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. 1. 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. Atomization techniques are used to make metallic powders which are used for making a desired object by pressing or by sintering [2]. The liquid metal generated as a stream breaks up into droplets by Rayleigh-Plateau instability, which subsequently solidify in a much colder medium. In the Impulse Atomization (IA) technique the liquid is pushed through a nozzle plate to form the liquid streams [3]. In order to deepen the understanding of the microstructure formation in the droplets, synchrotron X-ray micro-tomography was carried out at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Three-dimensional reconstructions of a large number of droplets were obtained, enabling the inner microstructure of the droplets to be statistically analysed for the first time. In a previous paper, 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. Indeed, while Rappaz and co-workers highlighted the <100>