Degassing, hydrogen and porosity phenomena in A356

The decrease in the solubility of hydrogen in liquid aluminium with temperature has been believed to be a major source of porosity. Therefore, degassing treatment is carried out in foundries to decrease porosity in castings. However, it was shown that it is difficult to nucleate hydrogen porosity in the absence of bifilms where bifilms simply aid the growth of pores. For this purpose, several casting experiments were carried out with commercial A356 alloy. In the first series of testing, the melt was degassed and then upgassed to three different levels. In the second series, the melt was first upgassed and then degassed gradually. In another test, hydrogen level was kept constant and sample collection was carried out. In all the trials, 10 bars were cast into sand mould to produce cylindrical samples for tensile testing. A step mould was used to investigate the porosity distribution and tensile samples were also collected from the same casting. For each casting experiment, a reduced pressure test sample was taken to check metal quality by using bifilm index. It was found that the turbulence and vortex (i.e. increase in bifilm population) during rotary degassing has a more significant effect on mechanical properties and porosity than the hydrogen content.