Study On The Fatigue Property Of MB8Magnesium Alloy Butt Welded Joints By Circulate UIT

Fatigue fracture is the main reason of causing the failure of welded structurewhich account for70%~90%of structure failure. Magnesium alloys have a series ofadvantages which are high strength ratio, high damping and easy cutting performanceand ease recycling, etc. Improve the fatigue strength of magnesium alloy welded jointfatigue and improve their performance has far-reaching significance to improving thereliability of magnesium alloy welded structure and applicability. In this paper, thefatigue properties of welded joints and treated by UIT were compared throughimpacting the toe area of MB8welded joints, and analyze the mechanism of surfacenano-crystallization by ultrasonic impacting MB8welded joint. Experiment resultsshow that:（1）The S-N of The MB8Magnesium butt joints after circulate UIT waslgN=21.13-8.67Δσ,the S-N of the butt welded joints was lgN=28.17-13.82Δσ,the S-Nof the butt joints after UIT was lgN=22.04-9.27Δσ. At the N=2x106cycles, the fatiguestrength of butt welded specimens treated by UIT was49.87MPa, which hadincreased by32%compared with the38.24MPa of butt welded specimens. Thefatigue strength of butt welded specimens treated by circulate UIT was51.33MPa,which had increased by3%compared with the49.87MPa of butt welded specimenstreated by UIT.(2) There were no fatigue striation on the fatigue fracture of the welded jointsand the welded joints treated after UIT by SEM, there were a lot of torn edges, it alsohad cleavage facets, cleavage steps, and a number of secondary cracks were found inthe extension area. It indicated that the technology of UIT couldn’t change the weldedjoints fracture mode, was both brittle cleavage fracture.(3) After UIT, the residual tensile stress in the area of MB8magnesium alloywelded joints was changing into compressive residual stress. The residual stress in welded joint area has a big degree of release after fatigue test.(4) The phenomenon of grain refinement in the region of MB8weld toe isobvious after UIT. With the increase of current by UIT using same time, the grain sizedecreases on the surface of the MB8weld toe.(5) The grain refinement of MB8magnesium alloy by UIT divided into thefollowing processes: there was a serious of plastic deformation in the region of weldtoe at the initial by UIT and the dislocation tangles and dislocation walls wasgradually formed. Then the dislocation and dislocation tangles wall gradually turnedinto the crystal structure of small Angle, the coarse grain split into multiple of theoriginal crystal structure. With the increase of deformation, the dynamicrecrystallization in the grain boundary, crystal and crystal will eventually turn intonano-crystalline.(6) The sample had a layer of plastic deformation layer after UIT and the layer ofgrain has been refined. With the increase of time by UIT, the corrosion resistance andwear resistance and surface hardness of magnesium alloy samples have differentdegrees of increase. When the current is1.2A and the time is6min by UIT, thecorrosion resistance improved49.1%and wear resistance improved52.2%andsurface hardness improved103%.