Grain Refinement of an Al-2 wt%Cu Alloy by Al3Ti1B Master Alloy and Ultrasonic
Treatment
Eric Q. Wang1, Gui Wang1, Matt S. Dargusch1, Ma. Qian2, Dmitry G. Eskin3, David H.
StJohn1*
1
Centre of Advanced Materials Processing and Manufacturing (AMPAM), The University of
Queensland, School of Mechanical and Mining Engineering, St Lucia, QLD 4072 Australia
2
RMIT University, School of Aerospace, Mechanical and Manufacturing Engineering, Centre
for Additive Manufacturing, GPO Box 2476, VIC 3083 Australia
3
BCAST, Brunel University, Uxbridge UB3 PH, United Kingdom
Abstract:
Microstructural grain refinement of cast metallic alloys improves the mechanical properties
of as-cast components and subsequent thermo-mechanical processing behaviour of semifabricated products such as billet and slab materials. The addition of master alloys containing
inoculant particles is standard grain-refining practice for most commercial cast aluminium
(Al) alloys where the most common grain refiners are based on Al-Ti-B and Al-Ti-C master
alloys. Alternatively, Ultrasonic Treatment (UT) has proved to be effective for the grain
refinement of various Al alloys. The effect of UT on microstructural evolution is derived
from its physical influence on the molten melt, such as cavitation, acoustic streaming and
radiation pressure.
The present study investigates the influence of UT on the grain refinement of an Al-2 wt%
Cu alloy with a range of Al3Ti1B master alloy additions. The effect of the application of
Ultrasonic Treatment (UT) over selected temperature ranges during cooling and solidification
on the grain structure and cooling behaviour has been investigated for an Al-2Cu alloy melt
cooled from 720°C. UT was applied over various temperature ranges before, during and after
the nucleation of primary aluminium grains. It was found that ultrasonic grain refinement was
achieved only when UT was applied from more than 20oC above the liquidus temperature
until below the liquidus temperature after nucleation has occurred.
When the alloy contains the smallest amount of added master alloy, UT caused significant
additional grain refinement compared with that provided by the master alloy only. However,
the influence of UT on grain size reduces with increasing addition of the master alloy.
Plotting the grain size data versus the inverse of the growth restriction factor (Q) reveals that
the application of UT causes both an increase in the number of potentially active nuclei and a
decrease in the size of the nucleation free zone due to a reduction in the temperature gradient
throughout the melt. Both these factors promote the formation of a fine equiaxed grain
structure.
The influence of ternary alloying elements on the
solidification and microstructure of Al-Si alloys
A.Darlapudi1, S.D McDonald and D H StJohn1
1
Centre of Advanced Materials Processing and Manufacturing,
The University of Queensland, St Lucia QLD 4072 Australia
E-mail: a.darlapudi@uq.edu.au
Abstract. The influence of alloying elements (Cu, Mg, Ni and Fe) on eutectic
nucleation, eutectic grain morphology and final microstructure of an Al-10Si
commercial purity alloy in the unmodified and Sr-modified conditions was
investigated. It was found that the nucleation and eutectic grain growth
morphologies of both the unmodified and Sr-modified Al-Si eutectic were
significantly influenced by the addition of ternary alloying elements and the
degree of influence varied depending on when the intermetallic phase formed
during the solidification of the alloy with respect to the Al-Si eutectic. In cases
where an intermetallic phase nucleated prior to the onset of the Al-Si eutectic
reaction, the eutectic nucleation frequency was affected by changes to the
available nuclei population. In cases where the intermetallic nucleated after the
Al-Si eutectic, segregation of the ternary solutes in front of the Al-Si eutectic
interface changed the nucleation and macroscopic growth dynamics. The
changes in nucleation and growth dynamics of the Al-Si eutectic due to the
presence of solute altered the morphology of the eutectic silicon considerably.
This study has revealed a number of insights into the mechanisms of
nucleation and growth of the Al-Si eutectic.
On the role of the melt thermal gradient on the size of the
Constitutionally Supercooled zone
Arvind Prasad1, Lang Yuan2, Peter D. Lee2, Mark Easton3, David StJohn1
1
Centre for Advanced Materials Processing and Manufacturing, The University of Queensland,
Brisbane, Australia
2
The Manchester X-ray Imaging Facility, School of Materials, The University of Manchester, Oxford
Rd., Manchester, M13 9PL, UK
3
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne
Australia
Abstract:
The Interdependence model analytically predicts the grain size taking into account the already nucleated and
growing grain, rejected solute diffusion from the growing grain and the inoculant particle distribution in the melt.
The model is predicated upon the idea that an inoculant particle will trigger a nucleation event when the local
undercooling in the melt where the particle resides, is larger than the undercooling required by the particle to
initiate a nucleation event. The undercooling at any point in the melt is the difference between the melt
temperature at that point and the temperature governed by the solute content of the melt at the same point
(defined by the phase diagram). The undercooling due to this solute content is called Constitutional
Supercooling (CS) and would be different at different parts of the melt due to solute diffusion ahead of the
growing grain. Likewise, the presence of a thermal gradient governs the local melt temperature. Thus it is clear
that prediction of nucleation by an inoculant particle requires quantifying the interplay between the thermal
gradient G, and the CS, both as a function of location.
The Interdependent model predicted grain size is proportional to the length of the solute diffusion zone to the
point where a maximum CS (TCSmax) is attained. Due to the exponentially decaying nature of the solute
diffusion profile, the location of TCSmax is some distance from the interface of the previously growing grain.
The thermal gradient present in the melt, G, is expected to affect the size of CS Zone (CSZ i.e. the length of CSZ,
x’CSZ, and the location of TCSmax, x’CSmax). This preliminary investigation assesses the effect of G on x’CSZ and
x’CSmax. A range of G values is introduced into both the analytical and a numerical model (MatIC) to obtain a
correlation between the value of G and the dimensions of CSZ. The result of a test case from the analytical
model shows that x’CSmax initially decreases rapidly and then decreases gradually approaching zero at very high
values of G. Independent of the analytical model, the results from MatIC replicate the trend obtained from the
analytical model. The MatIC simulations also shed light on the effect of solidification time on TCSmax.