Study On The Effect Of Property For Ultrasonic Impact MB8Cross Welded Joint

Ultrasonic impact treatment (UIT) has obvious effect on the surface of the material. Itcan improve the fatigue, corrosion, wear performance and so on. Magnesium alloy with theunique advantages is researched by many countries. But the study of magnesium alloy weldedjoint fatigue performance is still not too much. So study the property of magnesium alloywelded joint after UIT for its application in the field of structure has practical value.In this paper, MB8magnesium alloy plates with8mm thickness were welded by TIG ofmanual wire. Study the fatigue property of cross welded specimens and after UIT. The S-N ofthe cross welded joints was lg N=15.366.015lgΔσ，the S-N of the cross welded jointsafter UIT was lg N=23.1710.4lgΔσ。Under the condition of2×106, the fatigue strengthof cross joints after UIT was41.88MPa, but the fatigue strength of welded joints was32.07MPa. The fatigue strength of cross joints after UIT increased by30.59%. The fatigue ofMB8cross joints treated by UIT was5~17times than welded joints under the same levelcondition.The welded joints fatigue fracture in weld toe, which is closely related to themicrostructure. The weld toe in the columnar crystal and the coarse grain boundary, this leadsto the region become the weakest part of the welded joint. Under the SEM, the fatigue crackinitiation at the surface, but the surface of the specimens after UIT have residual compressivestress which can reduce the crack rate. The fatigue crack extension has many cleavage steps,and the secondary cracks. The fatigue final rupture has torn edge and a little dimple,especially the specimen after UIT more.UIT can effectively eliminate the bothway residual stress, which greatly improve thestress and the fatigue strength of welded structure. After UIT, the residual stress of weld zoneand weld toe can be reduced obviously. After fatigue fracture, the residual compressive stresswhich has a large release, but the release of the weld toe is bigger than weld zone, release rateof90%or more.UIT can refine MB8magnesium alloy grain. In the initial stage, the dislocation is themain way of deformation. With the increase of the dislocation, it will form dislocation tangles,dislocation wall and dislocation cell. When the UIT time increase, the dislocation annihilation,rearrangement and form the sub-grain. Finally, as the severe plastic deformation, thedislocation annihilation, rearrangement occur in the sub-grain, the sub-grain will be refinedinto nanocrystalline.In this paper, under different UIT time, study the MB8magnesium alloy in the3.5% NaCl solution. The result indicated that the longer the UIT time, the better of the corrosionresistance. After UIT, the surface obviously refined grains, and the bothway residual stress ofsurface decreased. When the UIT time is6min, the residual stress can reach-31MPa.Therefore, the surface corrosion resistance can be improved by UIT.