Owing to the severe segregation in the microstructure and the poor mechanical properties of the as-cast 7075 aluminum alloy, which fail to meet the application requirements of automotive products, the work aims to improve the segregated microstructure and mechanical properties of the alloy through solution and aging treatment, so as to satisfy the application requirements of automotive products. The as-cast 7075 aluminum alloy was adopted as the research object, and both single-stage solution + artificial aging (T6) and two-stage solution+artificial aging (T6) treatments were conducted. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to analyze the evolution of the microstructure and phase constituents of the alloy under different heat treatment regimes. An electronic universal testing machine was used to evaluate the room-temperature tensile mechanical properties and to observe the fracture morphology. The effect of the solution and aging treatment regimes on the microstructure and mechanical properties of the as-cast 7075 aluminum alloy was systematically investigated. The as-cast 7075 aluminum alloy was mainly composed of an α-Al matrix, MgZn2 phase and Al2CuMg phase, with pronounced segregation. After single-stage solution T6 treatment, the main phases of the alloy were α-Al, MgZn2 and Al2CuMg, and the secondary phases precipitated at grain boundary triple junctions with spherical and rod-like morphologies, exhibiting an average diameter of approximately 23 nm. After two-stage solution T6 treatment, only the diffraction peaks of the α-Al and Al2CuMg phases appeared in the XRD patterns, indicating that the MgZn2 phase was essentially dissolved into the matrix. The spherical and rod-like secondary phases at grain boundaries and within grains decreased remarkably, while granular phases increased with a tendency to coarsen, and a small amount exhibited a blocky morphology. During aging, the Al2CuMg phase continuously nucleated and grew by consuming the MgZn2 phase, leading to an increase in its volume fraction and a growth in the average diameter to approximately 44.5 nm. Mechanical property tests demonstrated that the alloy after two-stage solution T6 treatment achieved an ultimate tensile strength of 647.2 MPa, a yield strength of 605.2 MPa, a fracture toughness of 41.7 MPa·m1/2 and an elongation of 8.76%. The tensile fracture transformed from intergranular brittle fracture to transgranular ductile fracture, with a large number of dimples distributed on the fracture surface. Compared with the single-stage solution T6 treatment, the two-stage solution T6 treatment can more thoroughly eliminate the segregation of the as-cast 7075 aluminum alloy, promote the dissolution of the MgZn2 phase, and refine and homogenize the secondary phases, thereby significantly improving the comprehensive mechanical properties of the alloy (ultimate tensile strength of 647.2 MPa, yield strength of 605.2 MPa, elongation of 8.76%, and fracture toughness of 41.7 MPa·m1/2), and changing the fracture mode from intergranular brittle fracture to transgranular ductile fracture. After the two-stage solution plus artificial aging treatment, the mechanical properties of the as-cast 7075 aluminum alloy can satisfy the application requirements of automotive products, which provides a process basis for the application of this alloy in automotive structural components.
Key words
7075 alloy /
heat treatment /
microstructure /
mechanical properties /
fracture toughness
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