Influence Of Fe And Si Impurities On Microstructures And Mechanical Properties Of 7055 Aluminum Alloy

In order to meet the requirements of aircraft's weight reduction and reliability,the large-sized plate of ultrahigh strength Al-Zn-Mg-Cu alloy are developed to fabricate the main bearing components such as aircraft wall panels and beams.At present,the strength of large-sized 7055 aluminum alloy medium thick plate developed in domestic has reached the design requirement,but the fracture toughness and the homogeneity of mechanical properties need to be improved.Reducing the contents of impurity elements such as Fe and Si has been one of the important approaches to improve the fracture toughness.Whereas,a systematic research is lack that the evolution of the impurity elements during the processing and its effect on the microstructures mechanical properties of high strength aluminum alloys.In this paper,the factors affecting the microstructures,mechanical properties and homogeneity of the large-sized 7055 aluminum alloy medium thick plate in the thickness direction were studied.Based on this,researches focused on the influence of Fe and Si impurities on multi-microstructures,strength and fracture toughness.A series of 7055 aluminum alloys with the same Fe/Si ratio and different Fe and Si impurity contents were prepared by high purity raw material.The influences of Fe and Si impurities on the microstructures and mechanical properties of 7055 aluminum alloys were investigated and the mechanism of Fe and Si impurity contents on the microstructures and mechanical properties were revealed.The main conclusions are as follows:?1?For the large-sized 7055 aluminum alloy medium thick plate,the content and size of the residual coarse Al₇Cu₂Fe and Al₂CuMg at the center of the thickness are both greater than the edge,which reduces the ductility of the center.The higher degrees of recrystallization decrease the fractions of?fiber rolling texture at the edge,which leads to the lower strength at the edge than the center when loaded along rolling direction.?2?In as-cast microstructures of 7055 aluminum alloys,the area fraction of coarse?[Mg?Al,Cu,Zn?₂]phase gradually decreases from 4.65%to 2.65%,and the area fractions of coarse Fe-rich phase?Al₇Cu₂Fe/Al₃Fe?and Si-rich phase gradually increase from 0 to 0.572%and 0.7%respectively with increasing Fe and Si contents from 0.002%and 0.006%to 0.272%and 0.134%.The Fe content increases from 0.012%to 0.041%,the coarse Al₇Cu₂Fe phase in the alloy changes from granular to long needle shape and the average length increases.When the Fe content further increases to 0.144%,the Fe-rich phase transforms into Al₃Fe phase to make the average length decreases.With the Si content gradually increases from 0.024%to 0.134%,Mg₂Si phase changes from granular to long strip and the size increases.?3?After homogenization,extrusion,solution and aging treatment,the area fractions of coarse?phase in the alloys with different Fe and Si contents all reduce to less than 1%,while the area fractions of Fe-rich phase and Si-rich phase do not reduce.The sizes of coarse?,Al₇Cu₂Fe and Mg₂Si are largely reduced by crushing during extrusion.In homogenized and aged microstructures,the evolutions of the area fractions and sizes of coarse?,Al₇Cu₂Fe and Mg₂Si with Fe and Si contents increasing are the same with the as-cast microstructures.?4?The increase of Fe and Si contents scarely affects the Al₃Zr dispersoids precipitating during homogenization.While for??precipitates in grains precipitated during aging,the volume fraction and thickness reduce and the number densities increase in the alloys with 0.144%Fe,0.071%Si and 0.272%Fe,0.134%Si.After retrogression and reaging treatment?RRA?,discontinuous and stable?precipitates present at the grain boundaries and the precipitates free zones are all quite narrow in the alloys with different Fe and Si contents.?5?The critical particle size of?,Al₇Cu₂Fe and Mg₂Si phases is about 5?m to occur particle stimulated nucleation?PSN?in 7055 aluminum alloys.The effects of compositions and morphology of coarse second phases on PSN can be obtained as follows:the degree of non-uniform deformation of matrix caused by coarse second phases is?<rod-like Al₇Cu₂Fe<round-like Al₇Cu₂Fe?round-like Mg₂Si;the number of PSN recrystallized grains is??rod-like Al₇Cu₂Fe<round-like Al₇Cu₂Fe?round-like Mg₂Si;the distribution of PSN recrystallized grains is that the major orientations of recrystallized grains formed by PSN with coarse?phase are close to the extruded fiber textures,while the major orientations of recrystallized grains formed by PSN with coarse Al₇Cu₂Fe and Mg₂Si phases largely deviate from the extruded fiber textures.The numbers of recrystallized grains and recrystallized grains largely deviating from the extruded fiber textures formed by PSN increase with the number densities and size of coarse Al₇Cu₂Fe and Mg₂Si phases increasing.?6?In 7055 aluminum alloy,the evolution of the number and orientation of PSN recrystallized grains depends on the evolution of compositions,morphology,sizes and number of coarse second phases,that gives rise to the evolution of recrystallization and textures with Fe and Si contents increasing.When Fe content increases from 0.002%to 0.012%,a small number of 1-5?m Al₇Cu₂Fe phase forms in 7055 aluminum alloy cannot produce PSN recrystallization under deformation conditions,and the number of?phase larger than 5?m decrease to reduce the faction of high angle grain boundary by PSN with?phase and the volume fraction of largely deviating from the extruded fiber textures.The number densities of Al₇Cu₂Fe and Mg₂Si phases larger than5?m increase with Fe and Si contents increasing from 0.012%and 0.006%to0.272%and 0.134%,and Al₇Cu₂Fe phase transforms from long rod to nearly round shape when Fe and Si contents increasing to 0.144%and 0.071%,which lead to the numbers of recrystallized grains and recrystallized grains largely deviating from the extruded fiber textures formed by PSN increase.As a result,the fraction of high angle grain boundary increases and the content of extruded fiber textures reduce.?7?With the increasing of Fe and Si contents,the strength of 7055aluminum alloy decreases can be mainly attributed to the contents of extruded fiber textures reducing.Compared to the alloy with 0.002%Fe,the number of?phase larger than 5?m in the alloy with 0.012%Fe decrease can improve the ductility and fracture toughness through reducing the fractions of coarse second phases fracture and intergranular fracture with high angle grain boundary.The number densities and size of coarse Al₇Cu₂Fe and Mg₂Si phases increase and Al₇Cu₂Fe phase transforms from long rod to nearly round shape with Fe and Si contents increasing from 0.012%and 0.006%to 0.272%and 0.134%,which increases the fractions of coarse second phases fracture and intergranular fracture with high angle grain boundary to result in the reduce of ductility and fracture toughness.?8?Based on the above studies,it can be concluded that a small amount of Fe and Si impurities?0.002%<Fe?0.012%,0.006%<Si?0.024%?in ultrahigh strength 7055 aluminum alloy can decrease the content of coarse?phase,and form small-sized Fe-rich phase and Si-rich phase without producing PSN recrystallization,leading to high comprehensive mechanical properties of strength,ductility and fracture toughness.