Effect of forming parameters on the corrosion performance of retrogression- and warm-formed AA7075 alloy sheets

ABSTRACT

Retrogression forming (RF) and warm forming (WF) are used to remedy formability limitations of high-strength precipitation-hardened AA7075 alloy sheets. RF is a thermomechanical treatment combining retrogression heat treatment and the forming of initially peak-aged T6 temper. WF is a thermomechanical treatment of initially pre-aged temper. The present work examines the corrosion performance of retrogression- and warm-formed AA7075 alloy sheets by determining the optimal pre-aging temperature (80°C or 100°C), forming temperature (room temperature, 150–200°C), strain rate (1, 0.1, or 0.01 s⁻¹), and heating rate (1.1 or 20°C·s⁻¹) for conventional slow and novel fast-forming technologies. Several alternating- and direct-current electrochemical tests were administered, including electrochemical impedance spectroscopy, potentiodynamic polarization, and linear polarization resistance. Results showed that the corrosion performance (i.e., passive film and corrosion-resistant properties) of formed AA7075 sheets was directly proportional to pre-aging and WF temperatures, inversely proportional to the RF temperature, and mildly affected by strain/forming rate and heating rate during forming.

ABSTRACT

Le formage par rétrogression (RF, de l’anglais retrogression forming) et le formage à chaud (WF, de l’anglais warm forming) sont utilisés pour remédier aux limites de formabilité des tôles d’alliage AA7075 à haute résistance et durcies par précipitation. Le RF est un traitement thermomécanique combinant un traitement thermique de rétrogradation et le formage d’une trempe T6 initialement vieillie au maximum. Le WF est un traitement thermomécanique de la trempe initialement vieillie par précipitation. Le présent travail examine les performances en matière de corrosion des tôles d’alliage AA7075 formées par rétrogression et à chaud en déterminant la température optimale de pré-vieillissement (80°C ou 100°C), la température de formage (RT, 150-200°C), la vitesse de déformation (1, 0,1 ou 0,01 s^-1) et la vitesse de chauffage (1,1 ou 20°C·s^-1) pour les technologies conventionnelles de formage lent et les nouvelles technologies de formage rapide. Plusieurs tests électrochimiques en courant alternatif et continu ont été effectués, spectroscopie d’impédance électrochimique, la polarisation potentiodynamique et la résistance à la polarisation linéaire. Les résultats ont montré que la performance de corrosion (c’est-à-dire la couche passivante et les propriétés de résistance à la corrosion) des feuilles d’AA7075 formées étaient directement proportionnelles aux températures de pré-vieillissement et de WF et inversement proportionnelles à la température RF et légèrement affectées par le taux de déformation/formage et la vitesse de chauffage pendant le formage.

KEYWORDS:

• Aluminum sheet
• Corrosion current densities
• Corrosion potentials
• Electrochemical polarization
• Forming

MOTS-CLÉS:

• densités de courant de corrosion
• formage
• polarisation électrochimique
• potentiels de corrosion
• tôle d’aluminium

ACKNOWLEDGMENTS

The authors appreciate the support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC).

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

REVIEW STATEMENT

This article was reviewed and approved for publication by the Metallurgy and Materials Society of the Canadian Institute of Mining, Metallurgy and Petroleum.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

There are no ethical issues associated with this manuscript.

Additional information

Funding

The work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Foundation for Innovation (CFI).

Notes on contributors

S. S. Kope

Seyiwa Kope, B.Eng, is an MASc student in the Department of Civil and Mineral Engineering at the University of Toronto, with a B.Eng in Metallurgical and Materials Engineering, more than two years of academic research experience, and more than four years of industrial experience in a research laboratory, performing materials characterization and testing, quality control inspection, electrochemistry and corrosion testing and control, nondestructive testing, and failure analysis.

I. G. Ogunsanya

Ibrahim Ogunsanya, PhD, P.Eng, is a Metallurgist and Materials Scientist, a Professional Engineer in Ontario, and an Assistant Professor in the Department of Civil and Mineral Engineering at the University of Toronto. His research is focused on metals and alloys (processing-structure-property-degradation relationship), electrochemistry and corrosion, surface engineering and coatings, cement and concrete chemistry, and durability and sustainability of structures.