A fine grained area around a subsurface fatigue crack origin can usually be observed on fracture surface of a metallic material after very high cycle fatigue. This paper provides a fundamental study on the mechanisms to form this fine grained area using a martensitic stainless steel and advanced analysis instruments. The results show that the formation of a fine grained zone is a local behaviour. It is only a few micrometres in the transversal direction (cross section) and one micrometre in the longitudinal direction (crack propagation direction). High plastic deformation such as localized dislocation slip bands can be observed in this fine grained area. They interact with grain boundaries and cause the formation of damage by impingement cracking. The results indicate that occurrence of cyclic localized plastic deformation during very high cycle fatigue near the subsurface defect leads to the formation of fine grained area.

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Intrinsic instabilities in the depth of penetration achieved when laser welding aluminum alloys were investigated in this article. Four types of weld pool configuration, resulting in different welds, were identified, only two of which are associated with full penetration. A simple phenomenological explanation of the unstable welding process at the boundary of complete penetration was proposed. The nature and cause of the instabilities were discussed and suggestions were offered for process control.

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