Additive manufacturing of γ-TiAl based alloys

γ-TiAl based intermetallic alloys have attracted increasing attentions due to their attractive properties such as low density, high-temperature strength, and good resistance against oxidation and creep, making them a front-runner in replacing the currently used superalloys in aerospace sector. But the intrinsic room temperature brittleness of this type of alloys makes harder to fabricate parts with geometrically complexity, limiting their engineering applications. Additive manufacturing (AM) offers an effective approach to directly fabricate near-net-shape components with shape freedom. However, the complex thermal history and high thermal gradient during AM process lead to high residual stress, with which the brittle TiAl alloys are unable to tolerate, leading to serious cracking problem in the additively manufactured (3D printed) parts. This PhD project aims address this problem in order to 3D print crack-free parts with high density, followed by designing proper heat treatment process to further tailor the microstructure to optimize the mechanical property trade-off at room and high temperatures.

The most effective and practical approach to minimize internal defects is optimizing the AM processing parameters. It was found that the cracks can be mitigated by increasing the laser power through slowing down the cooling rate. In addition, higher laser power also facilitated the formation of nearly α₂/γ lamellar structure, inhibiting the forming of the brittle block α₂ phase. However, the cracks cannot be completely eliminated by simply controlling the AM processing parameters. In order to increase the AM processability of the TiAl-based alloy, conjugation of AM with conventional grain refinement technology was proposed. LaB₆ nanoparticles, for the first time, were identified as an effective inoculant to modify the β-solidifying TiAl-based alloy powder feedstock to grain refine the 3D printed parts. The present research has experimentally validated that addition of 0.5wt% LaB₆ nanoparticles enables to additively manufacture crack-free samples with a high density over 99.99%. The results showed that the average grain size was drastically reduced from 39.81 ± 9.12 µm in the as-printed samples without LaB₆ addition to 1.5 ± 2.07 µm, together with a microstructure change from nearly lamellar structure to duplex microstructure. As a result, the room temperature compressive strength and plasticity were simultaneously increased to be comparable with that produced using conventional manufacturing technologies.

To understand the underlying grain refinement mechanism, the TiAl alloys with wider range of LaB₆ nanoparticle additions up to 2 wt.% were prepared and characterized. It was found that, during AM heating, the added LaB₆ nanoparticles decomposed into La and B, leading to the formation of La₂O₃ and TiB. The B solute stemming from the added LaB₆ refined the primary β grains due to the B-induced constitutional supercooling mechanism. The in-situ formed boride precipitates were verified to be a nanoscale mixture of TiB and β₀, which were formed from the eutectic reaction L→β+TiB. During the subsequent solid-solid phase transformation, the refined β grains and the pinning effect of the La₂O₃ and TiB particles resulted the refinement of the microstructure. The experimental results also verified the 0.5 wt.% as the optimal additive level for the current TiAl alloy, which can not only fully eliminate cracks, but also achieve superior room temperature mechanical performance.

However, the grain refinement of the 3D printed TiAl alloy also led to an increase in creep rate at elevated temperatures due to the refined grains that facilitated the grain boundary gliding, and the reduction in volume fraction of α₂/γ lamellar structure that corresponds to higher creep resistance. The present work validated the feasibility and efficiency of proper postproduction heat treatment in restoration of the creep resistance through maximizing the α₂/γ lamellar structure in the alloy together with the optimization of room temperature compression strength and plasticity. Combined with the mechanical properties at room temperature and creep performance at 800 ℃ and initial applied stress of 150 MPa, an optimized two-step heat treatment process comprising of annealing at 1350 ℃ for 1 h followed by air cooling and a subsequent stabilization treatment at 850 ℃ for 6 h is developed. Upon this proper heat treatment, not only the compressive ultimate strength and plasticity at room temperature were simultaneously increased, but also the creep resistance is resorted to be compatible with the typical cast and wrought TiAl-based intermetallic alloys. In addition, although the 3D printed TiAl alloy without LaB₆ showed the lowest creep rate, the high densities of cracks and porosity led to the early creep failure at 92 testing hours. But, the TiAl alloy with LaB₆ inculcation after heat treatment was associated showed both low creep rate and longer creep life over the limited 100 testing hours.

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