The Evolution of As-cast Microstructure of Ternary Mg-Al-Zn Alloys: An Experimental and Modeling Study
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UCTION
THE Mg-Al casting alloys which include the Mg-AlZn alloys (AZ series) and Mg-Al-Mn alloys (AM series) are attractive choices as light structural materials in automotive industry.[1] The most common casting routes for these alloys are high-pressure die casting and direct chill casting. Recently, twin roll casting (TRC) process is an emerging technology to produce Mg alloy sheets[2–6] by employing solidification and deformation at the same stage. Although Mg alloys have been extensively investigated in areas of physical metallurgy, there are few fundamental studies that focus on the evolution of as-cast microstructure depending on solidification parameters. The local solidification conditions such as thermal gradient (G), solidification velocity (V), cooling rate (GÆV), and solute content influence the characteristics of as-cast microstructure such as secondary dendrite arm spacing (SDAS), primary dendrite arm spacing (PDAS), growth morphologies such as cellular, columnar, or equiaxed dendrite, microsegregation, and total amount of second phases. The authors[7] have recently published a systematic experimental analysis of the relationship between the as-cast microstructural development and the solidification parameters for Mg-3, 6, and 9 wt pct Al binary alloys. The experimental data were also MANAS PALIWAL, PhD Student, and IN-HO JUNG, Associate Professor, are with Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada. Contact e-mail: in-ho. [email protected] DAE HOON KANG, formerly Post-doctoral Fellow with Department of Mining and Materials Engineering, McGill University, is now Researcher with Novelis Global Research and Technology Center, Kennesaw, GA. ELHACHMI ESSADIQI, formerly Scientist with the Canmet Materials, Ottawa, ON, Canada, is now Professor with the Universite International de Rabat, Rocade de Rabat, Morocco. Manuscript submitted September 4, 2013. Article published online April 5, 2014 3596—VOLUME 45A, JULY 2014
reproduced by 1D microsegregation model[8] that incorporates the most robust solidification theories and accurate thermodynamic data. The present study is the extension of experimental and simulation work on microstructural solidification for ternary Mg-Al-Zn alloys. The objectives of the present study are to investigate the evolution of as-cast microstructure of Mg alloys depending on the solidification parameters. Directional solidification experiments were conducted for Mg-3Al1Zn (AZ31), Mg-6Al-1Zn (AZ61), and Mg-9Al-1Zn (AZ91) alloys (all in wt pct) to elucidate the relationship between the microstructural features, alloy chemistry, and solidification parameters. A multicomponent solidification model that incorporates solute back diffusion in hcp Mg phase, secondary arm coarsening, and dendrite tip undercooling is developed to predict as-cast microstructure of ternary Mg alloys. The model is dynamically linked with the thermodynamic library ChemApp V 6.0[9] for the input of accurate thermodynamic data from the Factsage thermodynamic database.[10]
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EXPERIMENT
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