Abstract
The understanding of rapid solidification behaviour, e.g. the undercooling versus growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, rapid solidification of single micro-sized Al-Si12 (mass%) particles of various diameters has been investigated via differential fast scanning calorimetry employing controllable cooling rates from 100 to 90,000 K s−1 relevant for AM. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles under controlled cooling rates, two different heterogeneous nucleation mechanisms of the primary α-Al phase are proposed. Surface heterogeneous nucleation dominates for particles with diameter smaller than 23 μm. For particles with diameter larger than 23 μm, the nucleation of the primary α-Al phase changes from surface to bulk heterogeneous nucleation with increasing cooling rate. The results indicate that at large undercoolings (> 95 K) and high cooling rates (> 10,000 K s−1), rapid solidification of single particle can yield a microstructure similar to that formed in AM. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.
Highlights
Rapid solidification of metal particles is an important process in many production chains of metal components
The rapid solidification process of single micro-sized Al-Si12 particles was successfully investigated in a wide range of cooling rates from 100 to 90,000 K s-1 by in situ differential fast scanning calorimeter (DFSC)
According to a modified theoretical model based on classical nucleation theory (CNT), the a-Al nuclei either form at the surface (Al2O3 layer) or on impurities inside the Al-Si12 particle
Summary
Rapid solidification of metal particles is an important process in many production chains of metal components. Attention has been given to understand the rapid solidification processes of micro-sized metal particles in AM processes and their influence on the developing microstructures [3,4,5,6,7]. Numerous studies have been reported on the rapid solidification of Al–Si alloys by melt spinning, gas atomisation, laser melting, spray forming, etc. The in situ investigation of the rapid solidification behaviour of powder particles under such conditions is not straight forward. Ex situ investigations and numerical simulation are commonly applied to study the transient, non-equilibrium states during rapid solidification under such extreme conditions far away from thermodynamic equilibrium [3, 4, 7, 13]. A better understanding of rapid solidification at such high cooling rates requires accurate measurements of the cooling rate and solidification temperature intervals and an accurate description of its mechanisms
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