Analysis of bubble behavior and microstructure of aluminum alloys welded by electron beam

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Aluminum alloys were welded by electron beam under both microgravity and terrestrial conditions to investigate the beam stability, the effects of welding parameters on the convection in a molten pool, microstructure and bubble behavior. First, a small-sized electron beam welding system was developed to be loaded in the capsule for the drop shaft type microgravity facility at JAMIC. The distance between the filament and the sample is just 370 mm. The beam diameter was set to be 3 mm to perform a thermal conductive welding as in arc welding.By performing transmission X-ray analyses, it is found that the number of bubbles decreases under microgravity. Butt-welding of 5NAl and Al-6 %Cu alloy was performed in a horizontal direction under both microgravity and terrestrial conditions and consequently, the convection due to surface tension is found to be very weak and even weaker that that due to gravity, although it is thought to be largest. This fact causes the number of bubbles to decrease under microgravity. When atomic oxygen is irradiated to the aluminum surface, the thickness of the oxide film increases, and consequently, the number of bubbles increases in the bead. These all results do not contradict the hypothesis that bubbles are generated due to the reaction between molten aluminum and Al_2O_s, producing Al_2O gas phase.When horizontal welding performed under terrestrial conditions, the molten pool falls downward, the solidification rate decreases in the middle and lower parts of the bead, and hence, a feather crystal structure is easily formed and the grain size becomes larger.