A new lubricant carrier for metal forming M. Arentoft (2) a, *, N. Bay (1) b , P.T. Tang a , J.D. Jensen a a IPU, Produktionstorvet, 2800 Kgs. Lyngby, Denmark b Department of Mechanical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark 1. Introduction The large surface expansion and high normal pressure combined with elevated contact temperature between workpiece and tool, which prevail in cold forging of metals cause the necessity of a high performance lubrication system [1–7]. The objective of lubrication is to reduce friction and thereby lower the deformation forces and increase tool life and more importantly to avoid galling, i.e. breakdown of lubricant film, metal to metal contact between tool and workpiece, pick-up of workpiece material on the tool surface leading to scoring of subsequent workpiece surfaces. An efficient lubrication system is therefore essential for ensuring production with satisfactory quality. Conventionally, a conversion coating of zinc phosphate with dual function is applied. Due to its topographic nature, a large surface area is created with pockets suitable for entrapment of lubricant. Dipping the zinc phosphated slugs in a liquid bath of sodium stearate, the conversion coating forms zinc stearate which is chemically bonded to the workpiece surface and covered with excessive sodium stearate as shown in Fig. 1. The total thickness of the coating is 10–20 mm. The costs of producing such a lubricant film amount in average to 3% of the total costs of the formed component. The major part of the costs is related to disposal of the sludge, which in case of alloyed steel slugs besides iron may contain chrome and molybdenum. These environmentally hazardous heavy metals, which appear in the sludge due to etching of the base metal during phosphating, have to be disposed by burying. 2. Novel method for creating porous lubricant carrier The authors have invented a new, patented lubrication system, based on a novel type of lubricant carrier, created by an alloy electrochemically deposited on the workpiece surface [8,9]. The alloying elements are specifically selected to ensure deposition of a two phase layer, consisting of a mixture of fine grains of two metals. After deposition, one of the two metals is selectively etched leaving a micro- or even nanoporous layer of the remaining metal on the workpiece surface. When a lubricant subsequently is applied to the porous coating, it will be trapped in the pores acting as numerous small lubricant reservoirs. In Fig. 2, the steps in creating the new tribological system are illustrated. The deposited two phase alloy in the present work is SnZn creating a film of typically 5 mm thickness. The film is subse- quently etched with diluted hydrochloric acid, whereby the Zn is removed, leaving a porous Sn layer. The diluted acid is considered environmentally harmless. If requested, the Zn can, by a simple precipitation procedure, be recycled. Deposition rate is 0.5–1 mm/ min. The topography of the etched surface depends on the chemical composition of the SnZn alloy and on the concentration, time and temperature used in the etching procedure. Fig. 3 shows the porous sponge-like coating. The cross-section in Fig. 3 left, clearly shows the pores for entrapment of lubricant. The new, porous coating may be used not only as lubricant carrier in conventional cold forging, but also in cold forging of microcomponents. For such components, the conventional solid film lubrication is often inappropriate due to packing of dies with excess lubricant and inability to obtain satisfactory close tolerances, since the total film thickness is of the same order of magnitude as the requested tolerances. Liquid lubricants are preferred, but due to risk of galling, when using liquid lubricants without a conversion coating, the combination of an ultra-thin, porous, metallic film and a liquid lubricant is proposed to overcome these problems. Other possible two phase alloys include: AgCo, AgFe, BiSn, CoCu and AgCu. Ag is corrosion resistant, bactericidal and has a significantly higher melting point as compared to Sn, properties which may be utilized in special cases. 3. Experimental work The capability of the new porous layer is tested by conventional friction tests in cold forging; a ring compression test [10] and a CIRP Annals - Manufacturing Technology 58 (2009) 243–246 ARTICLE INFO Keywords: Friction Lubrication Cold forming Alloy plating ABSTRACT A lubricant carrier for metal forming processes is developed. Surfaces with pores of micrometer size for entrapping lubricant are generated by electrochemical deposition of an alloy, consisting of two immiscible metals, of which one metal subsequently is etched away leaving 5 mm layers with a sponge-like structure. The pores will act as lubricant reservoirs during severe forming processes. The deposited microporous layer is evaluated by friction tests in the form of ring compression tests and double cup extrusion tests. Furthermore the anti-seizure properties are investigated by single cup extrusion at high reduction and excessive stroke comparing with conventionally lubrication using phosphate coating and soap. ß 2009 CIRP. * Corresponding author. Contents lists available at ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: http://ees.elsevier.com/cirp/default.asp 0007-8506/$ – see front matter ß 2009 CIRP. doi:10.1016/j.cirp.2009.03.062