Research On Surface Strengthening And Fatigue Failure Mechanism Of Magnesium Alloy With Gradient Nanostructure

At present,with the rapid development in the fields of aerospace,high-speed trains,and aerial weapons,increasingly stringent requirements have been put on key components in terms of lightweight,environmental protection,and long life.Magnesium alloy instead of aluminum alloy becomes an inescapable trend of development.However,the safe use of magnesium alloy structural parts has become a key factor restricting its wide application.In this paper,surface hardening treatment of AZ31 B magnesium alloys was performed using ultrasonic impacting and rolling technology,and the study also focused on the key scientific issues such as surface enhancement mechanism,micro-scale mechanical behavior and fatigue failure mechanism of magnesium alloy.(1)The surface strengthening mechanism of magnesium alloy after ultrasonic impacting and rolling treatment was studied in detail.The results show that twin deformation was the dominant deformation mechanism in the primary stage of deformation.As the strain increases,a large number of slip systems were activated,and dislocations interacted with twins.Simultaneously,dynamic recrystallization also would be induced.The coordinating effects of diverse refinement mechanisms finally achieved the nano-crystallization of the surface of the material.The grain size of the deformed layer of as-extruded AZ31 B magnesium alloy reached a minimum of 17.26 nm,indicating that the grain refinement effect was very significant.(2)According to the elasto-plastic power function constitutive model,the micro-scale mechanical behaviors of magnesium alloys were analyzed by finite element numerical simulation analysis methods.The elasto-plastic mechanical properties parameters and elasto-plastic constitutive relation of each gradient layer were obtained by this method.Meanwhile,the stress-strain curves were predicted.Finally,the mathematical relationship between grain size and mechanical properties of as-extruded AZ31 B was obtained.(3)The fatigue fracture mechanisms of magnesium alloy before and after the ultrasonic impacting and rolling treatment were studied.The results showed that the fatigue strength of magnesium alloy after surface hardening was increased from 140 MPa to 180 MPa,which was increased by 28.6%.The fatigue fracture analysis showed that the crack source position of the sample before and after the treatment was transferred from the surface to the subsurface,which indicated that the surface stress state of the material was improved after treatment,which inhibited the occurrence of cracks.The influence of the surface roughness,residual compressive stress,grain refinement and strain hardening on the fatigue performance of magnesium alloys were analyzed,which provided a theoretical basis for improving the service life of magnesium alloys.