Mechanical Properties And Very High Cycle Fatigue Behavior Of Aluminum Alloy Friction Stir Welded Joints

With the rapid development of national equipment and engineering,such as super highrise structure building,bridge engineering,high-speed train,etc.,the precision,durability,reliability and safety of components,especially alloy components,are required to be higher in major engineering projects.These requirements make it an urgent problem to study the mechanical properties of alloy components.In the past ten years,China has made great efforts to shift from "made in China" to "made in China intelligently",and has achieved certain results,such as prefabricated construction technology,the development of a new shield machine,the successful launch of a new long march rocket,etc.The former "usable" is now being transformed into "durable".In the application of civil engineering,the emerging engineering materials and structures no longer pursue cheap and fast,instead of good performance and long service life.Alloys are widely used in engineering.Due to its light weight and high strength,it is often used in the production of key building connection joints or building micro-components.During the engineering operation,the alloy joints or micro-components are often subjected to the highfrequency repeated effects of external loads and the alternating effects of environment and climate.Therefore,it is necessary to study the mechanical properties of alloys in engineering applications.In this paper,the full-field and local mechanical properties of friction stir welded joints of 6061 aluminum alloy in fatigue test were analyzed by using digital image correlation technology.The accumulation of plastic strain mainly occurred in the heat affected zone of the welded joints,and became more and more serious with the deepening of the experiment.In the elastic stage,the development of all joints is basically the same,but the yield stage is different.The heat affected zone first enters the yield and assumes most of the plastic deformation of the whole joint,which is directly cause material fatigue failure.Under the action of stress,the plastic deformation in the heat affected zone accumulates continuously,while in other regions having no obvious plastic accumulation even if the loaded stress value exceeds their yield limitation.During the loading process,the fatigue failure occurred in the heat affected zone,the crack initiation started seriously from the plastic accumulation of the joint surface,and the slip bond started and expanded.The very long fatigue life experiment of friction stir welding joint of 2024 aluminum alloy is carried out.The results obtained by the ultrasonic fatigue experiment system showed that friction stir welding,a new type of solid welding process,could effectively improve the fatigue performance of the joints.The initiation of fatigue cracks usually occurs in defects,pores or inclusions.Whether from sub surface of welded joints or on the surface,the main reason is that the defects of its microstructure or the surface defects affected by processing,the environmental factors produce a great stress concentration,which leads to crack initiation and fatigue failure.Thus the thermo-mechanically affected zone and weld defects become the main area of fatigue failure.The fatigue experiment of the friction stir welding of 2024/7075 dissimilar aluminum alloy was also studied.The experiment was carried out in the ultrasonic fatigue system.It is found that the fatigue failure modes of the two fatigue experiments are different.The crack propagation area of the welded joints of dissimilar aluminum alloy is small,while the instantaneous fracture area is large.The side of 2024 aluminum alloy with relatively poor mechanical properties is the main area for fatigue crack initiation,which generally occurs near the heat affected area.Compared with the conclusion of the 2024 aluminum alloy very long fatigue life experiment,the results of this experiment are more similar to the fatigue behavior under high stress,that is,the initiation of fatigue cracks tends to occur in the region with lower comprehensive mechanical properties and the heat affected zone on the side of the 2024 aluminum alloy.This experiment studied the mechanical properties of aluminum alloy as the main structure components in engineering applications,but due to welding ability of aluminum alloy is not strong,and easy to produce the problem such as porosity and thermal crack,lead to welded aluminum alloy has great limitations in practical applications.The poor performance limits the development of aluminum alloy welded joint application.Using this new type of welding,friction stir welding process,can be improved for aluminum alloy welded joint,the study of mechanical performance of aluminum alloy welded joint fatigue performance and long life in the actual engineering application and design of welded structure,and later forecast in such aspects as the service life of the welded joint is of great significance.