Abstract
This paper reports on rapid solidification of Al-Cu alloys. A heterogeneous nucleation/growth model coupled with a thermal model of a falling droplet through a stagnant gas was developed. The primary undercooling as well as the number of nucleation points was compared with Al-Cu alloy droplets produced by Impulse Atomization (IA). Based on experimental results from Neutron Diffraction, secondary (eutectic) phases were obtained. Then, primary and secondary undercoolings were estimated using the metastable extensions of solidus and liquidus lines calculated by Thermo-Calc. Moreover, Synchrotron X-ray microtomography has been performed on Al-4.5wt%Cu droplets. The undercoolings are in good agreement. Results also evidence the presence of one nucleation point and are in agreement with the experimental observations.
Highlights
Manufacturing of most metallic alloy products involves solidification at some stage
Several assumptions were made in the model formulation: 1 - Internal temperature gradients in droplets are negligible (Biot < 0.1 in the droplet size studied); 2 - The time for stream break up and spheroidization of ligaments emanating from the orifice plate is very small compared to the solidification time [15]; 3 - The initial velocity of the droplet exiting the orifice is 0.5 m/s [16, 17]; 4 - The ambient gas temperature remains constant during atomization; 5 - For radiation heat transfer, a droplet emissivity ε = 0.1 was used [18]; 6 - Thermal interaction between droplets is negligible; 7 - Droplet diameter decreases during solidification
A numerical prediction of the primary dendritic nucleation undercooling was achieved based on a thermal model coupled with a heterogeneous nucleation/growth model of a droplet falling through a stagnant gas
Summary
Manufacturing of most metallic alloy products involves solidification at some stage. Mechanical properties of these products are generally related to their solidification microstructures. Post-processing of the solidified materials is required to obtain the final product with desired properties. These postsolidification treatments are generally time-consuming and increase the production cost without fully eliminating solidification related defects such as segregation. It is important to understand all the dynamics involved in the formation of solidification microstructures in order to control the properties of the final products. IA is a single fluid atomization technique that is capable of producing droplets of controlled size having a relatively narrow distribution and a predictable cooling rate. The primary undercooling as well as the number of nuclei is considered and the results were compared to experimental results of a rapidly solidified Al-4.5 wt%Cu droplet, generated by IA
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