Study on the Relationship between High Temperature Mechanical Properties and Precipitates Evolution of 7085 Al Alloy after Long Time Thermal Exposures

Jinxin Zang,Pan Dai,Xian Luo,Bin Huang,Yanqing Yang,Jigang Ru,Shuai Liu

doi:10.3390/met11091483

Abstract

The requirement for 7085 Al alloy as large airframe parts has been increasing due to its low quenching sensitivity and high strength. However, the relationship between high temperature mechanical properties and the evolution of precipitates in hot environments is still unclear. In this work, thermal exposure followed by tensile tests were conducted on the 7085 Al alloy at various temperatures (100 °C, 125 °C, 150 °C and 175 °C). Variations of hardness, electrical conductivity and tensile properties were investigated. The evolution of the nano scale precipitates was also quantitatively characterized by transmission electron microscopy (TEM). The results show that the hardness and electrical conductivity of the alloy are more sensitive to the temperature than to the time. The strength decreases continuously with the increase of temperature due to the transformation from η′ to η phase during the process. Furthermore, the main η phase in the alloy transformed from V3 and V4 to V1 and V2 variants when the temperature was 125 °C. Additionally, with increasing the temperature, the average precipitate radius increased, meanwhile the volume fraction and number density of the precipitates decreased. The strengthening effect of nano scale precipitates on tensile properties of the alloy was calculated and analyzed.

Highlights

High strength 7xxx series Al alloys are often called aeronautical materials due to their wide applications in aerospace field
The 7085-T7452 samples were thermally exposed at various temperatures (100 ◦C, 125 ◦C, 150 ◦C and 175 ◦C) for 100–1000 h and the tensile tests were conducted at the corresponding temperatures, respectively
The number of η precipitates gradually increases as the temperature rises and the η phase becomes the dominant one after thermal exposure and tensile tests at 125 °C and above

Summary

Introduction

High strength 7xxx series Al alloys are often called aeronautical materials due to their wide applications in aerospace field. In order to meet the rapid development of large aircrafts, it has been desirable to fabricate large structural components using heavy forgings or thick plates [1,2,3,4]. The higher Zn/Mg ratio along with lower Cu content than other 7xxx series Al alloys means that the 7085 Al alloy possesses excellent properties, such as high corrosion resistance, outstanding workability and especially low quench sensitivity [5,6,7,8]. The 7085 Al alloy forging has been successfully used as the inner rear spar in advanced aircrafts A380 [1,9]. There have been many investigations on the microstructures and properties of 7085-T7452 Al alloy [5,10,12,13]

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Discussion

Conclusion