Fatigue Lifetime of Laser-MIG Hybrid Welded Joint of 7075-T6 Aluminum Alloy by in-situ Observation

The fatigue lifetime of the laser-MIG hybrid welded joint of 7075-T6 aluminum alloy was studied by in-situ observation. The results show that there exists a strong relationship between the fatigue crack initiation lifetime and fatigue failure lifetime. For the base metal specimens the ratio of fatigue crack initiation lifetime and fatigue failure lifetime is about 64.5%, while for the welded joint specimens it is about 20.2%. The observation of fatigue fracture surface indicates that there are a large number of dimples in the base metal specimens, while lots of gas porosities appear in the welded joint specimens instead. These gas porosities are regarded to be the reason of lower fatigue crack initiation lifetime.