Hard anodising is used to improve wear and corrosion resistance of aluminium parts. However, the hard anodised layer can cause a reduction in fatigue life if not taken into account at design and specification stage.
Why the Oxide Layer Causes Problems
A common misconception regarding hard anodising and fatigue, is that the hard surface is not able to scratch and therefore will not introduce stress concentration to the aluminium substrate. In reality the hard anodise layer is a brittle ceramic layer. Under cyclic loading of the component, micro-cracks will form on the surface of the oxide layer. These small cracks will act as stress concentrators at the surface of the component, with the potential for fatigue crack growth originating from the surface. As the thickness of the anodic layer increases, the effect of these surface cracks on the fatigue life becomes more pronounced.
Alloys Most at Risk
High strength alloys such as 7075 and 2024 are generally more sensitive to fatigue reduction due to hard anodising than lower strength alloys such as 6061 and 5083. So if you have a part made from a 7xxx or 2xxx series alloy that is subjected to cyclic loading then hard anodising will reduce the fatigue life and you must design your part with this in mind.
Geometry Choices That Help
Sharp internal radii and changes in section can concentrate stress on the surface. However, with a brittle surface layer, these areas become a greater concern. Use of a minimum radius of 0.5mm on internal corners, smooth transitions between sections and avoid of machined notches in areas subject to bending or vibration.
Shot Peen Before You Anodise
Shot peening the surface of the part before anodising of the aluminium section introduces compressive residual stress into the surface of the part. This tensile stress is what causes fatigue crack growth. Therefore by shot peening the surface of the part before anodising of the aluminium section, much or all of the fatigue performance loss can be recovered for fatigue critical parts.
Specify Selective Masking on Critical Features
Mask out areas such as bores, fillets and other stressed areas where you don’t need wear resistance. Hard Anodising is covered in more detail at www.poeton.co.uk/surface-treatments/anodising/hard-anodising/.
Managing the risk of fatigue of hard anodised parts is straightforward if taken into account in the part design and specification from the outset.