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Aluminum-finishingfor Attractive Appearance.SOLUTIONS with Effective, Practical Advice
Aluminum-finishing is generally the last process. It is applied to aluminum items or constructions after all other fabrication processes are completed. As such it comes also after welding. Welding is used for joining separate elements obtained by all available production methods. Among these are casting, forging, forming, machining etc. Aluminum-finishing provides an impervious protective coverage on the external surfaces of aluminum bodies. That assures long time standing in normal environments. The most known and used processes employed for Aluminum-finishing are described as anodizing. It consists in electrolytic passivation processes that increase the thickness of the natural oxide layer on the surface of aluminum parts. Anodizing can be done in a number of ways that convert the aluminum surface into a thin layer of aluminum oxide or alumina, while the parts are the anode in an electrolytic cell. Among the advantages of this Aluminum-finishing product one remarks:
Three are the main types of Aluminum-finishing anodizing, known as:
When Aluminum-finishing by anodizing of parts machined to final dimensions with tight tolerances, one should remember that the processes add a certain amount of thickness to the dimensions measured before processing. The measured growth is partially within the volume of the original part and is partially towards the outside. Although empirical indications are available, only actual measurements will suggest the best dimension to give to the finish machined part. Practical testing will permit to obtain the drawing dimension and tolerance after anodizing. In fact most drawings will indicate: "Dimensions after anodizing". The thickness of the anodized layer is best determined on a cross section ground and polished, by metallographic examination under an optical microscope with calibrated scale eyepiece. The thickness can also be determined by measuring with a calibrated micrometer the thickness of an anodized specimen. The Aluminum-finishing anodized coating is then stripped using the solution described in The thickness of the same stripped specimen is then measured again. If the original thickness of the specimen before anodizing is known, one can easily confirm that its thickness after stripping is less than the original one. The following Resources may provide additional information if required: http://www.anodizing.org/ http://www.matweb.com/reference/anodize.aspx Accepted Standards for the processes are: SAE AMS-A-8625 (previously known as MIL-A-8625)
DIN EN ISO 7599
SAE AMS-C-5541 (previously known as MIL-C-5541) These, sometimes known as Alodine, are extremely thin chemical coatings that are mainly used as local repair of damaged or removed anodizing. Conversion coatings provide enhanced corrosion resistance and improved bonding ability while remaining electrically conductive. These are not electrical processes, so that they are performed by immersion or by swabbing the surface with the prepared liquid solution. An Article on Filler Metals for welding aluminum alloys was published (4) in Issue 163 of Practical Welding Letter for March 2017. Summing up, Aluminum-finishing by suitable processes should be integrated into any fabricated product as a necessary part of design, not as an afterthought. Commercial sources can probably assist in the selection of useful processes if consulted before finalizing design.
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