Hot rolling emulsion
A material with a massive impact
As a premium provider with a strong focus on specialist products, excellent strength and forming properties are vital. However, surface quality also plays an important role for almost all our products. This not only determines their visual appearance but also influences their formability, solderability, slip resistance, bonding ability and much more.As we strive to keep pace with the rising demands our customers place on our flat-rolled products, it is essential that we clearly understand and continuously optimize the tribological system of aluminium hot rolling (see Figure 1). This tribological system comprises all factors that influence friction, lubrication and wear during the hot rolling process. Stringent monitoring and optimization of these factors is essential to achieving optimal rolling quality and high efficiency.Hot rolling of aluminium and aluminium alloys uses oil-in-water emulsions. These emulsions serve the following purposes during the rolling process:
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Lubrication: the oil portion provides lubrication between the aluminium and roll surfaces
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Cooling: the water portion cools the rolls and thereby protects against overheating
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Cleaning: removal of abrasion debris generated during the rolling process
An emulsion is a finely balanced mixture of two fluids that are usually immiscible. One example is water and oil: due to their different chemical properties, the two usually remain separate. An emulsion is created when the oil phase is dispersed into tiny droplets (dispersed phase) and distributed in the water (continuous phase) (Figure 2).The emulsifier is an important component of an emulsion: it facilitates droplet formation and prevents demixing. It acts as a link between the two phases and comprises a polar (hydrophilic) part and a non-polar (lipophilic) part. However, emulsions are metastable systems: unless energy is put into the system - such as through pumping or shearing forces - tiny droplets would combine to forge larger droplets and the emulsion would ultimately split.Emulsions are an important factor in the hot rolling process. Their quality (i.e. composition and stability) directly influences the surface quality of rolled products. Continuous analysis of emulsion composition and corresponding emulsion treatment are essential. Insufficient or improper monitoring can result in poor surface quality and, in the worst case scenario, production downtimes. The formulation of the emulsion concentrates procured by AMAG must remain within specific tolerances. Consequently, extensive analysis of incoming concentrates is vital for monitoring the main components (base oils, lubricating additives, EP additives, emulsifiers) and formulation performance. Furthermore, the composition of a hot rolling emulsion during use is never comparable to that of a fresh emulsion. It undergoes constant changes for multiple reasons, namely:
- Use and wear
- Modification to and degradation of substances (due to high mechanical and thermal loads)
- Formation of metal soaps (for fatty acid-based formulations)
- Contamination with hydraulic oils, greases, abrasion debris
- Degradation due to (uncontrolled) microbial activity
Our varied portfolio (AMAG produces goods in alloy classes 1xxx to 8xxx) and formats require a certain degree of flexibility in adapting the composition and properties of the hot rolling emulsion. The only way to achieve optimal results is by precisely tailoring the hot rolling emulsion to the specific mill and rolled material. This requires comprehensive emulsion treatment on a regular base.It relies on close cooperation between the AMAG CMI and the Production department to ensure high quality. Emulsion treatment can include adjusting its composition (by adding fresh oil or additives) and cleaning measures (e.g. skimming and (partial) dumps). Regular and comprehensive laboratory analysis is a fundamental part of monitoring emulsion quality (Figures 3 & 5). During treatment, a number of key parameters are determined (using around 20 analysis methods) and monitored, both in the emulsion itself and in the oil phase:
- General parameters (e.g. pH value, conductivity)
- Lubrication parameters (e.g. oil content, viscosity, lubricating additives, emulsion stability)
- Stability parameters (e.g. droplet size distribution)
- Contamination parameters (e.g. filtration residue, tramp oils and greases, abrasion debris)
- Microbiology
AMAG employ an entire spectrum of analysis methods, including viscosimetry, titration, laser diffraction analysis, infrared spectroscopy, gas chromatography and ICP spectroscopy. The results are collected in an emulsion database and provide a base for assessing emulsion quality for subsequent emulsion treatment and other quality assurance measures. Composition and stability are decisive factors in determining how the hot rolling emulsion behaves in the roll gap, where the oil phase is separated from the water phase (“plate-out”). The oil phase is vital for lubrication in the rolling gap (Figure 4). At AMAG, we are continuously expanding the range of methods we use to determine the composition of emulsions and their properties. Recent introductions include nuclear magnetic resonance spectroscopy and the measurement of emulsion stability using Turbiscan technology.
Nuclear magnetic resonance (NMR) spectroscopy is a spectroscopic method for examining the electronic environment of individual atoms and their interactions with neighboring atoms. It facilitates to access molecular structures and thereby enables to draw more precise conclusions about the composition of an emulsion’s oil phase than was previously the case. The Turbiscan allows us to determineemulsion stability in qualitative and quantitative terms over a prolonged period.
Customer advantages:
Emulsions are an important factor in the hot rolling process. Their quality (i.e. composition and stability) directly influences the surface quality of rolledproducts. It is therefore essential to continuously analyze emulsion composition, constantly refine analysis methods and, based on these insights, improve our emulsion treatment. Ongoing research aims to enhance theprocess management to safeguard and further improve the outstanding surface quality of AMAG products for our customers.
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