The effects of the melt holding time on the type, quantity and morphological distribution of aluminum alloy 2618 were investigated by means of OM, XRD and SEM. And the influence of the melt holding time on the hardness of alloy was explored. The results show that the heredity of the microstructure of the master alloy is not completely eliminated when the melt holding time is less than 10 min at 740 ℃. And it will make Al9FeNi phase coarse needle-like, and segregate with Al7Cu2Fe phase at the grain boundary or inside the grain. Properly prolonging the melt holding time can change the shape of Al9FeNi phase from coarse needle-like to fine needle-like and make the distribution of Al9FeNi phase and Al7Cu2Fe phase more uniform. Aluminum alloy 2618 was melted at 740 ℃ for 30 min and then poured into graphite molds at room temperature, which can make Al9FeNi phase fine and uniformly distributed in the matrix. With the extension of the melt holding time, the size of the nonequilibrium crystalline phase is basically unchanged, but the grain size will be slightly coarsened. The hardness of the alloy showed a trend of first increasing and then decreasing with the prolongation of the melt holing time within 10~60min, and when the melt was held for 30min, the hardness of the alloy reached the maximum HRB value of Rockwell hardness within the range of this test. The maximum value is 48.62HRB, and the hardness distribution of the alloy is relatively uniform. Therefore, it is beneficial to refine the precipitated phase, reduce the macro segregation, and refine the grain and improve the hardness of the alloy, while alloy 2618 was poured into the graphite casting at room temperature with melting at 740℃ and held for 30min.

The influence of melting-casting and heat treatment processes on the hardness of 2618 aluminum alloy was investigated by hardness testing. The results show that the hardness of 2618 alloy increases at first and then decreases with the prolongation of melt holding time, and the hardness reaches the peak value when the holding time is 30 min. With the increase of melting-casting temperature, the hardness of the alloy increases rapidly at first and then decreases slightly. The hardness of the alloy first increases and then tends to be stable as the homogenization temperature rises and decreases rapidly, and then tends to be stable with the extension of melt holding time. Moreover, the effect of homogenization temperature on the hardness is more obvious. With the increase of aging temperature, the hardness of 2618 alloy increases first and then decreases, and the hardness of the alloy increases sharply first and then decreases with the increase of aging time. The hardness of the alloy reaches the peak value of 68.88 HRB after aging at 190 °C for 12 h.

The thermodynamic-related physical parameters, precipitation amount of equilibrium phase, and initial precipitation temperature of Al-(1.9–2.5) Cu-(1.2–1.8) Mg-(0.9–1.5) Fe-(0.9–1.5) Ni alloy were simulated by using the thermodynamic simulation software JMat-Pro. The results show that with the change of element content, the liquidus temperature is in the range of 641.92–638 °C. The precipitation transition temperature of the eutectic phase with a low melting point ranges from 515 °C to 517.25 °C. The solidus temperature is closely related to Cu/Mg. The dendrite spacing decreases with the increase of Mg and Fe (Ni) contents, among which Mg has the greatest influence, followed by Fe (Ni) and Cu. The partial molar Gibbs free energy of Cu, Mg, Fe, and Ni is negative. The activity of Mg increases with the decrease in temperature, and the maximum activity is 0.10189, while the activities of Cu, Fe, and Ni are close to 0. The initial precipitation temperatures of S(Al2CuMg) and Al9FeNi phases are 424–450 °C and 635.4–643.01 °C, respectively. The initial precipitation temperature of the Al9FeNi phase decreases with the increase of Cu and Mg contents, while it increases with the increase of Fe (Ni) contents. With the increase of Cu and Mg contents, the initial precipitation temperature of S(Al2CuMg) increases and decreases with the increase of Fe (Ni) contents. The precipitation amount of S(Al2CuMg) mainly depends on the content and ratio of Cu and Mg, while that of Al9FeNi increases with the increase of Fe and Ni elements in the same proportion.