Forged aluminium parts are considered as special surface treatment articles and their surface treatment details need to be confirmed in the early stage to avoid any possible rework.
Why Alloy Grade Matters
The anodising process can be affected by the alloy composition, in particular by copper and zinc containing alloys (2xxx/7xxx series). These alloys are often used to produce forged aluminium parts and can produce a thinner and less consistent anodic oxide coating due to the alloying elements in the aluminium forming part of the oxide layer.
Account for the Grain Structure
As aluminium is forged the material’s grain structure will be compressed and aligned in the forging direction. As the anodic oxide forms, it will try to follow the direction of the grain flow on the surface of the part. This can lead to uneven coating formation where there are changes in the grain flow direction around radii or other areas of the part. You should check with your treatment supplier for their experience with forged aluminium stock, as this can differ from their experience with wrought plate or extrusions.
Allow for Desmutting Before Treatment
A forged surface will contain smut on its surface. Etching will cause this to rise to the surface of the anodic oxide film. The smut will cause poor adhesion and uneven coloration if it is not removed in a proper desmutting stage before anodizing.
Build Tolerance for Coating Growth into Your Drawings
Material is added by the anodising process and must be allowed for in drawings. Typically 50% of the coating thickness is incorporated into the surface and 50% forms up from the surface. Therefore a coating of 25 micron will add some 12-13 micron to each face of a bore etc. These figures must be included in drawings and allowed for in subsequent finish machining of parts after forging.
Agree Inspection Criteria Before Processing
When it comes to coating thickness, surface hardness and extended salt spray testing, ensure these requirements are written down and monitored. Where possible witness test pieces are processed at the same time as your parts and then inspected for adhesion and thickness. If you are looking for Anodising, see poeton.co.uk/surface-treatments/anodising/.
By considering the aluminium alloy, forging process, surface treatment process and the required dimensions before production, the chances of parts having to be inspected and possibly even destroyed can be greatly reduced.
Leave a Reply