| The cold extrusion strengthening technology based on open-slit bushings refers to putting the bushings with open slits into the fastening holes and passing the mandrel through the holes with overfilling,which causes a certain plastic deformation of the material inside the hole walls and generates a certain residual compressive stress to strengthen the fastening holes.In this paper,a combination of numerical simulations and experiments is used to improve the preparation process of open-slit bushings and to study the effect of strengthening metal holes based on cold extrusion of open-slit bushings under different conditions.In this paper,it is found that the open-slit bushing with forming defects not only generates residual tensile stresses near the fastening hole to reduce the fatigue strength,but also damages the structure of the fastening hole,causing destructive effects.Therefore,the preparation process of open seam bushings was optimized,and the forming stage was divided into two steps: strip pre-bending and coiling forming.The orthogonal experiments were designed for strip thickness,pre-bending radius,friction coefficient and crimping force in the strip pre-bending springback stage,and the ANOVA results showed that the strip thickness and pre-bending radius were the main factors affecting the strip pre-bending springback.It is found that when the pre-bending radius is consistent with the forming radius,the open seam bushing is prepared better and has better adaptability to the preparation of open seam bushings.The fitting relationship between bushing thickness and mandrel diameter was studied by numerical simulation when the initial hole diameter was 7.312 mm and the relative extrusion volume was fixed.The analysis of the residual stress distribution and the extrusion volume of the extruded hole shows that the stability of the cold extrusion effect and the uniformity of the residual stress distribution based on the open-slit bushing are better when the bushing thickness is 0.31mm～0.36 mm.On this basis,the thickness of openslit bushing is selected as 0.31 mm,and the bushing preparation and metal hole cold extrusion strengthening experiments are carried out through the improved process,and the comparison finds that the residual stress distribution around the hole is more consistent with the simulation results,and the fatigue experiments find that the fatigue life of the fastening hole is increased by about 5 times after applying the open-slit bushing cold extrusion technology.The cold extrusion strengthening effect of open-slit bushings with different hole margins and hole spacing is studied.When the hole edge distance is larger than 1.75 D,the hole circumference is uniformly displaced after extrusion,which has less influence on the cold extrusion effect.On the basis of determining the hole opening orientation and hole edge distance,the different hole spacing of adjacent holes was studied through the method of data transfer in numerical simulation.Extraction of tangential residual stresses in their transition regions revealed that when the hole spacing was small,positive stresses appeared at the edge of the transition region of the first cold extruded hole.When the hole spacing is greater than 3D,the cold extrusion effect of the open seam bushing of the adjacent holes has no significant effect.For a hole spacing of 1.5D,the cold extrusion effect was significantly improved by adding rigid filling blocks in the adjacent holes.By fatigue analysis software,fatigue life analysis based on Gerber fatigue life curve,the fatigue life after cold extrusion decreases when the hole spacing is 1.5D,and the fatigue life increases about 3.5 times after adding rigid filling blocks;when the hole spacing is 3D,the fatigue life increases about 5times. |
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