| Magnesium alloys have being widely used in many fields due to the unique physical and chemical properties,but there are some drawbacks as a result of the low melting point and the relatively more active chemical character in the process of the traditional fusion welding.Friction Stir Welding(FSW),a new kind of solid phase connection technology,leads a new road for the high-quality welding of magnesium alloys.Because of the non-uniform welding heat input during the process of FSW,the different zones of the welding joints exhibit the various deformation characteristics for the unsymmetrical microstructure and mechanical properties.Therefore,the studies on the fatigue fracture characteristics of magnesium alloys in different zones of the FSW joints are of great importance for both the application and scientific study,and the relevant studies can provide technical support and theoretical basis for the further wide application of magnesium alloys.In this paper,the influence of welding heat input on microstructure of AZ31 B magnesium alloy during FSW process was analyzed by infrared thermography technology.The micro-hardness test and EBSD data analysis on the FSW joint of AZ31 B magnesium alloy were carried out.The fatigue crack propagation behaviors in different regions of the FSW joints of AZ31 B magnesium alloy,namely weld nugget(NZ),heat affected zone(HAZ)in advancing side and HAZ in retreating side were studied,and the surface temperature evolution as well as the fracture morphologies during the deformation process were also analyzed.During the FSW process of AZ31 B magnesium alloy,complete dynamic recrystallization occurred in the nugget zone(NZ),the equiaxed fine grains were subsequently formed.Incomplete dynamic recrystallization occurred in thermo-mechanical affected zone(TMAZ)as a result of the slight agitation of the stirring needle,and resulted in the formation of non-uniform grains.HAZ was only affected by the welding temperature field,thus maintaining the same grain structure as the parent metal,but the average grain size of HAZ was slightly increased.The statistical results of EBSD of the FSW joints showed that compared with the grains in HAZ,the grains in NZ have a larger average Schmid factor,indicating that the NZ can be easily deformed under the same loading.The distinction between the Schmid factors of the HAZ in advancing side and the HAZ in retreating side was small.Compared with HAZ in AS and RS specimens under the same cyclic leadings,NZ specimens have the shorter fatigue life,the larger crack growth rate and the highest surface temperature evolution in the process of crack propagation.During the cyclic loading process,the surface temperature evolutions of specimens have corresponding relationships with their crack propagation behaviors.The surface temperature evolution curve can be divided into three stages.The temperature was slowly increased in stage I,namely the stage of slow crack propagation.In stage II,namely the stage of stable crack propagation,the surface temperature evolution reached the dynamic balance.In stage III,namely the stage of rapid crack propagation,the surface temperature was rapidly increased as results of the unstable propagation of the crack and the increased strain at the top of the crack.The fracture morphologies of AZ31 B magnesium alloy and the FSW joints in NZ and HAZ show a similar change rule,namely the transformation from the cleaved fracture to the quasi-cleavage fracture.The fracture morphology of NZ shows greater plasticity,indicating that the NZ material is more easily to be deformed.Besides,more secondary cracks and crack bifurcations were observed in HAZ specimens during the crack propagation process,thus weakening the propagation of the leading cracks,therefore,NZ specimens have the larger crack propagation rate than HAZ specimens. |
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