Study On The Fatigue Property Of MB8Magnesium Alloy Butt Weld By Ultrasound Impacting

Because of Magnesium alloys with a lot of advantages, such as high-strength ratio,high damping, rich mineral and easily recycled, it is called as the green structure materials ofthe21st century. But how do the magnesium alloy using in large and complex structures, thewelding technology is one processing technology of the most practical and the most importantmethods. However, the datas show that70%~90%of the weld structure’s failure caused bythe fatigue fracture of welded joints. So it is great significance of improving the fatigueproperties of welded joint in the application of magnesium alloys. In this paper, the fatigueproperties of weld joints and treated by ultrasonic impacting were compared throughimpacting the toe area of MB8welded joints, and analyze the mechanism of surfacenanocrystallization by ultrasonic impacting MB8welded joint. Experiment results show that:（1） The weld toe of MB8Magnesium alloy butt joints was treated by the ultrasonicimpacting, and the fatigue property of welded specimens and after ultrasonic impactingwelded specimens were studied. After statistical processing, the S-N of The MB8Magnesiumbase metal was lg N=48.822.7lgΔσ; the S-N of the butt welded joints waslgN=30.315.38lgΔσ; the S-N of the butt joints after ultrasonic impacting waslgN=31.414.28lgΔσ.（2） At the N=2x106cycles, The fatigue strength of the MB8Magnesium joints was74.13MPa, the fatigue strength of butt welded specimens treated by ultrasonic impacting was56.2MPa, which had increased by54.8%compared with the36.3MPa of butt weldedspecimens. It would generate the residual compressive stress improving the fatigue life afterultrasonic impacting in the weld toe. And the transition radius between the weld and basemetal were increased, making concentration of the stress reduced; Meanwhile the surfacedefects of welded joints were eliminated.（3） The fatigue failure of MB8magnesium base metal was brittle cleavage fracture bySEM. There were no fatigue striation on the the fatigue fracture of the weld joints and theweld joints treated by the ultrasonic impacting, but there were a lot of torn edges, it also hadcleavage facets, cleavage steps and few river pattern fracture. It indicated that the technologyof UIT could’t change the welded joints fracture mode, both was brittle cleavage fracture.（4）The weld toe of MB8Magnesium weld joints showed tensile stress，the value gotto σ x=191MPa， σ y=51MPa，Residual compressive stress formed in the weld toe by UIT,At the same time, the residual compressive stress would increased as the current increased, when the current was the1.5A and impacting time was15min, the residual compressive stresswould get to σ x=-47MPa, σ y=-72MPa.（5)Refined layer of the grains on the surface would be get after treated by ultrasoundimpacting.When the process parameters were1.0/10min,1.2/15min,1.5A/15min,2.0A/15min,the thickness of refined layer of the grains reached to10μm,30μm,60μm,60μm. ThroughTEM observation，grains of welded toe area all refined under four set impacting parameters,some even reached the nano level. when the impacting current was2.0A and the time was15min, the grains were refined to about100nm.（6）After treated by ultrasound impacting, the mechanical of surface nanocrystallizationfor MB8magnesium were: firstly, dislocation entanglement and wall formed in the coarse;secondly, as the internal stress and strain of the grain increasing, then the original coarsegrains were divided into sub grains; Thirdly, the dynamic recrystallization occurred byincreasing the strain and the temperature, the nano-grains formed with homogeneousdistribution and random orientation.