Research On Strengthening Mechanism Of Cryogenic Ultrasonic Surface Rolling And Fretting Wear Properties Of TB9 Titanium Alloy

Titanium alloy has high specific strength,good ductility,and great advantages in reducing structural weight,corrosion and protection costs,which is increasingly used in the field of aerospace structural components.Splines,springs,rivets and other components in the service process is usually in the tensile,compression,bending,torsion and other cyclic fluctuations in the load and other complex environment,and titanium alloy also has low surface hardness,wear resistance and fatigue resistance and other shortcomings,which seriously affects the reliability,service life and safety of the aircraft.In the face of the actual demand for wear and fatigue resistance of aerospace processing components,it is particularly important to improve the surface properties of the components by improving the manufacturing process.In the paper,the dynamic mechanical properties,microscopic plastic deformation mechanism and fretting wear properties of TB9 titanium alloy are systematically investigated by combining molecular dynamics theory and experiments with cryogenic ultrasonic surface rolling processing.A model of the rolling pressure and residual stresses exerted by ultrasonic impact force on the material surface was established,and the residual stress variation law induced by the ultrasonic rolling pressure on the surface of TB9 titanium alloy was obtained,and the mechanism of action of deep-cooling ultrasonic rolling on the macro and micro plastic deformation of TB9 titanium alloy was analyzed.Based on the crystal structure of β-Ti,a molecular dynamics model of cryogenic ultrasonic impact at different ambient temperatures and different impact velocities was established.The changes of the ultrasonic impact force on the internal induced temperature rise of β-Ti material and the process of dislocation twinning proliferation were extracted.The microstructure evolution law and temperature rise change law of β-Ti alloy under the effect of deep cold plastic deformation were elucidated.The experimental study of cryogenic ultrasonic impact of TB9 titanium alloy was carried out to verify the validity of molecular dynamics simulation of deep-cooled ultrasonic rolling,and to compare and analyze the changes of dislocation morphology and grain size under normal temperature and deep cooling.The surface properties of the machined workpiece were characterized by surface morphology,surface roughness and hardness analysis.The results showed that the surface roughness difference of cryogenic ultrasonic rolling was smaller compared with room temperature,but a finer grain layer organization with more excellent mechanical properties was formed and the surface hardness was increased to a small extent.At the same time,the cryogenic specimens produced phase transformation from β-Ti to α-Ti caused by dislocation,and the depth of residual stress layer was greatly enhanced by the synergistic plastic deformation of dislocation and phase transformation.Finally,micro-wear experiments were conducted on TB9 titanium alloy specimens before and after cryogenic ultrasonic rolling,and the mapping relationships between different static pressures,amplitudes,tumbling times and friction coefficients were established,and the surface morphology of the wear area was analyzed by ultra-deep three-dimensional microscope and scanning electron microscopy.The results show that the residual stresses and the microstructure of TB9 titanium alloy have a strengthening effect on the fretting wear properties.