## 1 Introduction  
The 7000 series Al-Zn-Mg (-Cu) alloy is a forged, heat-treated and high-strength aluminum alloy, which mainly achieves the precipitation strengthening effect by precipitating the second phase of the integration or semi-integration in the aluminum matrix. It is the strongest alloy among all aluminum alloys series and has high specific strength, high fracture toughness and fatigue resistance; it has been widely used in the aerospace and military industries [1, 2]. In the Cu-free 7000 series Al-Zn-Mg alloy, its strength is lower than that of the high-strength Al-Zn-Mg-Cu (e.g., 7075) aluminum alloy, but it does not produce a low melting point eutectic phase S(Al\({}_{2}\)CuMg). Therefore, its extrusion properties are better than those of the Cu alloy, and its welding and corrosion resistance are also among the best in the 7000 series alloys. It is one of the main alloys for making large thin walls with high-precision complex solid or hollow structural sections. It has been commonly used as the main material for large lightweight transportation vehicles, such as the manufacture of the extruded sections and plates for high-speed train bodies [3]. It is worthwhile to research and develop a new type of alloy with good weldability and strength without adding Cu atoms. When the aluminum alloy contains trace transition elements such as Mn, Cr, Zr, Sc, etc., whose partition ratios are greater than 1, a peritectic reaction is prone to occur during the casting process and to segregate the grains [4; 5]. During the homogenization heat treatment, dispersed phase grains such as Al\({}_{6}\)Mn, Al\({}_{4}\)Mn, E-(Cr\({}_{2}\)Mg\({}_{3}\)Al\({}_{18}\)), Al\({}_{7}\)Cr, Al\({}_{3}\)Zr, Al\({}_{3}\)Sc, etc., will precipitate in the grains. These dispersed phase grains have high temperature thermal stability [4], and can effectively inhibit the dislocations and grain boundary migrations of the alloy. In addition to increasing the recrystallization temperature of the alloy [5; 6], they can also significantly inhibit the growth of crystal grains [7]. Cr is prone to form E-(Cr\({}_{2}\)Mg\({}_{3}\)Al\({}_{18}\)), which will increase the quenching sensitivity of the alloy and affect the properties of the alloy [8]. Among the trace elements, the inhibitory effects of Al\({}_{3}\)Zr and Al\({}_{3}\)Sc are the best [4], so these two elements (Zr, Sc) were selected for the study. Zirconium has many benefits in aluminum alloys. The aluminum alloy matrix containing zirconium can easily form fine Al\({}_{3}\)Zr dispersed grains at high stable temperature, which can inhibit dislocations or grain boundary migrations and increase the recrystallization temperature of the alloy [9]. The addition of zirconium to the aluminum alloys also has a significant grain refining effect [7]. Zr has higher pore bonding energy than Mg and Zn. It can occupy the pores to suppress the main strengthening \(\eta^{\prime}\) phase (MgZn\({}_{2}\)) from prematurely precipitating in the solution quenching process, and transform the \(\eta^{\prime}\) phase into the \(\eta\) phase to reduce the quenching sensitivity of the alloy [9; 10]. Moreover, with the addition of scandium, it is extremely easy to form very small Al\({}_{3}\)Sc grains in the aluminum alloy matrix, which also have similar effects. When the adding equal amounts of Zr and Sc, certain effects of the Al\({}_{3}\)Sc grains are enhanced [7; 9; 11; 12]. The process of the alloy homogenization heat treatment is performed at a high temperature, which is below the eutectic temperature, and its purpose is mainly to reduce the micro-segregation of the as-cast alloy. Traditionally, in order to improve the solubility and accelerate the diffusion rate of the segregated elements, the homogenization will be performed at a temperature as high as possible. Due to the high temperature, it is easy to cause the aforementioned high-temperature thermally stable phase to coarsen, and to lower the precipitation volume fraction.