Isothermal Solidification of Micro Layers of Molten Aluminum Alloys

Isothermal solidification of micro layers of aluminum binary alloys has been analyzed experimentally and theoretically. An in-situ real time monitoring of the behavior of molten eutectic, and sub-eutectic Al-Si alloys on a reactive Al substrate have been conducted using the hot stage microscopy. All tests were conducted in a high purity nitrogen background atmosphere. The heating ramp rates were in the range of 20 K/min to 50 K/min, the peak temperature was in the range between 858 K and 873 K, and the cooling rate during quench was 100 K/min. A transient moving boundary diffusion model of Si transport across the liquid/solid interface has been corroborated with empirical findings. A heuristic explanation of the cause of isothermal solidification and a comparison of experimental data and numerical predictions convincingly illustrate that (1) slow solid state diffusion into the substrate and (2) a much faster diffusion within the liquid phase are both the key controlling factors of solidification kinetics. The margin of an agreement between modeling predictions and empirical data is strongly influenced by an uncertainty in the estimation of diffusion characteristics across the interface. Phenomena considered are critical for the magnitude of erosion/dissolution of substrate during controlled atmosphere aluminum brazing processes; hence they are essential for the integrity of resulting metal-metal bonds facilitated by brazing.