Influence Of Femtosecond Laser Processing Of Film-Cooling Holes On Recrystallization Behavior And High Temperature Fatigue Properties Of Single Crystal Superalloy

Single-crystal superalloys eliminate grain boundaries and have excellent high-temperature mechanical properties,while the occurrence of recrystallization significantly reduces the mechanical properties of the alloys,resulting in a decrease in turbine blade life.In this paper,the effects of different femtosecond laser processes(10w,30 w,"30w+ repair")on the residual stress,static recrystallization and dynamic recrystallization,and high temperature fatigue properties of the filmcooling holes in DD6 single crystal superalloy were systematically studied by electron backscattered diffraction(EBSD)and nanoindentation test,vacuum high temperature heat preservation test,high temperature fatigue test,optical microscope,scanning electron microscope,transmission electron microscope,etc..Among the three femtosecond laser processing processes of film-cooling holes,the "30w+repair" process has the smallest residual stress at the hole edge of the film-cooling,while its hole edge strain distribution was relatively uniform and its strain width range was the smallest.The processing effect of "30w+ repair" was the best,that of 10 w was the second and that of 30 w was the third.The "repair" process in "30w+ repair" effectively removed most of the strained areas left by the 30 w process and improved the appearance quality of film-cooling holes.The results of static recrystallization experiments at 1200°C,1250°C,and 1300°C for 1h,2h,3h,and 4h respectively showed that the recrystallized microstructure was only observed at 1300°C for 3h and 4h in all three femtosecond laser processing processes.Among them,the thickness of the recrystallized structure of the "30w+ repair" process was the smallest,followed by the 10 w process,and the thickest is the 30 w process in sequence.It showed that adding "repair" process could not only improve the quality of film-cooling holes,but also reduce the thickness of recrystallization.In all processes,the residual bulk eutectic around the holes of the film-cooling degenerated after holding 4 h at 1200 °C,1250 °C,and 1300 °C.With the increase of temperature,sunflower eutectic micostructure and lath-like microstructure appeared.A models of element diffusion among bulk eutectic regions,sunflower eutectic regions and lath-like structures were established.The high-temperature fatigue properties of the fatigue specimens with 30 w and "30w+ repair" processes at 1000℃ under three stresses of 450 MPa,500MPa and 600 MPa respectively were studied.The study found that under different stresses,the fatigue life of the “30w+repaired” fatigue samples was higher than that of the 30 w,because the film-cooling holes after the "repair" process were smoother and the stress of the area was more uniform in the fatigue test.Fatigue cracks originated from the edge of the hole and formed a fan-shaped oxide layer.With the increase of stress,the length of the first stage of crack propagation from the edge of the hole gradually decreased.Only under the low stress of 450 MPa,the rafting of the γ’ phase occurred at the edge of the film-cooling holes.There were no recrystallized microstructure observed under the three stresses.