The Study Of Friction Stir Processing On Microstructural Evolution And Strengthening And Toughening Of Mg-Zn-Y-Zr Alloy

Compared with the traditional Mg alloys,the Mg-RE alloys possess some advantages of high specific strength,high temperature resistance and high corrosion resistance,which are beneficial to the wide application in the field of light weight.The main fabrication method of Mg-RE alloys is casting at present.One of the tasks to promote the use of Mg alloys is the improvement of their strength and plasticity.As a kind of severe plastic deformation method,friction stir processing(FSP)has the advantages of simple operation,high efficiency and environment protection,which can realize the local or overall microstructural modification and improve the mechanical properties.In this study,Mg-Zn-Y-Zr alloys were subjected to FSP for the fabrication of high-performance Mg alloys.The influences of FSP parameters and initial states of Mg alloys on macro and microstructure,mechanical properties and fracture behaviors were investigated.The influence of artificial aging on microstructure and mechanical properties were analyzed.The strengthening and toughening mechanisms of the processing of FSP on Mg alloys were revealed and the fabrication of high-performance Mg alloys were achieved.As-cast Mg-Zn-Y-Zr alloy was firstly subjected to FSP and the influences of the coupling of thermal and mechanical effects on microstructural modification and mechanical improvement were investigated.After FSP,the stir zone could be divided into two parts.The upper part of the stir zone was called fiber zone,which contained fibrous grains.The middle and bottom part of the stir zone was called fine grain zone,which contained fine equiaxed grains.With the increase of the traveling speed,the area of the fine grain zone was gradually reduced with a decreasing slope.After FSP,the microstructure of as-cast Mg alloy was remarkably refined.The coarse grains underwent severe dynamic recrystallization(DRX)and grain size was refined to several microns.The coarse intergranular eutectic phase was broken into fine dispersed particles.Under the severe coupling of thermal and mechanical effects,the element diffusion rate was greatly improved,which promoted the transformation from I-phase(Mg3Zn6Y)to W-phase(Mg3Zn3Y2).The particle consisted of residual I-phase in the core part and W-phase in the edge part,forming the core-shell structure.The severe plastic deformation during FSP generated the annular basal texture in fine grain zone,in which basal planes of Mg grains were parallel to the pin surface.The microstructures of fine grains,dispersed particles and annular base texture were beneficial to the improvement of both strength and plasticity.With the increase of traveling speed,the grain size in the fine grain area decreased,the content of the dispersed particles increased,and the overall mechanical properties were improved.Under the FSP parameter of 800 rpm-200 mm/min,the yield strength,tensile strength and elongation were 171 MPa,300 MPa and 27%,respectively,which were increased by 92%,53% and 145% compared with the as-cast base metal.The microstructural evolution during FSP of as-cast and solid solution Mg alloys were studied by characterizing the microstructural characteristics of different parts in thermal mechanical affected zone.With the increase of the coupling of thermal and mechanical effects,twin dynamic recrystallization(TDRX),particle simulated nucleation(PSN),continuous and discontinuous dynamic recrystallization(CDRX and DDRX)occurred,which led to the refinement of coarse grains.Because of the low content of the second phase in the solid solution Mg alloy and the solute drag effect induced by large number of solid solution atoms,the extent of DRX was reduced.It made the average grain size in the fine grain zone of solid solution alloy larger than that of as-cast alloy.As a result,the strength of fine grain zone material in solid solution alloy was lower than that of as-cast alloy.For the materials in the fine grain zones of both as-cast and solid solution alloys under the FSP parameter of 800 rpm-200 mm/min,peak aging time were 8 h and 10 h under 250 ?,respectively.Due to the large number of thermal stable W-phase dispersed particles and precipitated nano particles along grain boundary,the growth of DRXed grains was hindered,which maintained the micron-level fine grains in the peak aging state.Rod-like ?1' precipitates were generated under peak aging state,which further improve the strength of FSP materials and maintain a relatively high plasticity.The yield strength,tensile strength and elongation of the materials in the fine grain zone of as-cast alloy were 201 MPa,330 MPa and 24%,respectively,which were increased by 126%,68% and 118% compared with the as-cast base metal,achieving the fabrication of Mg alloy with both high strength and high toughness.