Study On Microstructures And Mechanical Properties Of Al-Mg-Si-Li-(Cu) Alloys

At present,there are many studies on Al-Mg-Si alloys,adding Li to Al-Mg-Si alloy can change the morphology of Mg2Si particles,adding Cu to Al-Mg-Si alloy can enhance the hardening ability of Al-Mg-Si alloy.In this experiment,adding Li and Cu at the same time with a certain content of research in Li,study the effect of Cu content on the properties and Al-Mg-Si,and using the effect of Mn,Cr and Ti to improve the performance of the alloys.The experiment of different components of Al-Mg-Si-Li-(Cu)and Al-Mg-Si-Li-(Cu)-Mn-Cr-Ti two series alloys were investigated at cast and homogenization state,hot-rolled and coldrolled and Aging by Microstructure Analysis,analysis the microstructure change and performance of different components and different state alloy,on the basis of the different components of Al-Mg-Si-Li-(Cu)making the alloy solid solution and aging heat treatment process is reasonable.the tensile mechanical properties of the alloy,and the tensile fracture surface analysis of alloy,and the precipitation behavior of the alloys were studied.The microstructure of the as cast Al-Mg-Si-Li-(Cu)alloy are ?-Al,Si phase,AlLiSi phase,Mg2Si phase,Al2Cu phase,W phase,Al8FeMg3Si6 phase.With the additions of Cr,Mn,Ti,there exist mainly four kinds of phases based on their contrast,AlLiSi phase,Mg2Si phase,spherical W phase and bright dendritic phase complex phase.Without the addition of Mn,Cr,Ti,the hardness of the alloy with 0.8wt%Cu reaches to the peak value and the conductivity of the alloy with 0.4wt%Cu is minimum.With increasing Cu content,conductivity increased.After adding Mn,Cr and Ti,the performances of the alloys are more stable.With the content increasing of Cu,the hardness increases but the electrical conductivity decreases.After homogenization,the evolution of the second phases in the two seris oof alloys are similar,the Al8FeMg3Si6 phase is dissolved,the spherical W phase basically disappears,the number of striped W phase is obviously reduced,and the AlLiSi phase is rounded.For Al-MgSi-Li-(Cu)alloy,the hardness after homogenization is lower than those of as-cast alloys,and the conductivity after homogenization is higher than those of the as-cast alloys.With the addition of Mn,Cr and Ti,the hardness increases but the conductivity decreases.After hot rolling and cold rolling,it is found that the deformation is much easier for the alloys with the additions of Mn,Cr and Ti.Although the edge structure is different from the middle part,the banded structure is more slender than the alloys without the addition of Mn,Cr and Ti.After cold rolling,the banded structure is more slender and the distribution is more uniform.The hardness and electrical conductivity of the alloys with different compositions AlMg-Si-Li-(Cu)were solution treated and aged at different varying temperatures and durations.The solid solution treatment was performed at 530 ? ×1h and 180?×8h was selected as the aging treatment.Based on the tensile tests after solid solution treatment,with the increase of Cu content,the yield strength and tensile strength of the alloy increases gradually and reached the peak values in the 0.8Cu alloy,the yield strength and tensile strength are 145MPa and 295MPa,respectively.Compared to those of the alloys without the addition of Mn,Cr and Ti,the peak value of the strength is obtained for 0.4Cu alloy with the addition of Mn,Cr and Ti,its yield strength and the tensile strength are 160MPa and 316MPa,respectively.Based on the microstructure of the alloys with Mn,Cr and Ti additions after aging,The recrystallization process was inhibited resulting in finer grains after heat treatment.In the Al-Mg-Si-Li-(Cu)alloys after aging,the strength first increased and then decreased,reached to a peak value for the 0.8Cu alloy,the yield strength and tensile strength are 359MPa and 402MPa,respectively.With the addition of Mn,Cr and Ti,the strength variation showed the same tendency and a peak value of the strength for the 0.6Cu.Compared to the alloy without Mn,Cr and Ti additions,the yield strength and tensile strength,372MPa and 438MPa,increased by 8.73%and 14.93%respectively.Based on the DSC analysis,it was found in Al-Mg-Si-Li-(Cu)alloys,Cu promoted the formation of GP zone.The high strength for the 0.8Cu and 1Cu alloys came form the combined strengthening of both the GP zone and ?"phase.With the addition of Mn,Cr and Ti,and the formation of GP zone was further promoted.According to calculations using the DSC traces,the activation energy of each precipitates during aging process decreased without changing the precipitation sequence,as a result,the precipitation hardening is more significant due to the acceleration of the aging process.