Very-high-cycle Fatigue Properties Of Friction Stir Welded Aluminum Alloy Under Various Stress Ratios

Friction stir welding(FSW),a solid state welding technique,can be used to weld7 Series high-strength aluminum alloys with poor weld ability.7050 aluminum alloy is often subjected to alternating and complex loading in the actual condition,and the cycles are more than 10~7 times.Therefore,it is very important to study the fatigue behavior of aluminum alloy in the very high cycle fatigue(VHCF)regime under different stress ratios.The very high cycle fatigue tests of 7050 aluminum alloy friction stir welded joint specimens under three different stress ratios and the base metal specimens under stress ratio of-1 were carried out.The results show that the stress ratio has obvious influence on the fatigue strength,but does not change the shape of the S-N curve.The S–N curves always present multi-stage shapes under different stress ratios with the inflection point at about 10~8 cycles.After 10~8 cycles,the fatigue strength of the welded joint decreases significantly.Comparing the fatigue test data of base metal and welded joint at R=-1,it is found that when the surface condition of the joint specimen is good,its fatigue strength reaches the same level as the base metal.The effect of stress ratio on the crack initiation mechanism was clarified.The fracture failure modes can be divided into two types:surface induced fatigue fracture and sub-surface induced fatigue fracture.In the high cycle fatigue regime,the welded joint specimen always fractured from the surface under different stress ratio,and there was no internal initiation phenomenon.In the very high cycle fatigue regime,two kinds of crack initiation modes all appeared under the three differdent stress ratios.In fact,the fracture is the competitive process among surface initiation and internal initiation.When the stress level is high,the fatigue crack is related to the surface damage,and the fatigue fracture is dominated by the persistent slip bands.As the stress level decreases,the formation of surface fatigue cracks is suppressed.Under the loading condition of R=-1,the fatigue crack often initiate at the uneven internal structure.However,under the condition of high stress ratio,the maximum stress causes some inter grain to slip,so that the characteristic of the facet can be observed.Secondly,the distribution feature of fatigue fracture location was analyzed.The average stress increases with the raise of the stress ratio,which leads to the significant irreversible deformation at the advancing side,and the specimen were prone to fracture at the advancing side.The Goodman model,which describes the effect of mean stress,can still well describe the coordination of stress amplitude and mean stress at a certain life under ultrasonic frequency loading.The Goodman curves can be used to predict the fatigue strength under different stress ratios in a certain range.Fatigue strengths at different stress ratios were estimated by the Murakami’s model,and the calculated results are compared with real stress levels and the error is almost less than 10%.