The effect of heat treatment on Mg 2 Si coarsening in aluminum 6105 alloy
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20/1/04
2:48 PM
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The Effect of Heat Treatment on Mg2Si Coarsening in Aluminum 6105 Alloy M. USTA, M.E. GLICKSMAN, and R.N. WRIGHT The coarsening behavior of Mg2Si intermetallic particles was studied during homogenization of aluminum 6105 alloy. An industrially homogenized 6105 aluminum alloy contained large globular undissolved Mg2Si. With subsequent heat treatment at 590 °C over 1 week, it was found that the large Mg2Si precipitates coarsened with time. The coarsening of the large Mg2Si precipitates was quantified by statistical analysis of micrographs. A linear relationship between R 3.8 Ri3.8 and time fits the experimental measurements quite well, indicating that the coarsening is diffusion controlled, with grain-boundary diffusion dominating volume diffusion.
I. INTRODUCTION
ALLOYS of the 6xxx heat-treatable series in aluminum are the most widely used for the production of extruded sections. More than 80 pct of the aluminum alloys employed worldwide in the manufacture of extruded sections belong to the 6xxx series, i.e., to the Al-Mg-Si system.[1] In this alloy series, Mg and Si combine to form the compound Mg2Si, the primary hardening phase.[2] The alloy used in this study was 6105, principally composed of Al-Mg-Si. This is one of the dominant alloys in the aluminum extrusion industry. The strength of the alloy is dependent on the role of magnesium-silicide precipitates. Meanwhile, extrudability is strongly influenced by the amount of Mg and Si in solid solution, and the size of Mg2Si phase precipitate particles. The examination of commercially homogenized 6105 aluminum alloy by Usta[3] showed particles of Mg2Si as large as 10 m in the structure. From the standpoint of extrudability, coarse globular Mg2Si particles reduce extrudability, and they should dissolve before extrusion. In fact, it is observed that they do not dissolve. Moreover, they tend to coarsen with time when exposed to homogenization treatments undertaken at 590 °C for over 1 week. Studies have been done on the coarsening of particles in aluminum alloys. A recent investigation of coarsening kinetics of the intermetallic phases Al6Mn and Al20Cu2Mn3 in directionally solidified monocrystals showed that coarsening of Al6Mn intermetallic particles in the presence of liquid film, maintained by holding at a temperature above the peritectic or by the addition of 1 pct In to the alloy, follows an average particle size, d ~ t1/3 relationship much closer than it does a d ~ t1/2 relationship. This suggests that convection plays no role or only a minor role in coarsening and that coarsening is controlled by liquid diffusion.[4] For coarsening of Al6Mn in a solid matrix, the relationship d ~ t1/3 fits the experimental measurements reasonably well, indicating lattice diffusioncontrolled coarsening. Coarsening of Al20Cu2Mn3 intermetallic M. USTA, Assistant Professor, is with the Materials Science and Engineering Department, Gebze Institute of Technology, P.K. 141, Gebze-Kocaeli, Turkey. M.E. GLICKSMAN, John Tod Horton Professor of Materials
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