The Influence of Prolonged Natural Aging on the Subsequent Artificial Aging Response of the AA6111 Automotive Alloy
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UCTION
IN automotive manufacture, there is a storage time of typically a few months between the production of AA6111 aluminum alloy sheets and their further processing at industrial sites. During this storage period, a slight modification in the alloy microstructure accompanied with a slow increase in strength level may take place at room temperature. This phenomenon, known as natural aging, has been reported to suppress the precipitation kinetics and, thus, reduce the hardening response of AA6xxx alloys during subsequent paintbake aging.[1–5] In order to reduce the detrimental effects of natural aging, a pre-aging treatment at relatively low temperature is usually applied immediately after solution treatment and before the final artificial aging of heattreatable AA6xxx automotive alloy sheets.[6,7] For the AA6111 alloy, compared to the naturally aged (T4) condition, the microstructure of the pre-age temper (T4P) is more stable at room temperature; hence, its mechanical properties and aging response are less affected by the unavoidable naturally aging period.[8] Therefore, the material of the AA6111 sheets is usually supplied to automotive OEMs in the T4P temper, for forming into car panels and structures. ABDELHADI ABOUARKOUB, Research Associate, is with the The Libyan Petroleum Institute, Km 7, Gergarish Road, P. O. Box 6431, Tripoli, Libya. Contact e-mail: abdalhadi_aboargoub@yahoo. com G.E. THOMPSON, Professor of Corrosion Science and Engineering, X. ZHOU, Reader in Corrosion Science and Engineering, and T. HASHIMOTO, Experimental Officer, are with the School of Materials, The University of Manchester, Manchester M13 9PL, U.K. G. SCAMANS, Principal Scientist, is with the Innoval Technology Limited, Banbury OX16 1TQ, U.K. Manuscript submitted October 30, 2014. Article published online July 7, 2015 4380—VOLUME 46A, SEPTEMBER 2015
Although, the microstructure evolution in the pre-aged AA6xxx alloys during natural aging is agreed to take place at a much reduced rate compared to the T4 condition, the influence of prolonged periods of natural aging on the mechanical properties and aging response of the T4P tempers has not been clarified in the literature yet. The aim of this work was to study the effect of prolonged storage on the precipitation behavior of the AA6111 T4P alloy sheets during subsequent artificial aging treatment.
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EXPERIMENTAL
The material investigated was a commercial AA6111T4P automotive alloy, with the composition (in wt pct) listed in Table I. The alloy was provided in the form of 0.9-mm-thick sheet, which had been pre-aged at 85 C for 5 hours and then stored at room temperature for more than three years. In order to study the influence of prolonged storage on the microstructure evolution of the AA6111-T4P sheet, two sets of samples were prepared. Samples of the first set were left in the as-received condition (T4P + 3 years storage, hereafter referred to as T4PNA), whereas the second set was given a solution heat treatment at 823 K (550 C) for 22 minutes followed by water quenching (ST temper). Both s
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