The Fatigue And Fracture Behavior Of Friction Stir Crack Repaired Aluminum Alloy

In engineering structures and mechanical equipment,the phenomenon of fatigue damage spread all over the motion of parts.As the extension of service time,the micro-crack would present on the fractured surface,and the macroscopic crack,which derived the structural failure,had been developed eventually.There were various methods of crack repair,however,the results were not always satisfactory.A new approach to repair the crack was initially put forward,namely friction stir crack repaired.The experiment of friction stir repaired was conducted on pre–cracked 2A12 aluminum alloy plate,the process range of FSCR was obtained by theory analysis be combined with experimental observation.First of all,a reasonable interval was established through the repaired process tests,repaired aluminum alloy plates were introduced in the form of fatigue specimen and crack propagation specimen and these specimens were determined by the measurement of mechanical properties.OM(optical microscope)and SEM(scanning electron microscope)were applied to analysis metallographic and fracture surface morphology to repaired zone.Different process parameters including the fatigue life and crack propagation rate were measured by the fatigue testing machine.The results indicated that the optimized processing parameters such as the rate ?=70~110 mm/min and the corresponding ?=700~1100 rpm.With the optimized parameters,the fatigue life of S1100-V110 and S700-V110 could reach a high value of 44800 and 632713,respectively,while the fatigue life of pre–cracked samples were only 5147.Secondly,The heat treatment was applied to the repaired samples with the optimized processing parameters.The mechanical properties of the original repaired samples and these with the heat treatment were measured and compared.The obtained results of fatigue life and crack propagation tests showed that the fatigue life of repaired samples is 632713,which is 36.98% of while the fatigue life of these with the heat treatment could be dramatically increased to 853356 and the corresponding is 49.89%.Moreover,no obvious fatigue striation could be observed in the fracture morphology of the repaired samples,which could be ascribed to the destroyed grains induced by the friction stir.However,a quantity of fatigue striation occurred in the repaired samples with the heat treatment,which is presumably as a consequence of the solid solution of grains,second phase particles and inclusions during the heat treatment.It is also noted that the dimensions of the dimples in final rupture regions were also increased after the heat treatment.Last,the effect of different crack repair locations(i.e.in the advancing side,retreating side and the middle)on the fatigue life of the samples was also investigated.It is found that the fatigue life of the samples with crack repair locations in the advancing side,retreating side and the middle are 14928,153927 and 632713,respectively.Furthermore,in the case of samples with crack in the advancing side of the stir pin,due to the limited disruptive force,uncompleted destroyed grains formed and recrystallized in the deep zone,resulting in a flocculent morphology while the grains in the surface zone can be destroyed completed and recrystallized.Thus,double layer repaired regions could be observed in the fracture of these samples.Meanwhile,the fracture morphology of samples with crack in the retreating side and the middle samples are similar and no double layer repair regions could be observed.