Refreshing Industrially Processed 6xxx Series Aluminum Alloys after Prolonged Natural Aging

Abstract

Natural aging (NA) undesirably hardens 6xxx series aluminum alloys and hampers subsequent paint‐bake (PB) hardening, thus limiting the use of material after prolonged storage. It is presented that a refreshment treatment for seconds at a temperature between 230 and 290 °C can effectively lower the hardness/strength of two commercial alloys that have experienced NA for ≈3.5 years and enhance their PB hardening, thus enabling the reuse of the material with minimal energy input. The treatment is based on dissolving solute clusters formed during prior aging, but precipitation during refreshment can compromise its efficacy. The dependence of cluster dissolution and precipitation on the refreshment parameters as well as on the alloy composition is analyzed. A higher temperature is suggested for refreshing AA6014 alloy than for AA6016 alloy due to a higher thermal stability of the clusters in the former. Natural secondary aging (NSA) is investigated and it is proposed that the remaining undissolved clusters play an important role in controlling the mobile vacancy concentration. Refreshment experiments utilizing various heating media demonstrate that the treatment is hardly sensitive to the heating rate which facilitates its industrial implementation. The refreshed alloy can undergo further preaging to enhance the NSA stability and PB hardening.