Study On Aging Process Of Al-3.06Cu-1.57Li-0.57Mg Alloy

Al-Cu-Li-Mg alloys have low density,high elastic modulus and specific stiffness,So Al-Cu-Li-Mg alloys are widely used in the aerospace field.Al-Cu-Li-Mg alloys are an age-strengthening alloy.By tailoring a reasonable heat treatment process,the microstructure of the alloy can be optimized and the performance of alloys can be significantly improved.In this work,the effects of single-stage aging,two-stage aging with natural aging and predeformation followed by artificial aging on the microstructure and properties of Al-3.06Cu-1.57Li-0.57Mg-0.18Zr alloy were investrgated in detail using mechanical properties testing,XRD analysis,scanning electron microscope(SEM)and transmission electron microscope observation(TEM)and other experimental methods.The reasons for the changes in the mechanical properties of the alloys with different heat-treatment process were discussed after characterizing the precipitats,grain sizes and dislocation density.The main conclusions are as follows:(1)After solutionizing and queaching,the alloys were aged at 140 ?,160 ? and 180 ?,respectively,naming T6 process.It is found that the samples peak aged at 160 ? have the best comprehensive mechanical properties.The hardness is 145 HV,and the yield strength,tensile strength and elongation are 346 MPa,419 MPa,and 13.1%,respectively.In the alloy aged at140 ?,the mainest precipitates are??-phase precipitates except of alittle of T₁ phase precipitates,and lath-like S phase which are inducel by the dislocation.In the samples peak aged at 160 ?,the main precipitates are T₁ phase except of a few GPB zones.For alloy peak aged at 180 ?,there are a large number of dispersed GPB zones,but T₁ phase precipitates along the subgrain boundaries and other defects.In addition,the GPB zones in all of samples with peak-value hardness can be divided into two categories:conventional GPB zone and Li-GPB zone variants.(2)After solutionizing and queaching,the alloys wree natural aged for one week,and then artificial aged at 160 ?.Compared with the single-stage artificial aging at 160 ?,the two-stage aging process also change the mechanical properties and precipitation behavior of the alloy.The introduction of natural aging induces the hardness of the alloy to decrease.There are two hardness peaks in the two-stage aged alloys.A large number of GPB zones,rod-shaped S phase and T₁ phase are precipitated in the alloy at the first peak aging.Extending the aging time,the rod S phase is coarsened,some of the GPB zones are dissolved,and copper atoms are dissolved back,which decreases the hardness of the alloy.When the aging time continues to be extended,the Cu atoms re-precipitate in the form of lath S phase,and the strength of the alloy increases again.(3)After solutionizing and queaching,the alloys were immeliately deformed,and then aged at 160 ?,which is named as T8 process.With the increase of the pre-deformation level(0%,5%,15%),the strength of the alloy gradually increases,but the plasticity decreases.The microstructure characterization shows that with pre-deformation increasing,the grain size gradually decreases and the dislocation density gradually increases.The introduction of pre-deformation prior to aging prompts,a continuous composite phase and T₁ phase to form along the small-angle grain boundaries,but the dispersed GPB area is suppressed.