4 State and Overaging Effects
4. Conclusions
- The as-built state featured an extremely fine and equiaxed bimodal microstructure with grains 300–600 nm in size at the boundaries of the melt pools and grains 0.8–2.0 μm in size at their center. A complete columnar-to-equiaxed transition was therefore achieved by virtue of the combined action of Zr and Ti. Cubic nucleants corresponding to the Al3(Zr,Ti) phase were observed at the center of grains, while segregation of Cu and Mg was measured at the grain boundaries. No evidence of hot-cracks was detected in the investigated samples.
- A T4 treatment reached by holding the samples at 490 °C for 45 min, water quenching, and natural aging for at least 5 days allowed for achieving the highest microhardness value of 186.1 HV0.5. The T6 temper, consisting of the same solution-annealing treatment followed by artificial aging at 160 °C for 5 h, led to a microhardness of 173.9 HV0.5.
- Immediately after water quenching, the measured increase in microhardness from 109.7 to 147.4 HV0.5 was mainly related to the precipitation of additional Zr- and Ti-rich secondary particles during the annealing treatment, as predicted by Thermo-Calc ® simulations.
- The complex microstructure of the T4 temper was characterized by a large number of precipitates on the micrometer and sub-micrometer size scales. Coarse precipitates, 0.5–1 μm in size, corresponded to Al2Cu particles mainly disposed at the edge of the molten pools. Cubic precipitates of Al3(Zr,Ti) were observed at the center of the grains and were assumed to be the nucleants promoting their formation. Rod-shaped Al3(Zr,Ti) and quasi-spherical precipitates of about 100–300 nm rich in Cu, Mn, and Fe were arranged within the grains. Finally, particles smaller than 80 nm were assumed to be the expected Ω-phase (Al2Cu) strengthening precipitates.
- Substantial microstructural changes occurred in T4 samples when held for 50 h at 250 °C, with evidence of Cu and Mg segregation at the grain boundaries and slight grain coarsening (reaching a size of 0.8–2.2 µm), which contributed to a decrease in microhardness (from 186.1 to 119.3 HV0.5). The exposure for 50 h at 300 °C led to a further decrease in hardness down to 110.3 HV0.5 and stimulated the spheroidization of Cu and Mg at the grain boundaries.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Al | Cr | Cu | Fe | Mg | Mn | Si | Zn | Ag | V | Ti | Zr | |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Nominal | Bal. | <0.05 | 4.5–5.5 | <0.15 | >0.8 | 0.2–0.6 | <0.1 | <0.25 | 0.15–0.60 | <0.05 | Ti + Zr < 4.0 | |
Measured | Bal. | 0.02 | 5.41 | 0.11 | 0.70 | 0.57 | 0.04 | - | 0.44 | - | 1.08 | 1.80 |
As-Built | T4 | T4 + 50 h @ 150 °C | T4 + 50 h @ 250 °C | T4 + 50 h @ 300 °C | |
---|---|---|---|---|---|
HV0.5 | 109.7 | 186.1 | 167.9 | 119.3 | 110.3 |
Cu | |||||
Mg | |||||
Zr | |||||
Ti | |||||
Mn | |||||
Fe |
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Larini, F.; Casati, R.; Marola, S.; Vedani, M. Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion. Metals 2023, 13, 924. https://doi.org/10.3390/met13050924
Larini F, Casati R, Marola S, Vedani M. Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion. Metals. 2023; 13(5):924. https://doi.org/10.3390/met13050924
Chicago/Turabian StyleLarini, Federico, Riccardo Casati, Silvia Marola, and Maurizio Vedani. 2023. "Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion" Metals 13, no. 5: 924. https://doi.org/10.3390/met13050924