Effects Of Heat Treatment And Pre-Deformation On Microstructure And Properties Of 2195 Al-Li Alloy Sheet

Aluminum alloys are widely used to meet the need of lightweight structural metals in aerospace industries because of their benefits such as good corrosion resistance,high strength-to-weight ratio and advantageous structural performance,especially for the aluminum-lithium alloys with lighter weight and higher strength.In this paper,strengthening mechanism for the heat treatment and predeformation process of2195Al-Li alloy sheet will be investigated.The obtained results are supposed to provide theoretical guidance and technical support for forming actual structural components.The effects on different heat treatment conditions on the mechanical properties were studied by hardness and tensile tests.The optimum heat treatment processes was determined as the solution temperature of 520oC×30min,room temperature water cooling,single stage aging 160oC×48h,when the yield strength of 471 MPa,tensile strength of 493 MPa,elongation of 11.3%.The best process for step aging is100oC×8h+160oC×24h,when the yield strength of 534 MPa,tensile strength of 571 MPa,elongation of 12.8%.As for stepped aging process,the results show that with the increase of the pre-aging time,the time of arrival peak is shortened,but the peak intensity is reduced.The improvement of the final aging temperature has no obvious effect on the process of final aging,but the strength can be improved significantly.The effect of the final aging time on the mechanical properties is different according to the different temperature,but the intensity increases first and then decreases,and the elongation decreases.The tensile fracture surface and the microstructure were analyzed by scanning electron microscope(SEM)and transmission electron microscope(TEM).TEM shows that the strengthening phase of Al-Li alloy is mainly ?' phase and T1 phase,and ?' phase is the main precipitation phase of natural aging,T1 phase is the main precipitation phase of artificial aging.As for stepped aging process,the number of T1 phases increases and the distribution tends to be uniform with the prolongation of the pre-aging time.With the increase of the final aging temperature,the number of T1 phases increases but the distribution is relatively uneven.T1 phase of the smaller dispersion of a large number of distribution,the better the effect of strengthening.T1 phase more thick,the worse the effect of strengthening.So the mechanism of heat treatment and pre-deformation strengthening is to promote the dislocation and subgrain boundary behavior,to provides a nucleation position for the T1 phase,to promote theuniform dispersion of precipitation,thus increasing the strength.But,since the T1 phase is a brittle phase,the plasticity is lowered.SEM showed that the annealed materials were plastic fracture,and the materials after heat treatment was delamination cracking or quasi-cleavage fracture.The higher the layer thickness is,the better the plasticity is.The higher the proportion of quasi-cleavage fracture,the worse the plasticity.The grain size,grain boundary,sub-structure and texture before and after quenching were analyzed by electron backscattered diffraction(EBSD).The grain size statistics showed that the recrystallization occurred in different degrees after quenching,and 10% of the predeformation after quenching the grain size has grown abnormally.Grain boundary and substructure analysis show that the degree of dislocation reorganization increases due to the effect of heat activation during the quenching process,and the subgrain boundary gradually changes to large grain boundary,which reveals the reason of abnormal grain growth.The tensile test result shows a plastic instability-PLC caused by pre-deformation and solution treatment on the stress-strain curve,and it can be explained by DSA theory.The mobile dislocations are blocked by defects and solute atoms during the movement,resulting in stress fields,and a large number of solute atoms are pinned around the mobile dislocations.Only to increase the external stress,can be used to overcome the pinning dislocation,continue to move forward.The process of repeated pinning and de-nailing between the solute atoms and the movable dislocations reflects periodic zigzag fluctuations in the stress-strain curve.