Hydrogen and cooling rate effects on microporosity formation in the production of defect-controlled fatigue specimens

In experiments aimed at the production of fatigue specimens, the increased number of nearly identical specimens needed for each processing condition, together with the high sensitivity to pore size, call for very strict requirements of both the casting tool and the processing conditions. An experiment for producing aluminium alloy fatigue specimens by gravity casting with controlled microstructure and defects is presented here. The main requirements to be obtained on a set of specimens (extracted from a single casting block) were to have the near identical microstructure and gas porosity content. The main process parameters were the hydrogen level of the melt, the addition of oxides for improving the number of pore nucleation sites and the cooling rate within the casting mould. The distribution of the relevant properties (SDAS, %area of porosity) was measured throughout the casting plates in order to validate the design criteria of both the experiment and the mould.