Effect of gas-induced semisolid process on solidification parameters and dendrite coherency point of Al-4.3Cu alloy using thermal analysis

Semisolid processing of Al-4.3%Cu (A206) alloy was carried out using gas-induced semisolid (GISS) process in different conditions. The flow rate of inert gas, 1, 2, and 4Lmin(-1), starting temperature for gas purging (the temperature of superheated melt), 670, 660, and 650 degrees C, and the duration of gas purging, 10, 20, and 30s, were three key process variables which were changed during this investigation. Thermal analysis was successfully implemented through CA-CCTA technique for GISS samples as well as conventionally cast sample. The two-thermocouple thermal analysis technique was utilized to determine the dendrite coherency point of GISS sample and conventionally cast sample. The results showed that gas purging into the melt led to temperature drop of the melt to its liquidus temperature. In fact, copious nucleation is induced by cooling effect of inert gas bubbles. GISS process delays the dendrite coherency point from 644.2 to 637.3 degrees C which leads to increase the solid percentage from 9 to 21% at this point and therefore enhance the casting characteristics. It is found that inert gas purging into the molten metal, regardless of the process parameters, leads to the microstructural modification from fully dendritic to globular structure. Microstructural evaluation showed that the best sample which included fine grains of 76m in average size and with high level of globularity of 0.86 was achieved from a semisolid sample in which the gas purging started at 670 degrees C and its duration time was 20s with the gas flow rate of 4Lmin(-1).