| 2A97 alloy is a new type of aluminum alloy independently developed by China.It has low density,high strength,and high degree of alloying,which can reduce the weight of the structure and increase the rigidity of the structure.Therefore,it has been widely used in the aerospace field.Lugs is regarded as an important connecting element on the aircraft structure.There is a stress concentration area near the hole of the lugs,and it is easy to break under the action of cyclic load during service.In this paper,in order to improve the fatigue performance of the lugs,2A97-T84 alloy lugs of different sizes and shapes are cold-expanded,and the fatigue behavior of the lugs before and after strengthening has been studied.The microstructure of the lugs samples before and after cold-expansion were compared,the residual stress near the sample hole of the lugs was tested before and after cold-expansion,the residual stress field near the lugs was simulated by Abaqus finite element analysis software,and the fracture morphology produced by fatigue fracture was observed.The effect of cold-expansion on the fatigue properties of 2A97-T84 alloy lugs was investigated,and the fatigue fracture mechanism of 2A97-T84 alloy lugs was analyzed,then the mechanism of fatigue life gain of cold-expansion was discussed.The main research results are as follows.The fatigue life of 2A97-T84 alloy lugs is effectively improved by cold-expansion.The median fatigue life of small-sized straight lugs has increased by 81.5%;the median fatigue life of large-size straight lugs has increased by 65.2%;the median fatigue life of small-sized oblique lugs has increased by 77.5%;and the median fatigue life of large-size oblique lugs has increased by 71.9%.When the cyclic load is higher than 100 MPa,in the state without cold-expansion,the median fatigue life of small-sized straight lugs and oblique lugs is about 6 times and 4.8 times that of big-sized straight lugs and oblique lugs.In the state with cold-expansion,the median fatigue life of small-sized straight lugs and oblique lugs is about 7.4 times and 4.3 times of large-size straight lugs and oblique lugs.When the cyclic load is less than 100 MPa,the median fatigue life of big-sized lugs is much greater than that of the small-sized lugs.The fatigue crack of the 2A97-T84 alloy lug specimen without cold-expanded originates from the stress concentration on the inner wall of the hole and has multiple crack sources.The fatigue crack of the 2A97-T84 alloy lug specimen with cold-expansion originates from the secondary surface on the inner wall of the hole,and the fatigue source is single.Within the grain,the crack is propagated along the favorable slip plane.When there is an angle between the sliding surfaces of the two grains,the crack propagation path is deflected.When the slip surfaces of the two grains are consistent,the crack is not deflected.Instantaneous fracture zone is characterized by layered cracking and cracking along sub-crystal.Fatigue performances is improved by cold-expansion,which is related to the change of the microstructure and residual stress distribution in the hole edge of 2A97-T84 alloy lugs.On the one hand,the dislocation density in the microstructure is increased through cold expansion.The dislocation cell structure formed by tangling and cutting during dislocation movement,which pinned the dislocation during the fatigue crack propagation process.On the other hand,the peak tensile stress under alternating load is reduced by the residual compressive stress layer formed during cold-expansion,then the initiation time of fatigue cracks is extended,and the crack growth rate is slowed.Finally,the fatigue life of 2A97-T84 aluminum alloy lugs is enhanced.Abaqus finite element analysis results show that cold-expansion introduces a residual compressive stress field around the hole of the lugs,and the distribution range of residual compressive stress at the extrusion-end is larger than that of the crowding-in. |
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