Porosity induced fatigue damage of laser welded 7075-T6 joints investigated via synchrotron X-ray microtomography

The interaction between the pores and the fatigue damage in 7075-T6 joints was characterised via high resolution synchrotron radiation X-ray microcomputed tomography. The spatial distribution and growth behaviour of the pores were investigated and quantified. The gas porosity inside 7075-T6 hybrid welds was found to play a significant role in the fatigue crack nucleation and propagation process. The volume fraction and the total number of pores increased slightly with increasing number of fatigue cycles, which provides a key indicator for the damage evolution. Experimental results indicate that the micropores grew and distorted during fatigue cyclic loading, eventually resulting in fatigue cracks. Under cyclic loads, the maximum micropore is usually not a favourable site, in terms of physical suitability, for fatigue crack initiation and propagation, and the diameter of the cracked pores is in the range of 10-100 mu m.