Formation and disappearance of pores in aluminum alloy molten pool under microgravity

With the start of construction of the international space station, a welding technique in space is becoming essential for the construction and repair of space structures. We now describe the results obtained under microgravity by electron beam welding, which is thought to be the most probable candidate for space welding. The microgravity environments were achieved using the drop-shaft system at the Japan Microgravity Center. The system can produce a microgravity of 10–25 G and a duration of 10 s. The sample was an aluminum–copper alloy, 2219, which is commonly used in the aerospace industry. The effective diameter of the electron beam was set at approximately 3 mm in order to prevent the keyhole formation. A significant decrease in the number of the pores in the Al alloys was observed under microgravity. All of the pores smaller than 100 μm disappeared for both microgravity and terrestrial environments. These results indicate that there exists a new bubble formation mechanism, consisting of a reaction between the molten Al and Al2O3 forming Al2O gas, and that the convection due to the Marangoni flow is negligible in an Al molten pool.