Stress Aging Behavior And The Corresponding Properties Of The Novel 600 MPa Grade High-strength Al-Li Alloy

Creep age forming(CAF),which can simultaneously realize the shape and properties of components,is an advanced manufacturing technology that developed for the large integrally panels frequently used in aerospace industry.The stress aging process is the key factor for this manufacturing method.Compared with the conventional aluminum alloy,the Gen3 Al-Li alloys have been received more and more attractions in aerospace industry due to its high specific strength and stiffness.This thesis investigated the aging precipitation path,aging kinetic,creep behavior and the corresponding tensile properties,corrosion resistance,and plain strain fracture toughness KIC of the novel 600 MPa grade high-strength Al-Li alloy.This study can provide pre-research and basic theory for the replacement of conventional 7xxx high-strength Al alloys for the large integrally panels manufactured by the CAF method.The main points are as following:Stress is one of the most important stress aging parameters.Firstly,the effect of stress level on the microstructure,creep deformation,dislocation density,and tensile properties of the as-quenched and 2%pre-stretched samples were studied.For the as-quenched samples,the role of stress aging enhancing the precipitation of T1 phase was increasing the density and volume fraction as the stress level increased.Compared with stress-free aging,after stress aging for 24 h under 280 MPa,the density of T1 and θ’ phase increased to five times and decreased one order of magnitude,respectively.For 2%pre-stretched samples,the average diameter and volume fraction of T1 phase in the stress-free and stress aging samples were basically the same.The role of stress aging enhancing the precipitation of T1 phase was accelerating the coarsening of T1 phase.After stress aging for 24 h under 280 MPa,the thickness of T1 phase is 3-4 nm.The aggregation of T1 phase at sub-grain boundaries can observed both in the as-quenched and pre-stretched samples,but the aggregation alleviated as the stress level increased.The improvement of microstructure can bring out the improvement of mechanical properties.For asquenched samples,the optimal mechanical properties can be obtained when stress aged under 280MPa,the yield strength,ultimate tensile strength and elongation to failure were 585.8MPa,617.9MPa and 12%.For the 2%pre-stretched samples,the optimal mechanical properties can be obtained when stress aged under 100-150MPa,the yield strength,ultimate tensile strength and elongation to failure are 632.5MPa,655.5MPa and 10%.The evolution of dislocation density in samples as the stress level increased was quantitatively characterized based on the Williamson-Hall method.Combined with the strain hardening model and mechanical properties testing,the individual contribution of strain hardening and precipitation hardening made to the increasing of yield strength of the samples as the stress level increased was discussed.The time for the alloy reaching peak aging is another key variable for the stress aging process.Based on the evolution of microhardness,tensile properties and relative volume fraction of T1 phase with aging time,the aging kinetics of the novel high-strength Al-Li alloy under conventional stress-free aging and 220 MPa stress aging at 155 ℃ were further investigated(1-100 h).Stress aging can significantly accelerate the aging kinetics of the alloy,shortening the peak time to 12 h.The effect of stress on the aging kinetics was pronounced during under aging stage.Due to the accelerated aging response during stress aging,the thickness of the T 1 phase began to increase beyond 6 h,while the density gradually decreased.When the stress aging proceeded to stable stage,the T 1 phase in the alloy was significantly coarsened.At this time,its thickness was about 5-6 nm,which was equivalent to the thickness of five times of single-layer T1 phase.With regard to the precipitation mechanism of stress aging enhancing the precipitation of T1 phase,we carried out a preliminary discussion from the perspectives of thermodynamics and kinetics,respectively.Finally,the service properties of stress-aged alloys including intergranular corrosion and plane-strain fracture toughness KIC were also investigated.During early stage,the stress aged sample showed general intergranular corrosion model.As the stress aging proceeded,the intergranular corrosion sensitivity of the samples first decreased and then increased,and reached the minimum at the begin of peak aging stage.The evolution of the open circuit potential of the alloy with aging time was exactly opposite to the age hardening curve,which fully indicated that the corrosion potential of the alloy stress aged for different times was closely relating to the precipitation of T1 phase.Appropriate stress aging heat treatment process can make the novel high-strength Al-Li alloy have high fracture toughness while maintaining high strength.After 2%pre-stretch and stress aging at 155℃ under 100 MPa for 24 h,the plane-strain fracture toughness KIC and yield strength of the alloy are 33.3 MPa·m1/2 and 630 MPa,which is comparable to that of the conventional 7xxx series high-strength aluminum alloys.When the stress level increased to 220 MPa,the fracture toughness of the alloy decreased to 28.8 MPa·m1/2,indicating that high level stress would deteriorate the fracture toughness of the alloy.The improvement in fracture toughness of the stress aged samples can be attributed to the intrinsic factors such as the elimination of continuous grain boundary precipitation and precipitation free regions,the increase in the density of intragranular T1 phases,and the extrinsic factor such as the delamination toughening caused by secondary cracks perpendicular to the main crack plane.