VOLUME 3, ISSUE 3 MARCH • 2009 w w w. i p g . n o v e l i s . c o m
Aluminum Coil Manufacturing: Direct Chill Casting versus Continuous Casting
the tech spot Aluminum. The height of the chill block is very short, usually less than 5 inches deep. The second component, the bottom block is a pan attached to a hydraulic cylinder that moves down into a deep casting pit during casting.
Background of the Aluminum Sheet Ingot Casting Process Manufacturing of coiled Aluminum products begins with the casting of ingots or strips that are fabricated into coils of various widths and thicknesses. There are two principle methods of sheet casting. The first and more traditional method is called DC or Direct Chilled casting, and the second is CC or Continuous Casting. Both casting processes require mechanically rolling the ingot or strip down to a thin gauge. The final product characteristics are a result of specific process recipes. Each product recipe includes critical process factors such as the chemistry of the alloy, thermal processing/annealing, and the amount of work hardening resident in the finished coil. Modifications to these critical factors will create the variety of alloys and tempers that are commercially available.
Example of a DC mould assembly with the bottom block retracted down in the casting pit. The mould face or chill block can be seen in this photo along the perimeter of the open rectangular cavity.
The bottom block is positioned snug within the chill block. At the start of casting the molten Aluminum is allowed to fill the bottom block. The water cooling along the perimeter of the chill block causes the molten Aluminum to form a thin solid shell, while retaining a molten center. When the shell solidifies to a
DC Casting (Direct Chill) DC casting is a semi-continuous process involving the pouring of molten Aluminum into a short mould. The mould assembly for casting ingots has two principle components, an open rectangular chill block and a bottom block. The chill block is located in a fixed position around the mould and is internally water cooled. The chill block is the actual mould surface that makes contact with and freezes the molten
Photo of a DC ingot being removed from a casting pit. The standard manufacturing sequence for DC ingots includes: scalping to remove the surface layer, thermal processing to homogenize the ingot and to bring it to hot rolling temperature, hot rolling to reduce the ingot down to reroll thickness, cold rolling to reduce to final gauge, annealing to bring coil to required mechanical properties, and finishing to meet the customers requirements for shipment.
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predetermined thickness, the bottom block along with the semisolid ingot is lowered hydraulically into the casting pit. During this process no molten Aluminum is allowed to make direct contact with water. Additional quenching of the ingot surface is provided by a water curtain positioned beneath the mould. The water curtain sprays the hot surface of the newly formed ingot while it is lowered into the casting pit, causing the ingot to solidify completely through thickness within a short time period. When the cast ingot reaches its required length, the flow of molten Aluminum and the descent of the ingot are stopped. Typical dimensions of DC cast ingots vary between 18 to 30 inches in thickness, 40 to 100 inches in width, and 160 to 320 inches in length. Individual ingots can weigh between 15,000 and 50,000 lbs. To minimize manufacturing costs up to eight ingots can be cast simultaneously. CC Casting (Continuous Casting) The continuous casting process creates a continuous strip of Aluminum that can be coiled or fed directly into an in-line rolling process to reduce the strip’s thickness. There are three types of continuous casting processes commercially available. These include Block casters, Roll casters and Hazelett Belt casters. Each process applies a thin layer of molten Aluminum onto a water cooled metal belt, blocks or roller that allows the metal to quickly solidify. The solid cast strip is then immediately hot rolled to gauge and coiled. This is a near continuous process where interruptions usually occur only during maintenance. The typical finished gauge directly off of the continuous caster is between 0.060” and 0.30”. Traditional cold rolling in a separate operation can reduce the thickness further. In fact many cold rolling mills will run both Direct Cast and Continuous Cast products. (continued)
VOLUME 3, ISSUE 3 MARCH • 2009 w w w. i p g . n o v e l i s . c o m
Hazelett Belt Caster
Comparison between the two Casting Methods Capital and Manufacturing Cost: Manufacturing processes using traditional Direct Chill cast ingots are capital intensive. Capital requirements not only include the costs for the casting center, but also require manufacturing equipment for ingot scalping, ingot thermal treatment, and hot rolling. Processing of Direct Chill ingots also requires a high thermal input to reheat ingots for hot rolling. In comparison, the Continuous cast process only requires a casting unit with an in-line hot rolling facility. Energy costs are minimized by avoiding the scalping and ingot reheating process required for the Direct Chill cast product. Another distinct advantage of the Continuous cast process is that large coil sizes can be made available, which in turn also helps the overall efficiency of the manufacturing process. Metallurgical Characteristics: The Direct Chill cast process allows a high level of flexibility in establishing a variety of different metallurgical characteristics in the final coiled product. For this reason, a wide variety of alloy chemistries and tempers are commercially available for DC cast ingots. There is also a relatively large overlap in performance between the various alloy series and
tempers. This situation allows a high level of flexibility for the product and design engineers to apply Direct Chill cast Aluminum as a solution for their needs. Direct Chill cast Aluminum is the commercial standard for most industrial and commercial applications. The performance and physical properties of the final Aluminum sheet product is defined by its microstructure. The microstructure of the Continuous cast product is distinctly different than that of a Direct Chill cast product. This difference is created during the rapid rate of solidification inherent in the Continuous cast process. Typically, the microstructure in Continuous cast products is composed of fine particles that are not uniform in composition, size and distribution. This non-uniformity represents a challenge that, in many cases cannot be rectified. This is the reason why special alloys are allocated to Continuous cast products and why there remain only a small number of ideal applications for this product. Continuous cast products are generally not suitable for high magnesium bearing alloys, heat treatable alloys, high strength or high formability applications. Some exceptions to this rule do exist. In nearly all cases, however, products produced from Direct Chill cast products outperform Continuous cast products. Surface Cosmetics Characteristics: The solidification of Aluminum creates an oxide layer on the surface of the cast ingot or strip. Product applications that require a uniform or cosmetically appealing surface will be seriously affected by the presence of these oxides. Oxides will create a variety of deleterious defects on the surface of the final rolled product. Additional manufacturing problems can occur if the surface of the final product is chemically treated, etched or anodized. In some cases, the oxides can be so severe that defects can show through thick coatings of paint. For all product applications where surface is critical, Continuous cast products are to be avoided. Oxides are present on both Direct Chill and Continuous cast products. Direct Chill cast products can be scalped where the surface layer of the ingot which contains the oxides are physically machined off prior to rolling.
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This opportunity does not exist for Continuous cast products. The as-cast surface from the Continuous cast slab is immediately rolled following casting. The end result is that the Continuous cast product will have rolled in surface defects that are unacceptable for surface critical applications. Summary The selection criteria for determining the suitability of a product for a given application is dependent on a number of factors. If pricing is the only issue and a suitable product is commercially available, then Continuous cast products would be an ideal solution. If other product characteristics such as product performance and consistency are required, then costs become irrelevant. The comparative advantages and disadvantages of Direct Chill cast and Continuous cast products are defined as follows: Product Attribute
Continuous Cast
Cost.......................................lower cost Max Coil OD Size..................larger coils Product Attribute
DC Cast
Max Coil Width......................wider coils Product Consistency.............superior High Formability....................wide variety High Strength........................wide variety Alloy Selection.......................wide variety Temper Selection...................wide variety Surface Appearance..............superior Chem Treat Surfaces............superior Heat Treatable.......................exclusive Design Flexibility...................superior Formability.............................superior Anti-galling.............................superior High Magnesium...................wide variety
©2008, Novelis Corporation Authored by: Barney Costello, Product Development & Applications Mgr. Specialty Products Business Unit 3/09 Ver. 3
