Modification of Primary Si Crystals and Fe-rich Intermetallics in LM28 Al Alloy Fabricated Through Thixoforming

In the present research, thixoforming route was carried out in order to enhance the microstructural features of LM28 piston alloy. Typical microstructure of this alloy was composed of coarse, polygonal primary silicon particles, eutectic matrix and intermetallic phases. Thermal analysis was carried out to study the solidification path of the base alloy and determine the major arrest temperatures of metallurgical reactions. Continuous and iso-thermal mechanical stirring were utilized to produce non-dendritic LM28 alloy feedstock for further processing. The rheocast samples were subjected to a rotation speed of 450 rpm. The slugs machined from the solidified rheocast specimens were heated in the mushy zone temperature and then were thixoformed via a laboratory press. The thixoformed specimens show a relatively homogenous microstructure and present no evidence of porosities. Fine, blocky primary silicon and Fe-rich intermetallic particles were uniformly distributed in the matrix of LM28 alloy. Optical microscope and scanning electron microscope linked with EDX were used to investigate the microstructure of specimens.