Effect Of Zn Content On Mechanical Properties,Intergranular Corrosion And Quenching Sensitivity Of An Al-Mg-Si-Cu Alloy

Among the deforming aluminum alloys,6000 series Al-Mg-Si-Cu alloys have a widespread application due to the lower density,better corrosion resistance and welding performance.Considering that Zn element can combine with Mg to formη-Mg Zn₂ phase in 7000 series Al-Zn-Mg-Cu alloy,which produces a very high precipitation strengthening effect,a study on the effect of different content of Zn alloying on the properties of Al-Mg-Si-Cu alloy will provide an important reference for the optimization of alloy composition design.In this paper,an Al-Mg-Si-Cu alloy with different Zn contents（0-1.2 wt.%）was taken as the research object.Hardness,tensile and immersion corrosion tests were used to study the effects of Zn content on mechanical properties,intergranular corrosion sensitivity and quenching sensitivity of the alloys in T4and T6 states.In addition,differential scanning calorimetry analysis,X-ray diffraction,scanning electron microscopy and transmission electron microscopy observations were used to analyze the mechanism of the alloy properties from the perspective of precipitation behavior.Besides,the Zn-containing alloy with better mechanical properties was treated with solution+60%cold rolling deformation+low temperature aging to further improve the mechanical properties of the alloy and reduce the sensitivity of intergranular corrosion.The research results are as follows:（1）Low Zn content（0-0.8 wt.%）has no significant effect on the mechanical properties of the alloys in T4 and T6 states,the T4 state alloys have no intergranular corrosion sensitivity,while serious intergranular corrosion occurs in the T6 state alloys,and the maximum corrosion depth of the alloys in T6 state decreases with the increase of Zn content.When the Zn content is 0.8wt.%,the maximum corrosion depth is 320μm,which is 223μm less than that of the Zn-free alloy.When Zn content increased to 1.2 wt.%,the precipitation of GP zone andβ"phase has been promoted,the greatly improved tensile strength and yield strength of the alloy in T4 state are 363 MPa and 196 MPa respectively,which are 44 MPa and 30 MPa higher than those of the Zn-free alloy.The tensile and yield strength of the alloy in T6 state are 443 MPa and368 MPa respectively,which are 57 MPa and 44 MPa higher than those of the Zn-free alloy.Moreover,the elongation of the alloys in both state remains at a high level.And the intergranular corrosion sensitivity of the alloys in T4 and T6 states is significantly enhanced because of the higher Zn content,which increases the composition differences between the intergranular precipitations and the precipitation free zone（PFZ）at the grain boundary.In addition,Zn content has almost no effect on the tensile fractographies of the alloys,but mainly depend on the aging state:the T4 state alloys are the mode of dimple included transgranular fracture;whereas the T6 state alloys are characterized by the mixed mode of dimple induced transgranular fracture and intergranular fracture.（2）When quenching with lower cooling rate,Zn is always solidly dissolved in the matrix to produce solution strengthening effect,and inhibits the precipitation of the coarse Mg₂Si phase in the quenching process,so that the alloys can retain higher aging strengthening potential,and the quenching sensitivity of the alloys is reduced.Therefore,with the decrease of quenching cooling rate,the strength of Zn-containing alloys decreases less,and the higher the Zn content,the higher the alloys’strength under same quenching ways.But the tensile fractographies of the alloys are basically not affected by Zn content.With the decrease of quenching cooling rate,the fractographies of the alloys in T6 state changes as follows:the mixed mode of dimple induced transgranular fracture and intergranular fracture dominated by intergranular fracture→the mixed mode dominated by dimple induced transgranular fracture→the mixed mode dominated by intergranular fracture→the simple mode of dimple induced transgranular fracture.Accordingly,the elongation increases first,then decreases and increases aging.As for the corrosion resistance of the alloys,On the one hand,the continuity distribution for the grain boundary Q phase precipitated during quenching and aging changes due to the increase of precipitation amount,so the tendency of intergranular corrosion of the alloys in T4 state increase first and then weaken with the decrease of quenching cooling rate,while the intergranular corrosion sensitivity of the alloys in T6state increase first,then decrease and increase again.On the other hand,the interaction between the grain boundary Q phase and Zn atoms makes the tendency of intergranular corrosion of the alloys with lower Zn content（0-0.8wt.%）decrease with the increase of Zn content.However,the alloys with 1.2wt.%Zn content always have higher sensitivity of intergranular corrosion.（3）The alloy with 1.2 wt.%Zn content was treated by 530℃/1 h solution and water quenching+60%cold rolling deformation+low temperature aging（120℃,150℃）.The tensile strength and yield strength of cold rolled alloy are446 MPa and 418 MPa,respectively,which are 83 MPa and 222 MPa higher than those of the T4 state alloy,respectively.And the intergranular corrosion sensitivity of the CR alloy is obviously decreased,but the elongation of which is only 5.6%,and the corresponding tensile fractography is mainly intergranular fracture.At the early stage of low temperature aging,the proportion of dimple induced transgranular fracture modes on the tensile specimens increases.After continuous aging,the strength of the alloy aging at120℃increases slowly with the lengthen of aging time.The strength of the alloy aging at 150℃reaches the peak at 12 h and then gradually decreases,while the elongation of two aging alloys decreases continuously during the aging process,and the corresponding tensile fractographies tend to the mode of intergranular fracture gradually.On the other hand,with the aging time extenses,the intergranular corrosion sensitivity of the two aging alloys increases continuously,which is more serious in the alloy aging at 150℃.In addition,the alloy endowed with the best comprehensive properties by aging at120℃for 72 h.The tensile and yield strength of the alloy are 510 MPa and468 MPa respectively,which are 67 MPa and 100 MPa higher than those of the T6 alloy.The elongation is 9.0%and the maximum corrosion depth after immersion test is 233μm,which is 263μm lower than that of the T6 state alloy.