Düsseldorf, August 14, 2018

Heat treatment of aluminum strips for the automotive and aviation industry

Serving as high-quality lightweight parts, aluminum plates and sheets are used in numerous special applications. Particularly in automotive construction and in the aerospace industry, the material is used to an increasing extent due to its little weight with but high strengths. Without these innovative aluminum materials for structural parts or exposed applications it would be very hard to satisfy the condition of low weight. Many of these applications demand for the cold-rolled aluminum strip to be treated in a process of annealing combined with fast cooling to achieve formability and the necessary material strengths.  

From cold-rolled aluminum strip to finished lightweight material in one line

For the production of such grades, SMS group supplies suitable HCTLs (Heat and Chemical Treatment Lines). In addition to material heat treatment, the strip surfaces undergo an extra chemical treatment. The lines cover a vast range of materials. Material thicknesses are between 0.2 and 4.5 millimeters, while the strip width may be more than 2.5 meters.

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Typical layout of a Heat and Chemical Treatment Line.
Heat treatment in high-performance floatation furnace

The heart of the process is the annealing and cooling technology. At speeds of about 100 meters per minute, the strip is guided, without contact, through the over 100-meter-long floatation furnace in a sinusoidal wave. Here, the material is initially heated to a defined temperature and then held there for a material-dependent minimum of time, with temperature accuracy being of great importance for achieving the desired material property. To have the furnace certified by the NADCAP (National Aerospace and Defense Contractors Accreditation Program) for the production of aerospace grades, it is indispensable to provide evidence that, among others, air temperature accuracy is within +/- three kelvins over the entire width and length of the furnace. Complying with these conditions, the furnaces made by SMS group can be certified for the important supply industries.

SMS group developed a special water cooling system to achieve high cooling rates. The 2xxx and 7xxx series aerospace-licensed alloys, in particular, require high cooling rates of more than 400 kelvins per second. For the 6xxx series automotive alloys moderate but flexible cooling rates between 25 and 200 kelvins per second will do. In addition to achieving the needed cooling rate, extremely uniform cooling over strip length and width is crucial to largely prevent material distortion. A special feature in this connection is the predefined strip crossbow that has two major functions. On the one hand, the water may run off the strip in a controlled manner due to this curvature, and on the other, the stable strip shape ensures the required consistent cooling effect through a special spray bar system and the centered strip guidance.

Another highlight of the furnace is a mathematical model calculating the settings for the annealing and cooling processes with regard to the mechanical properties.

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Contactless strip travel on air cushions in the floatation annealing furnace.
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Specific cooling in the water quench due to pre-defined cross-bow.
Chemical treatment

It is especially the 5xxx and 6xxx series alloys used in the automotive sector that require chemical treatment. For this purpose, the surface is initially pickled by alkaline and acid media in an efficient spraying unit. This unit is designed to handle the sensitive surface with care, for example through equipment preventing the formation of droplets when individual areas are switched off.

To achieve a uniform chemical coating, SMS group prefers the installation of vertical roll coaters. These coaters are provided with sophisticated control technologies ensuring a uniform coating result in the layer thickness demanded in each case at low media consumption. Depending on the coating medium or the plant layout, however, it is possible to install horizontal roll or spray coaters respectively immersion baths.

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Vertical roll coater with drying section.
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The vertical roll coater comes with advanced controls that ensure uniform coating in the desired layer thickness. Equally convincing is the low media consumption.