| Ultrasonic impact treatment(UIT)is a new type of surface modification technology,which mainly achieves the purpose of improving the fatigue life of the material by introducing residual compressive stress and the hard layer on the material surface.At present,the technology has been widely used in construction machinery,marine ships,high-speed trains and other fields.However,the studies have shown that excessive ultrasonic impact treatment may cause fold defects on the surface of the material,destroying the integrity of the surface structure,and affecting the service life of components adversely.Therefore,it is necessary to study the formation mechanism of fold defects deeply and the effects on material fatigue behavior.Aiming at the two materials of Q355 steel and 6082-T6 aluminum alloy,this paper adopts the technical route of ABAQUS numerical simulation and experiments to carry out research on the formation mechanism of fold defects.We have explored the effect of ultrasonic impact process parameters and the surface roughness of samples on fold defects.It reveals the formation mechanism of fold defects on the corresponding lowalloy steel and aluminum alloy and its influence on the fatigue behavior of materials.In order to achieve the above goals,this article is mainly divided into the following three parts to carry out the related research:Firstly,we studied the formation mechanism and the influence of different surface roughness on the ultrasonic impact fold defects.The smooth surface(Sa=0.8 μm)and the rough surface(Sa>1 μm)of the two materials have been subjected to ultrasonic impact treatment respectively.The research has found that pits(100% coverage)will be formed on a smooth surface after one ultrasonic impact treatment.Unclosed holes will be closed completely due to the plastic flow of the metal(200% coverage)when impacted again.Due to the presence of machining marks on the rough surface,the peaks and valleys on the surface of the material will be superimposed on each other after UIT,and thus fold defects will appear.After UIT,the surface of the material has changed from elastic deformation to plastic deformation,causing internal dislocations and fold defects.The smaller the original roughness of the sample surface,the smaller the size of the fold defect and the simpler the shape of the fold defect.Fold defects are mainly thin strips.Secondly,we studied the influence of different ultrasonic impact times and amplitudes on the formation,the shape and the size of fold defects.The results show that the number of impacts increases from 1 to 4,and the shape of fold defects becomes more complex,the length of the defect increases by 1-2 times,and extends to the inside of the sample;When the impact amplitude increases from 20 μm to 30 μm,the fold defect changes from a single strip to the multiple strips and the size of the defects also increased significantly;Under the same impact parameters,Q355 steel is more prone to stack defects than 6082-T6 aluminum alloy.Finally,a three-point bending fatigue test was carried out on the Q355 steel and6082-T6 aluminum alloy samples treated by ultrasonic impact treatment.After that,we explored the influence of fold defects on the fatigue resistance of the treated materials.As the number of UIT increases,the fatigue life of the two materials increases firstly and then decreases.There is an optimal ultrasonic impact parameter.Compared with the original sample,the fatigue strength of Q355 steel has increased by 35% after ultrasonic impact treatment(100% coverage),while the fatigue strength of aluminum alloy only increased by 53%.It means Q355 steel is more sensitive to fold defects,so the increase in fatigue strength is small.Therefore,the change in the fatigue life of the material after ultrasonic impact treatment is the result of the combined effect of surface compressive stress and fold defects.The key to the ultrasonic impact treatment process is to increase the surface compressive stress while reducing fold defects. |
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