Study Of Microstructure, Properties And Precipitation Behavior Of Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr Alloy

A novel series of Al-Cu-Mg-Ag alloy with excellent elevated temperature properties and damage-tolerance was developed and became a candidate skin alloy for next generation of supersonic aircraft. The variation of microstructure and properties of a novel Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr alloy in solution and single ageing process was studied in this thesis. Furthermore, the effect of Muti-step ageing on properties and microstructure was also investigated. The fatigue and dynamic properties were estimated at the last part of this thesis. The main results were drawn as below:The effect of solution temperature and time on microstructure and properties on Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr alloy was studied. The rational single solution temperature is in the range of 520℃-525℃. The rational single solution time is about 2 hours.The influence of single ageing temperature and time on electrical conductivity, hardness and ambient tensile properties was investigated. Under the condition of natural ageing, the electrical conductivity of alloy decreases initially and then approaches to a stable value, while the hardness increases initially and then trends to a stable value. Under the condition of artificial ageing, the electrical conductivity of alloy increases initially and then trendes to a stable value, while hardness increases initially and then decreases after having reached the top value. When the ageing temperature reaches 160℃,the top value of hardness is the highest among all the ageing temperatures, i.e.HV=166.The focal discussion was put on the effect of ageing at 160℃and 180℃on the tensile properties of Al-4.8Cu-0.5Mg-0.3 Ag-0.15Zr alloy. When ageing at these two temperatures, the peak values of tensile property are not significantly different from each other. The peak value of UTS is about 510MPa, while YTS is about 475MPa, and the corresponding value of Elongation is about 14%. However, the responding of ageing-hardening is different. The higher ageing temperature, the faster increment of strength is obtained. The speed of reduction of strength after peak ageing increased with the increment of ageing temperature.The ageing behavior of Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr alloy in the process of single ageing was studied qualitatively. At low ageing temperature (65℃), the precipitate is mainly 0'phase without visible characteristic ofΩphase. At moderate ageing temperature (160-180℃), the precipitates areΩphase and small quantitative ofθ' phase. At elevated ageing temperature (>200℃), the type of precipitates is the same with that of the ageing at moderate temperature, but the speed of growth increases. The effect of moderate temperature on microstructure of the experimental alloy was studied emphatically. At the early ageing period, the size of precipitates is uniform and the precipitates are not crossed with each other. As increment of ageing time, the size of precipitates becomes heterogeneous. Apparent joining of precipitates and crossing zone starts to happen. Beside the field between the joining or crossing zone and the matrix, apparent dislocations are visible, which suggests the joining or crossing of precipitates requires dislocations to coordinate the different structure.The precipitation reactions corresponding to three exothermic effects in DSC experiment of Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr was studied. The peak temperatures of the three effects areⅠ=171℃,Ⅱ=230℃andⅢ=276℃.The corresponding reactions are arrangement of GP zone structure, the precipitation ofΩphase and the precipitation of 0' phase.The ageing behavior of Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr alloy at moderate temperature was semi-quantitatively investigated. The higher the temperature is, the greater the size of thickness and diameter ofΩphase is obtained. The peak value of fractions ofΩphase at 160℃and 180℃are nearly equal to each other, but the time corresponding to the peak values are different. The higher the temperature is, the shorter the time is required.Small angle X-Ray scattering was used to study the ageing behavior of the experimental alloy. Within the ageing temperature range of 160-200℃,as increase of the ageing time, the scattering patterns changed from concentric cycles to branches. The higher the ageing temperature is, the shorter the time for branches to appear is obtained. The branches are the result of Fourier transformation of plate-like precipitates. The corresponding precipitate of a branch can be identified by the angles between branches. Furthermore, the Guinier radius and integrated intensity were measured using SAXS. The tendency of the SAXS results is different significantly from that of HREM.The influence of typical Muti-step ageing processes on the microstructure and properties was studied. Muti-step ageing can rise the fracture toughness while keeping the strength and elongation of single ageing. The results indicate that the difference between Muti-step ageing and single ageing arises from 0'being separated out homogenously without any visible characteristic ofΩphase during the second low temperature ageing of Muti-step ageing,.The effect of ageing condition on fatigue and dynamic properties of Al-4.8Cu-0.5Mg-0.3Ag-0.15Zr was investigated. The alloy that undergoes ageing at peak conditions has an excellent fatigure property. The fatigue limit is about 291 MPa. The crack propagation resistant of the alloy under different ageing conditions is low. In addition, the results of Hopkinson showes that the strain hardening behavior are different when the alloy is aged at the condition of 160℃/2h,160℃/14h and 160℃/50h. Under low and high attacking speeds, YTS of the alloy aged at 160℃for 50h significantly varies while that of the alloy aged at 160℃for 2h and 160℃for 14h are indistinctively.