| Magnesium alloys are the lightest structure materials and has been widely applied in many areas,such as aerospace,automotive,computer,electronics,telecommunications and household electrical appliance and other industries,due to their advantages of low density,high specific strength and stiffness.In recent years,the automobile lightweight and environmental protection stimulated the rapid development of magnesium industry.As a structural material,the weldability of magnesium alloys is crucial for its application,however,the conventional fusion weld can't insure excellent mechanical and metallurgical properties of joints.As a solid-state joining method,Friction stir welding can effectively avoid the defects caused by the traditional welding method,which makes the performance of the joint greatly improved and the welding process is more environmentally friendly.Additionally,Friction stir welding is a more environmentally friendly method.In many cases,the structural materials are subjected to variable amplitude cyclic loading throughout their service lives,which resulted in fatigue failure.Additionally,fatigue failure often occurs at low stress level and has no obvious plastic deformation before the final fracture.Once the fracture take place,the consequences are serious.The fatigue process is caused by the accumulation of damage which eventually leads to formation of cracks and their subsequent propagation.When one of the cracks has grown to such an extent that the remaining net-section is insufficient to carry the applied load,a sudden fracture take place.The complex stress environment and the sudden failure behavior bring a great challenge to the study of fatigue fracture behavior.In present work,the fatigue fracture behavior of AZ31 magnesium alloy and its Friction stir welding joints was investigated using acoustic emission and Electron Backscattered Diffraction.The characteristics of acoustic emission signal and microstructure evolution during fatigue of AZ31 magnesium alloy were studied,and the relationship between the micro-size cracks and acoustic emission signals were determined,which provided a theoretical basis for early fatigue failure prediction of materials.At different stress levels,the fatigue limit of the material was rapidly predicted by the change of cumulative energy of acoustic emission signal,and the damage in the fatigue process of magnesium alloy Friction stir welding joints was located by acoustic emission.During the process of fatigue,different types of acoustic emission signals are generated by different types of damage within the material,the waveform and frequency of acoustic emission signals were determined.The waveform and spectrum distribution of acoustic emission signals are analyzed,and the characteristics of acoustic emission signals produced at various stages of the fatigue process are determined.Additionally,the fatigue damages were studied by Electron Backscattered Diffraction.To determine the acoustic emission signals of cracks,tests were finished when the particular acoustic emission signals occurred,the microstructure of the material is studied,and the characteristics of the corresponding acoustic emission signal corresponding to the micro crack are determined.The acoustic emission signals of cracks were determined,and the peak frequency of these burst signals locating around 100-200 Hz.Through the acoustic emission analysis system,the test results show that the cyclic hardening occurred during fatigue which is accompanied by a large number of acoustic emission signals and the plastic deformation.The amplitudes of acoustic emission accumulative energy during strain hardening under different load levels were linear fitted and the curve has obviously different slop when the load is higher and/or lower than the inflection point with abscissa of 97 MPa.Additionally,the fatigue limit determined by traditional is 90.43 MPa,therefore,the fatigue limit can be rapidly determined based on acoustic emission accumulative energy.Fatigue fracture failure of AZ31 magnesium alloy and its Friction stir welding joints was investigated using acoustic emission and the positioning of fatigue damage is monitored during fatigue.The test results show that the fatigue damage appeared in the Advanced Side(AS)heat-affected zone,and then accumulated on both sides heat-affected zone.Additionally,the microstructure of the Friction stir welding joints was investigated by Electron Backscattered Diffraction and the grain size of AS heat-affected zone is larger than Retreating Side(RS).Therefore,the results of the study on the welded joints using acoustic emission technology are in good agreement with the traditional results.In situ monitoring of fatigue damage provides a more direct and accurate way for the study of Friction stir welding joints of magnesium alloy. |
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