**3.3 Quenching**

210 Recent Trends in Processing and Degradation of Aluminium Alloys

**0 0.2 0.4 0.6 0.8 1 Fraction of solid, fs**

boundaries. However, it is difficult to distinguish between interdendritic and grain boundary melting in the microstructure. Interdendritic and grain boundary melting is shown in Fig. 13. The Fe-rich intermetallics melt at solution temperatures above 550°C leading to formation of spherical liquid droplets within the dendrites/grains. At high solution temperatures the width of the grain boundary melted zone increases, and spherical interdendritic liquid droplets enlarge and coalesce to form a large network of interdendritic liquid. On quenching this liquid, reprecipitation of silicon and other intermetallic particles may occur, and the average size increases. Quenching also leads to a large amount of shrinkage porosity adjacent to melted regions, which can coalesce and lead to the complete fracture of the casting, as seen in Fig. 14. The amount of liquid phase formed with high

Fig. 12. Solid fraction versus temperature of the A356 alloy used in the present work

temperature solution treatment depends greatly on the initial solidification rate.

Fig. 13. Interdendritic and grain boundary incipient melting

**520**

**540**

**560**

**580**

**Temperature (°C)**

**600**

**620**
