Fatigue crack initiation and propagation of wire arc additive manufactured Al-Mg alloy

•Fatigue damage mechanics was revealed by fatigue crack observation.•Multi-crack initiation is the fatigue damage characteristic for AM parts. Most small cracks are non-propagating cracks.•The small crack growth is affected by the periodic structure of AM.•The main crack tends to initiate from the coarse columnar grain boundary in the deposition zone.•Fatigue cracks initiate at the pores, non-melted particles, and grain boundaries.