Study On The Microstructure And Properties Of Al-Cu-Mg Heat-resistant Aluminum Alloy Containing Ag

The conventional Al-Cu-Mg series heat-resistant aluminum alloys such as2219and2618, are widely used in aeronautic and astronautic industries due to their high strength and better heat-resistant properties. But the working temperature is usually under200℃otherwise the mechanical properties of these alloys would decrease dramatically due to the coarsening of the strengthening phases, which can not meet the working temperature for the structural parts of new-generation supersonic aircraft and thrusters. Compared to the conventional Al-Cu-Mg series heat-resistant aluminum alloys, the Al-Cu-Mg heat-resistant aluminum alloys containing Ag possess higher strength at room temperature, better damage-resistant property as well as the excellent heat-resistant property and thermal stability. The new heat-resistant aluminum alloys could meet the heat-resistant property and the economical efficiency of the supersonic aircraft and would be used widely. As a result, Al-Cu-Mg alloys with and without Ag were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The effect of trace addition of Ag on the microstructure and properties of Al-Cu-Mg alloy was studied. And on this basis, the homogeneous treatment, the flow behavior and the corresponding microstructural evolution during hot compression deformation, the solution treatment, the aging treatment and the corresponding microstructure and property changes as well as the thermal-exposure and creep behavior were studied. This could provide basis for the composition design, the formulation of hot working process as well as the heat treatment process and its industrial applicationThe effects of trace addition of Ag to Al-Cu-Mg alloy on the microstructure and mechanical properties were studied by means of Vickers hardness tests, tensile testing at room and elevated temperatures, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results show that trace addition of Ag inhibited the precipitation of θ' phase and accelerated the precipitation of Ω phase. And it can also accelerate the age-hardening effect of the alloy, reduce the time to the peak-aging, enhance the peak hardness, make the major strengthening phases of the Al-Cu-Mg alloy turn θ' and less S' phases into Ω and less θ' phases and improve the mechanical properties of the alloy both at room temperature and at elevated temperatures.The effects of homogenization treatment on the microstructure of the Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the homogenization process of the alloy was also optimized. The results show that severe dendritic segregation existed in the ingot and the alloy elements were unevenly distributed both inside the grains and along the grain boundaries. With increasing the homogenization temperature or prolonging the holding time, the residual phases dissolved into the matrix gradually and all elements became more homogenized. The proper homogenizing process was500℃/16h, which was consistent with the results of homogenization kinetic analysis.The flow behavior of Al-Cu-Mg heat-resistant aluminum alloy containing Ag was studied by thermal simulation test. And the microstructural evolution during hot compression deformation was also studied by OM, SEM and TEM, respectively. The results show that the flow stress increased with increasing the strain rate or decreasing the deforming temperature. The relationship between ln[sinh(ασ)] and le ε, ln[sinh(ασ)] and1/T satisfied linear type. The constitute equation of flow behavior of the alloy can be expressed as ε=1.89×1013[sinh(0.0116σ)]7.29exp(-196000/RT). The flow stress during high temperature deformation was predicted by artificial neural network and the predicted stress-strain curves were in good agreement with the experimental results. With increasing the strain rate or decreasing the deformation temperature, the density of the dislocation decreased while the size of the grain as well as the volume fraction of the recrystallization increased and the main soften mechanism of the alloy transformed from dynamic recovery to dynamic recrystallization. According to the processing map analysis, flow instability occurs at two regions, the low temperature300～400℃with a strain rate of0.1～10s-1and the high temperature460～500℃with a strain rate of1.0～10s-1. The flow localization and the fracture are responsible for it. The suitable deformation condition for the alloy is385～460℃and0.001～0.003s-1.The effects of the solution treatment, single aging, the deformation aging and the interrupted aging on the microstructure and the properties of the Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the solution and the aging process of the alloy were also optimized. The results show that the suitable solution and single aging process was515℃/1.5h water-quenching+185℃/4h, treated by which, the tensile strength of the alloy was505MPa, the yield strength was443MPa and the elongation was12.2%. The deformation aging for the Al-Cu-Mg heat-resistant aluminum alloy containing Ag accelerated the aging process and the nucleation of Ω and θ' at dislocations and refined the precipitations both in the grains and on the grain boundaries. With increasing the deformation amount, the strength of the alloy decreased and then increased. And the suitable pre-deformation amount was4%. Treated by4%pre-deformation+185℃/2h aging, the tensile strength of the alloy was516MPa, the yield strength was453MPa and the elongation was12.1%. Lower secondary aging temperature for interrupted aging process accelerated the precipitations of θ' and increased the elongation of the alloy while higher secondary aging temperature accelerated the precipitations of Ω and refined its size, modified the distribution of the particles on the grain boundaries which led to the enhanced strength and plasticity of the alloy. The suitable interrupted aging process for Al-Cu-Mg heat-resistant aluminum alloy containing Ag was185℃/2h+150℃/6h, treated by which, the tensile strength of the alloy was518MPa, the yield strength was454MPa and the elongation was13.8%.The thermal exposure experiments of the peak-aged and the under-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag alloys were carried out and the corresponding changes of the microstructure and the properties with varying the thermal exposure temperature and time were studied. And the mechanism of the thermal stability of the alloys was also discussed. Both under-aged and peak-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag possessed excellent heat-resistant properties below250℃. With increasing the thermal exposure time or the temperature, the precipitations in the peak-aged alloy coarsed and the number of that decreased. Correspondingly, the strength of the peak-aged alloy decreased. The main strengthening phases Ω and θ' precipitated secondarily in the under-aged alloy exposed at100～150℃and the strength of the alloy increased and then decreased with prolonging the thermal exposure time. The strength of the under-aged alloy exposed at200～300℃decreased with increasing the thermal exposure time or the temperature.The creep behavior at elevated temperatures of both under-aged and peak-aged Al-Cu-Mg heat-resistant aluminum alloy containing Ag were studied. And the microstructural evolution was analysised by means of SEM and TEM. The Al-Cu-Mg heat-resistant aluminum alloy containing Ag possessed excellent creep resistant properties at elevated temperatures. The main strengthening phases Ω and θ' in the under-aged alloy precipitated dynamically during the creep process. And the coarsening rate of the strengthening phases and the steady creep rate of the under-aged alloy were much lower than that of the peak-aged alloy. The steady creep rate increased with increasing the temperature or the applied stress, which can be described by a constitutive equation ε=7.60x10-4σ3.60exp(-102000/RT). The main creep mechanism was dislocation mechanism at the early stage of creep and changed to the intracrystalline diffusion mechanism as the creep proceeded.