Mechanical Properties and Microstructure Evolution in 6061 Alloy Wire with Continuous Rheo-Extrusion and Heat Treatment

As a typical Al-Mg-Si alloy，6061 alloy has excellent plasticity，superior mechanical properties and corrosion resistance. 6061 alloy was widely used around the world as a common metal for automotive industry and aeronautical materials. In the process of application，6061 alloy prepared by traditional methods often has low hardness and low mechanical properties after aging. The require⁃ ments for automotive materials and aeronautical materials are becoming more and more stringent，its properties need further improved and the manufacturing cost need reduced. As a low-cost semisolid metal processing technique，continuous rheo-extrusion process was proposed in recent years. The continuous rheo-extrusion process has the characteristics of short process，high efficiency and energy sav⁃ ing，compared with the conventional metal processing methods，the energy consumption saved by more than 40% and the cost reduced by more than 30%. The basic principle of this technique is that mushy zone exists during continuous casting and extrusion process，melt is sheared and stirred by rotary work roll to refine and spheroidise solid phases during solidification，integrating slurry preparation with extrusion. During continuous rheo-extrusion process，the initial and final periods of casting operation are the nonequilibrium stag⁃ es. Technical parameters have great influences on the process stability and mechanical properties of product，for instance，excessive extrusion speed will lead to instability in the production process. In order to explore the short manufacturing process of aluminum al⁃ loy，6061 alloy wire with diameter 9.5 mm was prepared by continuous rheo-extrusion. 6061 alloy was melted at 700~760 ℃，the melt was refined by refining agent for 20~30 min to remove the oxides and impurities after being melted. The process parameters such as casting temperature and extrusion speed were strictly controlled in the experiment. After rheo-extrusion process，continuous solution treatment was applied for 6061 alloy at 535 ℃ for 3 h，and aging treatment at 150~190 ℃ for 4 h. The grain size and distribution of precipitation phase were systematically analyzed by means of X-ray diffraction analysis（XRD），microscope analysis，scanning elec⁃ tron microscope（SEM）and energy dispersive analysis（EDS）. In addition，the effects of casting temperature，extrusion speed and ag⁃ ing temperature on the mechanical properties of 6061 alloy were explored，the optimum conditions for continuous rheo-extrusion were determined. The results showed that the optimum casting temperature was 720 ℃ and the optimum extrusion speed was 0.157 m·s-1 for 9.5 mm diameter 6061 alloy wire by continuous rheo-extrusion. Heat treatment after rheo-extrusion process is an effective method to ad⁃ just the mechanical properties of aluminum alloys，which could be used to improve the mechanical properties of Al-Mg-Si alloys，and the heat treatment conditions had great influences on the microstructure and mechanical properties of the alloy. The effects of different heat treatment on microstructure，tensile strength and elongation of 6061 alloy by continuous rheo-extrusion were investigated. The re⁃ sults showed that the tensile strength and elongation of 6061 alloy by continuous rheo-extrusion were obviously affected by aging tem⁃ perature. When the aging temperature increased from 160 to 175 ℃，the tensile strength of 6061 alloy wire increased from 270.14 to 274.11 MPa，and reached a peak，its elongation decreased from 18.02% to 16.32%，when the aging temperature was increased to 190 ℃，tensile strength decreased from 274.11 to 265.12 MPa，and its elongation decreased from 16.32% to 13.16%. The tensile strength of 6061 alloy by continuous rheo-extrusion was the highest under the heat treatment conditions of solid solution at 535 ℃ for 3 h and aging at 175 ℃ for 4 h，compared with 6061 alloy after T6 heat treatment under standard of wrought aluminum and aluminum al⁃ loys extruded profiles for general engineering，its tensile strength and elongation were improved by 5.43% and 104%，respectively. Mg2Si was the main strengthening phase of 6061 alloy. Due to dislocations and grain boundaries could be pinned effectively by the dis⁃ persed Mg2Si phase，and tensile strength of 6061 alloy was improved accordingly. The aging treatment caused solute atoms to precipitate and form the second phase，strengthening phase Mg2Si homogeneously precipitated，which increased the mechanical properties and toughness of 6061 alloy. The amount and morphology of precipitated phase in 6061 alloy were influenced by aging temperature. As the aging temperature increased，the tensile strength of 6061 alloy increased and then decreased. When the aging temperature was lower than 175 ℃，the amount of precipitated phase increased with the increased of aging temperature，and then made the tensile strength of the alloy increased. However，when the aging temperature was higher than 175 ℃，as the aging temperature increased，the density of secondary phases became smaller due to the fast rate of atomic diffusion，and the tensile strength of the alloy decreased with the de⁃ creased of lattice distortion. The amount of precipitated phase increased as the aging temperature increased，which impeded dislocation movement，and the plasticity of alloy decreased，thus leading to the tensile of 6061 alloy decreased with increasing aging temperature. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.