Study On Microstructures And Mechanical Properties Of Al-Mg-Mn Alloys Prepared By Squeeze Casting

With the trend of emission reduction and green casting,the lightweight of safety structural parts in the field of transportation has become particularly critical,especially in the field of new energy electric vehicle,which demand for high strength,high ductility and light aluminum alloy safety structural parts is increasingly strong.Al-Mg alloys have found great applications in the automotive because of the low density,high specific strength,good formation ability,corrosion resistance,good weldability and so on.However,Al-Mg alloy as safety structure in traffic field is limited due to its poor casting performance and low strength.Microalloying and special casting are effective ways to improve the strength and ductility of Al-Mg alloy.Fe is the most harmful impurity element in aluminum alloy,which can not be avoided and will continue to accumulate in the process of scrap aluminium.Therefore,the non-heat treatable Al-Mg-Mn alloy with high performance,low cost and easy regeneration can be developed by squeeze casting,which can meet safety structural parts in transportation field with the high strength,high ductility and lightweight,and has great theoretical and engineering application value.In this paper,the as-cast Al-3.5Mg-0.8Mn alloys with different Fe contents were produced by neutralization modification and squeeze casting.Optical microscope(OM),scanning electron microscope(SEM),differential thermal analysis(DSC),X-ray diffraction(XRD)and synchrotron radiation X-ray tomography(SR-CT)were used to study the effects of Fe content and squeeze pressure on microstructure and mechanical properties of the alloys.The main conclusions are as follows:(1)The Fe element in Al-Mg-Mn alloys only forms Al₆(Fe,Mn)phase.The result show that Fe content and squeeze pressure have significantly changed on the morphology of Fe-rich phase,squeeze pressure can transform the Chinese-script Al₆(Fe,Mn)phase to the lath-like Al₆(Fe,Mn)phase,and refine the size of Fe-rich phase,and make it distribution uniform.The ultimate tensile strength and yield strength of the alloys but reduces the elongation significantly with the increase of Fe content.Squeeze casting can refine the microstructure of the alloy,reduce casting defects and improve the mechanical properties of the alloy.When the squeeze pressure increases to 75 MPa and Fe content is 0.5%,the as-cast Al-3.5Mg-0.8Mn alloy has excellent comprehensive mechanical properties.The ultimate tensile strength,yield strength and elongation of the as-cast Al-3.5Mg-0.8Mn alloy are 252 MPa,28 MPa and 28%,respectively.(2)There are Al₂Cu Mg phase and Al₆(Fe,Mn)phase in Al-3.5Mg-0.8Mn-0.6Cu-XFe alloy.With the increase of Fe content from 0.1%to 0.8%,the Al₆(Fe,Mn)phase transform the lath-like morphology to the Chinese-script morphology.When the squeeze pressure is 75MPa,the comprehensive mechanical properties of the as-cast Al-3.5Mg-0.8Mn-0.6Cu-0.5Fe alloy is the best,with the ultimate tensile strength of 300MPa,yield strength of 158 MPa and elongation of22%.The ultimate tensile strength and yield strength of the alloys have improved due to the solution strengthening of Cu element,but remaining Cu content will form the network of Al₂Cu Mg phase along the grain boundary,which reduces the elongation of the alloy.(3)The 3D morphology of Fe-rich phases and pores in Al-3.5Mg-0.8Mn alloys containing different Fe contents have been characterized by SR-CT.With the increase of Fe content,the distribution peak position(?)of Al₆(Fe,Mn)phase increases and then decreases,the average node length decreases and then increases.This is because when the Fe content increases from0.1%to 0.8%,the Al₆(Fe,Mn)phase in the alloy changes from the rod-like morphology to the Chinese-script morphology and then to the lath-like morphology.When the pressure increases from 0MPa to 75MPa,the distribution peak position(?)of Al₆(Fe,Mn)the peak increases,While the average node length decreases.Because the pressure improves the morphology of Al₆(Fe,Mn)phase and reduces the number of the lath-like Al₆(Fe,Mn)phase.With the increase of Fe content in the gravity casting Al-3.5Mg-0.8Mn alloy,the peak position(?)decreases.Because the curvature of pores near spherical shape changes greatly,while the surface of the shrinkage is smooth,so the?value decreases.The primary Al₆(Fe,Mn)phase in the as-cast Al-3.5Mg-0.8Mn-XFe alloys has two 3D morphology:rhombic prism and rectangular prism.During the growth process,the side length of the Fe-rich phase crystal increases gradually and elongates rapidly along the[001]direction,resulting in the existence of solute poor region in the central of the primary prism,thus forming the hollow inside the primary prism.Due to the lack of local solute atoms,it may cause the formation of hollow on the side of the primary prism.The size of eutectic Al₆(Fe,Mn)phase is very small and it is difficult to form solute poor region,so eutectic Al₆(Fe,Mn)phase is solid.