Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part II. Comparison be
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N AND PROCEDURES
THE background of this study on solidification and microsegregation of aluminum containing magnesium alloys is detailed in Part I of this work.[1] The main purpose of the present Part II is a comprehensive comparison of the Mg alloys AZ31 and AM50 with respect to their solidification behavior during directional solidification (DS), including a systematic variation of cooling rate. The microstructure and microsegregation developed during DS are in the focus of this comparison. The solute segregation behavior and corresponding solute profiles obtained by processing of electron probe microanalysis (EPMA) data were analyzed in depth in this work. Moreover, details are presented here for the alloy AM50 that may be compared to the exemplary data given for AZ31 in the methods-oriented Part I. These magnesium alloys are widely used, especially for automotive applications. The AZ31 is known as a good wrought alloy; it is commonly processed by rolling and forging/extrusion. The AM50 is a general casting alloy providing good strength and ductility.[2] For both applications, extrusion and casting, the primary solidification and apparent microstructure are crucial for DJORDJE MIRKOVIC´, formerly PhD Student, with the Institute of Metallurgy, Clausthal University of Technology, is Project Engineer, with Salzgitter Mannesmann Forschung GmbH, D-38239 Salzgitter, Germany. RAINER SCHMID-FETZER, Professor, is with the Institute of Metallurgy, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany. Contact e-mail: schmid-fetzer@ tu-clausthal.de Manuscript submitted July 14, 2008. Article published online February 20, 2009 974—VOLUME 40A, APRIL 2009
subsequent processing and final product quality, even though the microstructure formation and evolution during solidification are still not fully understood, as recently stated by Gertsman et al.[3] for AM50 and similar die-cast alloys. They reported on the microstructure and second-phase particles in low- and highpressure die-cast AM50 alloy.[3] Related work concerns the much more common magnesium alloy AZ91. Pettersen et al.[4,5] studied dendrite stem orientation relations of directionally solidified AZ91. They found that the stem direction depends on the interplay of the temperature gradient, G, and the growth velocity, V. Mathiesen[6] studied primary dendrite spacing as a function of GV of the AZ91 alloy. The methodology used for the comparison of the AZ31 and AM50 alloys is based on a customized DS technique, elaborated on in Part I of this work.[1] This improved experimental procedure comprises sample tube material selection and protective coating, filling of tubes and DS procedure, and finally microstructure and microsegregation characterization. The dedicated processing of the quantitative EPMA data, as well the calculation of the Scheil-total solute profiles, is specified in Part I. Details on the starting material and compositions of the alloy AZ31 are also given in Part I[1] of this work. The ingot chemical composition of the alloy AM50 is shown in Table I
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