Effect Of The Refinement Of Al₄Sr Phase And Co-modification Of Sr+Bi Combined With T6 Treatment On The Microstructure And Mechanical Properties Of Mg₂Si/Al Composite

Mg₂Si has the advantages of low density,high melting point,low coefficient of thermal expansion and high modulus,which makes it an ideal intermetallic reinforcement.The in-situ Mg₂Si/Al composite has a broad application prospect in the realization of material's lightweight high strength,stable service under high temperature conditions and the preparation of wear-resistant materials.However,the primary Mg₂Si usually shows a coarse dendritic morphology while eutectic Mg₂Si is Chinese-script morphology,and those morphologies are easy to dissever the matrix and generate severe stress concentration,leading to damage the mechanical properties.Modification treatment is a simple and effective method to change the morphology,size and distribution of Mg₂Si particles.The Sr element can change the morphology of primary Mg₂Si from dendritic to regular hexahedron,but its refinement effect is not significant,namely,the particle size is large and there are obvious sharp edges,so the Sr-modified Mg₂Si/Al composite has no remarkable improvement in mechanical properties.Therefore,it can be predicted that the spheroidization and refinement of primary Mg₂Si will greatly improve the mechanical properties of Mg₂Si/Al composite.Thus,firstly,based on the structural heredity effect of Al-10wt.%Sr modifier,this work improved the refinement effect of Sr on primary Mg₂Si by deforming Al-10wt.%Sr master alloy.And the effect of different sizes of Al₄Sr phase on the microstructure and mechanical properties of Mg₂Si/Al alloy before and after T6 heat treatment were also studied.Then,as an element with strong refinement ability,the Bi element was introduced into the Sr-modified Al-Mg₂Si system to further reduce the size of primary Mg₂Si and the morphology of primary Mg₂Si was changed to truncated octahedron.Taking this morphology as a precursor,the spheroidization of primary Mg₂Si was realized by combining with T6 heat treatment.Thus,this work explored a new method to prepare the high-performance spherical Mg₂Si reinforced Mg₂Si/Al composite.The main research results of this paper were as follows:?1?After rolling and hot extrusion of the as-cast Al-10wt.%Sr alloy,the Al₄Sr phase in the alloy became finer,which could improve its modification effect.With the decrease of the size of Al₄Sr phase,the average size of primary Mg₂Si decreased from 49.3?m modified by as-cast 0.1 wt.%Sr to 23.1?m modified by extruded 0.1 wt.%Sr.At the same time,the fine Al₄Sr phase was beneficial to the formation of highly bent and branched fine fibrous or point-like eutectic Mg₂Si,which promoted to obtain the ideal spherical eutectic Mg₂Si particles after T6 heat treatment.?2?The reduction of Al₄Sr phase size would further improve the mechanical properties of the alloy.The tensile strength,fracture elongation and brinell hardness of T6-treated Al-10wt.%Mg₂Si alloy modified by extruded Sr were 293 MPa,4.45%and 116 HB,respectively,which were 19.1%,38.1%and 21.5%higher than those of the T6-treated alloy modified by as-cast Sr.The tensile strength,fracture elongation and brinell hardness of the T6-treated Al-15wt.%Mg₂Si alloy modified by extruded Sr were 289 MPa,2.36%and 133HB,respectively,which were 22.9%,38%and 25.4%higher than those of the T6-treated alloy modified by as-cast Sr.?3?Bi could further enhance the refinement effect of Sr on the primary Mg₂Si and lead the morphology transformation of the primary Mg₂Si.After the addition of 1.0 wt.%Bi to Al-15wt.%Mg₂Si-3wt.%Cu-0.1wt.%Sr alloy,the average size of the primary Mg₂Si decreased to the lowest?11.5?m?,and its morphology also changed from cube to the truncated octahedron.However,when the content of Bi exceeded 2.0 wt.%,the over-modification effect would occur,namely,the primary Mg₂Si became coarse again.?4?Compared with the cube modified by single Sr and perfect octahedron modified by single Bi,the co-modified truncated octahedral primary Mg₂Si could realize the spheroidization after T6 heat treatment,while the single-modified morphologies were difficult to realize the spheroidization.Besides,the co-modification of Sr+Bi was more effective to promote the formation of spherical eutectic Mg₂Si during T6 heat treatment,while there were still some eutectic Mg₂Si in the form of coarse particle or irregular long rod under the condition of single modification.To achieve the spheroidization of primary Mg₂Si,the optimal T6 treatment parameters were as follows:the solution temperature was 530?for 6 h and the aging temperature was 175?for 2 h.?5?The spheroidization and refinement of primary Mg₂Si were successfully realized via Sr+Bi modification combined with T6 heat treatment,and the mechanical properties of Mg₂Si/Al composite were significantly improved.The tensile strength,fracture elongation andcomprehensiveperformancequalityindexoftheco-modifiedT6-treated Al-15wt.%Mg₂Si-3wt.%Cu composite were improved to 374 MPa,4.75%and 475.5 MPa,respectively,which were 21%,88.5%and 28.7%higher than those of the T6-treated alloy modified by single as-cast Sr.