Microstructure characterization and mechanism of fatigue crack initiation near pores for 6005A CMT welded joint

摘要

The fatigue test of 6005A cold metal transfer welded joint were carried out at room temperature and crack initiation near pores was systematically revealed. The fatigue strength of welded joint at 5 x 106 cycles based on S-N curve can be estimated as half of yield strength, which can be predicted reliably in engineering application. The fatigue test showed welded zone is the weakest zone due to the existence of micropores defects in the surface of the joint, even if there is softening zone in HAZ which is shown in hardness curve. Thinking about the existence of micropores, stress concentration will be produced near the pore even in smaller cyclic stress, which will drop down the crack initiation threshold, and the cupped grain boundary inside the pore may aggravate this condition. The pore in the equiaxed crystal microstructure would produce microcrack more easily than in columnar crystal structure. Moreover, there will be cracks with short-haul near the pore in the sample which isn't failed. These cracks won't impact the fatigue life of sample. But the microcrack occur in each of pores will converge into a macrocrack, when more than two pores gather in one region, then the fatigue property of the joint will be affected, seriously. Therefore, pores should be avoided in the process welding aluminum alloy as much as possible, especially the gathering of multiple pores.