Formation of concentration gradients due to temperature gradients in the 
mushy zone of Al-Cu alloys 
Claudia Wasmund, Markus Rettenmayr, Oliver Pompe 
Darmstadt University of Technology, Materials Science Department, Darmstadt, Germany 
Abstract 
nn Cylindrical samples of Al-7wt%Cu with two different initial microstructures (columnar and equi- 
rr axed) were melted in a steep temperature gradient. An initial gradient of solid fraction transforms in 
the course of the experiment into a macroscopic concentration gradient. Independently of the initial 
microstructure, a stationary concentration profile is attained after approximately one hour when the 
former mushy zone is completely resolidified. The measured concentrations in the former mushy 
zone correspond to the phase diagram (solidus concentrations) at the local temperature in the 
zon sample. In the initially equiaxed sample, in addition to the concentration gradient a gradient of grain 
aspect ratio develops. 
WS 
Introduction 
In contrast to solidification, melting processes have been studied very little in the past decades, 
mainly because it is assumed that the microstructure of a metal alloy which forms during 
solidification is not affected by the preceding process of melting. However, melting and 
solidification are not always separated in time. In the mushy zone of a metal alloy parts of the 
solidified microstructure can remelt locally, even if globally the enthalpy of the system is reduced. 
E.g., in semi-continuous castings of aluminum alloys, remelting occurring in regions near the 
surface due to the formation of an air gap between the cast piece and the mould can lead to inverse 
segregation and surface exudation [1]. Remelting at the root of secondary dendrite arms can lead to 
REV Bm defect (freckle) formation in directionally solidified nickel-base superalloys [2]. In this work, 
melting of a homogeneous sample in a temperature gradient, leading to segregation formation on a 
macroscopic level, is studied through the evolution of microstructure and concentration distribution. 
Ext.Abstr” 
Experimental 
Cylindrical samples (rods with 8mm diameter) of Al-7wt%Cu were used to investigate the melting/ 
resolidification process. Two different initial microstructures were used: an equiaxed fine grained 
microstructure (as cast), and a columnar dendritic microstructure, generated by directional 
solidification in a vertical Bridgman furnace. The samples were wrapped in carbon sheet (0.2mm 
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