The microstructural evolution of direct chill (DC) electromagnetically cast (EMC) 7075, 7050, 7020, 7475 and Titanal aluminum alloys ingot was investigated during homogenization. Investigations were performed utilizing metallographic analysis, scanning electron microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC), Transmission Electron Microscopy (T...
The microstructural evolution of direct chill (DC) electromagnetically cast (EMC) 7075, 7050, 7020, 7475 and Titanal aluminum alloys ingot was investigated during homogenization. Investigations were performed utilizing metallographic analysis, scanning electron microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC), Transmission Electron Microscopy (TEM) and Tomography. All dominant phases were analysed according to their chemical composition, shape and dissolution temperature. The analysis of recorded SEM images together with DSC and EDX analysis indicate that, below solidus temperature, five major intermetallic phases M, T, S, Mg-Si and Fe-rich particles (Al23CuFe4, Al7Cu2Fe) can exist. Moreover, the type and intrinsic character of intermetallic phases will differ in different alloys. Under the homogenization treatment, it is possible to dissolve the eutectic network (α–Al + M–Mg(Al,Cu,Zn)2 + S–Al2CuMg + T–Mg3(Al,Cu,Zn)2) totally into the matrix. But tomography results show that, after dissolution of eutectic in 7075, 7050 and Titanal alloys, some cavities remain in the microstructure. The thermodynamic modelling (Scheil simulation) results revealed that, by using the proper chemical composition, smaller S-phase fraction can be obtained after solidification, which leads to dissolution of S-phase and reduction/inhibition of pore formation throughout the homogenization process in 7075 and 7050 alloys. Moreover, the investigations confirm that the phase fraction of Mg2Si by homogenizing under 500°C is nearly unchanged over the whole process, while the phase fraction of the Fe-rich particles (Al23CuFe4, Al7Cu2Fe, and Al80(Cr,Fe)14–16Cu4–6 ) remains mostly constant. For the Fe-rich particles, it is also observed that these phases are not dissolved into the aluminum alloy during homogenization (<500°C), due to their high melting points. At 520°C, only spheroidization and thinning of Fe-rich particles is observed. For all alloys with Al3Zr dispersoids, the heating rate plays a crucial role in determining the size and distribution of the dispersoids. A low heating rate (TITANAL: 0.52K/min) leads to very fine and homogeneously distributed dispersoids, while a heating rate of more than 0.83K/min, which has been applied to the 7020 alloy, leads to a heterogeneous distribution and a mean radius of more than 14 nm (TITANAL: 4 nm).
Irrespective of the dispersoid type (Al3Zr or Al18Mg3Cr2), the number density significantly reduced at high temperatures with time. At the end of the homogenization process, the number density is found to always be significantly lower than the first calculated number density. Looking at the development of the mean radius and the number density at different temperatures, it is concluded that the highest nucleation rate for Al3Zr dispersoids is achieved at temperatures between 400-470°C. The ideal temperature also varies with Zr concentration.
The Al18Mg3Cr2 dispersoids nucleate very homogeneously at temperatures below 400°C, resulting in a number density increase every time this temperature level is crossed.
The published and available thermodynamic descriptions of the before-mentioned phases in the literature do not fully predict the effect of homogenization treatment procedures on these particles, Therefore, based on the acquired experimental knowledge, thermodynamic description of above phases have been reassessed and MatCalc thermodynamic database version “mc_al_v2.019.tdb” for the 7xxx series has been established. By using MatCalc software, it is now possible rapidly (approximately between 2 min to 6 hours, dependent on the particular setup of the simulation) to predict the evolution of phases with different chemical composition and arbitrary time/temperatures heat treatment during a homogenization process. A good quantitative agreement is found between the model predictions and the results from the experimental investigations.