Grain Refinement Of Al‐Si Hypoeutectic Alloys By Al3Ti1B Master Alloy And Ultrasonic Treatment

Abstract

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller, Jackson, Rutter, and Chalmers (1953). On that basis, Winegard and Chalmers (1954) further considered 'supercooling and dendritic freezing of alloys' where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

Ultrasonic Processing for Structure Refinement: An Overview of Mechanisms and Application of the Interdependence Theory

“…Considerable research has also highlighted the importance of cavitation and acoustic streaming on the activation of potent particles, nucleation of grains, altering convection patterns and reducing the temperature gradients during solidification [23,26,27,28,36,37,38,39,40]. While recent publications on advanced (in-situ) solidification studies have provided more detailed evidence on bubble dynamics during melt solidification [31,32,33,41,42], our research has been focused on other factors such as solute content, type of solute, constitutional supercooling, role of potent and impotent (oxide) particles, UST duration, origin and transport of grains, temperature range over which UST is applied, sonotrode preheating and other casting variables that could affect grain formation [6,37,38,43,44,45,46,47]. Using the Interdependence Theory of nucleation and grain refinement [15,48], we have revealed explanations for the role of solute [37,38,45,49], casting conditions, and micro [6,46] and nanoparticles [50] in assisting refinement by the application of UST.…”

The Poisoning Effect of Al and Be on Mg—1 wt.% Zr Alloy and the Role of Ultrasonic Treatment on Grain Refinement

“…The application of UST to alloys containing elements that poison nucleation has also resulted in improved refinement. For example, the exceptional grain refinement of Al alloys by the addition of Ti is poisoned by the addition of Zr (Atamanenko et al, 2010;Sreekumar and Eskin, 2016) or Si (Wang et al, 2016b). It has been reported that UST generated cavitation enhances nucleation by reducing the poisoning effect and this results in grain refinement (Atamanenko et al, 2010;Sreekumar and Eskin, 2016;Wang et al, 2016b).…”

“…For example, the exceptional grain refinement of Al alloys by the addition of Ti is poisoned by the addition of Zr (Atamanenko et al, 2010;Sreekumar and Eskin, 2016) or Si (Wang et al, 2016b). It has been reported that UST generated cavitation enhances nucleation by reducing the poisoning effect and this results in grain refinement (Atamanenko et al, 2010;Sreekumar and Eskin, 2016;Wang et al, 2016b). Similarly, in carbon inoculated commercial Mg-Al alloys the interaction of Fe or Mn with C is reported to cause poisoning of grain refinement (Easton et al, 2006;Du et al, 2010) while the application of UST results in excellent grain refinement (Ramirez et al, 2008;Nimityongskul et al, 2010).…”

Ultrasonic Processing of Aluminum–Magnesium Alloys

“…Alternative methods of grain refinement have also been explored using external fields, such as mechanically-induced and electromagnetic fields [ 22 , 23 ], and the use of ultrasonics. Some studies have applied UST to single Al–Mg compositions [ 24 , 25 ], as well as many other Al alloys [ 26 , 27 , 28 , 29 ].…”

“…All previous studies of the effect of UST on Al–Mg alloys only investigated a single Mg composition [ 29 , 33 ], rather than investigating the effects of UST over a range of alloy compositions. It is now well established that composition can have a significant effect on grain size when subjected to UST [ 27 , 28 , 33 , 34 , 35 ]. In these studies the effect of composition has been evaluated by relating grain size to the inverse of Q (i.e., 1/ Q ).…”