AMEPA’s innovation in aluminium mill finish roughness online monitoring

AMEPA GmbH, Germany, offers innovative measuring systems for companies in the steel, aluminium and automotive industry with a comprehensive range of services. With more than 400 installed systems in 34 countries the quality and process monitoring online measurement systems for slag detection and roughness measurement have made AMEPA systems to a worldwide standard. Offices in the USA and China as well as a global sales and service network guarantee comprehensive support.

The increase of complexity in metal forming together with thinner paint layers and even missing filler layer lead to higher customer requirements on the surface of aluminium strips for the automotive industry. But also the packaging industry is more and more demanding. These more stringent customer requirements and the necessity to increase productivity are some of the numerous reasons that challenge the aluminium manufacturers to a delivery problem of guaranteed high surface quality strips.

The appearance of the finished painted product has a high value for the end user but in the future not only the demands on brilliance are rising. Also due to cost and environmental reasons the paint layers become thinner as a result the structure of the base material becomes more important for the optical appearance of the final product. All this increases the need for a better control of the topography e. g. the roughness of aluminium and steel sheet.

So far, roughness is measured with mechanical stylus manually on samples taken right from the beginning and end of coil. This measurement does not provide representative values for the roughness distribution over the length of the coil because samples are taken at coil’s head or tail where production parameters like speed, rolling force etc. are not consistent.

To continuously control and through this to improve the surface properties the surface roughness sensor “SRM” has been developed by the Centre for Research in Metallurgy (CRM), Liège, Belgium and industrialised by AMEPA GmbH to measure online the roughness parameters in strip sheet production. The measurement is taken along the whole length of the strip with the possibility to position the sensor over the width of the coil. It gives valuable information about the roughness distribution across the strip. The selected measurement method is based on the principle of triangulation: a very thin line is projected on the surface and the relief is determined by analysing the deformation of the line. This measurement principle comes closer to the mechanical stylus reference measurement than any other optical measurement device and leads to a correlation between SRM and stylus in a range of +/-10 %.

The immediate availability of roughness data allows direct corrective action to process parameters if quality of surface roughness is out of its tolerance range. This avoids cost and time intensive second milling of the product.

The optical online roughness measurement gives access to process enhancements over a significant part of each coil and enables more detailed or complete documentation of this important product property in particular. The online measurement cannot only be used as highly selective production tolerance but also as tool for a more precise indication for the change of the texturing rolls.

The light sectioning principle uses the projection of a laser line onto the surface which is distorted by the topography. A microscopic image is taken and the surface profile can be recalculated from the image. The user receives not only the roughness parameters. He also directly gets an online display of the microstructure of his product. This helps to control the milling process and gives a tool to optimize the surface topography.

The example images of figure 1 show different surfaces with different textures which lead to different roughness values but also to differences in oil retention for lubrication during transport and deep-drawing. Also tribological behaviour and paintability are changing. The line in the middle of the images is the projected laser line which enables the system to calculate the roughness parameters like a stylus system. The background of the images shows the distribution, density and shape of the textures used to achieve the desired technological behaviour of the surfaces. The EDT textured aluminium sheet in a) consists of wide open structures with low roughness, peak count and only very limited oil retention; b) shows an EDT texture with higher roughness and peak count and also closed structures which will enable the surface to store more lubricant and keep it longer in position even during the deep-drawing; c) shows as a further example the typical mill finish structure for packaging.

Figure 1: Example images of different surfaces

a) aluminium EDT (Ra=0.75 µm, RPc=47 cm^-1)
b) aluminium EDT (Ra=1.10 µm, RPc=75 cm^-1)
c)aluminium mill finish (Ra=0.41 µm)

The SRM sensor (cf. figure 2) is installed on various lines which cover the whole variety of aluminium and strip steel products manufactured today: continuous annealing line, galvanising line (high reflective surface) but also on low reflective absorbing surfaces like electrolytically galvanized or galvanealed surfaces which are difficult to measure with other optical systems. Also coated, uncoated, random (e. g. EDT) and deterministic (e. g. EBT, laser textured) surfaces are considered.

Figure 2: SRM installation in production line

To complete customers’ requirements regarding sheet metal surface properties and process control AMEPA GmbH also provides an online oil film measurement installation. Working principle of the OFM measuring device is diffuse reflectance spectroscopy. For the measurement the surface is spotlighted by specific lamps. The light penetrates the oil layer on the sheet, is reflected by the sheet base and passes the oil layer for a second time. Part of the reflected light is caught and examined by the AMEPA-OFM sensor.

Using this method no further aid is needed. The measurement can be done at the installation location independently from the operation of the plant. The measuring range of the system covers oil film layers from 0.1 to 5 g/m² surface weight. Mineral oils are typically used as corrosion protection or preforming oils by the metal sheet industry or pressing plants. These lubricants feature a narrow range in their spectrum wherein they strongly absorb infrared light. This reduction of light is detected and determined by the OFM system which is available as an online or at-line system or as portable OFIS-handheld unit.

Besides the application in process lines German car manufacturers take measurements with the Amepa roughness and oilfilm measurement systems on aluminium and steel blanks to improve their efficiency and quality and for troubleshooting. Also all kinds of AMEPA-sensors are presently used in Europe in daily production by all major quality steel manufacturers e. g. ArcelorMittal SA, thyssenkrupp Steel Europe AG, Salzgitter Flachstahl GmbH, voestalpine AG and other production lines. Several systems are installed in China e. g. at Shougang Cold Rolling Co., Ltd. and Shagang Group on continuously galvanising and annealing lines as well as in a skin pass mills.

We appreciate our customers’ confidence in our technology and invite all interested parties to learn more about AMEPA GmbH, Germany and its products. Please visit us on the occasion of the “Aluminium 2018”, October 9^th-11^th, 2018, Duesseldorf, Booth 9I35.