The Microstructure And Mechanical Properties Of Laser Melting Deposited TA15/Ti₂AlNb Dual Alloys

In the late 20 th century, a quadrature O phase Ti₂ AlNb based alloy was developed, which possesses excellent high temperature strength, good high-temperature creep resistance, high fracture toughness, high oxidation resistance, low density, etc, and becomes one of the most potential aeronautical materials at 600℃⁸00℃. TA15 titanium alloy belongs to α + β two-phase titanium alloy with service temperature below 500 ℃, having excellent specific strength, good medium-low temperature thermal stability, etc. However, most of components are not working at a constant temperature because of the complex avation working environment. Hence TA15/Ti₂ AlNb dual alloy will be able to meet the requirement of different temperature working environment in the one component.Traditional preparation route of dual alloy material was based on welding of two different alloys, which could lead to weakened weld fusion zone and lower material utilization. Therefore, near-net-shaped TA15/Ti₂ AlNb dual alloy was fabricated by the laser melting deposition process in this research, which not only could improve material utilization, but effectively decreased the risk of weld fusion zone. The microstructure and tensile properties of the dual alloys prepared by laser melting deposition were investigated. Heat treatment and mixed zone were applied to improve material properties. The main conclusions are as follows:（1） The microstructure of thin wall samples from TA15/Ti₂ AlNb dual alloy fabricated by the laser melting deposition process was densification. The microstructure of as-deposited TA15 alloy consisted of lath α phase and β matrix, as-deposited Ti₂ AlNb based alloy displayed Wdimanstatten structure consisting of α2、B2 and O phase. Transition zone was divided into two zones, named TZ 1 and TZ 2, respectively. The phase evolution of the as-deposited TA15/Ti₂ AlNb dual alloy from the TA15 side to the Ti₂ AlNb side would be α+β→α+α2+β/B2→α2+β/B2+O→α2+B2+O. The phase in transition zone contained lath α phase, short and fine α2 phase, fine needle O phase. The phase evolution of the as-deposited Ti₂ AlNb based alloy along the build-up direction was α2+B2+O→B2 due to the difference of heat accumulation.（2） As-deposited dual alloy presented good tensile performance with the tensile strength of 1096 MPa and elongation of 5.2% at room temperature, better than welded TA15/Ti₂ AlNb dual alloy. The fracture position was in transition zone near Ti₂ AlNb based alloy, the fractograph contained typical river-like pattern and cleavage steps. The elongation of dual alloy was lower, therefore, heat treatment and mixed zone were applied and introduced to enhance the performance of transition zone and to improve the elongation of dual alloy.（3） The change of microstructure after three heat treatments was that α2 was coarsened and fine secondary α was precipitated in TA15 alloy side and TZ 1 side; the shape of α2 was changed from bulk to short rodlike in TZ 2; more O phase were precipitated in Ti₂ AlNb side with increasing aging temperature. After three heat treatments, the tensile strength and elongation decreased when increasing aging temperature. The fracture positions were all in TA15 alloy side. Heat treatment I（960℃/2 h OQ+700℃/8 h AC） presented better tensile strength and elongation than the other two heat treatments relatively, as 1053 MPa and 3.2%.（4） The mixing ratio of powder with 40%TA15+60%Ti₂AlNb, 50%TA15+50%Ti₂AlNb and 60%TA15+40% Ti₂AlNb（wt.%） was fabricated by laser melting deposition. 60%TA15+40% Ti₂ AlNb sample presented better tensile properties at room temperature, with tensile strength of 1105 MPa and elongation of 3.8%. Two different mixed schemes named mixed I and mixed II relatively were as followed: Mixed I: TA15/50%TA15+50%Ti₂AlNb/Ti₂AlNb; Mixed II: TA15/60%TA15+40%Ti₂AlNb/40%TA15+60%Ti₂AlNb/Ti₂ AlNb. Mixed II showed better performance at room temperature, with tensile strength of 1051 MPa and elongation of 9.0%. Compared to as-deposited TA15/Ti₂ AlNb dual alloy, the elongation of TA15/Ti₂ AlNb dual alloy with mixed zone had been significantly improved. The experiment result met the expectation of improving performance of as-deposited TA15/Ti₂ AlNb dual alloy by adding mixed zone.