Evolution of the dendritic morphology with the solidification velocity in rapidly solidified Al-4.5wt.%Cu droplets

M Bedel,A-A Bogno,H Henein,H Nguyen-Thi,Ch-A Gandin,G Reinhart

doi:10.1088/1757-899x/84/1/012016

Abstract

The microstructure morphology of Al-4.5wt.%Cu droplets formed by the Impulse Atomization technique is investigated. Three-dimensional reconstructions by synchrotron X- ray micro-tomography of several droplets reveal different morphologies in droplets of similar diameter and produced in the same batch. Moreover, microstructural features also indicate that the development of the dendrite arms occurs in some droplets along <111> crystallographic axes instead of the usual <100> directions observed in conventional casting for the same alloy. It has been observed that such an unusual growth direction of the dendrites is directly related to the solidification velocity. We underpin these results by carrying out comparisons with a solidification model. Predictions are used to discuss the change of dendrite growth direction, as well as the existence of a dendrite growth direction range for a given type of droplets. In addition, the effect of the droplet size and the cooling gas on the dendrite growth direction range observed experimentally is also investigated by using the model.

Highlights

Rapid solidification techniques have been developed as they enable to obtain a wide variety of structures which cannot be formed under conventional solidification processes [1]
These morphologies have been described in details in [4] by combining electron backscatter diffraction (EBSD) and X-ray micro-tomography analysis
When the grain envelope approaches the droplet periphery, the solute enrichment of the extra-dendritic liquid is not negligible anymore and growth is slowed down. From this time (t≈0.075s), we observe that the solid fraction evolution approaches the Gulliver-Scheil approximation as diffusion in the solid phase is limited and solute is well mixed in the liquid phase. These different growth regimes can explain the dendrite growth direction change observed in droplets by considering that the growth orientation is linked to the growth velocity

Summary

Introduction

Rapid solidification techniques have been developed as they enable to obtain a wide variety of structures which cannot be formed under conventional solidification processes [1]. They differ by the way to form the liquid as a strip or a droplet and by the method of heat extraction. We showed that four distinct morphologies could be identified in droplets of the same size and from the same batch [4] Such a range of morphologies can be linked to a range of solidification velocities for the droplets.

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