Abdoul-Aziz Bogno

Quantification of Primary Dendritic and Secondary Eutectic Nucleation Undercoolings in Rapidly Solidified Hypo-Eutectic Al-Cu Droplets

This paper reports on the quantification of primary dendritic and secondary eutectic nucleation undercoolings during rapid solidification of impulse atomized hypo-eutectic Al-Cu droplets. The procedure consists in determining the eutectic fraction of each investigated droplet from the fraction of intermetallic Al2Cu obtained by Rietveld refinement analysis of neutrons scattering data. The corresponding eutectic nucleation undercooling is then deduced from the metastable phase diagram of the alloy. The primary dendritic nucleation undercooling is subsequently determined using semi-empirical coarsening models of secondary dendrite arms. The two nucleation undercoolings are finally used as input variables to run a microsegregation model for binary alloys. The fractions of eutectic computed by the microsegregation model compare very favorably with the experimental results.

A General Formulation of Eutectic Silicon Morphology and Processing History

Understanding the influence of rapid solidification conditions on the formed microstructure is important to determine how process conditions affect the final mechanical properties. In this work the rapid solidification of hypoeutectic and hypereutectic Al–xSi (x = 10, 18 wt%) alloys was achieved using Impulse Atomization (IA). The resultant microstructures were examined and from this analysis a wide range of eutectic Si morphologies and length scales were observed. These results were further analyzed using 2 mathematical approaches to help quantify the experienced eutectic growth kinetics (i.e. growth rate and cooling rate), along with the thermal history of the powders. In the case of the hypereutectic Al–Si alloy distinct microstructure zones were observed. These distinct zones were used to determine the nucleation undercooling using combined concepts of the coupled zone in Al–Si alloys and the hypercooling limit of a melt. Here, a region of fine eutectic, α-Al+Si, is observed and is believed to be the first solids that precipitated directly from the undercooled melt and grew during recalescence. Following recalescence, a coarser structure of Si particles and eutectic developed. Within this work, the solidification paths of both hypo and hypereutectic Al–Si alloys will be outlined and a generalized approach to relate the shape of the eutectic Si to processing conditions will be presented.

Single Fluid Atomization Fundamentals

Atomization is simply defined as the breakup of a liquid into droplets. It can be achieved in many ways including spraying through nozzle, pouring on to a rotating disc, etc. Atomization practice and research usually involve materials processing in their liquid state either at or near room temperature (oil-based liquids, paint spraying, aerosol sprays, etc.) or at high temperature (metal melts). Most of the literature describing atomization mechanisms pertains to two fluid atomization in which a second fluid is applied to break up a melt stream into droplets. Two fluid atomization techniques for molten metals are described in Chap. 3.

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

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 crystallographic axes instead of the usual 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.