Effect Of Friction Stir Processing On The Long Period Ordered Stacking Structure And Mechaniical Properties Of Mg-Gd-Zn-Zr Alloy

Mg-Gd-Zn-Zr alloys with long period stacking ordered (LPSO)structure were subjected to a new porcessing—friction stir processing(FSP). The casting, extruded and soluted Mg-Gd-Zn-Zr alloys processedby FSP were investegated.The results indicate that different FSP parameters on as-castMg-Gd-Zn-Zr alloy result in different processing quality. The size ofwormhole decreases with the increment of travel speed of stir tool with therotation speed of600rpm. Defect-free microstructre can be achieved withthe travel speed of180mm/min. Grain was greatly refined (from54μm to4μm) and mechanical properties were enhanced remarkbaly with theparamerter of600rpm-180mm/min. However, non-uniformmicrostructure was achieved in the processing zone. It was characterisedby two different zones: shoulder-driving zone and pin-driving zone. Thegrain size was finer and yield strength (YS) was higher in theshoulder-driving zone while the elongation was lower than that in theshoulder zone. Moreover, some difference in the misorientation can beobserved in the two zones. Texture in the shoulder-driving zone is strongerand the c axes have a certain angle with the tensile direction while texturein the pin-driving zone is weaker and c axes are parallel to the tensiledirection. There may be two ways to recrystallize for the grains with LPSOstructure. The first one is that the recrystallized grains nucleate in theregion with less or without LPSO structure and grow subsequently. Theanother one is that kinking band was formed in the grain with large density of LPSO structure and the recrystallization nucleate in the kinking band. Itis indicated that the recrystallization is impeded by LPSO structure.The grain size was dramatically refined after FSP to the extruedMg-Gd-Zn-Zr alloy, decreasing from23μm to1-2μm. Meanwhile, the Xphase with LPSO structure was also broken into fine particle and wasdistributed uniformly in the stir zone (SZ). Thus, the mechanical propertieswere remarkably enhanced after FSP. Different processing parameter canalso lead to different processing quality. The size of tunnel defectdecreased with the increasing of travel speed or increasing of rotatingspeed. Moreover, the inceasing of processing passes can obtain moreuniform and finer microstructure.The coarse X phase at the grain boundary were broken remarkablyafter FSP on the soluted Mg-Gd-Zn-Zr alloy. A large number of X phasewas transformed into small second particles which were uniformlydistributed due to the local high temperature and high strain in the SZ. Theresearch results indicated that the X phase would be transfromed into othersecond phase with both high heat input and low heat input. And the secondphase would changed into dipersed rod-like X phase durign the subsequentheat treatment which indicates that the phase transformation is reversible.