An experimental study of the effect of Ag, Cu, Li, Mg, Ni, Ti, Sc, Zr on grain refining in binary and multicomponent Al-based alloys

Abstract

The objective of the current work is to establish the conventional mechanisms of grain refining, and the effect of the refiner-modifier interaction in Al-based alloys on the resultant grain refining obtained. The different castings were carried out in preheated metallic molds with solidification rates of around 0.15 °C/s and 7 °C/s. As the nucleation sites (Al3Ti) change in composition depending on the percentage of Si in the alloy, the term “poisoning” frequently used to explain the loss or weakening of the power of the refiner, is misused. Addition of L12 metals e.g., Zr and Sc would contribute to constitutional undercooling, restricting the coarsening of α-Al grains as well as act as suitable sites for heterogeneous nucleation. The use of transition or ultra pure (99.99) metals would increase the degree of undercooling as well as nucleation of new grains due to precipitation of the second phase leading to the formation of a heterogeneous grain distribution. Thus, grain refining of binary alloys - partially or fully, may not be produced only through constitutional undercooling. It is suggested that, the use of 0.18%Sc has more-or-less the same grain refining efficiency of 0.15%Ti added in the form of Al–Ti–B master alloy i.e., reduction in the grain size by about 85% and an increase in the alloy strength by about 30%. For a given solidification rate, there is a grain refining threshold beyond which no further refining can be achieved. Apparently grain refining mechanisms are independent of solidification rate, only the extent of refining.

Keywords

• Grain refining
• Aluminum alloys
• Alloying elements
• Undercooling
• Growth restriction factor