IN-SITU STUDIES OF DENDRITE FRAGMENTATION IN AL-CU

摘要

Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources opens for in-situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena will be presented. From directional solidification experiments it is found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network is strongly linked to the relative buoyant and settling motions of detached fragments and mush liquid, respectively. At the copper concentrations studied (20-30 wt%), primary aluminium dendrites are lighter than the melt and solidification experiments parallel- and anti parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations, however the results also demonstrate these models to differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions.