| Magnesium alloy is widely used in aerospace,automobile manufacturing,communication electronics and other fields due to its light weight,greater specific strength than stiffness,good shock absorption and heat dissipation,easy cutting,rich resources and recyclability.Welding problems will inevitably be encountered in the practical application of magnesium alloys.Friction stir welding is a novel and efficient connection technology,which is very suitable for welding magnesium alloys.The main failure mode of welded joints is fatigue fracture,and the resulting damage is often catastrophic.At the same time,with the increasing requirements for high-performance magnesium alloys in engineering applications,it is of great significance to conduct research on the fatigue properties of high-strength magnesium alloys and friction stir welding joints.In this paper,Mg-Zn-Se-Er high-strength magnesium alloy is used as the research object,but the alloy sheet is butt-welded by friction stir welding.By adjusting the rotation speed and welding speed of the stirring head,different friction stir welding heads were obtained and samples were prepared.Ultrasonic impact treatment was performed on the weld zone and the nearby surface of the original welded joint using ultrasonic impact equipment.The analytical method was used to design the type and size of base metal and friction stir welding joint fatigue samples.Each group of fatigue samples was obtained by milling machine processing,wire cutting and sanding with fine sandpaper.The test parameters were reasonably preset,and the ultrasonic fatigue test was carried out on the base metal sample,the original welded joint sample and the joint sample after ultrasonic impact,and the resonance of the sample was found to be good.At the same time,the fatigue life,fatigue limit and failure location of the samples under different stress conditions were recorded.The main research results show that:(1)The fatigue samples of the magnesium alloy base metal and the welded joints will all fail in the range of 10~5?10~9,and there is no traditional fatigue limit.Comparing the S-N curves of each group of samples,it is found that the fatigue properties of the original welded joints are worse than the base material.Ultrasonic impact can improve the fatigue performance of welded joints,and even make the joint fatigue performance close to that of the base material.At the same time,improper impact parameters or impact methods will cause macroscopic cracks on the surface,thereby reducing the fatigue performance of the joint.(2)Fatigue fracture analysis revealed that the fatigue crack sources of the base metal and the original welded samples were basically a single crack source,and a few samples had multiple crack sources.Fatigue crack sources are all generated on the surface of the sample such as corrosion pits,mechanical processing micro marks and other defects.The source of fatigue cracks in the ultrasonic shock state specimen is partly generated on the side of the sample and partly on the surface of the weld.(3)The initial expansion zone of the fatigue fracture of the base metal,the original welded joint,and the impact joint is quasi-cleavage fracture.The rapid expansion zone mainly shows brittle fracture,and the instantaneous fracture zone shows quasi-cleavage fracture.(4)The thickness of the deformed layer is positively correlated with the impact time.The microhardness of the surface layer after 2A high current impact treatment is generally higher than that of 1A small current at the same depth.As the depth increases,the hardness value gradually stabilizes,and eventually gradually approaches the strength of the base material,that is,about 70HV.(5)Extending the impact time or increasing the impact current value within a certain range can increase the residual compressive stress value.By comparison,it is found that the short-term,high-current impact has a more significant effect on the introduction of residual compressive stress.(6)The grain refinement effect of the surface layer of Mg-Zn-Se-Er magnesium alloy joint after ultrasonic shock treatment is obvious.When impacted at a current of 1A,the finest grain is about 150nm;when impacted at a current of 2A,the finest grain is about 100nm.(7)The mechanism of ultrasonic shock on grain refinement:the effect of ultrasonic shock at high frequency and high energy causes the surface of the magnesium alloy weld joint to undergo severe plastic deformation,a large number of dislocation lines are generated inside the grain,and the dislocation lines slip in different directions Entangled with each other,forming a dislocation wall at the grain boundary.With the accumulation of dislocation walls and dislocation tangles,subcrystals and subgrain boundaries gradually formed.The dislocations inside the unstable subcrystals are further transformed into randomly oriented nanocrystals.Under continuous strong ultrasonic impact,the surface grains are refined at the nanoscale. |
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