Solidification Curves for Commercial Mg Alloys Determined from Differential Scanning Calorimetry with Improved Heat-Tran
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AN understanding of the solidification processes and knowledge about as-cast microstructures are very important for the achievement of desired final cast components properties. This knowledge is also fundamental for the alloy development process, because pursuant material properties have to be developed from the as-solidified microstructure. This is valid especially for the casting process, which is one of the most important manufacturing methods of commercial magnesium alloys. The term ‘‘solidification curve’’ of an alloy is used for the solid (or remaining liquid) fraction vs temperature during solidification in a control volume with fixed overall alloy composition. The knowledge of this quantity is essential for the control of solidification and casting processes and is a very important input parameter for solidification simulation software. Also, welding technology could benefit from that information. Recently, Cao and Kou[1] reported the application of the solidification curves as a tool for the prediction and elimination of liquation-cracking occurring in partially melted zones in weldments of crack-susceptible Al alloys. The knowledge of solidification curves could also be very important for new semisolid material casting processes, such as New Rheocasting[2] and Thixomolding,[3] which has recently started to gain much interest for use with magnesium alloys. In this study, a differential scanning calorimetry (DSC) measurement with a mathematical heat-transfer DJORDJE MIRKOVIC´, Graduate Student, and RAINER SCHMID-FETZER, Professor, are with the Clausthal University of Technology, Institute of Metallurgy, Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted October 12, 2006. Article published online August 28, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
model (HTM) is proposed as an in-situ method for providing solidification curves; its validity is checked by application to the AM50, AZ31, AZ61, AZ62, and AZ91 commercial magnesium alloys. The selected alloys cover a wide range of typical applications such as rolling (AZ31), extrusion (AZ61), sand casting/welding wire (AZ62), and die casting (AM50 and AZ91). The alloys denoted as AZ61, AZ62a, and AZ91a with reduced Mn content (in addition to those denoted AZ62b and AZ91b) are also studied in order to investigate the effect of Mn on the alloy solidification path. Next to determining solidification curves for this wide range of Mg alloys, a main purpose of this work is to introduce a new method of desmearing involving the independent determination of a time constant as a function of temperature for the applied equipment. A further improvement is achieved through a more impartial interpretation of the measured DSC curves, where the termination of solidification is not being artificially defined by the operator. It is also shown that an improved Ta-capsule design including a dependable sealing method enables reliable and reproducible DSC measurements needed for the DSC-HTM approach, even for the very demanding magnesium allo
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