Abstract
• Faster hydrogen removal technology than conventional rotary degassing methods. • Effective elimination of oxide bi-films by innovative rotor-stator design. • High technology efficiency as very low gas flow and rotor speed required. • No covering flux required to maintain good melt quality for long periods after degassing. • Improved casting integrity and mechanical properties due to enhanced melt quality. The degassing of aluminium alloy melts is a crucial step in the production of high-quality casting products, as the presence of dissolved hydrogen and oxide bi-films is detrimental to the mechanical properties. Current rotary degassing techniques are effective, but they lack efficiency because of the high gas flow and long processing times required. This study aims to solve this problem by presenting an innovative rotor-stator degassing technology, that combines controlled inert gas injection with intensive melt shearing. It has been applied to the liquid metal treatment of an aluminium cast alloy to evaluate the effect on melt cleanliness, casting integrity and mechanical properties. The optimum conditions for an efficient bubble dispersion have been obtained by water modelling. The melt quality during and after degassing has been assessed by in-situ measurement of hydrogen concentration and by reduced pressure test sampling for oxide bi-films and porosity content evaluation. This new technology is faster, requires less gas flow consumption and produces higher melt quality than the existing degassing techniques, due to a characteristic combination of distributive and dispersive mixing flow. In addition, re-gassing is minimised, maintaining a high melt quality for longer time after processing. This results in castings with less defects and better mechanical properties. The improved degassing efficiency of this technology makes it an excellent alternative in industry to increase melt quality and casting productivity.
Highlights
Aluminium alloys are widely used in various industrial sectors for the purpose of weight saving and because of the excellent combination of properties, as reported by Polmear (1995) the presence of porosity in the final casting components is quite common and has a detrimental effect on the mechanical properties, especially the elongation and tensile strength, as reported by Caceres and Selling (1996) and the fatigue behaviour, as reported by Ammar et al (2008)
The degassing of aluminium alloy melts is a crucial step in the production of high-quality casting products, as the presence of dissolved hydrogen and oxide bi-films is detrimental to the mechanical properties
Degassing methodology The objective of the study was to evaluate the effect of the new high shear degassing technology, but to avoid bias and for comparison purposes, three degassing methods were used during this investigation, one on each crucible
Summary
Aluminium alloys are widely used in various industrial sectors for the purpose of weight saving and because of the excellent combination of properties, as reported by Polmear (1995) the presence of porosity in the final casting components is quite common and has a detrimental effect on the mechanical properties, especially the elongation and tensile strength, as reported by Caceres and Selling (1996) and the fatigue behaviour, as reported by Ammar et al (2008). It is necessary to ensure a good melt quality before casting by eliminating, or at least reducing, the causes of porosity. Campbell (2003) mentioned two major factors that determine the quality of an aluminium melt. One is the presence of oxide films with poor wettability and the other is the dissolved hydrogen. Both are consequence of reaction of the liquid aluminium with the ambient water moisture, as described in Eq 1, producing atomic hydrogen, that quickly dissolves in the melt, and alumina that is deposited in the surface as a thin layer. When the melt is disturbed during melt handling, the layer breaks and folds over, forming a so called bi-film oxide that becomes entrained inside the melt
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