Study On Thermal Relaxation Mechanism Of Residual Stress On Laser Peened TC6 Alloy After Cryogenic Treatment

With the increasing use of TC6 titanium alloy in the aviation industry,accidents caused by fatigue fracture are also becoming more and more serious.Surface deformation strengthening is an effective method to restrain fatigue crack growth and to improve the service life of parts.Laser peening(LP)is a new surface deformation strengthening technology,which can induce a high amplitude distribution of deeper depth compressive residual stress on the surface of a material.However,due to high temperature service conditions,the residual compressive stress will greatly relax.Therefore,this paper puts forward the technology of LP after Cryogenic Treatment(CT)in order to explore the mechanism and technology of CT combined with high strain rate deformation strengthening to improve the high temperature stability of residual compressive stress.The main works are as follows:(1)Based on dislocation reinforcement theory,second phase reinforcement theory,the interaction of laser and materials induced by plasma shock wave theory and fine grain reinforcement theory;the reinforcement mechanism during cryogenic treatment and laser peening was explained.According to the dislocation slip theory and creep theory,the macro mechanism of residual stress relaxation at high temperature was also explained.The micro mechanism of residual stress relaxation at high temperature was also explained according to the grain evolution,dislocation density evolution and precipitate.(2)The parameter optimization experiment of LP after CT was carried out.The surface integrity and mechanical properties of TC6 samples under different process parameters were tested.The results showed an obvious appearance of grain refinement in the C-0h-LP and C-12h-LP specimens,and the fracture dimples were deeper and wider.But the C-12h-LP specimen had higher surface micro-hardness,surface compressive residual stress and tensile strength.Although the hardness and strength of C-18 h specimen were higher than that of C-0h-LP specimen,its elongation decreased significantly.The fracture of C-18 h specimen was characterized by a large amount of quasi cleavage planes and the cleavage steps formed due to stable αphase transformed from metastable β phase being cut during the tensile process,indicating the brittle fracture mode.In summary,C-12h-LP was the best process parameter.(3)The C-12h-LP specimen and C-0h-LP specimen were exposed to different temperatures for different times.Then tensile tests at different temperature were carried out.The results showed that the residual stress stability of the two specimens was nearly the same under the condition of 400℃ and 600℃,and the residual stress of the C-12h-LP specimen was more stable under the condition of 500℃.At 500℃,the grain growth was slow,and fractures were characterized by the quasi cleavage plane formed due to strengthening phases being cut,and the quantity in C-12h-LP specimen was larger and the distribution was more uniform.This phenomenon could be explained.C-12h-LP process could induce higher dislocation density,resulting in a higher precipitate dislocation nucleation rate.More interaction between precipitates and dislocations led to greater resistance to dislocation motion,so the residual stress was more stable.