Physically Based Selection of the Characteristic Cooling Rate Interval for Predicting SDAS Under Conventional and Accelerated Solidification Conditions

The secondary dendrite arm spacing (SDAS) is a critical microstructural parameter that directly influences the local mechanical performance of Al-Si casting alloys. Although the power-law relationship between cooling rate and SDAS is widely acknowledged, the existing literature lacks a standardized framework for selecting the most appropriate solidification thermal interval for calculating this characteristic cooling rate. This study introduces an optimized methodological approach by evaluating solidification kinetics across two distinct thermal regimes: a standard non-isothermal sand mold (no chill) and an accelerated cast iron insert configuration (with chill). Cooling curves of a primary hypoeutectic AlSi7 Mg0.3 alloy were continuously recorded via ten calibrated K-type thermocouples along a wedge-shaped casting profile, and the corresponding local SDAS values were quantified using light optical metallography. By mathematically evaluating six distinct thermal analysis boundaries, the results demonstrate that conventional global intervals (e.g., liquidus-to-eutectic) yield lower predictive accuracy () due to the accumulation of thermal “noise” from the early fluidic stage and the final eutectic reaction. In contrast, the localized interval bounded strictly between the Dendrite Coherency Point and the Rigidity Temperature () achieved excellent correlation for both the baseline slow-cooling regime () and the accelerated regime (). This optimized window successfully isolates the exact kinetic timeframe of secondary dendritic evolution and effectively eliminates localized calculation anomalies across the entire spectrum of cooling rates. Thermodynamic analysis reveals a sharp cooling asymmetry in the wedge geometry, where the local solidification timescale collapses by a factor of 34 at the chill base. This extreme kinetic restriction limits the characteristic solute diffusion distance ()) by a factor of nearly six (using representative solute diffusivities for Mg and Si in liquid aluminum), providing a quantitative physical basis for the significantly flatter coarsening slope (−14.85) under accelerated cooling compared to the baseline regime (−32.63). The proposed methodology establishes a physically grounded, highly accurate microstructural prediction tool suitable for advanced foundry engineering and casting simulations.