Effects of Solution Treatment on Microstructure Transformation, Tensile and Exfoliation Corrosion Properties of 7136 Alu
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Effects of Solution Treatment on Microstructure Transformation, Tensile and Exfoliation Corrosion Properties of 7136 Aluminum Alloy Junlong Zhang, Jingyuan Li, Shaokun Tian, and Dan Lv (Submitted May 30, 2018; in revised form January 17, 2019) The effects of solution treatment on microstructure transformation and tensile properties of 7136 aluminum alloys were investigated by means of scanning electron microscopy, electron backscatter diffraction, differential scanning calorimeter, x-ray diffraction and tensile test, electrochemical evaluations, exfoliation corrosion tests, respectively. The results indicate that solution treatment had a significant effect on the microstructure transformation and tensile properties. Soluble constituents of 7136 aluminum alloys were more likely to form the g(MgZn2) phase rather than the S(Al2CuMg) phase. With increasing the solution temperature, the g phase was re-dissolved into the a(Al) matrix gradually. After solution treatment of 475 °C/1 h, the g phase was completely dissolved into the matrix, with insoluble Fe-rich phases remained. The dissolution of g phase would increase the strength and the corrosion resistance of the alloys. When continuing to increase the solution temperature, the recrystallized fraction increased obviously, resulting in the decrease in the strength and the corrosion resistance. Consequently, the strength and the corrosion resistance of the alloys increased firstly and then decreased with the increase in solution temperature. After solution treatment of 475 °C/1 h, the tensile strength and yield strength of the aged alloy reached their peaks, 738 and 715 MPa, respectively. The elongation of the alloy solution treated at 475 °C/1 h reached 11.2%, and its exfoliation corrosion rating got EB level. Furthermore, after the double-stage solution treatment of 450 °C/1 h + 475 °C/1 h, the recrystallized fraction decreased, resulting in a slight increase to 12.6% of the elongation, a slight increase in the corrosion resistance and a slight decrease in the strength. Keywords
7136 aluminum alloys, microstructure transformation, recrystallization, solution treatment, tensile properties
1. Introduction The Al-Zn-Mg-Cu (7xxx series) aluminum alloys are extensively used in the aerospace and transportation industries due to their excellent mechanical properties, high strength-toweight ratio and good corrosion resistance (Ref 1-3). In recent years, many novel 7xxx aluminum alloys such as 7136, 7056 and 7081 have been applied to the aerospace field. Compared with conventional 7050 and 7075 aluminum alloys, their higher Zn content and Zn/Mg ratio will make their mechanical properties increased to a higher level (Ref 4). Generally, to obtain improved mechanical properties, 7xxx aluminum alloys are often subjected to different heat treatment, including homogenization, solution and aging treatment. Due to the bulk of undissolved eutectic in the ingots, there still exist copious amounts of residual phase after the homogenization and hot defor
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