To whom it may concern:
The purpose of this memorandum is to discuss some of the sources of variation in anodizing response of aluminum plate products. Anodizing can be influenced by local variations in composition, precipitate distribution, and grain structure that develop naturally during fabrication.
Aluminum wrought plate is fabricated from thick, direct chill cast ingot. During the solidification process, solute elements (in the case of 7075, predominantly Zn, Cu, and Mg) are pushed out of the initial metal to solidify and into the remaining liquid, which results in a composition gradient through the thickness (and across the width) of the ingot. This gradient persists through fabrication, and influences the subsequent formation of precipitates, such that the final plate will have differences in composition and precipitation throughout.
Another factor is that grain structure within the plate also varies through the plate thickness and across the width. These gradients are due in part to composition differences (in this case, the dispersoid former, Cr, is the important element) and in part due to the thermomechanical processing to which the plate is subjected during hot rolling. As gauge decreases, temperature control is more difficult to maintain, and gradients in grain structure may become more pronounced.
Finally, when aluminum plate products are quenched after solution heat treatment, the quenching medium (generally water) impinges on the metal surface, efficiently removing heat. In the interior of the plate, heat must be transferred outward to the surface, resulting in a through thickness gradient in quench rate. This gradient in quench rate also contributes to local variations in precipitate distribution and anodizing response.
Given the gradients discussed above, it is not unusual to have different parts from within the same lot respond differently to anodizing, depending on their location within the lot.
Because it is a natural outcome of the manufacturing process, anodizing performance in plate is not guaranteed, although Kaiser Aluminum has done a great deal of research over the past several years in an effort to minimize through-thickness differences in anodizing behavior. This research has culminated in the recent introduction of Kaiser Select® 6061-T651 plate, offering improved uniformity in through-thickness anodizing response.
Brian A. Cheney
Senior Staff Product Engineer
Kaiser Aluminum – Trentwood Works