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

Abstract

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 The ram is then steadily withdrawn, pulling the shell with it.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.