An alternative to zinc stearate could make aluminum extrusion safer.
Nature endowed elemental aluminum with many desirable characteristics: It’s a metal with a favorable ratio of strength to density, good thermal and electrical conductivity, corrosion resistance and suitability for alloying with other elements. It is also present in ample quantities in the earth’s crust.
Since the early 19th century, humans have invented technologies for extracting this metal from bauxite ore and preparing dozens of alloys for aviation, aerospace, automotive, marine, construction, packaging and consumer applications. Many vital technologies rely on advances in metallurgy, metalworking and lubricants for aluminum extrusion processes.
Extrusion refers to mechanical processes used to push material through a die and produce objects with a cross-sectional shape or profile defined by the die. Squeezing a tube of toothpaste, for example, extrudes a tidy cylinder of paste, and coincidentally, the tube itself probably was manufactured by extruding aluminum.
Advantages of extruding aluminum components include more precise tolerances than many competing processes, relatively low tooling costs for dies, and sustainability, according to the Aluminum Extruders Council’s Aluminum Extrusion Manual. Sustainability benefits include infinite aluminum recyclability, low environmental impact of extrusion processes and eco-friendly applications of the finished products, such as weight reduction of conventional vehicles, new battery casings, and housings for engines and electronics in electric vehicles.
Aluminum extrusion is one of the fastest growing segments of the metalworking industry. One particular process, cold impact extrusion, is popular for high-speed mass production of aluminum parts with high dimensional accuracy, good surface quality and the favorable material properties and microstructure required for automotive applications. However, the success of the extrusion process depends on metalworking fluid performance.
Hot and Cold
Aluminum extrusion typically requires feedstock in the form of long billets. For hot extrusion, each billet is preheated before it is introduced into a hydraulic press. A dummy block, which is the part of the press that comes in contact with the billet, forces the aluminum through a steel die and then rapidly separates from the extruded workpiece. The extrusion may be stretched, cut to a specified length, heat-treated to harden, and finished with surface treatments. Products include standard T-, C-, H- and I-shaped cross-sections as well as a limitless variety of custom shapes.
Metalworking fluids serve as release agents to prevent adhesion of the face of the billet to the dummy block. Lubricants are also used on billets and dies to extend tool life and ensure good surface finish and quality of workpieces. On account of the high temperatures (the melting point of aluminum is 655 degrees Celsius or 1,215 degrees Fahrenheit), many operators rely on solid lubricants such as graphite, carbon black and boron nitride, either in powder form or formulated in organic solvents or greases.
In cold extrusion processes, billets are at room temperature when they enter the die, and the metal temperature rises as it deforms and passes through the die. Cold processes require less energy than hot processes and may produce workpieces with less surface oxidation and better mechanical properties.
Cold impact extrusion is a specialized technique that processes short metal slugs instead of long billets. A hydraulic or mechanical press drives a high-speed punch that applies high pressure to push a slug into a die or mold. The hot metal softens and fills the gap between the punch and the die or mold to form a workpiece that is ejected from the extruder. Punches, dies and molds are custom-designed for production of aluminum bottles, gas cartridges, battery shells, nozzles and so forth. Zinc stearate powder, with a melting point of 120 C, is commonly used to lubricate cold impact extrusion processes.
A Dust-free Approach
Wilhelm Rehbein, senior manager of applications technology of the lubricants additives business for Lanxess Deutschland GmbH, explained at the 2019 annual meeting of the Society of Tribologists and Lubrication Engineers why proper lubrication is vital for the extrusion process.
“An insufficient amount of lubricant causes an inadequate flow of the aluminum as it is extruded. Imperfect separation properties of the lubricant film can cause adhesion of workpiece material on the punch or die. Too much lubrication will cause too fast a flow of material. This often results in an orange peel-like, rough surface on extrusions. An unevenly applied lubricant film will also lead to an unevenly shaped workpiece.”
Zinc stearate, a zinc soap or salt of stearic acid, is used as a solid lubricant and release agent in cold extrusion processes. This powder is typically mixed with slugs in an industrial-scale tumbler. Because zinc stearate has a very low density—approximately 1.1 gram per cubic centimeter—it readily forms dust, forcing operators to use exhaust systems to attempt to control the dust.
