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

在微重力和地面条件下对铝合金进行了电子束焊接，研究了电子束的稳定性、焊接参数对熔池对流、显微组织和气泡行为的影响。首先，研制了一种小型电子束焊接系统，并将其装载在JAMIC跌落轴型微重力设施的太空舱中。灯丝和样品之间的距离仅为370毫米。梁的直径被设置为3毫米，以执行热导焊接在弧焊。通过透射x射线分析，发现微重力作用下气泡数量减少。在微重力和地面条件下，5NAl和al - 6% Cu合金在水平方向上进行对接焊，因此，表面张力引起的对流非常弱，甚至比重力引起的对流更弱，尽管它被认为是最大的。这一事实导致在微重力下气泡的数量减少。当原子氧照射到铝表面时，氧化膜的厚度增加，因此，珠中的气泡数量增加。这些结果都与铝液与Al_2O_s反应产生气泡，产生Al_2O气相的假设不矛盾。在地面条件下进行水平焊接时，熔池向下下降，焊头中下部的凝固速度减小，容易形成羽状晶体结构，晶粒尺寸变大。

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.