Thermodynamic evaluation of the thixoformability of Al–Si alloys

Abstract

The goal of this work is to introduce a simple method for evaluating the thixoformability of Al–Si alloys based on semisolid transformation curves obtained by differential thermal analysis (DTA), differential scanning calorimetry (DSC) and Thermo-Calc® simulation. The Al–Si alloys evaluated had a silicon content ranging from 2 to 7wt% Si and were produced by grain refining with the addition of Al5Ti1B master alloy. The semisolid transition for each alloy was analyzed according to classical thermodynamic thixoformability criteria, i.e., by looking at the highest “knee” on the liquid fraction vs. temperature curve, the sensitivity of liquid fraction (dfL/dT0.4–0.6) at 40/60% liquid fraction and the solidification interval of each alloy. The results showed that thermodynamic characterization using DTA, DSC and Thermo-Calc® was effective in predicting semisolid behavior but that use of the commonly adopted criteria reduces the number of alloys considered suitable for semisolid metal (SSM) processing. The criterion proposed here, “The sensitivity of the liquid fraction, dfL/dT, at the desired liquid fraction, fL, exclusively for the primary phase must be as low as possible (<0.03°C−1)”, increases the number of thixoformable alloys by focusing on the control of SSM processing. Thixoforming or Rheocasting must be avoided at temperatures at which the common secondary phase, i.e., usually the eutectic phase present in SSM raw materials, undergoes transformation as this transformation is more complex and less stable than the corresponding one for the primary phase. The paper also demonstrates that kinetics parameters, such as heating (or cooling) rate, have a major effect on semisolid transformation and should be incorporated in the control of SSM processing so that the best processing window can be determined more accurately.