Study On Melt Degassing And Grain Refinement Of Al - Si Alloy By Electromagnetic Directionally Solidification

In this paper, for developing a clean and environmentally friendly degassing route, we design herein a strategy for degassing of molten aluminum alloys under electromagnetic field via directional solidification. We evaluated the degassing feasibility and mechanism for the formation and evolution of the porosity. As expected, it enabled the migration of porosity to the upper end instead of the distribution in the whole sample. Furthermore, we investigated the effects of various Al-Si ratios (Al-35wt% Si, Al-45wt% Si, Al-55wt% Si, Al-65wt% Si), induced current intensity (12 A,15 A,18 A), pull-down distances (4 cm,8 cm,12 cm,14 cm,16 cm), pull-down rates, i.e., solidification rates (25 μm/s,20 μm/s,15 μm/s,5 μm/s) and re-melting on the degassing efficiency, porosity area fraction, and grain refinement of alloys, etc. Clearly, the experimental results show that with the reducing of pull-down rates, the degassing efficiency increases and the porosity area fraction decreases, respectively. The lowest porosity area fraction is down to 0.08% under the 5 μm/s pull-down rate. Simultaneously, there are almost no porosity and other defects in the lower part of the final product. We observed the characteristics of the sample from the perspective of macro and micro morphology, such as, SEM, OM, SEM-EDS, XRD, Image Analysis Software (Image Analyzer V1.36.1), Electronic Universal Testing Machine (TE-XWW-20), and Digital camera, etc.In summary, we have successfully developed an efficient method for degassing and grain refinement of Al-Si melt via the electromagnetic directional solidification. This is the first example of degassing and grain refinement for aluminum alloys which combined with electromagnetic stirring and directional solidification technology. It was shown to have attractive features, including environmentally benign conditions and high degassing efficiency. Overall, we can conclude advantages for this method into two separately parts: for the electromagnetic stirring, (i) fully release hydrogen atom which is initially present in the solid into the melt and combined with each other to turn into hydrogen molecules; (ii) convert electromagnetic oscillation signals into mechanical oscillations which would produce a lot of cavitation bubbles as the hydrogen degassing carrier; (iii)generate a flowing within the melt which is beneficial for improving the degassing rate; for the directional solidification, to produce a temperature gradient which will cause hydrogen solubility variation, the cavitation bubbles will move and enrich to the higher temperature region gradually. Additionally, our further investigations into the effect of different cooling system on the fabrication of aluminum alloys and their potential applications in aeronautic and space industry are currently ongoing.