Effects Of Pre-treatments And Composition On Ageing Precipitation Behavior And Corrosion Property Of Al-Cu-Li-X Alloys

Al-Cu-Li alloys are widely used in key structural components of spacecraft and aircraft,due to low density,high specific strength,high specific modulus and good corrosion resistance.With further development of aerospace industry,the demand for weight reduction in aerospace vehicles has become more and more urgent.In order to use aluminum-lithium alloys to replace traditional aluminum alloys on a larger scale,it is urgent to further improve overall performance of Al-Cu-Li alloys.Heat treatment and alloying composition can directly determine the performance of aluminum-lithium alloys.However,at present,the influence mechanisms of heat treatment process and composition on microstructures and properties of Al-Cu-Li alloys are still unclear.The reason is that the influence mechanisms of heat treatment process and composition on the atomic segregation behavior at early ageing stage are still unclear.Based on above,this paper first takes 2050 Al-Li alloy as research object,systematically investigated the pre-treatments(pre-deformation treatment,natural ageing treatment after pre-deformation and cold rolling and annealing treatment)on the ageing precipitation behavior and strengthening of 2050 Al-Li alloy and revealed the influence mechanisms.Then,the effects of Mg content and Ag addition on the ageing precipitation behavior of Al-Cu-Li-Mg alloy were comparatively studied.Moreover,the effects of the above pre-treatments and composition on the corrosion characteristics of Al-Cu-Li-X alloys was systematically studied,and the mechanisms of precipitate microstructures on the intergranular corrosion sensitivity and pitting corrosion resistance of Al-Cu-Li alloys was explained.Pre-deformation can inhibit the formation of GP zone in the early stage of aging.This was due to significant reduction of the density of Mg-Cu clusters.The Cu-Mg segregation enriched with Li and Ag atoms was the key to promote the heterogeneous nucleation of T₁ phase on dislocations.Pre-deformation can provide more nucleation sites for T₁ phase by increasing the number of dislocation structures occupied by the Cu-Mg segregation in early ageing stage.Pre-deformation can inhibit the formation of?phase,which was benefical to promote the precipitation of T₁ phase.Pre-deformation can significantly accelerate the ageing strengthening response for 2050 Al-Li alloy and increase the peak strength from 551MPa to 557MPa,which was mainly due to the increase of the contribution of deformation strengthening.Pre-deformation can reduce the precipitation strengthening contribution of the peak-aged alloy,which was mainly due to the large reduction in the diameter of T₁phase.Natural ageing treatment after pre-deformation can further promote the precipitation of T₁ phase.It was because of the formation of large numbers of Mg-Ag clusters during the natural ageing process,which further increased the nucleation sites of T₁ phase.Natural ageing treatment accelerated the ageing strengthening response during artificial ageing,and increased the peak strength from 557MPa to 583MPa,which was caused by the increase of the volume fraction of T₁phase.Cold rolling and annealing treatment delayed the growth of T₁ phase and reduced the number density of T₁ phase,and could slow down the ageing hardening response and reduce the strength and plasticity of the alloy after ageing.The increase of Mg content can promote the formation of S'phase,which hindered the nucleation and growth of T₁ phase,resulting in a decrease of the number density of T₁ phase,slower growth rate and slower precipitation kinetics.Compared with low-Mg alloys,high-Mg alloys had a faster ageing hardening response in early ageing stage.This was due to the formation of higher density of Mg clusters in high-Mg alloys.In subsequent aging stage,the high-Mg alloy had a slower ageing hardening response,which was caused by the hindering effect of the formation of S'on the precipitation of T₁ phase.The increase of Mg content had little effect on the peak strength,but could reduce the plasticity and ductility.The addition of Ag inhibited the formation of GP zone in the early ageing stage and promoted the uniform precipitation of spherical?'phase,which was caused by the segregation of Ag-Li hindering the occurrence of Cu-Li segregation.Ag addition can make the T₁ phase nucleate on dislocations earlier,and accelerate the growth rate and precipitation kinetics of T₁ phase.Ag addition can increase the density of T₁phase in the peak-aged alloy,while significantly reduced the density of?'phase.Ag addition accelerated the ageing hardening response and improved both strength and elongation for the peak-aged alloy.The enrichments of Mg and Ag atoms were found in the T₁ precipitates in the Al-Cu-Li-Mg-Ag alloy.DFT calculation results showed that the substitutions of both Mg and Ag atoms for the Li atom at the outermost layer of T₁ phase could result in decreases in the formation energy and the interfacial energy of T₁ phase,which could increase the driving force for T₁ nucleation and lower the nucleation potential barrier,thereby facilitating the nucleation of T₁ phase.Pre-treatments had great influences on the corrosion resistance of 2050 Al-Li alloy.Results showed that both pre-deformation and natural ageing treatments can decrease the intergranular corrosion(IGC)sensitivity of the under-aged 2050 Al-Li alloy.This was because both of them could promote intragranular T₁ precipitation,which reduced the potential difference between the grains and subgrain boundaries,so resulting in the improvement on the IGC resistance.Cold rolling and annealing treatment can significantly increase the IGC sensitivity,which was caused by the formation of continuous solute segregation on the grain boundaries in the sample subjected to cold rolling and annealing treatment.Both pre-deformation and natural ageing treatments can decrease the pitting corrosion resistance of the peak-aged 2050 Al-Li alloy,and increase the degree of the pitting damage of the alloy after exposure in salt spray environment.This was because they could increase the content of T₁ phase,which resulted in decreasing the compactness of the oxide film on the sample surface,and accelerating the growth rate of the corrosion area,and increasing the potential difference between the matrix and the Cu-rich particles formed by T₁ phase after dealloying.Cold rolling and annealing treatment increased the pitting corrosion resistance of the alloy,which was caused by the reduction of the content of T₁ phase and the decrease in the grain size.The addition of Ag can decrease the IGC sensitivity of the under-aged Al-Cu-Li-Mg alloy,which was mainly because Ag addition inhibited the formation of GP zone and promoted the precipitation of T₁ phase.Ag addition can decrease the pitting corrosion resistance of the peak-aged alloy,which was mainly due to the higher content of T₁ phase and the change in the composition of T₁ phase for the Ag added alloy.