| Surface modification technology is a key process in the research of anti-fatigue manufacturing research.Water jets,as a means of modifying energy concentration,is receiving more and more attention.The purpose of water jet shot peening is to use the high-speed kinetic energy of the water flow to drive the pellets to achieve high-speed movement.The water and the pellets act together on the surface layer of the material to realize the plastic deformation of the surface layer and induce the residual stress field.In this process,the incident angle of the jet is one of the important parameters,which directly affects the modification effect of the jet.In the engineering application,the parts often have various narrow surfaces and slots,in which the water jet could not be peening.Therefore,it is important to study the jet process under non-normal target conditions.In this paper,18CrNiMo7-6 carburized alloy steel was used as the modified material,and the modification mechanism and related experimental research of water jet peening under non-normal incidence conditions were carried out.The following is the main research work:1 In the theoretical research,the geometric model of non-perpendicular jet was constructed,and the impact force of different incident angle jets is theoretically derived,and the analytical relationship between impact force and incident angle was obtained;2 The study of the parameters of the jet stream in water was developed.The weights of the factors affecting the residual stress of the modified samples were analyzed by orthogonal test.The results show that the weight of the water jet pressure is the largest,the weight of the jet target and the moving speed of the nozzle are the smallest,and the influence of the two on the residual stress can be neglected.And the optimal process parameters obtained by range analysis are injection pressure 300 MPa,incident angle 75°,injection number 3 times,jet moving speed 400mm/min and stand off target distance 70 mm.Under this condition,the residual compressive stress of the sample can reach-1292.14MPa;3 The study of the number of injections and the final residual stress was carried out.The results show that under the water pressure of 300 MPa,for the incident angle of 75° to 90°,the residual compressive stress value of about-1300 MPa can be achieved at one round,and the number of injections is increased.The effect of improving the residual stress is not obvious;for an incident angle of less than 75°,the too small injection angle will significantly reduce the residual stress value,but by increasing the number of peening round,the final residual stress is close to the value under the vertical injection condition;However,the effect of multiple injections on incident angles below 45 degrees is not significant,and it is not recommended to use too small incident angles in engineering applications;4 The optimal combination relationship between the size of the nozzle and the pellet was studied.The results showed that the 1.2 mm diameter nozzle is suitable for 0.2 mm diameter pellet,and the 1.5 mm diameter nozzles were matched with the 0.1 mm pellet diameter.The compressive stress value is higher than other combination,and the peak value of residual compressive stress can reach-1347.87 MPa;5 The tendency of the hardness change of the sample after peening was studied.After surface modification,the hardness of the sample first increased and then decreased in the near surface layer of 0-500?m.The peak value reached 67.9HRC at 20?m and stabilized at 60 HRC at 300-500?m.The test shows that the two rounds jet strengthening can significantly improve the hardness of the surface layer,but the hardness value after three rounds has no obvious change,which proves that under the current conditions,the hardness of the material has an maximum value,and the method of multiple jet modification can effectively improve the material hardness. |
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