Rehbein presented a dustless alternative to zinc stearate powder for lubricating cold aluminum extrusion processes, explaining that the initial motivation to research substitutes for zinc stearate came from a working group consisting of aluminum manufacturers and universities. During the group’s discussions, aluminum manufacturing companies complained about the strong dust formation that resulted from using zinc stearate. The powder “can cause irritation of the skin, eyes and respiratory tract. There is also risk of a dust explosion caused by electrostatic ignition. Toxic and irritating vapors are formed by thermal decomposition. Therefore, our idea was to develop a substitute to zinc stearate which does not form dust,” he said.
Rehbein’s team first evaluated model formulations of extreme pressure and antiwear additives in an ester base oil. They used a Muller-Weingarten hydraulic press with maximum pressing force of 2,000 kilonewtons or 450,000 pounds, to form cup-shaped work pieces. When compared to a zinc stearate control formulation, all five model formulations required greater pressing forces to form shorter cups.
Rehbein’s team then tried a different approach and developed aqueous dispersions of solid lubricant particles. They applied each dispersion to slugs by tumbling for fifteen minutes until the water evaporated and left an extremely thin lubricant film on the slugs.
The next step was to compare model dispersions formulated with different solid lubricant particles in laboratory tests. A “spike test” was carried out in which a slug was pressed into a die to form a long, narrow prong. This test simulates the tribological conditions of cold extrusion processes. (See graph on Page 42.)
Rehbein’s team compared the effects of the dispersions on spike height to zinc stearate, which gave an indication of material flow during extrusion.
“In particular, the material flow properties of the new lubricant have to be similar to those of the zinc stearate to avoid the expensive reconstruction of the cold extrusion tools,” said Rehbein. “Compared with zinc stearate, the spikes that were produced with the new lubricants showed nearly the same height. The temperature of the spikes after the cold extrusion process was slightly reduced; however, the pressing force was slightly increased. Pressing forces were nominally identical [between the new lubricants]. Surface finish with Lubricant 1 was superior to Lubricant 2, which formed residues in the die. We selected Lubricant 1 for further testing.”
Tested and True
Field tests were performed at an aluminum processing company that specializes in cold extrusion processes. Lubricant 1 was used to manufacture 350 DK-series pistons from slugs of EN AW-6082 aluminum alloy. A complex sequence of extrusion processes with variations of pressing speed and force was used to manufacture each piston, which resembled a cylindrical cup with a spike inside. (See graph on Page 43.)
The maximum pressing force was 220.0 kN with zinc stearate and 223.6 kN with Lubricant 1. The shell (die) temperature was 4.2 percent lower with Lubricant 1—43.1 C versus 45.0 C with zinc stearate. “Material flow and surface quality were excellent with Lubricant 1,” Rehbein noted. “We observed no formation of dust during the tumbling and pressing processes. And there was no adhesion of aluminum on the tool. Compared to zinc stearate, there was less lubricant residue on the punch and in the die.”
The best results in terms of material flow, surface quality and tool cleanliness, said Rehbein, were obtained with a dry lubricant film of approximately 0.2 mg of Lubricant 1 per square centimeter of aluminum, while approximately 0.3 mg of zinc stearate were needed per square centimeter for an optimal material flow.
“During our development work, we were surprised that the new lubricant also offers further advantages, like … a better compatibility with aqueous and solvent based cleaners,” Rehbein noted.
Lubricant 1 was easily removed by washing or heat treatment and was highly compatible with acidic, neutral and slightly alkaline water based cleaners as well as solvent based cleaners such as alcohols and tetrachloroethylene.
Rehbein stated that the most important takeaway from the project is that there are viable lubrication alternatives to zinc stearate in aluminum cold impact extrusion processes that do not require a change in the manufacturing process or the tool design.
Furthermore, “The new dust-free lubricant is a suspension of solid lubricant particles in water,” said Rehbein. “The solid lubricant particles are derived from renewable organic raw materials and do not contain any metals or petroleum products.”
The success of the new lubricant “shows potential for the cold impact extrusion of aluminum alloys, particularly the manufacturing of aerosol spray cans or squeeze tubes, but also technical parts like fuel filter housings or pin bloc coolers,” Rehbein concluded.z
Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at email@example.com or 267-567-7234.