Study On Microstructure And Mechanical Properties Of Mg-Zn-Al-Cu-Mn Series Alloys With High Zn Content

Magnesium alloys have been widely used in the fields of transportation,aerospace and 3C due to their high specific strength,specific stiffness and easy recovery.However,the flammability,poor plasticity and poor corrosion resistance of magnesium alloys limit the development of magnesium alloys.Composition design and optimization is the basic method for the development and application of magnesium alloys to improve the basic properties of magnesium alloys and develop new series of magnesium alloys.Therefore,the composition design and optimization of new high strength magnesium alloys has always been one of the hot spots and themes in the research field of magnesium alloys.In this dissertation,on the basis of the preparation of Mg-Zn(6,8,10,12 wt.%)binary alloy and Mg-10Zn-1Al-0.5Cu-0.5Mn new magnesium alloy with high zinc content,the emphasis is on the study of the alloy in the structure,mechanical properties and fracture behavior of the as-cast,heat treatment,semi-solid heat treatment and extrusion deformation.It provides experimental and theoretical basis for the development of high performance magnesium alloy.With the increase of Zn content,the grain size of Mg-xZn(x=6,8,10,12 wt.%)alloy becomes finer and then coarser.When the Zn content is 10 wt.%,the grain size is the finest.The microstructure of Mg-xZn alloy is composed of?-Mg and Mg Zn₂ phases.With the increase of Zn content,the morphology and distribution of Mg Zn₂ phases in the alloy gradually change from point-like dispersion distribution to elongated semi-continuous and continuous aggregation distribution,resulting in the tensile deformation of the alloy.The tensile strength and elongation decreased with the increase of Zn content.After the solution treatment of Mg-xZn(x=6,8,10,12 wt.%)alloy,the second phase Mg Zn₂ was partially dissolved in the matrix,while the remaining Mg Zn₂ group remained in the microstructure.After solution treatment,the tensile strength and elongation of the alloy decrease gradually with the increase of Zn content.Mg-xZn alloy has certain effect of solution strengthening and aging strengthening.The tensile strength and elongation of Mg-6Zn alloy decrease gradually with the increase of Zn content after solution treatment.The tensile strength of Mg-6Zn alloy after solution treatment is the highest,reaching288.6 MPa.Compared with the casting alloy and solution treated state,the tensile strength of Mg-xZn alloy after bipolar aging treatment is significantly increased,and the tensile strength of Mg-10Zn alloy after aging treatment is increased to 334.6 MPa compared with the solution treated state,which is increased by 28.25%.During the semi-solid isothermal treatment of Mg-10Zn alloy,with the increase of the semi-solid treatment temperature,the morphology of the primary phase gradually spheroidized and the degree of rounding increased,and the volume fraction of the remelted liquid phase increased and became more continuous.When the temperature rose to 610?,the sample was directly remelted and liquefied,and then solidified to form dendrite.On the basis of Mg-10Zn magnesium alloy,a new type of Mg-10Zn-1Al-0.5Cu-0.5Mn magnesium alloy was prepared experimentally.The as-cast microstructure of the alloy shows typical coarse dendritic morphology.The microstructure is composed of dendritic primary?-Mg and Al₈Mn₅,Mg₃₂(Al,Zn)₄₉ and?(Mg Zn Cu)phases distributed within and between dendrites.After solid solution treatment,the degree of microsegregation of alloying elements is weakened,and the dendrite morphology disappears.Mg Zn Cu and Mg₃₂(Al,Zn)₄₉ decompose and are dissolved into the matrix,and the number of Mg₃₂(Al,Zn)₄₉ decreases significantly.However,the massive Al₈Mn₅phase does not decompose or dissolve due to its high thermal stability.After solution,the tensile strength and elongation of Mg-10Zn-1Al-0.5Cu-0.5Mn alloy increase to 292.6MPa and 20.64%,respectively,increasing by21.14%and 70.58%.After aging,fine second phase precipitates from the supersaturated solid solution of the alloy matrix,which increases the tensile strength and decreases the elongation,reaching 336.5 MPa and11.2%,respectively.The semi-solid microstructure of Mg-10Zn-1Al-0.5Cu-0.5Mn alloy after being held at 610?for 90 min is composed of primary?₁-Mg,?₂-Mg and gray eutectic phase ?₂-Mg and gray Mg₃₂(Al,Zn)₄₉,Mg Zn Cu and Al₈Mn₅ phases are distributed around?₁-Mg,and Zn content in?₂-Mg is higher than that in primary?₁-Mg.After semi-solid treatme Mg-10Zn-1Al-0.5Cu-0.5Mn alloy billet has better deformation ability.The Mg-10Zn-1Al-0.5Cu-0.5Mn alloy can be extruded at a low temperature of 225?after solution treatment,and the extruded rod surface is good.The dynamic recrystallization of the alloy occurs during extrusion.The grain size of the alloy is less than 10?m,and the streamline microstructure is obvious.A large number of large second phase distribution occurs in the microstructure,and a large number of fine precipitated particles are formed in the matrix.With the increase of extrusion temperature,the tensile strength of Mg-10Zn-1Al-0.5Cu-0.5Mn alloy decreases gradually,and the tensile strength reaches the highest value(357.8 MPa)at 225?.With the increase of extrusion temperature,the elongation has the same rule as the tensile strength.At 225?,the elongation reaches the best value,reaching 23.8%.Mg-10Zn-1Al-0.5Cu-0.5Mn extruded alloy has significant aging strengthening effect.After aging,the tensile strength and elongation of extruded bars without semi-solid treatment and semi-solid treatment increase,reaching 365.2 MPa,360.2 MPa and 16.32%,17.36%,respectively.After aging,the second phase in the supersaturated solid solution after homogenization before extrusion will precipitate.These second phases will increase the tensile strength of the alloy,but the elongation will decrease slightly.