| In response to the national call for energy saving,emission reduction and light weight,aluminum alloy materials are widely used in the aviation manufacturing industry due to their good mechanical properties and processing performance.The working conditions of a large number of aluminum alloy components on the aircraft fuselage are very severe,and the service process Inevitably,crack defects and fatigue damage will occur.The traditional surface treatment process itself has certain shortcomings,which can no longer meet the needs of high-performance use.In order to effectively strengthen the aluminum alloy components,the two methods of laser shock processing and ultrasonic extrusion strengthening can be combined with each other to form a brand new composite strengthening method.At present,only a few mathematicians have carried out relevant exploration,so the mechanism of ultrasonic extrusion assisted laser shock composite strengthening on the fatigue resistance of aluminum alloy components is worthy of further study.In this paper,the research idea mainly based on experimental verification,supplemented by theoretical analysis and numerical simulation is used to propose a method of ultrasonic extrusion assisted laser shock composite strengthening.The effect of composite strengthening on7050-T7451 aluminum alloy is studied and analyzed.The influence of relevant process parameters on the composite strengthening effect is discussed,and the mechanism of the composite strengthening effect on fatigue life is discussed.The main research contents of the paper are as follows:(1)By the ABAQUS FEA software,the composite strengthening process is numerically simulated,the three-dimensional stress distribution on the surface of the aluminum alloy component under different strengthening processes is compared,and the effect of relevant process parameters on the ultrasonic extrusion assisted laser shock composite strengthening is studied influence.After composite strengthening,the entire surface appears as a residual compressive stress layer,and the stress distribution is significantly better than laser shock processing.Both laser power density and pulse width will affect the three-dimensional stress distribution on the surface after composite strengthening.Compared with the ultrasonic extrusion strengthening parameters,when the amplitude fluctuates by a few microns,it has almost noeffect on the surface three-dimensional stress distribution,but when the amplitude is too large,it produces a negative effect;the change in static load will also have an impact on the surface three-dimensional stress distribution.(2)Using reasonable process parameters,perform strengthening test on aluminum alloy components,and measure the surface morphology and roughness of unreinforced specimen,laser shock processing specimen and composite strengthening specimen.By comparison,it can be found that different strengthening methods have a greater influence on the surface morphology and roughness of 7050-T7451 aluminum alloy samples.The laser shock processing increases the surface roughness and maximum height difference of the sample,while the composite strengthening can effectively reduce the surface roughness and maximum height difference of the sample.By measuring the residual stress of the samples under different strengthening methods,it can be found that there is a tensile stress area in the edge area of laser shock strengthening,while ultrasonic extrusion assisted laser shock composite strengthening can eliminate the tensile stress in the edge area of single laser shock strengthening,the surface of the samples shows residual compressive stress,the maximum value is far greater than that of the laser shock strengthening specimen,and the depth of the action layer is deeper.At the same time,the residual stress test results also verify the reliability of numerical simulation.(3)Through the fatigue test,it can be found that the fatigue life of the reinforced end of the laser shock processing specimen and the composite strengthening specimen far exceeds that of the unreinforced end,but the average fatigue life gain of the reinforced end of the composite strengthening specimen largely outweighs that of the laser shock processing specimen.This fully embodies the advantages of ultrasonic extrusion assisted laser shock composite strengthening.In addition,when the amplitude changes only slightly between 1?m~10?m,the fatigue life gain of the reinforced end of the sample after composite strengthening is relatively stable;but when the amplitude is too large,the fatigue life gain of the reinforced end of the sample is significantly reduced compared with before.When the static load increases,the fatigue life gain of the reinforced end of the sample also increases.(4)Observing the morphology of the fatigue fracture,it can be seen that multiple fatigue sources appear on the surface of the unreinforced specimen,which are mostly concentrated in the middle of the fracture edge.The fatigue cracks extend radially to the inner region of the sample;there are no friction marks and stepped pattern in the fatigue source area,the fatigue cracks propagate smoothly inside.There are only one source of fatigue at the fracture edge of the laser shock processing specimen and the composite strengthening specimen;there are obvious slip steps in the fatigue source area,but the fatigue source area of the composite strengthening specimen is flatter than that of the laser shock processing,and the the slip trace is thinner and shallower,indicating that the fatigue cracking of the fatigue source area of the sample after composite strengthening is more difficult and the germination time is longer.The fatigue crack growth rate of strengthening specimen is significantly lower than that of unreinforced specimen,which shows that different strengthening methods have played a role in inhibiting the initiation and growth of fatigue cracks.But the effect of composite strengthening is the most excellent. |
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