Microstructure Comprehensive Regulation And Mechanical Properties Of Al-Mg₂Si Alloys With Graded Distribution By Compound Treatment

The automotive industry developed rapidly,and the engine is the main device that provides power for automobiles.Improving the comprehensive mechanical performance of the engine cylinder liner is of great significance.However,due to the inherent brittleness of the intermetallic compound Mg₂Si in Al-Mg₂Si alloy,the comprehensive mechanical properties of the alloys are not high,making it difficult to meet the requirements for microstructure and properties of engine cylinder liner.Therefore,the morphology and size and distribution of Mg₂Si,and matrix in Al-Mg₂Si alloys are regulated by compound treatment including modification treatment,magnetic field treatment and heat treatment in this paper.A lightweight Al-15Mg₂Si graded material with Mg₂Si graded distribution and high hardness at the edge is prepared,and the mechanisms of microstructure regulation are also revealed.First-principles calculations method was used to select the modification element.The thermodynamic stability of high melting point intermetallics formed by adding different modification elements into Al-Mg₂Si alloys were calculated.The calculation results of formation enthalpy and cohesive energy showed that Sr,La,Y and Eu could form stable intermetallics when added into Al-Mg₂Si alloys.The adsorption energy of different modification elements on Mg₂Si surface were also calculated.The results showed that among the four elements,Eu had the lowest adsorption energy and the strongest adsorption ability at the Fcc₀ site on the Mg₂Si（100）surface.The projected density of states results showed that among the four elements,Eu had the strongest bonding ability and adsorption inhibition growth ability on Mg₂Si（100）surface.From the perspective of energy and electronic structure,Eu element was selected theoretically cause the best modification ability among the four elements.Modification treatment with Eu was used to regulate the morphology and size of Mg₂Si.The alloy composition was optimized and the modification mechanisms were also revealed.Experimental study about the modification treatment of Al-x Mg₂Si（x=18,15,13wt.%）alloys with Eu were carried out.Eu could modify both primary Mg₂Si and eutectic Mg₂Si in Al-Mg₂Si alloys.Eu regulated the size and morphology of primary Mg₂Si effectively.The mechanism of size refinement with Eu addition is the formation of Al₂Si₂Eu,which acted as the heterogeneous nucleation substrate to Mg₂Si.The mechanism of morphology improvement is the adsorption of Eu.Due to the strong adsorption and bonding ability of Eu on the surface of Mg₂Si{100},the relative growth rate of different crystal plane changed and thereby the morphology of Mg₂Si improved.The mechanism for morphology regulation of Eu on eutectic Mg₂Si is the selective adsorption of Eu,resulting in the transformation of eutectic Mg₂Si from lamellar to fibrous.The comprehensive tensile properties of three kinds of matrix alloys without modification were compared,and the Al-15Mg₂Si alloy is optimum.Among the researched alloys,Al-15Mg₂Si-0.1Eu alloy modified with0.1wt.%Eu had the optimal comprehensive tensile properties.The yield strength was90 MPa,and the ultimate tensile strength reached 166 MPa and the elongation reached14.3%.Effect of traveling magnetic field on distribution of Mg₂Si phase in Al-15Mg₂Si alloys with Eu modification were researched.And the regulation mechanisms of magnetic field were also revealed.Independent application of traveling magnetic field on Al-15Mg₂Si alloy was studied.When without traveling magnetic field,primary Mg₂Si phase distributed uniformly on the cross-section of the sample with hardness of 55 HV at the outer region.After appling 6 A excitation current,Mg₂Si formed graded distribution on the cross-section of the sample.The content of the Mg₂Si at outer region was 14.42 vol.%,while the content at the center was only 0.3 vol.%.And the size of primary Mg₂Si particle reduced to 10μm.The combined effect of modification treatment and traveling magnetic field on the refinement of primary Mg₂Si particle in Al-15Mg₂Si-0.1Eu alloy was better than applying traveling magnetic field merely.The size of primary Mg₂Si refined to 9μm.The hardness at outer region increased to 72 HV.The regulation mechanism of traveling magnetic field on the distribution of Mg₂Si derived from the electromagnetic body force.Under the traveling magnetic field,primary Mg₂Si particle was subjected to the resultant force pointing to the side wall.And the force drived the Mg₂Si particle moving closer to the side wall and formed graded distribution.The refinement mechanism of Mg₂Si particle under traveling magnetic field is the electromagnetic stirring effect.Compound treatment containing modification and magnetic field and heat treatment were used to improve the comprehensive mechanical properties of Al-Mg₂Si alloys.And the strengthening mechanisms during heat treatment were also revealed.Al-15Mg₂Si-0.1Eu alloy strengthened with micron Mg₂Si particles and nanoβ"precipitates and with high comprehensive tensile properties was prepared by modification and heat treatment.The yield strength of the alloy reached 188 MPa,and the ultimate tensile strength reached 281 MPa and the elongation is 8.4%.Orowan strengthening plays a major role in improving the yield strength of the alloy during the heat treatment process.The size of Mg₂Si particle was further refined to 8μm at the outer region of the cross-section after compound treated Al-15Mg₂Si alloy with combination of modification and magnetic field treatment and heat treatment.The hardness increased to 115 HV.Lightweight Al-15Mg₂Si graded material with collaborative strengthening by micron Mg₂Si particles and nanoβ"precipitates along with graded distribution of Mg₂Si and high hardness was prepared.