Direct Chill Cast Aluminum Alloy Research Articles

A Simplified Three-Phase Model of Equiaxed Solidification for the Prediction of Microstructure and Macrosegregation in Castings

Macrosegregation is a result of the interplay of various transport mechanisms, including natural convection, solidification shrinkage, and grain motion. Experimental observations also indicate the impact of grain morphology, ranging from dendritic to globular, on macrosegregation formation. To avoid the complexity arising due to modeling of an equiaxed dendritic grain, we present the development of a simplified three-phase, multiscale equiaxed dendritic solidification model based on the volume-averaging method, which accounts for the above-mentioned transport phenomena. The validity of the model is assessed by comparing it with the full three-phase model without simplifications. It is then applied to qualitatively analyze the impact of grain morphology on macrosegregation formation in an industrial scale direct chill cast aluminum alloy ingot.

Centerline Depletion in Direct-Chill Cast Aluminum Alloys: The Avalanche Effect and Its Consequence for Turbulent Jet Casting

Avalanche dynamics of sedimenting grains in direct-chill casting of aluminum ingots is investigated as a primary driving force for centerline segregation. An analytical model predicting the importance of avalanche events as a function of casting parameters is proposed and validated with prior art results. New experimental results investigating the transient and steady-state centerline segregation of DC casting with a turbulent jet are reported.

Application of a Pore Fraction Hot Tearing Model to Directionally Solidified and Direct Chill Cast Aluminum Alloys

Hot tearing susceptibility is commonly assessed using a pressure drop equation in the mushy zone that includes the effects of both tensile deformation perpendicular to the thermal gradient as well as shrinkage feeding. In this study, a Pore Fraction hot tearing model, recently developed by Monroe and Beckermann (JOM 66:1439–1445, 2014), is extended to additionally include the effect of strain rate parallel to the thermal gradient. The deformation and shrinkage pore fractions are obtained on the basis of the dimensionless Niyama criterion and a scaling variable method. First, the model is applied to the binary Al-Cu system under conditions of directional solidification. It is shown that for the same Niyama criterion, a decrease in the cooling rate increases both the deformation and shrinkage pore fractions because of an increase in the time spent in the brittle temperature region. Second, the model is applied to the industrial aluminum alloy AA5182 as part of a finite element simulation of the Direct Chill (DC) casting process. It is shown that an increase in the casting speed during DC casting increases the deformation and shrinkage pore fractions, causing the maximum point of pore fraction to move towards the base of the casting. These results demonstrate that including the strain rate parallel to the thermal gradient significantly improves the predictive quality of hot tearing criteria based on the pressure drop equation.

Recrystallization microstructures and textures in AA 5052 continuous cast and direct chill cast aluminum alloy

Commercially produced hot bands of AA 5052 continuous cast (CC) and direct chill (DC) cast aluminum alloys were cold rolled to (thickness) reductions of 70%, 80%, and 90% followed by annealing at different conditions. The recrystallization kinetics are found equivalent for both the CC and DC materials. Recrystallization microstructures are different between the CC and DC materials. Evolution of recrystallization texture in the CC and DC materials were investigated by using three-dimensional orientation distribution functions (ODFs) that were determined by X-ray diffraction. The recrystallization texture was correlated with cold rolling reduction (prior to annealing), annealing temperature, and annealing time. Results showed that the R {124}〈211〉 and cube {001}〈100〉 are dominant recrystallization texture components in both CC and DC materials. During annealing, the intensity and volume fraction of the cube component strongly depend on the prior cold rolling history. In contrast, the intensity and volume fraction of the R component remains almost constant regardless of the different cold rolling reductions prior to annealing. After complete recrystallization, the intensity and volume fraction of both R and cube components appear to be independent of the annealing temperature and annealing time.

Recrystallization microstructures and textures in AA 5052 continuous cast and direct chill cast aluminum alloy

Commercially produced hot bands of AA 5052 continuous cast (CC) and direct chill (DC) cast aluminum alloys were cold rolled to (thickness) reductions of 70%, 80%, and 90% followed by annealing at different conditions. The recrystallization kinetics are found equivalent for both the CC and DC materials. Recrystallization microstructures are different between the CC and DC materials. Evolution of recrystallization texture in the CC and DC materials were investigated by using three-dimensional orientation distribution functions (ODFs) that were determined by X-ray diffraction. The recrystallization texture was correlated with cold rolling reduction (prior to annealing), annealing temperature, and annealing time. Results showed that the R {124}〈211〉 and cube {001}〈100〉 are dominant recrystallization texture components in both CC and DC materials. During annealing, the intensity and volume fraction of the cube component strongly depend on the prior cold rolling history. In contrast, the intensity and volume fraction of the R component remains almost constant regardless of the different cold rolling reductions prior to annealing. After complete recrystallization, the intensity and volume fraction of both R and cube components appear to be independent of the annealing temperature and annealing time.

