PSI - Issue 65

I.S. Kamantsev et al. / Procedia Structural Integrity 65 (2024) 109–113

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I.S. Kamantsev et al. / Structural Integrity Procedia 00 (2024) 000–000

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thermodynamic instability - Antipov et al. (2017). In turn, heat treatment of deformed semi-finished products serves as a powerful means of influencing the structure and, as a consequence, a regulator of physical and mechanical properties. One of the common types of heat treatment is annealing. Thanks to annealing, it is possible to achieve a homogeneous structure, which allows one to significantly intensify subsequent processing - Filippov et al. (2013). The temperature of recrystallization annealing in literary sources varies and is in the temperature range of 300…500 °C, the holding time is from half an hour to several hours - Kolachev et al. (1999). There are many different methods for assessing changes in the structure of a material during thermal and deformation treatments, one of which is determining the grain form factor - State standard 5639-82. In accordance with this method, the grain shape is assessed and the degree of deviation from the ideal (equiaxed) shape is determined. Microhardness is an independent characteristic by which one can assess the level of strength properties of a material - Shveikin at al. (2023). Comparison of the results of the obtained grain form factor values with microhardness measurements allows one to relatively reliably judge the completeness of the recrystallization process. This paper presents the results of the combined use of the grain form factor calculation method and microhardness determination in order to select an optimal temperature range for recrystallization annealing of the deformed aluminum alloy 2024.

2. Material and research methodology

A plate made of the deformable aluminum alloy 2024 was used as the initial material, the chemical composition of which is presented in table 1.

Table 1. Chemical composition of the aluminum alloy 2024, wt.%. Si Fe Cu Mn Mg Cr Ni

Zn

Ti

V

Al

0.069

0.243

4.030

0.596

1.592

0.007

0.017

0.016

0.037 0.010 93.36

To obtain a homogeneous structure, preliminary annealing was carried out at a temperature of 495 °C, which is higher than the temperature of complete dissolution of intermetallic compounds in the aluminum solid solution, but lower than the equilibrium solidus temperature for the three-component Al-Cu-Mg system - Berezovskaya et al. (2016). The holding time was 12 hours, which was sufficient for the transition of intermetallic phases to a solid solution and the precipitation of dispersoids (manganese aluminides). As a result of further cooling in air, the precipitation of secondary intermetallic compounds of a smaller size (1 μm) and more uniformly distributed than before diffusion annealing is observed - Maltseva at al. (2019). Hot rolling was carried out in a single-stand, reversible rolling mill "Duo-Quarto", with preliminary heating of the plate to 430 °C for 0.5 hour. Rolling was carried out in 3 passes with a change in the height of the plate from 0.18 cm to 0.1 cm. The cube-shaped (0.1x0.1 cm) samples for the studies were subjected to recrystallization annealing in a laboratory electric furnace PKL-1.2-36 in an air atmosphere, the temperature in which was controlled by a thermocouple with an accuracy of 5 °C. Annealing was carried out with the aim of forming and growing new grains with a perfect crystalline structure. Heat treatment was carried out at temperatures of 300, 350, 370, 400, 450, 495 °C, the holding time was 1 and 2 hours, followed by cooling in air. Sections for microstructural studies were prepared on cut blanks in a plane parallel to the rolling plane. Etching to determine the grain form factor was carried out using a reagent of the following composition: 1 ml HF, 26 ml HNO3, 1.6 ml HCl, 100 ml H2O for 0.4 hours. Metallographic studies of the structure were performed using a Neophot-21 optical microscope with magnification from 50 to 1000 times. As part of the study, at least 150 grains were analyzed for each temperature in accordance with - State standard 5639-82. The measurement and form factor calculation error was no more than 1%. The microhardness of the obtained samples was measured using a Shimadzu microhardness tester with an indenter load of 0.05 kg at room temperature.