The Effect of Holding Time and Solidification Rate on Porosity of A356

During casting of aluminum alloys, the final microstructure and mechanical properties are strongly affected by the ambient atmosphere. The liquid aluminum reacts with water vapour to form aluminum oxide on the surface and hydrogen is dissolved in the melt. The intrusion of these defects into the cast part will result in degradation of the properties. Excess hydrogen is typically removed by degassing operations. Considering the density difference of liquid and oxide, holding of the liquid for a certain period of time is assumed to be the solution for the removal of oxides. However, the possible change in the morphology of the oxide structure (thin or thick film) has been disregarded. Therefore, in this work, two different charges of A356 alloy was melt at 750 o C in a resistance furnace and held for 3 hours. One of the melts were left untreated, and the other was degassed. Hydrogen level was measured by AlSPEK. At certain time intervals (30 and 120 minute), samples were collected for melt quality change by using reduced pressure tests. A mould that consisted of various thickness was used to cast parts in order to check the porosity distribution at different cooling rates.