Research On The Regularity Of Friction Stir Welded Joint Damage Accumulation

Frcition Stir Welding (FSW) as a solid state welding technology, has behaved many significant advantages in welding field of Al alloys. In order to improve the fatigue performance of welded joint, that researching the influence of the parent metal, joints types, welding flaws, joints structure and fatigue load distribution on the fatigue strength and life of fatigue testing specimens can provide important reference to friction stir welding technology for the promotion and application and build a reasonable fatigue assessment norm.5083 and 6061 aluminium alloy friction stir welded butt, lap joints and parent metal fatigue specimen was tested with Amsler 250 HFP 5100 high frequency fatigue-testing machine,in this paper. The p-S-N curve of the welded joint with confidence r=90%, survival rate p=99%,90% and 50% was figured out by single point method, up-down method and group method, on the basis of reliable theories.By fatigue test, we find that butt joint fatigue performance was smaller than that of butt 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 zones 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. Penetration defects were also discovered in weld root of the butt joint. By SEM we also found that the fatigue fracture was smooth for 5083 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.Based on damage curve method, an nonlinear fatigue cumulative damage model is put forward in this paper. According to three steps fatigue test of butt joint the aluminum alloys of 5083, this model is verified and the error is less, Therefore, the model has better prediction precision and practicality value.