Metallography of aluminum and it's alloys solidified under high hydrostatic pressure

This paper reports macroscopic and microscopic metallography of aluminum and its alloys solidified in a cylindrical die under high hydrostatic pressure of 1000-6000kg/cm2. When the pressure was applied at 50-60°C higher than the melting temperature, columnar crystals, which had originated on the surface of die, easily developed towards the center of die, although other conditions were selected to allow the formation of equi-axed crystals under normal pressure. Less superheating, on the other hand, often generated a center zone of equi-axed crystals, and the zone was wider with the increase of pressure. Preferential linear growth of dendrite stems in columnar zone remarkably occurred under high pressure, and exclusion of solute to liquid side of solid/liquid interface was facilitated by the application of high pressure. It resulted in normal segregation in aluminum-silicon and aluminum-copper alloys. When the pressure was constant and the temperature at which pressure was applied was lower, the equiaxed zone was wider, and the crystals were finer by the effects of undercooling (accompanied by the rise of the melting temperature) and rapid cooling. It was found that pure aluminum solidified at 11°C higher than normal melting temperature under applied pressure of 1500kg/cm2.