Effects of hydrogen and bifilms concentrations on the porosity and mechanical properties of an AlSi11(Fe) foundry alloy

The aim of this work is to investigate the relationship that exists between the hydrogen content of an aluminum casting alloy and the assessment of melt quality. Defects, and most importantly porosity, affect the microstructure and the mechanical properties of cast alloys to such a degree that defects analysis, control and prevention has become one of the main fields of research in casting engineering. Porosity can be divided into gas porosity, correlated to the Hydrogen dissolved into the melt, and shrinkage porosity. It has recently been demonstrated that hydrogen cannot nucleate homogeneously nor heterogeneously into liquid aluminum, a result that forces to abandon old beliefs that attributed the origin of gas porosity into cast aluminum alloys to the nucleation of hydrogen bubbles into the melt during the solidification phase. This thesis tries to clarify the role of hydrogen in pore formation in reduced pressure test (RPT) samples, the analysis of which is often used to assess the quality of the melt. To do so, six different melts were prepeared, each one characterized by different quality of the raw material, type of damage induced during melt handling, and hydrogen content. RPT samples and tensile test bars were collected and analyzed in order to correlate the melt conditions to the development of porosity under reduced pressure and mechanical properties. The first chapter of this thesis focuses on the study of the literature, scrutinizing the work of other authors. The second chapter is dedicated to the discussion of the experiment’s purpose and experimental procedure. The third and conclusive chapter has the objective of discussing the results of the experiment.