THREE-DIMENSIONAL IMAGING OF GAS PORES IN FUSION WELDED Al ALLOYS BY SYNCHROTRON RADIATION X-RAY MICROTOMOGRAPHY

Abstract

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 diame- ter and denser in heat affected zone and lower welds. Besides, there are numerous pores with diameter less than 20 pm, 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.