The Correlation Between Intergranular Corrosion Resistance and Copper Content in the Precipitate Microstructure in an AA
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NTRODUCTION
AL-MG-SI(-CU) (6xxx) alloys are widely used in automotive and construction industries due to their high strength, ductility, corrosion resistance, and low weight. Usually, such alloys are cast and homogenized, during which dispersoidal AlSiMnFe particles with sizes in the order of ~ 100 nm form in the Al matrix, controlling grain size evolution during a subsequent extrusion step.[1,2] Large (~ lm) primary particles containing the same elements as the dispersoids are also present in the microstructure.[3] As the temperature during extrusion reaches more than 500 C, most of the Mg, Si, and Cu elements are in solid solution. However, a further solution heat treatment (SHT) is sometimes performed before the final artificial aging (AA).[4] Al-Mg-Si(-Cu) alloys are predominantly used in an aged state because they are strengthened by the formation of high numbers of nano-sized metastable precipitates in the Al matrix during the AA. This is a very complex process, and everything that occurs after extrusion or after the SHT influences the numbers, size distribution and types of
CALIN D. MARIOARA, OTTO LUNDER, and SIGURD WENNER are with the SINTEF Industry, 7465 Trondheim, Norway. Contact e-mail: [email protected] ADRIAN LERVIK and RANDI HOLMESTAD are with the Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. JULIE GRØNVOLD is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491, Trondheim, Norway. TROND FURU is with the Norsk Hydro ASA, 0283 Oslo, Norway. Manuscript submitted March 2, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
metastable precipitates.[5–9] Therefore, parameters such as cooling rate from extrusion or SHT, room temperature (RT) storage time and pre-deformation before AA, as well as AA temperature and time are crucial for the material properties. To be able to optimize properties and design new alloys, the processes happening at the micro- and nanoscale must be studied and understood. Cu additions to Al-Mg-Si alloys in general increase strength and thermal stability,[7,10] but often at the expense of a reduced intergranular corrosion (IGC) resistance.[11,12] Hence, this work investigates possible ways of improving IGC resistance of Cu-containing Al-Mg-Si alloys by manipulating the thermo-mechanical processes leading to the condition of the final product. Recent works indicate that IGC propagates due to the presence of a continuous Cu film along the grain boundaries (GBs), and that IGC resistance increases at over-aged conditions due to induced discontinuity in this film.[13,14] On the other hand, Cu additions modify the precipitation sequence by suppressing the b¢¢ phase responsible for the peak hardness in Al-Mg-Si alloys and new, Cu-containing phases are created.[7] Therefore, the idea behind the present work is to maximize Cu absorption in the bulk precipitates, thus leaving less Cu available to form a continuous Cu film at the GBs. Ideally this should occur near the pe
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