Research On The Mechanism Of Surface Strain Strengthening In Thermo-Mechanical Coupling High Speed Two-Dimensional Ultrasonic Rolling Process

Two-dimensional ultrasonic rolling process is a new technology which combines the longitudinal and torsion composite ultrasonic vibration with the surface rolling process technology to realize the strengthening of workpiece surface.High speed machining technology can improve the processing efficiency and the surface quality,while it might produce higher processing temperature,and then induce the change of stress-strain field and surface micro-hardness of the work piece.The combination of two-dimensional ultrasonic rolling processing technology and high speed machining technology may probably acquire the better surface quality with higher processing efficiency.In this thesis,the surface strengthening mechanism of high speed two-dimensional ultrasonic rolling process is studied under thermo-mechanical coupling effect by theoretical analysis,finite element analysis and processing experiments.The main work is as follows:(1)Based on the processing principle of the two-dimensional ultrasonic rolling process,the transition relationship between plastic deformation and temperature,and the relationship between temperature gradient effect and internal microstructure evolution were studied.The results show that temperature field and stress field affect each other in the two-dimensional ultrasonic rolling process.The workpiece surface temperature increases with the effect of plastic deformation and friction,and the temperature field influences the stress field by changing the correlative material properties.(2)The thermo-mechanical coupling numerical simulation was carried out to study the time-dependent change of the surface temperature field,equivalent strain and stress field in high speed two-dimensional ultrasonic rolling process.The results show that the surface temperature field,equivalent strain and stress field increase with the increment of time.The equivalent stress value gradually keeps a constant level,and residual compressive stress value diminishes with the depth increasing.With thermo-mechanical coupling effect,the surface residual compressive stress are obtained,and its thickness is about 1.7 mm,and the temperature field,stress field and strain field influence each other.(3)The two-dimensional ultrasonic rolling process experiments were carried out by using single factor experiment to study the influence of rolling process parameters on surface roughness,micro-hardness,residual stress and surface micro-topography.The results show that after the two-dimensional ultrasonic rolling process,the obviously improved surface micro-topography;greatly reduced roughness value and obviously increased micro-hardness are obtained,and the residual compressive stress are formed.The surface micro-topography shows that the rolled surface of aluminum alloy is smoother.The surface microstructure is obviously compressed,and at the same time the hardened layer is formed.The surface quality can be improved when the suitable static force,spindle speed,feed rate and amplitude are selected.