Effect Of Cold-expansion On Fatigue Behavior Of Domestic 7050 Alloy Lugs

7050 aluminium alloy is widely used as structural parts in aviation industry because of its high strength,good plasticity and excellent fatigue property.7050 alloy lugs is an important connecting element of aircraft structure and an important link in the structural force transmission channel.The fastening holes in the lugs are typical structural details of stress concentration,and fatigue cracks are prone to occur under fatigue loads.Therefore,it is of great significance to take some measures to reduce the stress concentration at the hole of lugs and improve the fatigue performance for ensuring flight safety and prolonging the service life of the aircraft.In order to improve the fatigue performance of the fastening holes,the cold-expansion was carried out for different sizes of lugs to change the stress distribution and microstructure of lugs.Then the fatigue test was carried out on the specimens,and the fatigue fracture properties of the specimens were explored.The influence of microstructure on the fatigue properties of lugs was also analyzed.The finite element analysis of the hole extrusion process was carried out,and the effect of residual stress on the fatigue properties of the lugs was discussed.The main results are as follows:Cold-expansion can improve the fatigue performance of lugs,especially for small lugs.When the peak stress is 45 MPa,the fatigue life of the small lugs without cold-expansion reach 10~6.When the peak stress is 55 MPa,the fatigue life of small lugs with cold-expansion reach 10~6.The fatigue life of large lugs strengthened distinctly when the peak stress is 55 MPa,and the fatigue life increases from 198027 to 334500.Without cold-expansion,the low cycle fatigue performance of small lugs are better,while the high cycle fatigue performance of large lugs are better.After cold-expansion,the fatigue performance of small lugs are improved to a certain extent.In low-cycle fatigue area,the fatigue performance of small lugs are better than that of large lugs;in high-cycle fatigue area,the fatigue performance of small lugs are similar to that of large lugs.In low cycle fatigue cycle stage,the fatigue load is larger,more defects and increasing stress concentration factor are the main factors affecting the low cycle fatigue performance of big lugs.In high cycle fatigue stage,the fatigue load is small,the distance between the inner wall of the hole and the outer surface of lugs is 20 mm,and that of small lugs is only 5 mm.When fracture occurs,the crack of big lugs have to go through a longer journey,so the high cycle fatigue performance of the large lugs is better.The fatigue crack of the specimen without cold-expansion mainly originate at the surface of the hole,and there are many fatigue sources.While the fatigue sources of specimen with cold-expansion are mainly near the inner surface of the hole wall.The average width of fatigue striation of small lugs without cold-expansion is 16.36?m,and that of small lugs with cold-expansion become 7.7?m.The average width of fatigue striation of big lugs without cold-expansion is 7.28?m,and that of big lugs with cold-expansion become 4.8?m.The above phenomena show that cold-expansion can reduce the stress concentration factor on the surface of the hole,and change the crack source location.The crack growth life can be increased by cold-expansion.Abaqus finite element analysis shows that:after cold-expansion,the residual compressive stress distribution of small lugs is larger than that of big lugs;the residual compressive stress of the extruded rod surface of two-size lugs is larger that of the squeeze into surface of two-size lugs.