Effects Of Multi-phase Microstructure Regulation On Anisotropic Tensile Properties Of 2219 Wrought Aluminum Alloy

Due to its high specific strength and sound weldability,2219 wrought aluminum alloy has been extensively used to fabricate propellant tank of large launch vehicle.In order to meet the development needs of lightweight and high reliability of the new generation launch vehicle,manufacturing higer performance2219 aluminum alloy with larger rings become an important trend.However,the manufacturing method of the transition ring has a significant effect on anisotropic microstructure.The larger the diameter,the more obvious the anisotropic microstructure.At a result,the anisotropic mechanical properties,especially the anisotropic ductility of 2219 aluminum alloy with larger rings is prominent.Thus,the in-depth study of the relationship between microstructure and anisotropic mechanical properties has a great guiding significance in manufacturing such a huge ring with high performance.In order to find out the influence of multi-scale second-phases on anisotropic tensile properties of 2219 aluminum alloy,this paper takes 2219 wrought aluminum alloy as the research object.The number,shape,size and distribution of multi-scale second-phases were controlled by composition design and heat treatment process.The formation and evolution of multi-scale second-phase particles,fracture behaviors and anisotropic tensile properties were examined by using metallographic microscopy（OM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,tensile test and micromechanical analysis,etc.The main conclusions are as follows:（1）The morphology and distribution characteristics of the coarse Al₂Cu and Al₇Cu₂（Fe,Mn）constituents in 2219 aluminum alloy ingots with different Cu and Fe contents were systematically analyzed.The results show that the Al₂Cu constituents with tri-pole junction morphology precipitated along the grain boundary,while a small amount of spherical Al₂Cu particles were mainly distributed inside the grains.The needle-like Al₇Cu₂（Fe,Mn）constituents mainly distributed across the dendrite.The constituents increase with the increase of Cu and Fe content.The area fraction of Al₂Cu constituents increased from 5.29%to7.61%with the increase of Cu content from 5.56wt.%to 6.52wt.%,increased by43.9%.（2）The evolution behavior of Al₂Cu and Al₇Cu₂（Fe,Mn）constituents with different morphologies via multi-directional forging and extrusion,and their effect on anisotropic tensile properties were studied.The results show that the main evolution mechanisms of Al₂Cu constituents were shear-fragment and dissolution-diffusion,while the main evolution mechanisms of Al₇Cu₂（Fe,Mn）constituents was only shear-fragment.Increasing the deformation amount,the coarse particles were dissolved fully.With the increase of forging temperature,the diffusion-dissolution of Al₂Cu constituents increased,while the shear-fragment reduced.The streamline distribution of the constituents along the deformation direction makes crack initiation easier to aggregate along the deformation direction,resulting in a lower elongation in T direction.When the Cu content increased from 5.56wt.%to 6.52wt.%,the elongation anisotropy index of alloy increased from 34.90%to 56.38%,while the tensile strength and yield strength anisotropy index of alloy was less than 7%.（3）The formation and evolution behavior of precipitates and their effect on anisotropic tensile properties were studied.The results show that the quenching precipitates in ellipsoidal or spherical shape precitate,preferentially precipitated at grain boundaries.The higher the quenching water temperature,the more the quenching precipitates.Quenching precipitates were very sensitive to the elongation of alloy in T direction.Increasing the quenching water from 20℃to80℃,the elongation anisotropy index of alloy increased from 10.32%to 17.73%,while the tensile strength and yield strength anisotropy index of alloy was less than 7%.More over,with the increase of pre-deformation,the dislocation density increased,resulting in increasing the number of fineθ’precipitates with a much higher density of uniformly distribution.Meanwhile,the discontinuous distribution characteristics of precipitates on grain boundaries are gradually prominent,as well as narrower the precipitate-free zone width with increasing the pre-deformation.The optimum overall performance was achieved with pre-deformation 3%,and the tensile strength,yield strength and elongation anisotropy index of alloy was 3.59%,5.90%and 37.11%,respectively.With the increase of aging time,the strength increase first and then reduce slowly,while the elongation decrease first and then increase slowly.The tensile strength and yield strength anisotropy index was less than 7%.The elongation anisotropy index reached the maximum of 37.11%at peak aging.（4）The sensitive multi-scale second-phase particles on elongation and anisotropic elongation are as follows:constituents,grain boundaries quenching precipitates and grain boundaries aging precipitates.Based on the precipitation kinetics,strengthening mechanism and fracture theory,the strength models and elongation anisotropy models have been built for 2219 wrought aluminum alloy.The theory prediction is in agreement with the experimental results.