Effect Of Cu And Heat Treatment Process On Microstructure And Mechanical Properties Of Hypoeutectic Al-Mg₂Si Alloy

Al-Mg₂Si alloy is an important manufacturing material for lightweight parts,has great market potential and wide application prospect.However,its application in practical production is limited because of its thick structure and poor mechanical properties.In this experiment,the influence of Cu and heat treatment process on microstructure and mechanical properties of hypoeutectic Al-Mg₂Si alloy was studied,hoping to provide reference and help for future production and application.The results show that:With the addition of alloying element Cu,the primary?-Al and eutectic Mg₂Si phases of Al-10Mg₂Si alloy can be refined obviously,and the average size and secondary dendrite arm spacing(SDAS)of the primary?-Al phase in the microstructure can be reduced obviously,and the morphology of the primary?-Al phase is changed to near spherical shape.When the Cu content is 1.5wt%,the grain size and SDAS decreases to 8.58?m and 2.54?m,respectively.Compared with the alloy without Cu,the grain size decreases by 49.5%and75.9%,and the refining effect is most obvious.At the same time,the addition of Cu can make the eutectic Mg₂Si phase change from thick lamellar and Chinese character to fine fibrous.The reason why Cu refines the microstructure of the alloy is that Cu is enriched in the growth front of primary?-Al and eutectic Mg₂Si phases,which results in supercooling of the components and impedes their growth.In the as-cast state,the yield strength and tensile strength of the alloy increase with the increase of Cu content,while the elongation first increases and then decreases.When Cu content is 1.5wt%,the yield strength,tensile strength and elongation of the alloy are increased from 136 MPa,223 MPa and 1.5%to 169 MPa,317 MPa and 2.3%,respectively.At the same time,the fracture mode of the alloy is changed from brittle fracture to ductile fracture by adding 1.5wt%Cu.After solution treatment,the microstructure of the alloy can be obviously changed.The thick lamellar and Chinese shaped eutectic Mg₂Si transform into spherical shape,and the Cu-containing intermetallic compounds dissolve in the matrix,and the size and volume fraction of the alloy decrease.When the Cu content is 3.5wt%,the length-width ratio of the eutectic Mg₂Si phase in the microstructure decreases to 1.71,which is 84.6%lower than that in the as-cast alloy.The aging treatment can significantly improve the mechanical properties of the alloy,and the hardness and tensile strength of the alloy are significantly improved.There are two peak values in the aging hardening curve of the alloy containing Cu,and the peak hardness of the alloy all appears at the second peak.When the addition of 3.5wt%Cu is added,the peak hardness of the alloy after heat treatment can reach 205.3 HV,which is120.0%higher than that of the alloy without Cu,and the tensile strength of the alloy at room temperature can be increased to 351 MPa.The microstructure refinement and strengthening of the alloy after heat treatment is due to the dissolution,crushing and spheroidization of eutectic Mg₂Si phase in the solid solution stage,and the supersaturated solid solution of alloying elements in the matrix.Under the combined action of solution strengthening and aging strengthening,the hardness and tensile strength of the alloy are significantly increased.