ON THE MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF SQUEEZE-CAST AL-7 WT-PERCENT SI ALLOY

Specimens of commercial Al-7 wt% Si alloy have been solidified under d ifferent pressures, varying from atmospheric to 70 MN/m2, into rods 10 mm in diameter and 59 mm in length. Two types of porosity have been e xamined in these specimens, macro-porosity (observed at magnifications < 100 x ) and micro-porosity (observed at magnifications > 100 x), an d the pressure was found to have reduced the volume fraction and size of both. Critical pressures have been found for each type of porosity, above which the pores disappear. Different features of the microstruc ture have been examined. Whilst the type of structure does not change with variation of pressure, the volume fraction of primary dendrites i ncreases and their secondary arm spacing decreases. A more homogeneous structure with more perfectly arrayed dendrites is obtained in squeez e-cast specimens. The improvements in soundness and microstructure in squeeze castings are reflected in their mechanical properties, which a re found to be superior to those of gravity-cast castings. By increasi ng the pressure, the tensile fracture strain and strength and the impa ct toughness are increased. Correlation between the fracture tensile s trength and the microstructure is established.