Effect Of Microstructure Heterogeneity On Very-high-cycle Fatigue Properties Of Friction Stir Welded Aluminum Alloy

Friction stir welding(FSW),a solid state welding technique,is applicable even to non-fusion-weldable aluminium alloys such as precipitation hardening alloys of7xxx series and results in high strength and defect free joints.Therefore,it is widely used in aerospace,rail transportation,new energy and other fields.In this study,self-developed TJU-HJ-I ultrasonic fatigue test equipment was performed on the 7050-T7451 aluminum alloy and FSW joint to investigate the fatigue behavior in the very high cycle fatigue(VHCF)regime.The effect of microstructure heterogeneity on fatigue property of aluminum alloy FSW joint was mainly discussed.It is found that the S–N curves at the VHCF regime for the base material and the FSW joint of 7050 aluminum alloy present multi-stage shapes with the inflection point at about 10~8 cycles where the transition from short life at high stress level to long fatigue life appears as a relatively horizontal plateau,and no classical fatigue limit exists.It can be observed that the fracture positions of the base material specimens are randomly distributed along the gauge.As to the FSW joint,most failure occurs preferably at thermo-mechanically affected zone(TMAZ).Moreover,in the regime of 10~6-5×10~7,fatigue failures of the FSW joints mainly occur at the TMAZ on the advancing side;on the other hand,in fatigue regime between5×10~7 and 10~9cycles,most of failures occur homogeneously on both the advancing side and the retreating site of TMAZ.Digital image correlation(DIC)technique and optical microscope were adopted for the full-field strain measurements during static tensile tests and microstructure observation in order to capture the effects of heterogeneity on strain distribution of FSW joint.It is found that as the loading increases,the strain starts to concentrate on the advancing side and the retreating side of the TMAZ.The difference in strain distribution becomes much more noticeable when the stress is higher.Furthermore,at higher stress,the cyclic strain localization at the TMAZ on the advancing side tends to exceed the PSB threshold,and induces irreversible plastic strain accumulation and final failure initiation;if the stress amplitude decreased below the PSB threshold but above the irreversibility threshold,the cyclic slip retain a small but non-negligible irreversible component which is distributed randomly at the TMAZ on the advancing side and the retreating side.As a result,the fatigue crack originates from specimen surface without preferential site on TMAZ.The two-dimensional hardness contour across the transverse cross section of the FSW joint was obtained and it is found that the hardness distribution is not uniform.Moreover,it is shown that the fatigue failures were mostly initiated at the surface or subsurface of the base material and the FSW joint specimens through fractographic observation by SEM.Fatigue strengths in the VHCF range was predicted by the modified Murakami’s model,and the estimated fatigue strengths of the FSW aluminum alloy joints and base material are compared with stress levels and all the error is less than 10%.