Study On Laser Remelting Process And Microstructure And Properties Of Multiphase Ni₃Al-based Alloy

Due to the limitation that the dissolution temperature ofβphase is close to the melting point of the alloy,the control of the microstructure of multiphase Ni₃Al-based alloy by traditional heat treatment is still very limited,especially the size and volume fraction ofβphase in the alloy cannot be effectively reduced.The laser remelting process has the characteristics of high cooling rate and high temperature gradient.Therefore,the temperature gradient and cooling rate of multiphase Ni₃Al-based alloy under different laser remelting conditions（laser power and scanning rate）were calculated by Rosenthal equation.Then,according to the calculation results,the surface laser remelting treatment of the experimental alloy was carried out under different laser remelting parameters of 3000～4500 W laser power and 5～20 mm/s laser scanning rate.The optimum laser remelting parameters and energy density range of the alloy were determined,and the multiphase structure（especiallyβphase）,room temperature/high temperature tensile and creep behavior of the alloy after laser remelting were systematically studied.The temperature gradient and cooling rate of the alloy under different laser remelting parameters were calculated by Rosenthal equation.Increasing the laser power and decreasing the laser scanning rate can increase the temperature gradient.On the contrary,increasing the laser power and decreasing the laser scanning rate can reduce the cooling rate.The corresponding temperature gradient and cooling rate under the condition of 4000 W laser power and 10 mm/s laser scanning rate are 0.86×106 K·m^-1,the energy density is102.6 W/cm²,respectively.The laser remelting process can effectively improve the temperature gradient and cooling rate of the multiphase Ni₃Al-based alloy,and the cooling rate is significantly higher than the water cooling rate(138 K·s^-1)of the alloy.The effects of different laser remelting parameters（3000～4500 W laser power and 5～20 mm/s laser scanning rate）on the microstructure of multiphase Ni₃Al-based alloy were analyzed.The results show that the appropriate parameters（4000 W-10 mm/s）laser remelting treatment can effectively reduce the size and volume fraction of interdendriticβphase in the alloy and the size of cubicγ′phase in the dendrite two-phase structure,and significantly refine the alloy structure.After laser remelting,the average area of singleβphase in the remelting zone of the alloy decreased from 216.82μm² in the as-cast state to2.55μm²（The average width decreases from 7.68μm to 1.36μm）,the volume fraction ofβphase decreased from 19.4 vol.%in the matrix zone to 13.6 vol.%,,and the average size of cubicγ’phase decreased from 0.48μm in the unremelted matrix zone to 20 nm.At the same time,there are a large number of non-twinned structures with lamellar structure in theβphase of the remelting zone,and a large number of nano crystals in the lamellar structure.The effect of laser remelting process on the tensile properties of multiphase Ni₃Al-based alloys was studied.It was found that the room temperature tensile properties of multiphase Ni₃Al-based alloys can be effectively improved by laser remelting with appropriate parameters.Under the optimal laser remelting parameters with a laser power of4000 W and a laser scanning rate of 10 mm/s,the tensile strength of the alloy at room temperature and 980°C high temperature can reach 1215 MPa and 280 MPa,respectively,and the elongation of the corresponding sample is 14.09%and 6.41%,respectively.After remelting with the optimal laser parameters of 4000 W-10 mm/s,the high temperature creep life of the alloy can reach 107.5 h at 1000°C/120 MPa.