| Mg alloys with icosahedral quasicrystal(IQC)or long-period stacking ordered(LPSO)strengthening phases have received considerable attention,because of their superior mechanical properties at both the ambient and elevated temperatures.IQCs and LPSO structures are usually quite coarse and distributed mainly at grain boundaries(GBs)in as-cast Mg alloys.Severe plastic deformation(SPD)is an effective way in refining microstructures of metals and alloys.Friction stir welding(FSW)/friction stir processing(FSP)is emerging as a very effective solid-state joining/processing technique,which has been demonstrated in various industries Recently,Mg alloys with IQC or LPSO have been successfully processed or welded by friction stir methods.But,dissimilar FSW of Mg-TM-RE alloys with IQCs and LPSO structures has not been investigated comprehensively to date.In this study,Mg95.8Zn3.6Gd0.6 and Mg97Cu1Y2(at.%)as-cast alloys containing respectively IQC and LPSO strengthening phases were joined by Friction Stir Welding.The microstructural evolution during FSW was investigated in detail using various electron microscopy techniques,with more efforts focused on evolution of IQCs and LPSO structures in the stirred zone(SZ)which experienced complex material flow and mixing as a result of SPD under high strain rates.The mechanical properties of SZ at room temperature were investigated.A FSW joint without macroscopic defects was obtained under proper processing parameters.The welded joint can be obviously divided into stir zone(SZ),thermo-mechanical affected zone(TMAZ)and base materials(BMs),without appearance of any obvious heat affected zone(HAZ)in both BMs.A laminated onion-ring structure composed of alternative distribution of layers with significantly refined microstructures from different alloys was formed in the SZ.During FSW,materials in the SZ experienced complex flow under high strain rates.Significant grain refinement occurred due to the dynamic recrystallization during FSW.Additionally,coarse IQCs were broken up and dispersed with most of them being transformed into cubic W-phase particles,and thick 18R-LPSO plates were fractured and transformed into fine 14H-LPSO lamellae.Fine W-phase particles and 14H-LPSO lamellae formed during dissimilar FSW usually have no specific orientation relationship with surrounding Mg matrix.Chemical measurements demonstrated occurrence of interdiffusion between dissimilar layers in the SZ.Phase transformation was also observed for some particles of IQCs and LPSO in regions slightly outside the SZ.The mechanical properties of SZ of the FSW joint were investigated.The SZ exhibited increased microhardness,compared with the α-Mg matrix in both the as-cast alloys.An ultimate tensile strength of～415.4 MPa and an elongation to failure of～27.8%,both exceeding those of base materials,were obtained in the SZ.The microstructural refinement(grain size refinement,fragment and dispersion of strengthening particles)should play an important role in improvement of the mechanical properties of SZ.In addition,the formation of an onion-ring laminated composite structure with alternative arrangement of layers rich in particles and LPSO lamellae respectively in the SZ,is also conducive to the simultaneous increase in tensile strength and ductility of the present dissimilar FSW SZ. |
