Microstructure controlled bending response in AA6016 Al alloys

A contemporary approach in the car weight reduction is the use of low weight and high strength Al alloys sheets for hang-on body panels production. The final step in the forming route of such panels is the attachment of the outer skin to the inner part of the panel by applying a hemming operation. This joining method is cheap, easy to perform and environment-friendly, but requires severe 180 bending of the edges of the outer skin which quite often results in cracking or complete tearing of the bend surface. Such kind of failure restricts the further application of the hemmed products. The microstructures after solution heat treatment and pre-aging (T4P temper state) of two grades age-hardening AA6016-type aluminium alloy sheets were studied in this work by means of optical microscopy, scanning electron microscopy and electron backscatter diffraction. The obtained results were related to the hemming response of the grades. It was found that the alloy composition is one of the main parameters controlling the bendability of these grades through the amount of the formed strengthening phases. However, the applied thermal treatment remains the key factor responsible for the favorable distribution of these phases into the microstructure. The grain size and the volume fraction of the constituent particles were found to play secondary role in forming the material bending properties and can be only used for their fine tuning. The presence of Mg(2)Si (beta-phase) and/or Al(1.9)CuMg(4.1)Si(3.3) (Q-phase) particles in the grain boundaries structure was recognized as a critical microstructural feature causing severe reduction in the bending ability of the sheets by promoting an intergranular fracture. The possibility of grain boundaries failure exponentially raises with the time passed due to the natural aging. (C) 2011 Elsevier B.V. All rights reserved.