Author(s) |
Chen Shen, Xueming Hua, Lin Wang, Wenlu Zhou, Yuelong Zhang, Zengxi Pan, Huijun Li, Fang Li, Linjun Huang |
Abstract Scope |
1. Introduction
Due to the low-density, excellent high temperature material/mechanical properties, titanium aluminides, also referred as γ-TiAl based alloys, have been continuously attractive to lightweight desired fields such as aviation, aerospace and automotive industries. However, the room temperature brittleness of titanium aluminide significantly increases the fabrication and shape forming costs of the corresponding parts, thus the high efficiency and low-cost fabrication methods have been considered as one important area for the titanium aluminide further development. Since 2015, a twin wire-arc additive manufacturing (T-WAAM) technique has been developed to fabricate titanium aluminides by feeding two different wires into a single molten pool. Although the room temperature properties of T-WAAM fabricated titanium aluminides have been enormously investigated, few works have been conducted to the performance under elevated temperature. In the present research, a Ti-48Al wall-shaped deposit is fabricate using a twin-wire plasma arc additive manufacturing (TW-PAAM) system, and subsequently tensile specimens are extracted from the component for 650 ℃ tensile testing. According to the obtained results, under 650 ℃ the tensile strength of as-fabricated TW-PAAM Ti-48Al binary alloy can achieve over 450 MPa, which is competitive to the binary titanium aluminide fabricated using casting methods.
2. Experimental
During the TW-PAAM process, the Ti and Al wires were independently fed into the plasma torch generated molten pool at certain wire feed speed ratio to produce the target Ti-48Al binary alloy in-situ. The deposition current and voltage were set at 120 A and 20 V, respectively. Inert gas shielding (99.99% purity Ar) of the TW-PAAM process includes both the PAW torch and the trailing gas cover. The total gas flow rate of the process was 45 L/min, thus both the molten pool and the as-deposited TiAl alloy can be protected properly from the oxidation and nitriding. The filler materials fed into the in-situ alloying pool were 0.8 mm diameter pure Ti and Al wires. The specific wire feed speeds of the two wires producing the Ti-48Al binary alloys are 130 cm/min and 113 cm/min, respectively. The travel speed of the TW-PAAM process was 120 mm/min. To get rid of the cold cracks during the buildup process, the interpass temperature of the present TW-PAAM was set over 500 ℃. In total, 50 layers were deposited on the Ti-48Al buildup wall. The dimensions of the buildup walls were approximately 150 × 50 × 8 mm (length × height × width).
High temperature tensile specimens were extracted from the middle of wall component along the longitudinal direction using the electric discharge machining. The gauge volume of the designed tensile specimen was 5 × 3 mm. The moving speed of the testing jig was set at 1 mm/min. In total, five specimens were extracted from the buildup wall and carefully polished before testing. Here it should be mentioned that microscopic characterizations were also performed and will be included in the formal manuscript.
3. Results and Discussion
The tensile strengths of the testing specimens were 455, 400, 370, 434, 410 MPa, with the corresponding elongations at 2%, 3.5%, 2.5%, 3.0%, 1.5%. The average tensile strength and elongation are 414 ± 29 MPa and 2.5 ± 0.7%, respectively. All the specimens were fractured in the gauge volume section, thus excludes the influence of initial defects in the as-fabricated Ti-48Al deposit. Compared to the room temperature stress-strain (SS) curves, the SS curves obtained at 650 ℃ exhibit obvious yield phenomenon. Such finding can be confirmed by the fracture surface analysis that dimples are found occupying almost 1/3 of the surface. In general, three fracture modes are found in the fractured surfaces: brittle fracture along the γ-TiAl lamellae, inter-layer fracture, and ductile dimple fracture.
Compared to the tensile properties of the binary Ti-48Al alloys fabricated using the traditional casting methods, the present TW-PAAM as-fabricated deposit show competitive tensile values at 650 ℃. Although the layer-by-layer microstructure in the as-fabricated sample significantly influences the tensile property stability under room temperature, the results under elevated temperature are quite concentrated. As it is known, for the titanium aluminides, the optimal microstructure for the material strength and ductility should be the duplex-phase microstructure, which contains both the γ-TiAl/α2-Ti3Al lamellae and equiaxed grains, while the present as-fabricated Ti-48Al deposit is full-lamellar. Therefore, with the help of post-production heat treatment, the tensile properties of the TW-PAAM fabricated titanium aluminides can be effectively improved.
4. Conclusion
The high temperature tensile properties of the titanium aluminide alloy fabricated using the innovative TW-PAAM technique is firstly reported in the present work. The obtained 650 ℃ tensile strength and ductility are competitive to the casting binary titanium aluminides. Compared to the room temperature testing, obvious ductile performances are indicated by the SS curves and dimple fracture surfaces. Further research works will be focused on mechanical properties under different elevated temperatures. And according to the existing literatures, by post-production heat treatment, the present full lamellar structured titanium aluminides can be transferred into the duplex structured alloy thus reach better mechanical properties. Also, the TW-PAAM fabricated titanium aluminide will include more alloying elements to achieve mature compositions such as the TiAl-4822 and TiAl-45XD. |