Evolution of semi-solid microstructure of 7075 aluminum alloy during isothermal heat treatment

[China Aluminum Network] Semi-solid molding process has many advantages over traditional molding processes, and has broad application prospects. The key to this technology is how to obtain semi-solid tissue, which is also a recent research hotspot. There are several methods for obtaining semi-solid tissues, of which the isothermal heat treatment method appears later, but it is very practical. Compared with other methods, this method has the advantages of low cost, simple process, and easy promotion. In addition, there have been many methods for obtaining semi-solid microstructures of cast aluminum alloys and other non-ferrous metals, and there are relatively few studies on high-strength aluminum alloys. In this paper, the semi-solid microstructure of high-strength 7075 aluminum alloy after isothermal heat treatment was studied, and the process parameters of isothermal heat treatment were determined, which provided a reference for the application of semi-solid forming technology in the future. The material used in this experiment is a high-strength 7075 aluminum alloy. The experimental temperature range is in the solid-liquid two-phase region. The solid and liquid temperatures are 477 and 635 °C, respectively. Fifteen specimens of 4 mm×4 mm×8 mm were taken from the original metal bar, and then kept at 580, 600 and 615° C. respectively, and the holding time at each temperature was 5, 15, 30, 45 and 60 min. When the furnace temperature rises to a predetermined temperature, the corresponding set of samples is placed in a box resistance furnace, and the temperature error is controlled at ±1°C. After the sample is finished with the predetermined heat treatment process, the water quench is quickly taken out. After the sample is embedded and polished, the corresponding tissue is observed under an optical microscope. In the high solid phase temperature range, when the holding time is the same, the size and spheroidization degree of the crystal grains increase with the increase of the holding temperature. When the heating temperature is the same, the grain size increases gradually with the holding time, and the roundness first decreases and then increases. From the grain size and roundness comprehensive consideration of the more reasonable process parameters: heating temperature 615 °C, holding time 15min.

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