Synchrotron Radiation Imaging Of Bubble Behavior In Aluminum Alloy Melts

Aluminum alloy is a kind of metal material of adsorbing hydrogen easily andusually analyzed with methods of solidification microstructure analysis, numericalsimulation and physical simulation to make research on bubble nucleation, growthand distribution and other issues, which is expected to solving problems of hydrogenpores in castings and welding products, etc. Hydrogen pore is one of the most seriousdefects in aluminum metallurgy, and seriously affect the mechanical properties,apparent quality and corrosion properties of materials. Due to the opacity of the metalmaterial, it is difficult to directly observe the solidification process. A possiblesolution to this problem is provided by the appearance of synchrotron radiationimaging, whose high-brightness, high time and spatial resolution advantages can beused to in situ observations of bubble behavior in aluminum alloy melt, which isbeneficial to get knowledge of melt aluminum pore formation process and mechanism,so developing appropriate measures to reduce or eliminate hazards process pores isprovided by strong scientific basis.Compositions of Al-5.0wt.%Ca and Al-10wt.%Zn are employed in inductionmelting and melt is injected into dimethyl silicone to prepare spherical samples.Because it is difficult to completely avoid moisture, Al reacts with water easily andgenerates hydrogen, which is retained in the form of pores near the surface of alloyballs after solidification. With synchrotron radiation imaging technique of ShanghaiSynchrotron Radiation Facility (BL13W1Line Station), observed are phenomenonof the evolution of residual pores into bubbles in melting，and bubble motion, growthand consolidation in superheating, as well as the evolution of bubbles into the poresin the solidification process, revealing the growth behavior and distribution of bubbles.In the heating and melting process, irregular pores is gradually spheroidizing withdecreasing strength of dendrite network nearby. For the cooling and solidificationprocesses, the situation is reversed. Except for individual bubbles remaining spherical,most bubbles are squeezed by the dendrite network and limited into irregular shapes.The initial diameters of the bubble group present Gaussian distribution characteristic,which is because a random nucleation of bubbles presents a Gaussian distribution.The initial growth of the bubbles, controlled by diffusion, does not alter thecharacteristics, so that the initial diameters present a Gaussian distribution. However,with the movement collision and merging of the bubbles, the distribution of bubblediameters is no longer maintained stochastic. Individual bubbles grow in a stepwisemanner, which is because the growth is a competition process for hydrogen, therebyhydrogen depleted peripheral region is established around the bubbles, so that thehydrogen supersaturation decreases, leading to bubbles stop growing. When thehydrogen supersaturation accumulates over a period of time, reaching a certain criticalvalue, the bubble continues to grow, so a stepwise growth manner is shown.After comparisons of Al-Ca and Al-Zn alloys, alloying elements are found tohave a greater influence on the behavior of bubbles. Due to differences in the surficialoxides, gas permeability of Al-Zn alloy is better than Al-Ca alloy. Because of theescape of hydrogen bubbles, bubble growth becomes harder, which makes sense forthe degassing of aluminum alloy. Related work needs further careful study.