基于同步辐射X射线成像技术, 对激光复合焊7020-T651铝合金接头内部的微气孔进行了研究, 对气孔的体积、圆整度、扁平度及气孔形心至自由表面的距离等三维特征参数进行了统计分析与拟合. 结果表明, 铝合金熔焊微气孔主要为近球形的冶金型气孔, 圆整度在0.65以上, 以焊缝中心近似呈现对称分布, 且焊缝上部气孔尺寸较大, 热影响区和下部气孔密集且尺寸较小. 等效直径在20 mm范围内的气孔, 在接头上部和下部的频率分别高达65%和85%, 并且100 mm以上的大尺寸气孔较少见. 此外, 由于熔池的下塌倾向和快速凝固, 残留于枝晶网络间液相导致焊缝下部形成垂直于焊缝且层叠分布的形状复杂的热裂纹, 少部分气孔之间以及气孔和热裂纹之间存在着连通现象, 从而使得下部微气孔的圆整度平均值变小. 此外, 焊接速率越大, 整个接头内部气孔体积分数越小, 但对气孔形貌和位置的分布影响并不明显。

Large numbers of complicatedly distributed gas pores are inevitably formed during the hybrid fusion welding of aluminum alloys because of the sharp reduction of supersaturated hydrogen. However, there is no consistent and explicit view on how these gas pores are distributed and influence the static and fatigue property of welded aluminum joints. In this work, pores in hybrid welded 7020-T651 were characterized by high-resolution synchrotron radiation X-ray computed microtomography. The volume, sphericity, flatness and distance of pores centroid to free surface of samples were statistically measured and fitted. From the 3D characterization, micropores inside hybrid welds are mainly metallurgical pores, which are symmetrically distributed about the seam centerline, giving a mean sphericity larger than 0.65. Moreover, pores inside upper welds appear to be larger in effective diameter and denser in heat affected zone and lower welds. Besides, there are numerous pores with diameter less than 20 μm, with a frequency of 65% and 85% in the upper and lower weld, respectively. It seems that hot cracks with complicated morphology form in the lower weld due to shrinkage and rapid solidification of the molten pool. Furthermore, it is found that the connections of a few pore-pore and pore-hot-crack together with the hot cracks result in the smaller sphericity of gas pores in the lower welds. Finally it can be indicated that the higher welding speed gives rise to the smaller pore volume fraction, but has little influence on the distribution of pore position and sphericity。