Abstract
Al-Mg based alloys are widely used in industrial applications and grain refining is used for structural refinement to achieve high strength and ductility. The aim of this work is to investigate the effects of two grain refiners of Al-5Ti-1B and Zr on the solidification transformations of Al-6Mg alloy and dendrite coherency point (DCP) using two thermocouple cooling curve thermal analyses (CCTA). The microstructure and intermetallic phases were studied through scanning electron microscopy (SEM), EDS, and X-ray mapping. Microstructural evaluations showed the main secondary phases of Al-6Mg alloy are Al3Fe, Al8Mg5, and Mg2Si. The CCTA results indicated that the addition of Al-5Ti-B and Zr causes an increment of nucleation temperature, TN,α,(from 622.4 to 628.4 and 625.7 °C, respectively). The grain refining potential is an inherent property that depends on the growth restriction factor (GRF) and mostly relates to solute concentration and liquidus line slope. The refining power of Al-5Ti-1B for Al-6Mg is more effective because of its larger growth restriction factor (GRF) in comparison with Zr. Therefore, the grain size of the alloy used Al-5Ti-1B or Zr grain refiner was 105 µm and 185 µm, respectively. This phenomenon has been proved when recalescence undercooling (ΔTR,α) was calculated. It has been shown that ΔTR,α reaches zero when Al-5Ti-1B master alloy is added to Al-6Mg alloy. Zr caused the decrement in the first derivative peak intensity of eutectic phases which may be eventuated from extending the solid solubility by the addition of Zr. These grain refiners also affected the dendrite coherency point (DCP). Although Zr changed the TN-TDCP and fsDCP, Al-5Ti-1B increased the TN-TDCP interval by about 14.8 °C and delayed up the coherency solid fraction by about 89.7 pct which can decrease the casting defects.
Published Version
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