Analysis And Inhibition Of High Temperature Brittleness On Laser Welds Of Ti₂AlNb-based Titanium Aluminides

Ti2Al Nb-based titanium aluminides has excellent properties, such as high room-temperature strength, high temperature oxidation resistance, fracture toughness, which make it the most promising material substituting Ni-based alloy. Laser welding with high energy density, high precision, high automatic degree, small heat affected zone, has a significant application in aerospace field, which calls for strict accuracy and quality. Laser welding joint of Ti-22Al-25Nb(at.%) alloy can obtain good room temperature performance, however, the high temperature brittleness tendency is seriously affecting its application in high temperature environment.2mm thick Ti-22Al-25Nb(at.%) alloy of laser welding experiment was firstly conducted, resulting in good weld seam performance with no defects such as porosity, incomplete fusion. Laser welding joint had high mechanical properties at room temperature with 868.2MPa tensile strength and 4.5% elongation rate at room temperature. At 650℃, high temperature brittleness tended to be extremely serious with 650.1MPa tensile strength and only 2.6% elongation. Heat affected zone can be divided into near heat affected zone(near-HAZ) with B2 +α2 phase and far heat affected zone(far-HAZ) with B2+O+α2 phase. The fusion zone consisted of single B2 phase. A high-temperature confocal laser scanning microscopy experment indicated that single B2 phase and coarse grains in fusion zone resulted in the decrease of weld properties; in the process of high temperature tensile test, O phase nucleated and grew up at grain boundary of B2 phase, increasing the grain boundary width and causing the high temperature brittleness of the weld joint.Based on the theory of the second phase strengthening, changed the composition of laser weld seam by adding Ti-6Al-4V element. When the adding rate was 2g/min, the fusion zone consisted of single B2 phase. When adding rate increased to 5g/min and 10g/min, the fusion zone changed into dual phase of B2 + acicular martensitic α′. Compared with laser joints, properties of added alloy joint at room temperature changed rarely, but the high temperature tensile strength and elongation improved with 760.3MPa and 3.3%, which were caused by martensite passivation under the high temperature environment.According to the theory of fine-grain strengthening, conducted laser welding experience with Ti-22Al-25 Nb alloy powder. The fusion zone was still composed of a single B2 phase. The HAZ width increased in the joint of three-layer welding, and single B2 phase appeared in near HAZ. In multilayer weld, grain dendrite segregation in layer one and two was improved due to the effect of thermal cycle of the multilayer welding. The adding powder increased nucleation particles, augmenting equiaxed grains in theweldseam. Compared with the grain size of laser weld, grain sizes were between 50~130μm, reducing by 50%. Room temperature tensile strength of three-layer welding with powder reached 900.5MPa and elongation rate reached 5.2%, the fracture mode was cleavage fracture. High temperature performance improved significantly with about 780.5MPa tensile strength, 5.1% elongation rate and intergranular fracture mode.