Study On Microstructure And Mechanical Properties Of Microalloyed Al-Mg-Cu Alloy

Al-Mg alloy is a kind of non heat treatable alloy.Addition of small amount of Cu to Al-Mg alloy can lead to rapid hardening during the paint process?annealing at175?for 30 minutes?.This phenomenon provides a possibility for the application of Al-Mg-Cu alloy as car body plate.In this paper,small amount of Si?Er?Zr were added to Al-Mg-Cu alloy to investigate the effect of microalloying elements on the aging behaviors.The effect of natural aging on the subsequent artificial aging,and the effect of cold deformation and subsequent annealing on the mechanical properties and microstructure of the alloy were also investigated.Conclusions are drawn as follows:The primary phase in the as-cast Al-Mg-Cu alloy is mainly S phase.The Mg₂Si primary phase is formed in the microalloyed Al-Mg-Cu alloy when the Si content is too high,the AlCuEr primary phase in the as-cast Al-Mg-Cu alloy is formed after the addition of Er.After solid solution,the S phase in the alloy was completely redissolved into the matrix,but in the alloy with high Si content of 0.5wt.%Si,there was still Mg₂Si phase.AlCuEr phase remained in the matrix of the alloy after solution treatment,but the Cu content in the phase was lower than that in the as-cast alloy.Part of Cu was dissolved back into the matrix.All of the Al-Mg-Cu alloys show the characteristics of rapid aging strengthening,and the strengthening effect of the alloys increase significantly with the addition of a small amount of Si?0.15wt.%?.The rapid increase of alloy strength after aging treatment is due to the formation of Mg-Cu clusters and S phase.Si can greatly increase the rapid strengthening increment of the alloy,because the addition of Si makes the precipitated phase in the alloy refine obviously.With the increase of Si content,the natural aging hardening rate of the alloy increases and the hardening level of the alloy increases.After a long period of natural aging,the hardness in the early stage of artificial aging is higher than that of the alloy without natural aging,and the peak hardening level of artificial aging is lower than that of the alloy without natural aging.The strength of the as-solutionized alloy is lower and the elongation is higher.Because of solid solution strengthening,the strength increases with the increase of Si?Mg content.As a result of the strengthening effect of Al₃?Er,Zr?precipitated formed during the solution,the yield strength of the alloy with addition of Er and Zr is 27Mpa higher than that of the alloy free with Er and Zr.The yield strength of Al-2Mg-0.8Cu alloy increases by 35MPa after aging at 175?for 30minutes.The yield strength of alloy with 0.15wt.%Si is increased by 83MPa.Although the hardness of the alloy with Er is lower at the initial stage,the yield strength of the alloy increased about 80MPa after aging at 175?for 30 minutes,which is comparable to the yield strength of the alloy without addition of Er.As a result of cold work hardening,the strength of the alloy increases with the increase of the amount of deformation,but the elongation of the alloy decreases significantly.The yield strength of the alloy decreases slightly due to the joint effect of precipitation strengthening and recovery after annealing at 175?for 30 minutes,while the tensile strength and the elongation increase obviously.With the increase of annealing time,the strength of the alloy decreases due to coarsening of precipitated phase.The 40%deformed alloy has the most excellent properties after annealing for30min,its strength and elongation are both high.Because of the effect of Mg on cold hardening,the strength of cold rolled alloy increases with the increase of Mg content.By studying the tensile properties of 40%?60%and 80%cold deformed and reannealed alloys,it can be seen that Si microalloyed Al-2Mg-0.8Cu alloys increase with Si content.The yield strength of the alloy annealed at 175?for 30 minutes is higher than or equal to that of the cold rolled state,and the elongation increased.There is no significant decrease in yield strength as that in the Al-3.5Mg-0.8Cu-0.15Si alloy.It can be inferred that Si can significantly enhance precipitation strength and make up for the decrease of yield strength caused by low temperature recovery.Er and Zr composite microalloying can significantly improve the strength of the alloy.Especially,the tensile strength and elongation of the alloy after annealing at 175?for 30 minutes under large amount of deformation?80%?are greatly improved,thus the alloy with high strength and good plasticity can be obtained.