Continuous Rheo-Extrusion and Double Aging Heat Treatment of 7075 Aluminum Alloy Wire

With the proposal of the national strategic goals of emission peak" and "carbon neutrality"，the demand for energy conservation and emissions reduction has been increasing urgent. To date，aluminum is the second most-produced metal. Its light weight，high strength，and high conductivity make aluminum to be a good choice for many products，such as the automobile，aircraft and so on. High strength lightweight 7000 series aluminum alloy is an important material to instead of steel with small density，high strength，and good corrosion resistance. As a typical representative of 7000 series aluminum alloy，7075 aluminum alloy has a high tensile strength which generally reaches more than 500 MPa after heat treatment and is widely used in high strength structural parts due to its excellent performance. However，due to the high degree of alloy content and wide temperature range of solidification characteristics，the segregation，cracking and other defects are generally appear in the process of forming of 7075 aluminum alloy. Therefore，it is of great significance to explore the high quality forming process of 7075 aluminum alloy. The continuous rheo-extrusion has been proposed as a low-cost semi-solid metal processing technology with the advantage of short process，high efficiency，energy saving，strong adaptability，low production cost，and producing infinitely long products in theory. Compared with the traditional metal processing method，the energy consumption and cost is reduced by more than 40% and 30%，respectively. The technology combines the slurry preparation and extrusion with a mushy zone between the continuous casting and extrusion process. The melt is sheared and stirred by the rotating extrusion wheel during the solidification process，and the solid phase is refined and spheroidized. In the process of continuous rheological extrusion，the process parameters have a great influence on the process stability and mechanical properties of the product. When the 7075 aluminum alloy wire is prepared by continuous rheological extrusion，the different process parameters have a great influence on the forming quality. Based on the continuous rheological extrusion forming technology with the advantages of refining the grains and reducing the forming defect，the continuous rheological extrusion process was applied in this work. The microstructure was observed by optical microscopy（OM，Leica DMi8A，GER）and a field emission-scan electroscope microscope（SEM，Zeiss SUPRA55，GER）with an energy dispersive X-ray spectrometry（EDS）detector and an electron back-scattered diffraction（EBSD）detector. Tensile tests were conducted using an electronic universal tensile machine（UTM，AG-IC100kN，JPN）at a constant tensile speed of 0.0015 mm·s-1. Compared with the different process parameters on the surface quality and organizational performance of 7075 aluminum alloy wire based on the continuous rheological extrusion，the optimized parameters were obtained. Based on produced material，the experimental scheme of heat treatment of 7075 aluminum alloy with solution treatment and double aging heat treatment were investigated. The results showed that the forming quality of 7075 aluminum alloy could be effectively improved by preheating the cavity and the die to 300 ℃ and the temperature wasmaintained for a period of time. The uniform grains were obtained and the segregation of the second phase was reduced for the alloy products. Alloy products with excellent surface quality and mechanical properties are obtained in the range of extrusion wheel speed of 0.039~0.063 m·s-1 and pouring temperature of 750 ℃. The results demonstrated as the speed range of 0.039~0.063 m·s-1，with increasing the rotation speed of the extrusion wheel，the deformation of the microstructure was larger and the uniformity of the grain was lower，and there was an increase in the tensile strength while decreasing in the elongation. The optimized process parameters of 7075 aluminum alloy based on continuous rheo-extrusion were obtained，which included that preheating cavity and mold to 300 ℃ ，pouring temperature of 750 ℃ ，and extrusion wheel speed of 0.047 m·s-1. Under these process parameters，the tensile strength and the elongation of 7075 aluminum alloy were 373.73 MPa and 12.09%，respectively. At 470 ℃ for 90 min of solution treatment，the second phase was undergoing obvious redissolution，but the second phase was not dissolved obviously and the grains continue to grow after prolonging the solution time. The main precipitation phases were η' phase and η phase under the double aging heat treatment，and grain boundary precipitates were coarse and widely spaced，which improved the corrosion resistance. The second-stage aging temperature had the most obvious effect on the tensile strength. Under the same first-stage aging system and the second-stage aging time of 14 h，the second-stage aging temperature of 150 ℃ was higher than that of the 7075 aluminum alloy under the second-stage aging temperature of 180 ℃. The tensile strength increases 50.90 MPa while the elongation decreased 0.44%. The optimized double aging heat treatment process parameters of 7075 aluminum alloy were obtained，which included that solution treatment at 470 ℃/90 min and double aging at 110 ℃/5 h+150 ℃/14 h. The tensile strength and the elongation of 7075 aluminum alloy wire products under this heat treatment process were 523.44 MPa and 9.96%，respectively. Compared with the continuous rheological extruded 7075 aluminum alloy wire，the tensile strength is increased by about 40.2%. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved."