| Friction stir welding(FSW),as a solid-state welding technology,avoids the defects caused by other welding technology due to solidification and has a great advantage in aerospace,shipbuilding and other fields,especially in the connection of high strength aluminum alloy.7075 aluminum alloy is a kind of hard aluminum alloy material which can be strengthened by heat treatment.It has excellent comprehensive properties and typical high strength.Fatigue damage is the main reason for failure of mechanical parts.For FSW structure,the joint is often the most vulnerable part to fail under complex fatigue load.Therefore,it is of great significance to study the fatigue performance and life prediction method under variable amplitude loading for aluminum alloy stir welded joints.The fatigue performance of 7075 FSW joints under variable amplitude cyclic block loading is systematically studied by combining experiment with theory using high strength aluminum alloy 7075 as the research object.A fatigue life prediction method based on grain size,hardness distribution is proposed considering the load order effect of FSW joints.The main research work is as follows:Firstly,the fatigue properties and fracture locations of the alloy 7075 stir welded joints were obtained under varying amplitude cyclic block loading under high and low cycle sequences.The fatigue life under low and high cycle loading is longer than that under high and low cycle loading.The number of cycles of load block has a certain influence on the fatigue life of the specimen.With the increase of the number of load blocks,the fatigue life of the specimen shows a downward trend.Secondly,the cyclic block load interrupted fatigue experiment under low and high and low cycle load sequence was carried out for 7075 alloy stir welding joint.It is found that the reason why the fatigue life under variable amplitude cyclic block loading is higher than that under variable amplitude multi-stage block loading is that under variable amplitude cyclic block loading,the vulnerable damaged area of the specimen is constantly changing,which makes multiple cracks showing the characteristics of continuous transformation and propagation.It is found that the crack growth rate under high and low cyclic block loading is higher than that under low and high cyclic block loading,which results in that the fatigue life under low and high cyclic block loading being higher than that under high and low cyclic block loading.Thirdly,it is found that the weak area of the joint model will change with the change of the load level,and the position of the weak area under the load level is taken as the failure position of the main crack through the finite element simulation method.The crack propagation characteristics of 7075-T651 aluminum alloy friction stir welded joints under high and low cycle loading sequences are simulated.It is found that the crack length in the nugget zone is longer than that in the base metal zone under high and low cycle loading.The crack in the base metal zone stops propagating before the crack length in the nugget zone reaching a certain length.Under the low and high cycle loading conditions,the two cracks that originated in the base metal zone and the nugget zone propagate together until the end.And the crack length in the base metal zone is greater than that in the nugget zone.The main crack length under low and high cycle loading is longer than that under high and low cycle loading,which is consistent with the results of interrupted fatigue test.Finally,a fatigue life prediction method based on grain size,hardness distribution was proposed considering the load order effect of stir welded joints by studying the law of crack initiation and propagation under variable amplitude cyclic block loading.The experimental results show that the prediction results of the model are in good agreement with the experimental life and the error of predicted lives is within a factor of 2. |
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