Potential of an Al-Ti-MgAl 2 O 4 Master Alloy and Ultrasonic Cavitation in the Grain Refinement of a Cast Aluminum Alloy
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GRAIN size is one of the most important factors that influences the processing and properties of Al alloys, and grain refinement is considered to be the foremost technique for achieving improvement in the quality of cast products, eliminating coarse and columnar grains, minimizing casting defects such as hot tear or macrosegregation, and facilitating downstream processing of the cast products.[1–5] From the various grain refinement techniques, the most common practice is the addition of specialized master alloys containing potent nucleation substrates (TiB2, TiC, AlB2), primary intermetallics (Al3Ti, Al3Nb, etc.), and/or growth-restricting alloying elements (Ti, V, Nb, etc.).[6–10] In current industrial practice, Al-Ti-B (Al-Ti-TiB2) and Al-Ti-C (Al-Ti-TiC) master alloys are the most common grain refiners used in cast and wrought Al alloys.[2,3] The grain refinement mechanisms as well as some technological issues related to Si or Zr poisoning of Al-Ti-B grain refiners are thoroughly scrutinized by theoretical models and experiments.[11,12] Despite the commercial success of Al-Ti-B and Al-Ti-C grain refiners, some drawbacks are related to their application such as reduced performance in high-Sior Zr-containing alloys and agglomeration of borides in Al-Ti-B master alloys and a narrow process-parameter
V.M. SREEKUMAR, Research Fellow, and N. HARI BABU, Reader, are with the Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, Waterside House, Cowley Business Park, Uxbridge UB8 2HP, United Kingdom. Contact e-mail: [email protected], Sreekumar.VadakkeMadam@brunel. ac.uk D.G. ESKIN, Professor, is with the Brunel University London, and with Tomsk State University, 634050, Russia. Manuscript submitted July 5, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
window for Al-Ti-C master alloys.[1,2,5,6,12] A search for new grain refining systems that may substitute that currently used is ongoing. In recent times, interest has grown on using oxides such as Al2O3, MgAl2O4, and MgO as nucleation substrates in Al.[13–15] It is well known that these oxides are formed naturally on the aluminum melt surface as stable layers, being thermodynamically stable and having sufficiently good crystallographic match with solid aluminum to act as substrates for its nucleation.[13] Similarly, these oxides can be formed in situ as a reaction product of externally added oxide particles and liquid aluminum.[16] In the case of thermodynamically unstable oxides (e.g., volatile oxides such as SiO2) added externally in liquid aluminum, formation of stable oxide crystals can be maximized by complete conversion of the volatile oxides.[17] The same reactions were demonstrated upon conversion of volatile oxides into interconnected fibers of MgAl2O4 (spinel) phase in an Al alloy by self-propagating, high-temperature synthesis of powder compacts.[18,19] It was reported that oxide particles work better as nucleating substrates when the melt is subjected to an external physical field. Atamanenko et al.[20] reported improved nucl
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