Materials Science Forum Vols. 519-521

Abstract: Factors controlling the recrystallization of a continuous cast Al-Fe-Ni alloy were investigated by analyzing microstructure, grain structure, texture, and electron backscattering patterns. The texture of an as-rolled sample heated at 400°C was similar to that of a sample at the reduction ratio of 50% after intermediate annealing at 550°C. The as-rolled samples had a strong (213)[36-4] (S-type) orientation that was changed to a strong (130)[3-12] (QQ-type) orientation by heating at 400°C. This recrystallization looks like continuous recrystallization. Discontinuous recrystallization due to particle-stimulated nucleation is suppressed by the fine dispersion of Al-Fe-Ni-Si particles that form during casing and intermediate annealing, and stabilize the grain and subgrain structure. We think that a course initial grain structure and moderate level of strain level are the important factors promoting continuous recrystallization in this alloy.

Abstract: An investigation was conducted on the softening behaviour of cold rolled continuous cast (CC) AA5754 Al alloy and compared to the results for the ingot cast (IC) material. The present study suggests that the CC material exhibits greater resistance to softening as compared to the IC AA5754 for the same amount of cold deformation. The differences in the softening kinetics become more noticeable with increasing level of cold deformation and from a processing point of view can be attributed to the absence of the homogenization stage during the processing of the CC material. Resistivity measurements were carried out during the annealing treatment of the CC materials to examine the possibility of concurrent precipitation, which could potentially retard the softening kinetics for these materials. In addition, the current research reveals that the CC material produces a finer recrystallized grain size as compared to the IC material.

Abstract: The addition of scandium severely retards the recrystallization of Al-Sc alloys when it is present in the form of fine Al3Sc precipitates. Though many studies have focused on the role of Al3Sc in the deformation and recrystallization of pre-aged or hot deformed Al-Sc alloys, recent studies on the annealing response of solutionized and cold rolled material have shown various possibilities for microstructural stability depending on the relative kinetics of precipitation and recrystallization. In this study, the microstructural evolution of solutionized and cold rolled Al- 2.9wt%Mg-0.16wt%Sc has been followed in order to evaluate the role of imposed strain and annealing temperature on the recrystallization kinetics.

Abstract: An experimental AA5182 sheet was cold rolled 80%, and tensile specimens removed with orientations 0, 45 and 90 degrees to the rolling direction. Room temperature monotonic tensile tests were performed on the specimens in different recovered states obtained by isothermal annealing at 230°C from 0.1 to 10h. The tests were instrumented to measure instantaneous plastic strain ratio, and unloaded just after incipient necking, but before failure if possible. With annealing the flow curves are characterized by lower strains to the onset of jerky flow, the reappearance of yield point elongation, decrease in work hardening, and increasing ductility. The recovery in substructure was described using a constitutive parameter proportional to the mean slip distance.

Abstract: Specimens of commercial purity aluminum were subjected to a strain path change test during high temperature deformation. Specimens were deformed at 4000 C and strain rate of 0.1 s-1 up to various strains of 0.2, 0.5, and 1. Then in a strain path change test, specimens were first deformed to a strain of 0.5, and subsequently deformed to strains of 0.2 and 0. In order to further the understanding of the deformation mechanisms in aluminum, the subgrain sizes and misorientations were characterized in detail by comparative studies using optical microscopy in polarized light (POM), orientation imaging microscopy (OIM/SEM) and transmission electron microscopy (TEM). The analysis revealed that while subgrain size is relatively insensitive to strain, overall misorientations increased with increasing strain. These analyses confirmed a strong bimodal distribution of boundaries during deformation coupled with a low fraction of medium angle boundaries. The results contribute to the understanding that dynamic recovery in aluminum maintains subboundaries with low misorientation but as grains elongate and more subgrain become adjacent to grain boundaries the fraction of high angle boundaries rises.

