Study On Vhcf Properties Of Base Metal And Welded Joints Of MB8 Wrought Magnesium Alloy

With the rapid development of automobile and railway locomotive,long-term service parts need to understand its fatigue performance in very high cycle regime.However,the researches on the fatigue properties of magnesium alloy and its welded joints are mainly focused on low cycle regime and high cycle regime properties,and few studies on the very high cycle fatigue properties of magnesium alloy and its welded joints are involved.Therefore,facing new demand of transportation,it is necessary to carry out a in-depth and detailed study on the fatigue properties of magnesium alloy and its welded joints in very high cycle regime.In this paper,the analytic calculation and finite element analysis of ultrasonic fatigue specimen of MB8 deformed magnesium alloy were carried out,and the basic rule and the optimization method of ultrasonic fatigue specimen were discussed.After that,crystallography observation,static mechanical properties testing,ultrasonic fatigue testing,fatigue crack initiation and propagation observation and stress concentration of finite element simulation were carried out on the MB8 magnesium alloy base metal,welded joints and welded joints without residual high.S-N curve characteristics and fatigue fracture mechanism were discussed.The influence of welding process and stress concentration on very high cycle fatigue properties of MB8 magnesium alloy was analysised.Results show:(1)Basic rule and optimization method of ultrasonic fatigue specimenThe sizes of ultrasonic fatigue specimen of the base material can be calculated by analytical method.The shape of ultrasonic fatigue welded joint specimen is irregular.It was simulated and analyzed by finite element method.The welded joint specimen has the following rules: when residual high in the side face of welded joint specimen,It has no effect on the first-order of natural frequency;when the residual high in the right side of specimens,the first order natural frequency increases non-linearly with the increase of the height of residual high,and the first-order natural frequency increases linearly with the increase of the width of residual high.The effect of residual high on first-order of natural frequency can be reduced by changing the resonant length.The first-order natural frequency decreases linearly with the increase of resonant length.(2)S-N curve characteristics and fatigue fracture mechanism of base metalThe S-N curve of MB8 deformed magnesium alloy shows a continuous downward trend,and there is no fatigue limit in traditional sense.The S-N curve equation of base metal iss-(28)(35)lg06153.045187.2)lg(N.The conditional fatigue limit of it is 104.99 MPa in 1×107cycles.The fatigue ratio of MB8 wrought magnesium alloy rolled sheet is 0.30.Comparedwith other wrought magnesium alloy,MB8 wrought magnesium alloy has lower very high cycle fatigue properties.The fatigue mechanism of MB8 wrought magnesium alloy sheet is stress-corrosion fatigue.The fatigue crack originates from the corrosion pits on the surface of the sample.In the initial expansion region,the effect of corrosion is still evident on the fatigue crack growth.In medium extension area,fracture morphology has trans-granular cleavage.In late expansion area,the fracture morphology is candy like pattern and a small amount of trans-granular cleavage.Fatigue crack extends along grain boundary,and accompanied by a small amount of trans-granular propagation.(3)S-N curve characteristics and fatigue fracture mechanism of welded jointsThe strength of MB8 wrought magnesium base metal and welded seam is lower,while the strength of heat affected zone is higher.The strength of fatigue limit of S-N curve is35.2MPa.In high cycle regime,the S-N curve equation is s-(28)(35)lg1764.08559.2)lg(N.The fatigue strength coefficient of welded joint is 0.398.The fatigue crack initiates in weld toe.In the weld toe,the micro-structure distribution is not uniform,and there has high stress concentration.The formation of fatigue crack is caused by the "squeezing in" and "crowding out" of the slip band.In the initial expansion zone,the fracture morphology was randomly oriented trans-granular cleavage.In transition expansion area,the fracture morphology is candy like pattern and a small amount of trans-granular cleavage.Fatigue crack extend along grain boundary,and accompanied by a small amount of trans-granular propagation.In the coarse grain expansion area,the fracture morphology exists only candy pattern,the fatigue crack propagates along coarse grain boundary of heat affected zone.In columnar crystal extended area,fracture morphology is a bar shaped candy pattern.The fatigue crack propagates along the columnar grain boundary.(4)S-N curve characteristics and fatigue fracture mechanism of welded joints without residual highThe S-N curve of MB8 wrought magnesium alloy without residual high also has the fatigue limit.The fatigue strength value of fatigue limit is 40.6Mpa.It's S-N curve iss-(28)(35)lg1391.07398.2)lg(N.The fatigue strength coefficient is 0.556.The fatigue crack originates in exposed columnar grain after polished.Under alternating stress,fatigue crack is formed by the "squeezing in" and "crowding out" of slip band.Fatigue crack expands along the 45 degrees with principal stress.After extending to the columnar grain boundaries,fatigue crack grows along the columnar crystal boundary.Compared with the welded joint,the morphology of transition zone still has random orientated trans-granular cleavage step,but the coarse grain growth area and columnar crystal extension area is not only along the grain boundaries.A small mount of trans-granular steps are suggested that fatigue crack extend along grain boundary,and accompanied by a small amount of trans-granular propagation in coarse grain extension area and columnar grain extension area.(5)The influence of stress concentration and micro-structure change on very high cycle fatigue propertiesThe main reason for the decrease of fatigue performance of welded joints is the stressconcentration and the weak micro-structure of welded joints.The coefficient of stress concentration at the toe of the welded joint is 1.129.The fatigue strength coefficient changed from 0.398 to 0.556 after the removal of residual high.The fatigue strength is reduced by44.4% due to welding process.And the fatigue strength is decreased by 15.8% for stress concentration.So the decrease of fatigue properties of welded joint mainly results from weak micro-structure.