Research On Microstructure Tailoring And Properties Of Micro-alloyed Laser Welded Ti₂AlNb-based Alloy Joints

Ti₂AlNb-based alloys are a kind of lightweight high-temperature structural materials with great prospect for applications due to their high specific strength,creep resistance,fracture toughness and low coefficient of thermal expansion.Their applications require materials joining.Laser welding has been increasingly widely used because of advantages such as high level of automation,low heat input and large depth-to-width ratio.It is found that laser welded Ti₂AlNb-based alloy joints are highly brittle at 650?.Based on the fact that Ti₂AlNb-based alloys are usually used at high temperatures,it is necessary to conduct study on suppression of the high-temperature brittleness.Ti-22Al-25 Nb alloy sheets with the thickness of 1mm were used in this study,and mechanical properties of laser welded Ti-22Al-25 Nb joints were systematically studied at different temperatures.It was found that the tensile strength of laser welded joints reached around 50% of that of base metal,and no plastic deformation was observed at 650?.The tensile strength of joints was lower than that of base metal,and the elongationto-failure of joints was significantly decreased at 800?.However,brittleness was not found at other temperatures,and superplasticity was even observed at 1000?.The base metal experienced recovery at 1000?,which could be accelerated by deformation.As ?-stabilizers,V and Ta powders were added to the weld metal to inhibit the phase transformation from B2 to O using laser welding technology with powder feed.By using matched process parameters,good weld appearance was obtained,but unmelted powders were found if excess Ta powders were added.The fusion zone consisted of single B2 phase,and columnar grains disappeared,which were replaced by equiaxed ones.In the heat affected zone,the amount of O phase decreased and that of B2 phase increased with increasing distance from base metal.Tensile property tests showed that micro-alloyed joints fractured at base metal at room temperature.The tensile strength and elongationto-failure of micro-alloyed joints were greatly increased compared with those of laser welded joints at 650?.The micro-alloyed joints showed a combination fracture mode of intergranular and cleavage fracture.Based on preparation of composites,TiB₂ powders of two dimensions were added to the weld metal to produce in situ formed needle-like TiB.TiB fibers tend to gather along grain boundaries with effects of grain refinement,but the grain size decreased slowly when more TiB₂ powders were added.When TiB₂ powders of the large dimension were added,unmelted powders were found in the fusion zone.Because of the diffusion of B,TiB of relatively small dimension was also observed in the heat affected zone.Joints with addition of TiB₂ of the small dimension showed best comprehensive mechanical properties.The joints fractured at base metal at room temperature,and their strength and elongation-to-failure were obviously increased at 650?.