| Owing to its high specific strength,high specific stiffness,low density and good machinability,damping and electromagnetic shielding properties,magnesium alloys have a wide range of applications in aerospace,automobile manufacturing,electronic devices and rail transit.However,due to the dense hexagonal crystal structure,magnesium has the characteristics of low strength,poor plastic deformation ability,high chemical reactivity and easy to be corroded,resulting in poor fatigue and corrosion properties,which restricts the further development of magnesium alloys.In this paper,the effects of ultrasonic impact treatment(UIT)on the microstructure and properties of magnesium alloys were studied from the perspectives of microstructure,residual stress and fatigue properties.Especially in the practical application,it is found that UIT will cause material surface damage and microcracks.This paper innovatively proposes the re-ultrasonic impact treatment(re-UIT)technology,which can eliminate the harmful effects of microcracks that may be induced by UIT.Therefore,this paper carried out fatigue test and immersion corrosion test on untreated samples,UIT samples and re-UIT samples respectively to study the effects of re-UIT on the fatigue properties and corrosion properties of magnesium alloys,the following is a summary of the results:After UIT,the morphology of the weld toe of AZ31B magnesium alloy welded joint is improved,presenting a smooth transition,the stress concentration is reduced,and the surface microstructure is refined.By ABAQUS finite element simulation,it is found that the stress state of weld toe changes from tensile stress to compressive stress.The fatigue test results show that the fatigue life of AZ31B magnesium alloy welded joint after UIT is about 1.6-4.7 times than that of untreated joint.At 2×106cycles,the fatigue strength is increased by 28%.After re-UIT,the high roughness on the surface and microcracks induced by the UIT were eliminated,the fatigue performance was further improved.Fatigue test were carried out on the untreated samples,UIT samples,and re-UIT samples.The results show that the fatigue life of UIT samples of AZ31B magnesium alloy base material is about 2.1-2.3 times of the untreated samples,the fatigue strgength is increased by9.4%under 2 x 106cycles;the fatigue life of the re-UIT samples is about 19-26 times that of the untreated sample.At 2×106cycles,the fatigue strength of the untreated base metal sample can be increased by 31.3%,indicating that the fatigue performance of the sample after re-UIT is significantly improved.By analyzing the corrosion test results,it is clear that the UIT sample has the highest corrosion rate,yet the re-UIT sample's corrosion rate is the lowest.The reason for the deterioration of corrosion resistance of UIT dues to the influence of larger surface roughness and surface microcracks,which made the corrosion reaction easily occurs at the surface undulation and the initiation of microcracks.The improvement of corrosion resistance by re-UIT is attributed to the elimination of microcracks and the finer grain.The mathematical analysis results show that the first-order exponential function modely y=y1+Aex/t can better fit and predict the dynamic variation of corrosion pit diameter/depth distribution of AZ31B magnesium alloy. |
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