Favorable Environment for a Nondendritic Morphology in Controlled Diffusion Solidification
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INTRODUCTION
A nondendritic morphology of the primary phase in a solidified binary alloy could be achieved by controlling the solute redistribution and thermal fields in the solidifying phases both individually and together. The first such efforts were in the rapid solidification process wherein a nondendritic (cellular) microstructure could be formed in the solidified part, because the extraordinarily high growth rates experienced by the solid-liquid interface lead to a stable growth of the same.[1] In the latter part of the twentieth century, the thixoforming and rheocasting processes were developed as casting methods wherein a nondendritic morphology of the primary phase could be obtained in the cast part with a binary eutectic alloy along with the benefits in improved mechanical properties and performance. Unfortunately, these processes have proven prohibitive in capital and manufacturing costs.[2–7] Aluminum alloys have been the most significantly researched material to obtain a nondendritic cast microstructure, and near-net-shaped casting of Al wrought alloys along with their superior properties and performance have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification as caused by the presence of a pronounced dendritic network,[8] which renders the cast components ineffective. To circumvent the disadvantages of thixoforming and rheocasting processes and to enable a cost-effective near-net-shape casting of Al wrought alloys, the controlled diffusion solidification (CDS) process was innovated to enable casting aluminum alloys with a nondendritic morphology of the primary Al phase in ABBAS A. KHALAF, Post Doctoral Fellow, and SUMANTH SHANKAR, Associate Professor and Director, are with the Light Metal Casting Research Centre (LMCRC), Department of Mechanical Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada. Contact e-mail: [email protected] Manuscript submitted October 30, 2010. Article published online March 11, 2011 2456—VOLUME 42A, AUGUST 2011
the resultant cast microstructure by circumventing the problem of hot tearing and obtaining a product with improved mechanical properties.[9,10] The CDS is a simple process involving the mixing of two precursor alloys of different thermal masses (temperature and solute content) and, subsequently, casting the resultant mixture as a near-net-shaped cast product. The process lends itself to easy commercialization with a marginal capital cost required for set up of an additional melt holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al-based wrought alloys into near-net-shaped components without additional processes and cost.[11,12] The success of the CDS process in yielding a nondendritic primary Al phase morphology stems from the initial favorable environment for such an event created during the mixing of the two precursor alloys: copious nucleation and forced convection. In this article, evidence is presented to elaborate the specific thermal and s
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