Study On Formation Mechanism Of Metal Materials Defect Induce By Ultrasonic Peening Treatment

Ultrasonic peening treatment(UPT)is a cold-work surface treatment that is developed on the basis of traditional shot peening.During UPT,an ultrasonic generator produces mechanical vibrations to drive a peening needle towards the treated surface,thus inducing surface hardening and plastic deformation.Compared to the other peening methods,UPT offers a number of benefits in terms of its equipment easy to carry,high productivity,low cost,adapt to deal with complex shape workpiece and local treatment.It has been used to improve the mechanical properties of metallic materials in aerospace,marine,automotive,railway,and bridge structures.Moreover,it has been development the new direction of surface nanocrystallization,rehabilitation and maintenanceand.Recent work has proposed that extensive UPT can introduce small cracks that could lead to poor structural integrity,leading to a significant reduction of material quality and service life.In this work,relevant work was carried out through combining method of experiment and numerical simulation,based on the goal of sample surface quality after UPT especially the defects.The main research results were listed as follow:Firstly,the UPT defects can be divided into two types: fold defects and clusters defects.We obtain the formation process of fold defects and proposed the mechanism of UPT defects formation.The fold defects was the micro short cracks result of dislocation movement induce by material surface plastic deformation and the short crack growth in compressive residual stress field.Clusters defects was the fatigue crack result of cyclical impact loading.Secondly,we studied the influence of initial surface topography and UPT parameters to surface quality,especially for UPT defects.Test result show that defects could not avoid in UPT process.The initial surface morphology of material,striking velocity,cycle of impact time and impact angle can significant influence the type,dimension and morphology of UPT defects.Impact angle is the key factor of forming defects during UPT process.Thirdly,three group of samples were prepared using different UPT procedures(asreceived(AR),UPT,and UPT + vibration finishing(UPTP)ones),their fatigue and corrosion properties were evaluated.Test result show that samples with fold defects induce by UPT skipped the stages of crack initiation and early propagation,leading to a significant reduction in the fatigue life.The fold defect preferentially initiates fatigue cracks because of the high concentration factor at the ends of the fold.UPT defects leading to material corrosion potential rising,electrochemical impedance spectroscopy and corrosion resistance decreasing.Finally,we proposed that vibratory finishing processing as UPT defects removal method.We optimization the defects removal process through adjust finishing machining time and vibration frequency.After finishing for 3h under vibration frequency of 30 Hz,45# steel contained a compressive residual stress layer(545 μm below the sample surface),plastic deformation layer(45 μm below the surface)and grain refinement layer(120 μm below the sample surface)without UPT defects.Moreover,the volue of surface roughness was reduces to 3.06 μm.Realize the goal of contained layers with refined grains and residual compressive residual stress,while the UPT defects were eliminated.