Study on Stability of Residual Stress Induced by Laser Shock Processing in Titanium Alloy Thin-Components

Because the compressor thin-blades of aero-engine often fractured in service, laser shock processing was suggested to be applied as a surface strengthening technology. Aim at the problem of compressive residual stress relaxation in laser-peened compressor thin-blades, TC11 titanium alloy thin-components were treated by laser shock processing and then conducted in axial tensile-tensile fatigue test and thermal insulation in vacuum. X-ray diffraction tests were carried out to obtain the relaxation rules of residual stress under fatigue loading and thermal stress loading. In addition, the relaxation mechanisms of residual stress were indicated. Experiment results demonstrate that surface compressive residual stress relaxes by 53%, and 95% of stress relaxation occurs in the previous 5 fatigue cycles under the fatigue loading (maximum stress σ_max=500 MPa, stress ratio R=0.1). The surface relaxation degree and severely-relaxed depth increase with fatigue loading, and the relaxation mechanism is that plastic deformation of local area material results in residual stress redistribution. Surface compressive residual stress relaxes by 3%, 29% and 48% respectively after thermal insulation for 120 min under the constant temperature of 200℃, 300℃ and 400℃. Surface compressive residual stress relaxes by 18% and 58% respectively after thermal insulation for 120 min under the altering temperature of 200℃+400℃ and 300℃+400℃. The relaxation all occurs in the previous 60 min. There is a similar trend with temperature in the aspect of severely-relaxed depth. The relaxation mechanism under thermal stress loading is that dislocations and grain-boundaries are activated to move and annihilated, and then plastic deformation recovery occurs. Due to the distinction of relaxation mechanisms, there is an obvious superimposed effect under the combined action of fatigue loading and thermal stress loading.