The role of the roll separating force in the high-speed twin-roll casting of aluminum alloys was examined. In horizontal-type twin-roll casting, as the casting speed increased upon decreasing the roll separating force, the strip texture changed from a shear and rolling texture to a random texture. Direct temperature measurements during high-speed twin-roll casting showed that the roll separating force played a significant role in maintaining a good contact between the strip and the roll surface. This resulted in a high cooling rate around the roll nip and enabled the fabrication of a sound strip with a fine microstructure. Moreover, the high casting speed and lowered roll separating force gave a band structure consisting of fine globular grains in the mid-thickness region of the strip, which could be considered beneficial in the formation of a well-dispersed center segregation.

We herein suggest a metallurgical method using pure aluminum with no freezing temperature range to derive appropriate roll/melt interfacial heat transfer coefficients in simulation of twin-roll casting process. This method is inspired by the concept that the position of the kiss points where two solidifying shells encounter and the roll nip coincides under the condition where the roll load becomes zero as the roll rotation speed decreases. The conditions where the roll load becomes zero under various melt supply temperature conditions in the actual TRC process are found experimentally. These conditions are then applied to simulation models to derive heat transfer coefficient values. When comparing these values with those derived previously from the empirical relation for roll rotation speed and heat transfer coefficient, the conclusion is drawn that the deviation was reasonably low, around 10%.

Recent advances in the leisure boat industry have spurred demand for improved materials for propeller manufacturing, particularly high-strength aluminum alloys. While traditional Al-Si alloys like A356 are commonly used due to their excellent castability, they have limited mechanical properties. In contrast, 7xxx series alloys (Al−Zn−Mg−Cu based) offer superior mechanical characteristics but present significant casting challenges, including hot-tearing susceptibility (HTS). This study investigates the optimization of 7xxx series aluminum alloys for low-pressure die-casting (LPDC) processes to enhance propeller performance and durability. Using a constrained rod-casting (CRC) method and finite element simulations, we evaluated the HTS of various alloy compositions. The results indicate that increasing Zn and Cu contents generally increase HTS, while a sufficient Mg content of 2 wt.% mitigates this effect. Two optimized quaternary Al−Zn−Mg−Cu alloys with relatively low HTS were selected for LPDC propeller production. Simulation and experimental results demonstrated the effectiveness of the proposed alloy compositions, highlighting the need for further process optimization to prevent hot tearing in high Mg and Cu content alloys.

The twin-roll casting (TRC) process has gained significant attention for aluminum sheet production due to its cost-effectiveness and high processing efficiency. However, controlling the initial grain structure of TRC strips remains challenging due to the absence of a hot rolling stage, necessitating an advanced predictive modeling approach. In this study, a cellular automaton–finite element (CA-FE) model was developed to predict the grain structure and texture of aluminum strips fabricated via TRC. Both pure Al and AA7075 alloys were cast under identical conditions using a pilot-scale horizontal twin-roll caster, and their microstructures were characterized experimentally. The developed model incorporated a Gaussian nucleation distribution function and an equivalent binary approach to account for the solidification behavior of multicomponent alloys. The CA-FE simulation results successfully reproduced the key aspects of solidification, grain structure, and texture evolution of TRC strips. The predicted temperature distribution and solid fraction evolution showed distinct differences between the alloys, with pure Al forming columnar grains and AA7075 developing a fully equiaxed structure, which closely matched the experimental findings. Additionally, texture analysis using inverse pole figures (IPFs) and pole figures (PFs) revealed a clear orientation in pure Al, whereas AA7075 exhibited a random texture, both of which were well captured by the CA-FE model. The findings indicate that the developed model offers a reliable prediction of the solidification microstructure and texture evolution in TRC strips, making it a valuable tool for optimizing continuous casting processes.

The twin-roll casting (TRC) process is widely used in the aluminum industry due to its cost efficiency and continuous production capability. However, maintaining consistently high surface quality remains challenging due to complex heat transfer behavior at the roll/strip interface. This study examines the critical influence of roll surface conditions, especially the formation of an Al coating layer, on solidification behavior and resulting strip quality in the TRC of an Al-5Mg alloy. Experimental results demonstrated that casting without an Al coating layer led to surface defects such as hot tears and porosity due to insufficient cooling. In contrast, strips produced with a stable Al coating layer exhibited excellent surface quality with no surface defects. Numerical simulations further indicated that a stable Al coating enhanced the interfacial heat transfer coefficient (up to 30,000 W/m²K), ensuring effective cooling and complete solidification before the strip exited the roll nip. Moreover, simulations validated the feasibility of using steel rolls in industrial applications, provided the coating layer was consistently maintained. This research highlights the significance of roll surface control in improving TRC product quality.

With the increasing demand for high-performance leisure boat propellers, this study explores the development of high-strength aluminum alloy propellers using the low-pressure die-casting (LPDC) process. In Part I of the study, we identified the optimal alloy compositions for Al-6Zn-2Mg-1.5Cu propellers and highlighted the challenges of hot tearing at the junction between the hub and blades. In this continuation, we developed a coupled thermal fluid stress analysis model using ProCAST software to optimize the LPDC process. By adjusting casting parameters such as the melt supply temperature, initial mold temperature, and curvature radius between the hub and blades, we minimized hot tearing and other casting defects. The results were validated through simulations and practical applications, showing significant improvements in the quality and structural integrity of the propellers. Non-destructive testing using X-ray CT confirmed the reduction in internal defects, demonstrating the effectiveness of the simulation-based approach for alloy design and process optimization.