Uniaxial Compression Behavior Of Aluminum-Lithium 2A66 Alloy And Evolution Of Its Microstructure And Properties During Multiaxial Forging

The new Al-Cu-Li-X alloy is widely used in the fabrication of skin,floor beam,fuel storage tank,shell and other aircraft and spacecraft structural parts in the form of plate,forging and profile due to its advantages of low density,high elastic modulus,high damage tolerance and excellent fatigue property.However,the properties of these structural parts are the result of the complex action of different parameters during processing.Therefore,it is necessary to build a more accurate constitutive model and processing map to obtain better process parameters.Multiaxial forging technology is an important technology for forming and performance control of Al-Cu-Li alloy forgings.It has great potential in solving the problems of high anisotropy and low short-transverse plasticity of rolling plate.This subject takes the new Al-Cu-Li 2A66 alloy with high alloying content as the research object,and uses Gleeble thermo mechanical simulation equipment to carry out the high-temperature uniaxial compression tests under multiple conditions to study the flow behavior,uniaxial deformation mechanism and to build the strain compensation constitutive model.According to the modified data,the processing maps based on four typical instability criteria are drawn,and the most suitable one is determined to optimize the forging process parameters.Triaxial forging and upsetting and drawing forging were carried out for 2A66 alloy with different passes,followed by subsequent heat treatment.By means of OM,SEM,EBSD,XRD and TEM structure analysis technologies and hardness and tensile property testing methods,the mechanism of microstructure refinement in forging process was explored and the model of grain refinement was constructed,the evolution mechanism of precipitates in heat treatment process was analyzed,and the influence of microstructure evolution on mechanical properties was explored.The main results are as follows:(1)The exact strain compensated constitutive model of 2A66 alloy was constructed.Based on the modified rheological stress of friction and temperature rise,the material constants?,Q,n and ln A under different strain values were calculated.The optimal fitting of material constants and strains is sixth order polynomial fitting.The constitutive model considering the effect of strain is obtained.The model has high correlation coefficient and low average relative error,which shows that the model has good prediction ability of flow stress behavior.(2)In the uniaxial compression test at 500?,the coarsened T₁ phase in the as-homogenized 2A66 alloy undergoes static dissolution and static re-precipitation.When the alloy is heated to 500?,the solid solubility of solute atoms in the matrix increases,and the coarsened T₁ phase is unstable at this temperature,and the Cu and Li atoms in the particles diffuse to the Al matrix,making T₁ particles dissolve;in the compression stage,dislocation and other defects a re introduced into the alloy,which provides a convenient position for the precipitation of fine T ₁ phase;in the cooling stage after compression,heterogeneous nucleation and growth of T₁ phase is preferentially carried out on the dislocation,sub-boundary and dispersed particles,and the T₁ phase was obviously refined.(3)The comparison and test results show that for 2A66 alloy,the prediction ability of K-P criterion and M-R criterion is better than that of Gegel criterion and M-R criterion,and M-R criterion is more accurate than K-P criterion,so M-R processing maps is most suitable for 2A66 alloy.Two forging process parameter windows are optimized according to the M-R processing map.The first one is 410??480?/0.03s^-1?3s^-1,in which the?value is relatively stable and the main mechanism is dynamic recovery;the second one is 480??500?/0.001s^-1?0.03s^-1,and the corresponding mechanism is dynamic recovery and partial dynamic recrystallization.(4)After triaxial forging and T8 heat treatment,the anisotropy of mechanical properties of 2A66 alloy decreased and its short-transverse plasticity increased significantly,and the tensile property is the best when forging pass is 9.The grains are refined and distributed more evenly by the alternating action of multidirection load,and many strengthening phases are precipitated in the matrix by T8 heat treatment.When the forging passes are increased to 9,the ultimate tensile strength,yield strength and elongation of T8 heat treated alloy are 494 MPa,412 MPa and14.8%respectively,and the differences between the corresponding longitudinal mechanical properties and mechanical properties are 0.2%,1.2%and 5.7%respectively.The fracture mode is the combination of intergranular ductile fracture and dimple fracture.(5)The second phase and grain of the as-homogenized 2A66 alloy were refined significantly and the maximum texture strength was decreased obviously after the upsetting and drawing forging.With the increase of forging passes,the Al₂Cu phase with chain distribution at the grain boundary and a large number of coarsened T₁ phase in the grain are broken and dissolved back.The corresponding breaking mechanism is dislocation shearing mechanism,and the volume fraction of the second phase particles reduces from 10.10%at two passes to 3.08%at 6 passes.The grain size first decreases from 450?m at as-homogenized state to 20?m at two passes,and then slowly decreases to 4.4?m at six passes.The grain size distribution is more uniform,and the recrystallization ratio increases to 84%.The grain refinement mechanism is mainly continuous dynamic recrystallization.The deformation texture gradually changes to recrystallization texture,and the texture strength decreases from 19.287 in 2 passes to 7.294 in 6 passes.(6)With the increase of upsetting and drawing forging passes,the yield strength first increases and reaches the maximum value at 4 passes,and then decreases at 6 passes.The increase of yield strength of as-forged 2A66 alloy is mainly due to the effect of dislocation strengthening mechanism and grain boundary strengthening mechanism.With the increase of forging passes,the grains are continuously refined and the grain boundary area increases.The contribution of grain boundary strengthening increases from 38.2 MPa in two passes to 81.0 MPa in six passes.The dislocation density increases first,and the contribution of dislocation strengthening increases from 139.0 MPa in two passes to 146.2 MPa in four passes.After 6 forging passes,a large number of dislocations transformed to large angle grain boundary,and the dislocation density decreased,which reduced the contribution of dislocation strengthening to 101.9 MPa.The increase of the contribution of grain boundary strengthening is not enough to compensate for the decrease of the contribution of dislocation strengthening,resulting in the decrease of yield strength.(7)The strength of 2A66 alloy increased greatly and the elongation decreased slightly after different passes of upsetting and drawing forging and T8heat treatment,among which the tensile properties at four passes were the best.After T8 heat treatment,a large number of T₁ strengthening phases were precipitated in the matrix,which increased the yield strength by about 2 times as much as that of the forging state.When the forging passes were four,there were more dislocations and sub-boundary defects,which provided more favorable positions for the nucleation of T₁ phase,resulting in an increase in the number of T₁ phases and a greater strengthening effect.The corresponding tensile strength,yield strength and elongation were 516 MPa,427 MPa and 9.7%,respectively.(8)The grain refinement model of 2A66 alloy in the multiaxial forging process was constructed.The alternating transformation of loading force axis promoted the formation of deformation band in the grain and local grain boundary sliding,and accelerated the continuous dynamic recrystallization,so that the coarse original grains were replaced by many fine new grains.