Metallurgical Parameters Controlling Fragmentation and Spheroidization Processes of Eutectic Si Particles in Al-Si Cast Alloys

The present study was performed on 356 and 413 alloys solidified at 8 °C/s. The samples from 413 alloys were solutionized at 510 °C, whereas samples from 356 alloy were solution heat treated at 555 °C, for times up to 1200 h. All polished samples were deeply etched using Keller’s etchant to dissolve the aluminum matrix and expose the eutectic silicon particles. Etched samples were investigated using field emission scanning electron microscopy (FESEM). It was observed that Sr-modified 356 alloy can nearly reach complete spheroidization in that all eutectic Si particles will be individually distributed throughout the matrix after a solutionizing time of 1200 h. Non-modified 356 alloy, under the same condition, still revealed partial fragmentation similar to Sr-modified 413 alloy. There was no complete spheroidization in non-modified alloys which explains the reported large standard deviation when polished samples are examined using optical microscopy. In other words, observations made from deeply etched samples (3D) disagree with those obtained from polished samples (2D). In addition, it is impossible to achieve coarsening of the spheroidized Si particles under industrial conditions. The present study was supported by statistical analysis as well as tensile testing.