| Aluminum alloy materials can meet the needs of lightweight,large-scale and complex structures,so their applications are second only to steel in modern industrial production.Aluminum alloys have characteristics of low density,high specific strength,good plasticity,strong conductivity for heat and electricity,excellent processing and forming performance and good corrosion resistance,thus they have been widely used in aero-space,military equipment,transportation,construction industry,metallurgical chemical industry,energy industry and other fields.6061 aluminum alloy is one of the most important alloys in Al-Mg-Si(6XXX)series,which is a kind of medium strength aluminum alloy and can be heat-treated by aging.It has excellent corrosion resistance,weldability and coloring property,leading to its wide use.Although 80%of aluminum extrusion products in the world are made of 6XXX aluminum alloy,yet its application is restricted for its strength.To expand the application field of 6XXX series aluminum alloy,its strength needs to be further improved.In this paper,four kinds of experimental alloys were designed,with 6061 alloys and 6061 alloys with additional Mn and Zr as the research object.By means of inductively coupled plasma direct reading spectrometer(ICP),differential scanning calorimeter(DSC),X-ray diffractometer(XRD),scanning electron microscope(SEM)with energy spectrum analysis(EDS),etc.,properties such as hardness,tensile property,corrosion property and microstructure of the alloy were characterized.The effects of both addition of Mn and Zr and different casting solidification methods on mechanical properties and microstructure of the alloy were studied systematically.The mechanism of influence of aging precipitation and rolling deformation on material strength was studied.By XRD analysis,the lattice constants of A1 and the solid solubility of main alloy elements in Al-Mg-Si alloys were calculated,and the apparent activation energies of ?" in 6061 alloy and 6061-0.15Mn-0.05Zr alloy were calculated.In the meantime,their kinetic equations were determined.By studying the microstructure and solution calculation of the original casting state,the effects of both addition of Mn and Zr and casting in water-cooling copper mold on the microstructure and mechanical properties of the alloy was analyzed.Casting in water-cooling copper mold reduces the segregation of elements and increases the solubility of the main alloy elements:the solubility of Mg,Si,Mn is increased by 7%-12%,10%-14%,10%respectively.At the same time,by way of EDS energy spectrum analysis and other means,the precipitations of the original cast alloy was determined:in 6061 alloy cast in water-cooling copper mold,the precipitations are granular phase?-Al8(MnFe)2Si,?-Al8(MnFeCr)2Si and strip phase ?-Al9(MnFe)3Si2;while the precipitations in alloy cast in Fe mould are skeleton phase Al5(MnFe)Si and Al5(MnFeCr)Si,granular phase ?-Al(MnFe)2Si and ?-Al8(MnFeCr)2Si as well as strip phase ?-Al9(MnFe)3Si2.For the sake of eliminating the inhomogeneity and nonequilibrium microstructure in the alloy,the as-cast alloy was treated by homogenization.The overburning temperature of the alloy was determined by DSC analysis,and the heat treatment method of the alloys was determined to be 560?×3h according to metallographic observation.After homogenization treatment,the precipitation phase of the alloy changed to strip phase ?-A19Fe2Si2 and granular phase?-Al8(FeMnCr)2Si.The dispersion distribution of the crystalline phase improved the mechanical properties of the alloy.The solution degree of Mg in homogenized alloy is increased by about 30%than that in as-cast alloy,together with the solution degree of Mn and Si increasing by more than 50%,which explained the effect of homogenization treatment theoretically.At the same time,the effects of homogenization and casting in copper water-cooled mould casting on the mechanical properties of the alloy were analyzed by tensile test.The results show that the mechanical properties of homogenized alloy are better than that of as-cast alloy,and that of alloy cast in copper water-cooled mould casting is better than that of alloy cast in iron mould.However,6061-0.15 Mn-0.05Zr alloy homogenized and cast in water-cooling copper mold has the best mechanical properties with tensile strength of 286MPa,yield strength of 127MPa and elongation of 18%.The main strengthening phase of the alloy is ?"phase,whose precipitate activation energy is calculated.In the alloys 6061(Fe),6061(Cu),6061-0.15Mn-0.05Zr(Fe),6061-0.15Mn-0.05Zr(Cu),the apparent activation energy of ?" phase is 147KJ/mol,217KJ/mol,185K/mol,235KJ/mol,respectively.At the same time,the aging kinetics equations of the alloys were fitted,which provides theoretical basis for further study of aging precipitation behavior.The tensile strength,yield strength and elongation of 6061-0.15Mn-0.05Zr alloy cast by water-cooling copper mould after aging are 356MPa,230MPa,21%,respectively.The tensile strength,yield strength and elongation of 6061-0.15Mn-0.05Zr alloy cast by copper water-cooled mould casting after rolling are 363MPa,217MPa and 9%,respectively.Both methods improved the mechanical properties of the alloy and greatly improved the strength of it.The corrosion properties of rolled 6061 and 6061-0.15Mn-0.05Zr alloys in 3.5%(mass fraction)NaCl solution were studied.The results of immersion weightlessness test show that the corrosion rate of water-cooling copper mold casting alloy is lower than that of iron mould casting alloy,and the corrosion resistance of 6061 alloy added with Mn,Zr is better than that of 6061 alloy.The polarization curves show that the self-corrosion potential of 6061-0.15Mn-0.05Zr alloy cast by water-cooling copper mold shifts to be more positive,which indicates that the joint effect of Mn and Zr and casting in water-cooling copper mold improves the corrosion resistance of the alloy.The microstructure observation showed that the corrosion degree of the alloy was large and the local corrosion pit appeared,while the 6061-0.15Mn-0.05Zr alloy was mainly pitting corrosion.Combine experimental results and literature research,we can confirm that the corrosion mechanism of the alloy was to be in accordance with the rupture mechanism of passivation film and the adsorption theory. |
PDF Full Text Request