Direct chill casting is the most common semi-continuous method used to fabricate rectangular or cylindrical ingots of non-ferrous metals, particularly aluminium, copper, magnesium metals and alloys. The process uses a water-cooled mould with an open bottom. Molten metal is poured into the mould, where the outer layer solidifies taking the shape of the mould while retaining the molten metal in its centre. As the ingot emerges from the mould, water is sprayed directly on the ingot surface, completing the solidification process.
In DC casting, centreline segregation (macro-segregation) is a critical problem. In this region, significant divergence from nominal alloy composition can occur, leading to considerable variations in mechanical properties. Various mechanisms are involved. These include convection, shrinkage induced flow, grain sedimentation and accumulation of solid crystals. The overall macro-segregation pattern is dependent on the relative contributions of these mechanisms.
Until recently, the effect of ultrasonic melt treatment on centreline segregation was not fully explored. Generally, ultrasonic cavitation and acoustic streaming assist in the redistribution of the various elements involved in centreline segregation and certainly weaken it though without necessarily eliminating it.
A significant problem with conventional ultrasonic treatments using fixed frequency technology is the formation of standing waves which are thought to prevent the elimination of centreline segregation. To overcome this, we have recently developed multiple frequency ultrasonic treatment. This eliminates standing waves and can eliminate centreline segregation at an industrial scale.
“A significant benefit of direct chill casting is that solidification occurs in a narrow layer and is relatively easy to control at a macro-level. However, there are significant problems to be overcome, particularly in terms of micro-segregation and grain structure. An approach to managing these phenomena is to use ultrasonic treatment during solidification. The process involves the introduction of one or more sonotrodes into the melt, and ultrasonic energy provides significant benefits for degassing, micro-segregation reduction, and grain structure refinement” Nico van Dongen, project manager ultrasonic degassing, micro-alloying and grain refinement.
Gas porosity is a critical defect in aluminium castings. Porosity degrades mechanical properties, including fatigue resistance. It occurs when the gas concentration exceeds its solubility in the metal. In aluminium the principal gas is hydrogen. This is formed by a chemical reaction between the liquid metal and atmospheric water vapour producing alumina and hydrogen gas. Hydrogen is more soluble in the liquid than it is in the solid, so it precipitates out from the liquid during solidification creating porosity. Thus, to achieve a high-quality casting, dissolved hydrogen must be removed from the molten metal before it solidifies.
While various methods of achieving this are available, ultrasonic degassing in molten aluminium has many advantages, including its relatively low cost and minimal environmental impact. The process involves introducing ultrasonic waves into the melt. Primarily, as the ultrasonic wave propagates through the melt, cavitation bubbles are generated. Dissolved gas then diffuses into these bubbles which rise to the surface and escape into the environment.
Ultrasonic grain refinement
Ideally, a non-dendritic fine grain structure in which the grains are equiaxed (with equal dimensions in all directions) is preferred. Such a structure reduces shrinkage and hot tearing while providing a more even distribution of secondary phases and micro-porosity. The overall result is improved mechanical properties.
Many methods of grain refinement are employed, with their common aim being to increase the number of nuclei by heterogeneous nucleation.
Ultrasonic grain refinement in molten aluminium, which involves introducing acoustic waves with a frequency higher than 17 kHz into the melt, induces compression and expansion waves which can form cavities; a phenomenon known as cavitation. When the cavities collapse, intense heating occurs, resulting in localised high-pressure zones. The effects of cavitation include heterogeneous nucleation, cavitation-assisted fragmentation and grain multiplication, transport of dendrites, and intense mixing.
The effect of ultrasonic energy on grain refinement is critically dependent on the frequency and amplitude of the applied ultrasonic energy.