Research On The Mechanism And Inhibition Method Of High Temperature Brittleness Of Laser Welded Ti₂AlNb-based Alloys

There has been a constant and urgent demand from the aviation and aerospace industry for the materials of excellent high temperature performance.On account of high strength at elevated temperatures,fine antioxidant capacity and excellent fracture toughness,Ti₂AlNb-based alloys have been deemed as one of the most promising materials to realize the requirements of high temperature resi stance and lightening from structural components in aviation and aerospace industry.Research on the Ti₂AlNb-based alloys has gradually shifted from the optimization of microstructure and properties to engineering applications as the manufacturing process of Ti₂AlNb-based alloys continuing to mature.Since the manufacturing of structural components often involves the welding process of these alloys,a reliable welding technology for Ti₂AlNb-based alloys is tightly connected to their further applications.Laser welding is deemed as an ideal procedure to weld the components in aerospace industry because of its high welding precision,high depth-width ratio weld and ability of being automated.Research on the laser welding process of Ti₂AlNb-based alloys is therefore crucial to promote their applications.This article mainly focused on the cause and inhibition of the high temperature brittleness of laser welded Ti₂AlNb joints.Two typical Ti₂AlNb-based alloys,Ti-22Al-25Nb?at.%?and Ti-22Al-24.5Nb-0.5Mo?at.%?,were adopted in the experiment,and the overall laser weldability,tensile behavior of both the base metal and welds,cause of high temperature brittleness and the improvement mechanism of microalloying for the laser welds were extensively discussed.The hope is provide some theoretical support for the laser welding technology of Ti 2Al Nb-based alloys.Firstly,the laser weldability of Ti₂AlNb-based alloys were systematically studied.The fusion zone was composed of columnar,single B2 phase because of that the cooling rate of laser welding process was much higher than the critical cooling rate at which the B2?O transformation would occur.In the HAZ,a gradient microstructure was observed because O?B2 and ?₂?B2 transformation occurred on heating.The results of numerical simulation,conducted using Marc software,of the thermal cycles in laser welding process proved that the phase composition in the fusion zone stayed the single-B2 phase composition despite the increase of heat input.While growth mode of B2 grains gradually shifted to equaixed dendritic growth as the heat input increased.And also,the O?B2 and ?₂?B2 transformation was enhanced and the volume of O and ?₂ decreased significantly as the heat input increased.Both the Ti-22Al-25 Nb and Ti-22Al-24.5Nb-0.5Mo joints exhibited excellent tensile properties at room temperature.However,obvious high temperature brittleness was found in the joints in tensile tests at 650 °C.The high temperature strength of Ti-22Al-25 Nb joints was around 300 MPa and the fracture occurred in the tensile deformation stage.In contrast,the high temperature strength of Ti-22Al-24.5Nb-0.5Mo joints was above 800 MPa,but the plastic deformation ability was poor.The high temperature brittleness of Ti-22Al-24.5Nb-0.5Mo joints was identified with their low elongation-to-failure.In situ tensile tests were applied to study the deformation behavior of the base metal and fusion zone of Ti-22Al-25 Nb joints.The fusion zone of Ti-22Al-25 Nb joints deformed through dislocation slip at room temperature.While cracks initiated and propagated along the B2 grain boundaries at 650 °C.Tensile test at different temperatures showed that the high temperature brittleness mainly existed in the B2+O phase field.The results of FIB+TEM analysis proved the existence of B2?O transformation along the B2 grain boundary in the fusion zone.The morphology of O phase was continuous thin layer in the fusion zone Ti-22Al-25 Nb joints.The formation of O phase layer provided an ideal environment for the cracks to grow and therefore caused the high temperature brittleness.For Ti-22Al-24.5Nb-0.5Mo joints,the in situ tests at 650 °C showed the propensity to plastic deformation of the B2 grains in the fusion zone.The results of FIB+TEM analysis also demonstrated the existence of O phase along the primary B2 grains after the thermal simulation experiment.However,the O phase was in the discontinuous particle shape,compared with the morphology of O phase in the fusion zone of Ti-22Al-25 Nb joints.The morphology of O phase varied with the B2?O inhibition effect of Mo element.The results of thermal analysis kinetics showed that the activation energy of Ti-22Al-24.5Nb-0.5Mo fusion zone was higher than that of T-22Al-25 Nb.With higher content of ?-phase stabilizers,the B2?O transus increased to some extent.The sub-structure and preferred orientation of the B2 grains in the fusion zone also aggravated the high temperature brittleness of Ti₂AlNb joints.Laser welding with filler powder process was proposed to realize the microalloying of the fusion zone in order to improve the high temperature brittleness of Ti₂AlNb joints,on the basis of the cause of high temperature brittleness and its inhibition mechanism.Compared with single laser welded joints,the tensile strength and ductility of the microalloyed Ti-22Al-25 Nb joints were above 600 MPa and around 4%,respectiviely.The reasons for the improvement of high temperature brittleness were twofold: Firstly,the morphology of granular O phase was transformed to discontinuous particles to blunt the crack propagation as a result of B2?O inhibition.Another reason was the columnar structure in single laser welded joints,which was disadvantageous for dislocation slip,was transformed into more equiaxed structure which was more favorable of dislocation slip.