Study On The Microstructure And Fatigue Fracture Behaviors Of Aluminum Alloy Friction Stir Welded Joints

Aluminum alloy, one of the lightweight materials, nowdays has been widely used in aerospace manufacturing. Because of its particularity of the products, there are high demands to the fatigue property of the welded joints, Friction stir welding (FSW), a new solid state welding technology, has many significant advantages in welding Al alloy, especially in hard aluminum alloy.Nowdays people have done much research on welding parameters, joints structure and welding defects , but seldom on the fatigue property. In order to popularize FSW technology, it's necessary to study the fatigue property of the welded joints..In this paper, We mainly studied the aluminum alloys of 2024-T4,5A06-H112,6061-T6 and 7075-T6,and by tests we found some information about the fatigue fracture behavior and microstructures of these alloys ,besides we also studied the factors affecting the fatigue behaviors, such as joints types,welding effects,stress ratio and so on. By fatigue test, we find that the fatigue crack source lies in the overlap area where hook flaw always exists for overlap joints, and in the center of the weld root for butt joint. The fatigue performance when stress ratio is 0.5 was better than the one when ratio is 0.1 for all of alloys, and the greater the strength is, the more obvious this effect is; the fatigue strength for the lap joint was 17.94MPa. it was smaller than that of butt joint ,which was 100.63MPa, the fatigue performance of the butt joint was better than that of overlap joint. In the short life range ,the aluminum alloy with good plasticity has better fatigue property, but this effect is not so obvious in the long life range. Welding defects have significant effects on fatigue performance.By metallographic test, it was found that the welded joints for butt and lap ioints all could be divided into four zones, that is ,welded nugget zone,thermal-mechanical affected zone,heat affected zone and base material. Further more, we found that BM is made up of fine equiaxial recrystallized grains, TMAZ contains large reformed grains, and the grains grew up in HAZ. There are welded defects between the sheets of the lap joint, such as"hook flaw"or kissing bond , similarly, the incomplete penetration defects were also discovered in weld root of the butt joint. By Vickers hardness test , we found the hardness distribution of the top sheet of 2024,5A06 and 6061 Al alloy was all"W"-type, the smallest hardness lay in heat affected zone,the bottom sheet didn't have obvious distribution;The hardness distribution of 7075 Al alloy was similar to the top sheet of the overlap joints.By SEM We also found that the fatigue fracture was coarse for 2024 and 7075 aluminum alloy, but smooth for 5A06 and 6061 aluminum alloy; Fatigue crack for all aluminum alloys started from the weld defects where stress intensity existed. Many dimples appeared in instantaneous fracture zone, which means the final fracture mode is plastic fracture.