Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A
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NTROLLED diffusion solidification (CDS) is a novel process wherein a nondendritic microstructure is obtained by controlled mixing and solidification of two precursor liquid alloys into a cast component.[1,2] The process enables the casting of Al wrought alloys into near net-shaped components. The process aims to improve the mechanical properties and performance of the castings while reducing the cost of the casting process. The primary Al phase in castings made by conventional processes such as with sand and metal molds has a dendritic morphology. Al-based wrought alloys could not be easily cast into shaped components by conventional processes because they are prone to hot tearing wherein the inter-dendritic liquid at the end of solidification cannot effectively feed the shrinkage cavities created by the solidification of the primary dendrites caused by the large and complex dendritic network that is created. Al-based wrought alloys can be cast with the ABBAS A. KHALAF, PhD Candidate, PEYMAN ASHTARI, Postdoctoral Fellow, and SUMANTH SHANKAR, Associate Professor, are with the Light Metal Casting Research Centre (LMCRC), McMaster University, Hamilton, ON, Canada L8S 4L7. Contact e-mail: [email protected]. This article is based on a presentation given at the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online August 25, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
CDS process because of the absence of a large and complex dendritic network and thus alleviating the hot tearing tendencies. CDS is a method of semi-solid metal (SSM) processing, specifically rheocasting. The advantages of the SSM process are low gas porosity, low pouring temperature, longer metal mold life, and improved properties and performance of the cast part. There have been significant advances in the field of SSM in the last 30 years. There are typically two processing routes in SSM: rheocasting and thixoforming.[3–9] In rheocasting, the liquid metal is cooled to a specific temperature in the semi-solid region and subsequently held for a while to homogenize the temperature and the microstructure of the primary nondendritic phase before casting into a shaped component by a pressure-induced process. At the liquidus temperature during cooling of the liquid, it is ensured that there is copious nucleation of the primary phase and forced convection to spread the nuclei evenly in the melt to enable a nondendritic microstructure. Recently, various commercial rheocasting processes have been developed for Al alloys.[3] MIT, UBE rheocasting, THT, CRP, and SLC are a few examples of commercial rheocasting processes.[3,9,10] The most time- and energy-consuming step in all the currently available commercial processes for rheocasting is the holding period at the semi-solid stage to create a billet for casting. This step in the processes can take
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