| Warm laser peening technology is one kind of surface treatment technology,which is based on force coupling action of high temperature and high strain rate,this technology can significantly improve the process stability and effect of prolonging of fatigue life of traditional laser peening technology.In this paper,AA6061-T6 welded joints were served as the research objects,and the strengthening process of warm laser peening was studied which was based on high,low cycle fatigue life.With the comprehensive application of theoretical analysis,finite element simulation and experimental research methods,the dislocation morphology,precipitate and other microstructure induced by warm laser peening as well as residual stress field and release rules of residual stress field were analyzed,the evolution rules of microstructure and stress-strain relationship in the process of warm laser peening were explored,on the basis of that,the mechanism of prolonging of fatigue life and fracture criterion of warm laser peening technology were analyzed accroding to the influence rules of the effect of warm laser technology on aluminum alloy welded parts.(1)Evolution rules of microstructure of welded joint subjected to warm laser peening.The microstructure of welded joint and welded joint subjected to warm laser peening was observed by the test methods of SEM and TEM,the microstructure includes dislocation morphology,precipitate and grain morphology,and the evolution process of microstructure of welded joint subjected to warm laser peening was analyzed.The evolution rules of microstructure of welded joint subjected to warm laser peening and its strengthening mechanism were studied theoretically based on the physical basis of dislocation dynamics,impact dynamics and plastic deformation as well as the combination of aging model of dynamic and model of dislocation density with force couppling action and also the reinforcement theory of dynamic precipitation.(2)Residual stress field and release rules of residual stress field of welded joint subjected to warm laser peening.The residual stress field of welded joint surface and surface of welded joint subjected to warm laser peening was tested by the test method of XRD,the influence rules of effect of WLP on welding stress field were analyzed,the estimation models of plastic deformation induced by WLP,depth of plastic deformation and amplitude of residual compressive stress were deduced based on the combination of impact dynamics and the influence rules of effect of WLP on welding stress field.The release rules of residual compressive stress induced by WLP were studied by testing the residual compressive stress of welded metal and heat affected zone during fatigue process,and the analysis model of residual stress relaxation under cyclic loading was deduced based on J-C constitutive model.(3)The numerical analysis of residual stress field and fatigue life of welded joint subjected to warm laser peening.The finite element analysis software ABAQUS was used to establish the finite element model of welded joint(base material is AA6061-T6,welding wire is ER4043),Gaussian heat source was used to simulate TIG welding process,the temperature and stress fields of welded joint were studied in the process of TIG welding,the influence rules of thermal incubation effect on temperature and stress fields of welded metal and heat affected zone were explored.On the basis of welding stress field,the analytical method of residual stress of welded joint surface was established by analysis step of thermo-mechanical coupling effect,the influence rules of peening temperature on residual stress induced by WLP and the improvement mechanism of effect of WLP on stress fields of welded joint were analyzed.On the basis of that,the optimal temperature of aluminium alloy welded joint subjected to warm laser peening was selected.Finally,the fatigue life analysis of aluminium alloy welded joint subjected to warm laser peening was carried out with the combination of ABAQUS and MSC.Fatigue software,life extension of fatigue effect of WLP on welded joint was verified compared with the as-machined and welded joint subjected to warm laser peening,and the life extension of fatigue mechanism of WLP was analyzed from a perspective of stress strengthening.(4)Surface roughness and microhardness of welded joint subjected to warm laser peening.The surface roughness and microhardness which were the measure of changes of material macrostructure and microstructure were measured,the influence rules of effect of WLP on surface roughness and microhardness of welded joint were studied.The effect of WLP on work-hardening of materials was studied with the combination of influence rules of effect of WLP on surface roughness and microhardness and microstructure of welded joint subjected to warm laser peening,and the hardening mechanism of welded joint subjected to warm laser peening was concluded.(5)The life extension of fatigue and fracture mechanism of welded joint subjected to warm laser peening.The fatigue life of welded joint subjected to laser peening was tested under the conditions of low cycle and high cycle,the influence rules of effect of peening energy,peening area and peening times on fatigue life were studied,and the best combination of process parameters was selected.The fatigue fracture morphology under the conditions of low cycle and high cycle was observed by the method of SEM,and the fracture process of joint subjected to warm laser peening was deduced,the influence rules of effect of WLP on crack initiation,propagation rate of fatigue cracks and fracture toughness in the fracture process.The estimation formula of fatigue crack propagation rate and fatigue life of welded joint subjected to warm laser peening was deduced based on fatigue and fracture mechanics as well as the microstructure and residual stress field of welded joint subjected to warm laser peening.In this paper,AA6061-T6 welded joints are served as the research objects,the microstructure and residual stress of surface of welded joint subjected to warm laser peening and fatigue life of welded specimens were mainly studied,the surface strengthening mechanism and the life prolonging of fatigue mechanism of welded joint subjected to warm laser peening were explored.The results show that WLP can promote the effect of dislocation strengthening and grain refinement in the process of plastic deformation through dynamic strain aging and dynamic precipitation,and the better microstructure can be obtained compared with traditional laser peening technology.At the same time,WLP can improve the residual compressive stress amplitude and its stability of welded joint surface by improving microstructure,and the fatigue life of welded joint can be improved dramatically. |
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