Bubble formation in aluminium alloy during electron beam welding

Electron beam (EB) welding and gas tungsten arc (GTA) welding were performed on aluminium alloys in both terrestrial and microgravity environments. A new mechanism of bubble generation is proposed, while hydrogen is generally the major source of the porosity in aluminium alloys. It is deduced from the following results that the bubbles are formed through a reaction between the molten Al and Al2O3 forming Al2O. (1) The pores are formed only in a vacuum, as during electron beam welding. (2) The pores are distributed only in the upper part, while the pores due to hydrogen are widely distributed in the upper half. This result indicates that the pores are formed due to a chemical reaction occurring at the highest temperature, and not due to the decrease in the solute species. (3) The number of pores is significantly decreased under microgravity during electron beam welding, although this number is increased for GTA welding using a shielding gas containing hydrogen. (4) The number of pores is increased as the thickness of the oxide film is increased.