Homogenization of EN AW 6005A Alloy for Improved Extrudability
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HOMOGENIZED billets extrude easier and faster than cast billets that are simply heated to the extrusion temperature.[1] Higher throughput rates, lower breakout pressures, superior shape accuracy, and surface quality as well as higher mechanical properties thus obtained all justify the additional cost of the homogenization treatment.[2] A homogenization treatment consists of an isothermal soaking step followed by controlled cooling.[3] The interdendritic network of the plate-like b-AlFeSi intermetallic particles, often held responsible for surface defects and poor extrudability, are replaced by the more spherical discrete a-AlFeSi particles during soaking.[4–9] In addition, the Mg2Si particles and coarse eutectics are solutionized while the coring inherited from the as-cast microstructure is leveled out in favor of a more homogeneous solute distribution.[5,10,11] A fully solutionized billet, however, is difficult to extrude.[12] This is taken care of during cooling of the billet to room temperature at the end of soaking. Slow cooling tends to produce coarse b-Mg2Si particles that are difficult to solutionize at typical preheating temperatures and at high heating rates and may even survive the extrusion process. Such particles lead to incipient melting and surface tearing during extrusion, giving
YUCEL BIROL, Senior Scientist, is with the Materials Institute, Marmara Research Centre, TUBITAK, Kocaeli, Turkey. Contact e-mail: [email protected] Manuscript submitted March 8, 2012. Article published online August 11, 2012 504—VOLUME 44A, JANUARY 2013
poor surface quality and poor tensile properties and reduced productivity.[13] Rapid cooling, on other hand, traps the Mg and Si in solution, avoids Mg2Si precipitation, and impairs extrudability.[14–16] Hence, the cooling cycle has to be executed so as to precipitate as much Mg2Si as possible, in a form and size easily solutionized during subsequent billet heating.[17] b¢ precipitates are preferred as they readily dissolve above 773 K (500 C) during the extrusion process leading to good hardness levels after aging. The current study was undertaken to identify the optimum homogenization practice for EN AW 6005A, a medium strength AlMgSi extrusion alloy, suitable for structural applications in the automotive industry owing to a very attractive combination of functional and mechanical properties in addition to light weight.[18]
II.
EXPERIMENTAL
The EN AW 6005A alloy used in the current investigation was cast industrially using a vertical DC caster in the form of 6-m-long billets with a diameter of 203 mm. Its chemical composition is given in Table I. Samples for homogenization experiments were sectioned from a transverse slice of the cast billet, at least 10 mm away from the surface, to avoid possible microstructural and compositional variations from one sample to the other. The first set of homogenization experiments were undertaken to identify the optimum soaking time and temperature. The billet samples were heated to the soaking temperatures between 833 K and 853 K (
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