| 7000 series Aluminum alloys used in aerospace and aircraft are heat treatable aluminum alloy. Comparing with 2000 series Al alloys and Al-Li alloys, 7000 series Al alloys have relatively good mechanical properties and corrosion resistance. It is widely used in aerospace and aircraft industries and transportations or vehicles. It's a very crucial kind of light and high-strength structural materials. Optimizing the existing heat treatment process to improve the comprehensive properties is the most practical way to save cost.The present work was carried out on commercial AA7075 Al alloy sheets, produced by Southwest Aluminum(Group) Co. Ltd. The chemical composition of this alloy is Al-5.40Zn-2.20Mg-1.38Cu-0.16Cr-0.08Mn-0.03Ti-0.26Fe-0.11 Si. 7075 Al alloy sheets were solution treated and artificial aged in air furnace and sand furnace. Effects of heating rate and quenching rate of 7075 Al alloy sheets were studied. The progress of precipitate during heating stage was investigated by using 200oC/h for the heating rate. The different quenching rates were got by different quenching medium, such as cold water, boil water and still air. The thermocouple, optical microscopy, hardness tester, scanning electron microscopy(SEM), electron backscattered diffraction(EBSD), transmission electron microscopy(TEM) are used to test the mechanical properties and observe the microstructure.The results show that electrical conductivity decrease with the increase of temperature during heating stage. The defeats disappear and most soluble phases dissolve into matrix during this stage. The hardness increase because of the solution strengthening and the change of hardness is huge between 350 oC and 450 oC due to the fast diffusion rate. Recrystallized structures can be observed in the specimens in the heating stage of solution treatment, but the recrystallized structures are still elongated grain instead of equiaxed grain because of the drag effect of dispersoids. The {001}<100> texture is still exist after solution treatment. When the temperature is lower than 450 oC during heating stage, proportion of low angle grain boundaries and texture intensity increase but the high angle grain boundaries decrease with the increase of temperature. There are opposite changes when the temperature is higher than 450 oC. BSE and TEM images show that soluble phases dissolve into matrix when the temperature higher than the solvus temperature and the amount of defects decrease with the increase of temperature. The insoluble phases, such as constituent phases and dispersoids, are left after solution treatment.When the 7075 Al alloy samples are heated with different heating rate during solution heat treatment, grain structure is elongated and the amount of elongated grain structure decrease with the increase of heating rate. The amount of elongated grain structure of sample heated with 50oC/h is maximum, but the amount of elongated grain structure of sample heated with 20484oC/h is minimum. Most grain structure of sample heated with 20484oC/h is equiaxed grain. The intensity of {001}<100> texture and proportion of low angle grain boundaries decrease with the increase of heating rate.For 7075 Al alloy sheets, the cooling rate of cold water, boil water and still air is 270oC/s, 30oC/s, 0.5oC/s, respectively. The cooling rate of cold water is faster than critical cooling rate so that the supersaturated solid solution have no time to resolve and the degree of supersaturation is highest among the three quenching medium. The electrical conductivity of sample quenched in cold water is the lowest. The cooling rate of boil water is lower than critical cooling rate and the supersaturated solid solution resolve at a narrow temperature range. Only small amounts of precipitation precipitate in the grain and fine precipitation precipitate on the grain boundary. The cooling rate of still air is much lower than critical cooling rate so that a large number of precipitation precipitate in the grain and grain boundary. Precipitation free zones occur on the grain boundary. The electrical conductivity of sample quenched in still air is very highest. Many fine and dispersed precipitations precipitate in the sample quenched in cold water during artificial aging because of the highest degree of supersaturation. So, the tensile strength is the highest. |
PDF Full Text Request