The nature of surface cracking in direct chill cast aluminum alloy ingots

The vertical direct chill (DC) semicontinuous casting process has been the mainstay of the aluminum industry for Fig. 1—One quarter of a horizontal cross section of a DC cast aluminum 3004 alloy ingot. Surface crack locations are indicated by A and B. the production of billets and ingots since the late 1930s due largely to its robust nature and relative simplicity. Unfortunately, the process can produce distortions in the ingot and Table I. Nominal Composition (in Weight Percent) of cracks can form[1,2,3] owing to the nonuniform, high rate of 3004 Alloy heat removal imparted by the direct contact of the bottom block or water with the partially solidified ingot. The surface Mg Mn Si Fe Cu Zn Al crack is one type of crack that increases the cost and reduces 1.0 1.25 0.3 0.7 0.2 0.25 balance the yield of the DC casting process. This article is concerned with the nature of the surface cracking in large cross-section ingots produced using the DC casting process. In the DC casting process, molten metal flows into a short, rectangular, water-cooled mold, which is initially closed by a plug (called a starter or bottom block) on a movable ram. The metal freezes against the bottom block and forms a shell on the mold surface. The ram is then steadily withdrawn, pulling the shell with it. As the shell exits the bottom of the mold, cold water is sprayed directly on it for cooling. In this manner, a cast ingot of a desired length is produced. Figure 1 shows one quarter of a horizontal cross section of a 3004 alloy ingot with overall dimensions of 0.7 1.85 7.6 m (28 73 300 in.). The composition of aluminum 3004 alloy is given in Table I. The section was obtained approximately 0.15 m (6 in.) from the bottom of the ingot. Small surface cracks occur near the middle and quarter point of the long side of the ingot. The locations of the surface cracks are marked A and B. Research[4–7] has been carried out to control the formation of surface cracks, but the nature of the crack formation is unclear. During DC casting, the ingot surface is in direct contact with either a metal mold or cooling water. This often leads to expectations that the Fig. 2—A surface crack, shown in the middle of the figure with enlarged views at the top and bottom indicating the surrounding microstructure. surface crack could be a cold crack,; [7] i.e., one that occurs Note that the crack lies between dendrites or along grain boundaries. below the solidus temperature of the alloy, rather than a hot tear, i.e., one that occurs above the solidus temperature. It is difficult to believe that the stresses or strains generated along the crack and scanning electron microscopy (SEM) during DC casting due to the uneven shrinkage of the ingot images of the crack surface were obtained. can produce cracks in solid aluminum, which is highly ducThe middle photograph in Figure 2 shows the entire crack, tile at elevated temperatures. In order to clarify the nature indicating that it is approximately 50 to 80 mm deep. The of the surface crack, i.e., whether it was a cold crack or a top micrograph in Figure 2 shows the crack and the microhot tear, we examined cracks on samples taken directly from structure near the surface of the ingot. The bottom microthe surface of a DC cast ingot. The microstructure around graph in Figure 2 shows the tip of the crack and the the crack was examined. The samples were then fractured microstructure near the tip. The aluminum alloy grains are equiaxed throughout the ingot due to the addition of a grain refiner. If we track the crack from the ingot surface all the Q. HAN, Research Staff Member, and S. VISWANATHAN, Senior way to its tip, we see that the crack is formed along grain Research Staff Member, are with the Oak Ridge National Laboratory, Oak boundaries. No transgranular cracking was observed. At the Ridge, TN 37831-6083. D.L. SPAINHOWER, Technical Manager, is with tip of the crack, small secondary cracks invisible to the Logan Aluminum, Inc., Russellville, KY 42276. S.K. DAS, President, is naked eye are evident. These secondary cracks are discontinwith Secat, Lexington, KY 40511. Manuscript submitted June 6, 2001. uous but are also formed along grain boundaries.

Determination of thermophysical properties and boundary conditions of direct chill-cast aluminum alloys using inverse methods

In order to quantify the cooling conditions undergone by an ingot during direct-chill (DC) casting, thermocouples were immersed in the liquid pool and consequently entrapped in the solid, thus monitoring the temperature of the metal during its descent. Assuming steady-state thermal conditions, the time-dependent temperatures measured by these thermocouples were then converted into spacedependent temperature profiles. These values were the input of a Maximum A Posteriori (MAP) inverse method described by Rappaz et al.,[1] which has been adapted in this case to steady-state thermal conditions. This MAP method permits the deduction of the temperature-dependent thermal conductivity of the alloy, initially, and then of the highly nonuniform heat-flux distribution along the ingot rolling faces, in a second step. The obtained values are in good agreement with literature and clearly reflect the widely different boundary conditions associated with primary cooling (contact with the mold) and secondary cooling (water jet).

The effect of free-floating dendrites and convection on macrosegregation in direct chill cast aluminum alloys: Part II: predictions for Al–Cu and Al–Mg alloys

In Part I, a binary mixture model of the DC casting process was proposed, that accounts for fluid flow in the melt and mushy zone, as well as in a slurry zone characterized by the transport of solute-depleted, free-floating dendrites. In this paper, the model is applied to the DC casting of Al–4.5 wt% Cu and Al–6.0 wt% Mg billets, and the predicted surface-to-centerline distribution of macrosegregation is consistent with trends observed in DC cast ingots. These trends include the development of negative segregation at the centerline, subsurface solute enriched and depleted regions, and positive segregation at the billet surface. Negative segregation at the centerline increased with an increase in the packing fraction at which free-floating dendrites are presumed to coalesce into a rigid dendritic structure. Likewise, negative segregation at the centerline and positive segregation in the enriched region increased with an increase in the characteristic diameter of the free-floating dendrites.

The effect of free-floating dendrites and convection on macrosegregation in direct chill cast aluminum alloys: Part I: model development

To obtain a better understanding of the evolution of macrosegregation during the direct chill casting of aluminum alloys, a binary mixture model has been developed that accounts for the redistribution of alloying elements through the transport of free-floating dendrites and fluid flow in the melt and mushy zones of a solidifying ingot. Separate and distinct mixture momentum equations are employed to account for momentum transfer in two-phase regions which include a slurry of free-floating dendrites and a rigid solid matrix. A scaling analysis revealed that the flow of liquid and free-floating dendrites is controlled by a balance between buoyancy forces arising from mixture density variations and the development of an adverse pressure gradient. In a companion paper, the model is used to predict macrosegregation in Al–4.5 wt% Cu and Al–6.0 wt% Mg billets.