Contraction of aluminum alloys during and after solidification
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3/6/04
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Contraction of Aluminum Alloys during and after Solidification D.G. ESKIN, SUYITNO, J.F. MOONEY, and L. KATGERMAN A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.
I. INTRODUCTION
THE process of direct-chill (DC) casting is the most common way to produce ingots and billets for further deformation processing. Despite the fact that this technology has been used in the aluminum industry since the 1950s, the cause of some common defects is still under discussion. Hot tearing, porosity, macrosegregation, and distortion of billet geometry (e.g., butt curl) are the major defects that occur during casting. The feeding of the growing solid phase with the liquid (hence, the permeability of the mushy zone), the structure formation, the development of strength in the mushy zone, and the solidification shrinkage together with thermal contraction being in complex interaction may result in the formation of defects. Hot tearing or hot cracking is one of the most common problems encountered in DC casting of aluminum alloys. The main cause of this defect is that stresses and strains built up during solidification are too high compared to the actual strength of the semisolid material. This type of defects occurs in the lower part of the solidification range, close to the solidus, when the solid fraction is more than 0.9.[1] At this point, the mushy zone is definitely coherent, but the liquid film still exists between most of the grains. The term coherency (or coherency temperature) should be used with caution. If the coherency is understood as a temperature at which a continuous dendritic network is formed, and the material starts to develop strength and retain its shape,[2,3] then this point can be better defined as a rigidity point. At temperatures above the rigidity point, the grains are free to move with respect to each other and so do not transfer any forces. Moreover, before the rigidity temperature is reached upon solidification, the liquid phase can easily flow between grains and, therefore, the melt feeding and D.G. ESKIN, Senior Scientist, is with the Netherlands Institute for Metals Research, 2628AL Delft, The Netherlands. Contact e-mail:
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