Abstract: Using modifications to the Rappaz-Drezet-Gremaud hot tearing model, and using empirical equations developed for grain size and dendrite arm spacing (DAS) on the addition of grain refiner for a range of cooling rates, the effect of grain refinement and cooling rate on hot tearing susceptibility has been analysed. It was found that grain refinement decreased the grain size and made the grain morphology more globular. Therefore refining the grain size of an equiaxed dendritic grain decreased the hot tearing susceptibility. However, when the alloy was grain refined such that globular grain morphologies where obtained, further grain refinement increased the hot tearing susceptibility. Increasing the cooling decreased the grain size and made the grain morphology more dendritic and therefore increased the likelihood of hot tearing. The effect was particularly strong for equiaxed dendritic grain morphologies; hence grain refinement is increasingly important at high cooling rates to obtain more globular grain morphologies to reduce the hot tearing susceptibility.

Abstract: Aluminium alloys during solidification change their density. This process can be conditionally divided into two stages: solidification shrinkage due to the density difference between liquid and solid phases and thermal contraction due to the temperature dependence of the solid density. Solidification shrinkage is the main cause of porosity in castings and also plays an essential role in the development of macrosegregation, whereas thermal contraction is important for the development of hot and cold cracks and is responsible for shape distortions during casting. An experimental technique has been developed and applied to binary Al–Cu alloys in order to quantify the thermal contraction in the solidification range and at subsolidus temperatures. It is shown that thermal contraction of aluminium alloys starts at rather high fractions of solid, between 80 and 95%. The experimentally determined temperature of contraction onset agrees well with the temperature at which the mushy material acquires the ability to transfer stresses. The magnitude of contraction accumulated in the solidification range corresponds well to hot tearing susceptibility of the alloy. Factors that decrease the temperature of contraction onset and the magnitude of contraction, e.g. grain refinement, are also known to decrease hot tearing. The data on the temperature at which the thermal contraction starts, on the magnitude of the contraction, and on the thermal contraction coefficient are used to model hot tearing and shape distortions during casting.

Abstract: Accurate knowledge of the boundary conditions is essential when modeling the Direct-Chill (DC) casting process. Determining the surface heat flux in the secondary cooling zone, where the greater part of the heat removal takes place, is therefore of critical importance. Boiling water heat transfer phenomena are quantified with boiling curves which express the heat flux density as a function of the surface temperature. Compilations of boiling curves for the DC casting of aluminum alloys present a good agreement at low surface temperatures but a very poor agreement at higher surface temperatures, in the transition boiling and film boiling modes. Secondary cooling was simulated by spraying instrumented samples with jets of cooling water. Quenching tests were conducted first with a stationary sample, and then with a sample moving at a constant “casting speed” in order to better simulate the DC casting process. The ejection of the water film in quenching tests with a stationary sample and the relative motion between the sample and the water jets both lead to an Advanced Cooling Front (ACF) effect, in which cooling occurs through axial conduction within the sample rather than through boiling water heat transfer at the surface. The heat flux density and surface temperature were evaluated using the measured thermal history data in conjunction with a two-dimensional inverse heat conduction (IHC) model. The IHC model developed at the University of British Columbia was able to take into account the advanced cooling front effect. The effect of various parameters (initial sample temperature, casting speed, water flow rate) on the rate of heat removal in the film boiling and transition boiling regimes was investigated.

Abstract: Vertical direct chill (VDC) casting of aluminium alloys is a mature process that has evolved over many decades through gradual change to both equipment design and casting practice. Today, air-pressurised, continuous lubrication, hot top mould systems with advanced station automation are selected as the process of choice for producing extrusion billet. Specific sets of operating parameters are employed on these stations for each alloy and size combination to produce optimal billet quality. The designs and parameters are largely derived from past experience and accumulated know-how. Recent experimental work at the University of Queensland has concentrated on understanding the way in which the surface properties of liquid aluminium alloys, e.g., surface tension, wetting angle and oxide skin strength, influence the size and shape of the naturally-stable meniscus for a given alloy, temperature and atmosphere. The wide range of alloyand condition-dependent values measured has led to the consideration of how these properties impact the stability of the enforced molten metal meniscus within the hot top mould cavity. The actual shape and position of the enforced meniscus is controlled by parameters such as the upstream conduction distance (UCD) from sub-mould cooling and the molten metal head. The degree of deviation of this actual meniscus from the predicted stable meniscus is considered to be a key driver in surface defect formation. This paper reports on liquid alloy property results and proposes how this knowledge might be used to better design VDC mould systems and casting practices.