Relationship of microstructure to fatigue strength loss in anodised aluminium-copper alloys

Aluminium-copper alloys AA2219 and AA2024 are commonly anodised. The fatigue strength declined by 41 and 29%, respectively, following sulphuric acid anodisation of large cross-section material. The fatigue strength loss was significantly less (3 and 12%, respectively) for sulphuric acid anodised small cross-section material. For large cross-section material, fatigue fractures originated at pits formed from Cu rich constituent dissolution during anodising. No pitting and less fatigue strength loss were observed for small cross-section material compared with large cross-section material. The microstructure of large cross-section material containing a network of Cu rich constituent particles compared with small cross-section material where the particles are small and disconnected relates to the extent of working during mill processing, Limited data on chromic acid anodised material is given.