Porosity formation in aluminium alloy castings under quasi-directional solidification

There is a significant variation in the amount of porosity found in directionally solidified castings. The porosity is least at the chill and increases towards the riser. A recent publication has suggested that this porosity distribution is caused by the rejection of hydrogen into the liquid during freezing, which results in chemical macrosegregation of dissolved gas along the length of the casting. Experiments were conducted using five aluminium-based alloys in order to test this hypothesis. The results show that chemical macrosegregation of dissolved hydrogen does not occur to any measurable degree. Instead, the observed distribution of porosity appears to be caused by effects related to changes in solidification rate and dendrite arm spacing. A simple “thermodynamic” model, which includes the effect of surface tension of the pore bubble, accounts for general features of the porosity distribution.