LIGHT METAL AGE, DECEMBER 2012 18 Introduction Lubrication is an essential part of all aluminum fabri- cating operations, either directly through metal working processes, or indirectly to lubricate machinery. One of the most important ways of converting aluminum from cast slab/ingot into a usable industrial form is by rolling. The rolling process makes it possible to reduce an ingot of metal, weighing up to 20 tonnes and measuring 2 m x 8 m x 600 mm thick, to plate gauges (typically 250 mm to 6 mm), sheet gauges (typically 6 mm to 250 µm) and, ultimately, foil gauges (typically 250 µm to 6 µm). 1 In many parts of the industrialized world, approximate- ly 50% of all aluminum alloys used are in the form of fat rolled product and over the last 35 years major changes have taken place in rolling lubricant development and understanding. It is now accepted that the rolling lubri- cant not only infuences mill productivity, but also signif- cantly affects the quality of metal produced. The range of processes involved in the aluminum rolling and the types of lubricant used are presented in Table I. For all types of aluminum rolling the role of the lubri- cant is threefold: to prevent direct contact between the roll and aluminum surfaces, to extract the heat gener- ated by friction and deformation, and to take away fnes and debris from the roll bite area to the flter. The load bearing capacity, cooling effciency, and abil- ity to provide a clean annealed product must be consid- ered when formulating a rolling lubricant and, in addi- tion, it is essential to ensure chemical stability to mini- mize changes during use. Lubricant performance depends on a balance of prop- erties between the base stock and additive components. Correct selection of lubricant components, in terms of both their rheological properties and compositional fea- tures, has a signifcant impact on per formance. Rolling additives are critical to the provision of optimum fric- tional characteristics, preventing problems of skidding or roll bite refusals that can occur with excessively low friction or poor surface quality that may arise with exces- sively high friction. 2 Hot Rolling The hot rolling of aluminum occurs over a wide tem- perature range (Table I) and lubrication and thermal control of the work rolls is achieved by the use of oil-in- water emulsions (suspensions of oil droplets in a continu- ous water phase), which are sprayed onto the rolls in con- trolled patterns (Figure 1). Spraying the emulsion onto the rolls also removes any loose debris from the roll bite area, enabling it to be transported to the flter where it is removed. Emulsions are complex mixtures of components and most commercial blends remain proprietary. In general, however, hot rolling lubricant formulations contain a base oil component (paraffnic, naphthenic, mineral, or synthetic) into which are blended a range of additives including lubricity improvers, antioxidants, emulsifers and wetting agents. Additionally, the formulation may also contain corrosion inhibitors, biocides, and coupling agents that help provide stability during storage and as- sist the emulsifcation process. Emulsifers stabilize the sur face of the oil droplets to- wards the water phase. The two most common types of emulsifer used in hot rolling are anionic and non-ionic in nature. Anionic emulsifers are principally metal or al- kanolamine soaps (the reaction product of an organic acid and a metal or alkanolamine). Non-ionic emulsif- ers are ethylene oxide condensation reaction products where the length of the polymerized ethylene oxide chain determines the degree of water solubility (increas- ing chain length increases water solubility) and, ultimate- ly, the amount of oil separated at the roll bite. The additives are polar in nature, a feature that facili- tates physical/chemical bonding onto the roll or roll- coated surface, providing load bearing and protecting the freshly generated aluminum surface. Commonly used additives are organic acids and esters. Even modest amounts of highly reactive organic acids in formulations can have a dramatic effect on the surface quality of the rolled sheet; however they generate metal soaps during use, which can infuence emulsion stability. 3 Esters, being less reactive, are relatively stable in the emulsion and are now extensively used in commercial “acid-free” formulations. 4 In general, the greater the polarity of the additive, the stronger the bonding force, providing more effective lubrication. Some formulations may contain extreme pressure additives, which react di- rectly with the surface of the roll and help minimize sur- face defects caused by localized welding of the aluminum to the steel roll. Aluminum Rolling Lubrication By Mark Foster and Chris Pargeter, Innoval Technology Ltd. Process Lubricant Temperature (°C) Gauge Range (mm) Hot Rolling Oil-in-water Emulsion 270–560 2–600 Cold Rolling Oil-based/ Water-based Ambient–170 0.15–6 Foil Rolling Oil-based/ Water-based Ambient–140 0.005–0.6 Table I. Aluminum rolling process lubricants. Figure 1. Hot rolling mill coolant sprays (courtesy of Lechler Ltd.).