CIRP Annals - Manufacturing Technology 58 (2009) 243–246

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CIRP Annals - Manufacturing Technology

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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, Denmarkb Department of Mechanical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

A R T I C L E I N F O

Keywords:

Friction

Lubrication

Cold forming

Alloy plating

A B S T R A C T

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.

� 2009 CIRP.

1. Introduction

The large surface expansion and high normal pressurecombined with elevated contact temperature between workpieceand tool, which prevail in cold forging of metals cause the necessityof a high performance lubrication system [1–7]. The objective oflubrication is to reduce friction and thereby lower the deformationforces and increase tool life and more importantly to avoid galling,i.e. breakdown of lubricant film, metal to metal contact betweentool and workpiece, pick-up of workpiece material on the toolsurface leading to scoring of subsequent workpiece surfaces. Anefficient lubrication system is therefore essential for ensuringproduction with satisfactory quality. Conventionally, a conversioncoating of zinc phosphate with dual function is applied. Due to itstopographic nature, a large surface area is created with pocketssuitable for entrapment of lubricant. Dipping the zinc phosphatedslugs in a liquid bath of sodium stearate, the conversion coatingforms zinc stearate which is chemically bonded to the workpiecesurface and covered with excessive sodium stearate as shown inFig. 1. The total thickness of the coating is 10–20 mm. The costs ofproducing such a lubricant film amount in average to 3% of the totalcosts of the formed component. The major part of the costs isrelated to disposal of the sludge, which in case of alloyed steel slugsbesides iron may contain chrome and molybdenum. Theseenvironmentally hazardous heavy metals, which appear in thesludge due to etching of the base metal during phosphating, haveto 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 alloyelectrochemically deposited on the workpiece surface [8,9]. Thealloying elements are specifically selected to ensure deposition of a

* Corresponding author.

0007-8506/$ – see front matter � 2009 CIRP.

doi:10.1016/j.cirp.2009.03.062

two phase layer, consisting of a mixture of fine grains of twometals. After deposition, one of the two metals is selectively etchedleaving a micro- or even nanoporous layer of the remaining metalon the workpiece surface. When a lubricant subsequently isapplied to the porous coating, it will be trapped in the pores actingas numerous small lubricant reservoirs. In Fig. 2, the steps increating the new tribological system are illustrated.

The deposited two phase alloy in the present work is SnZncreating a film of typically 5 mm thickness. The film is subse-quently etched with diluted hydrochloric acid, whereby the Zn isremoved, leaving a porous Sn layer. The diluted acid is consideredenvironmentally harmless. If requested, the Zn can, by a simpleprecipitation procedure, be recycled. Deposition rate is 0.5–1 mm/min. The topography of the etched surface depends on thechemical composition of the SnZn alloy and on the concentration,time and temperature used in the etching procedure. Fig. 3 showsthe 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 lubricantcarrier in conventional cold forging, but also in cold forging ofmicrocomponents. For such components, the conventional solidfilm lubrication is often inappropriate due to packing of dies withexcess lubricant and inability to obtain satisfactory closetolerances, since the total film thickness is of the same order ofmagnitude as the requested tolerances. Liquid lubricants arepreferred, but due to risk of galling, when using liquid lubricantswithout a conversion coating, the combination of an ultra-thin,porous, metallic film and a liquid lubricant is proposed toovercome these problems. Other possible two phase alloysinclude: AgCo, AgFe, BiSn, CoCu and AgCu. Ag is corrosion resistant,bactericidal and has a significantly higher melting point ascompared to Sn, properties which may be utilized in special cases.

3. Experimental work

The capability of the new porous layer is tested by conventionalfriction tests in cold forging; a ring compression test [10] and a

Fig. 1. Conventional lubricant film of phosphate coating reacted with sodium soap.

Fig. 2. (A) Deposited SnZn alloy. (B) Coating after selective etching of Zn. (C) Porous

coating lubricated with oil. (D) Porous coating lubricated with grease.

Fig. 4. Ring test of silver.

M. Arentoft et al. / CIRP Annals - Manufacturing Technology 58 (2009) 243–246244

double cup extrusion test [11]. Both tests are based on the dividedflow principle, where the material flow is friction sensitive. Asearlier stated, the main purpose of the tribological system is toavoid galling. A high reduction cup extrusion with large punchstroke is therefore carried out to test the lubrication system undersevere conditions.

3.1. Ring compression test

In the initial ring compression tests, pure silver is chosen asbase material, since deposition on silver is expected to be a non-complicated procedure. The stress–strain curve is determined byuni-axial compression test and found to follow the Hollomonexpression: s = 295e0.27 [MPa]. The initial ring has the prescribedratio 6:3:2 between outer diameter (OD), inner diameter andheight, with OD = 15 mm. The rings are compressed betweenoverhanging tool steel planes (AISI M3:2 PM, 62 HRC) polished to asurface roughness Ra = 0.03 mm. Height reductions are chosen as20, 40 and 60%. Four different lubrication systems are tested: thenew, porous coating lubricated with Molykote DX paste (in thefollowing abbreviated MoDX) or mineral oil and the samelubricants without any coating. The MoDX paste is a high viscositygrease containing distilled, heavy naphthenic petroleum, lithiumsoap, solid lubricants and additives in a paraffinic base oil. Themineral oil is made as a mixture of two oils, i.e. 50 wt.% Sunoco Sun60 (low viscosity naphthenic mineral oil) and 50 wt.% HoughtonPlunger CR5 (high viscosity mineral oil). A resulting viscosity of60 cSt at 40 8C is obtained. No boundary lubrication additives areadded to the oil.

Fig. 3. Cross-section and top view of porous coating.

A set of calibration curves corresponding to the workpiecematerial (strain hardening exponent n = 0.27) are determined by2D FE simulations using DEFORM1 and adopting the Coulombfriction model (t = mp). Fig. 4 shows the calibration curves togetherwith the experimental data.

The friction coefficient for the coated and lubricated surfaces isfound to m � 0.04, whereas the non-coated surfaces for highreductions gives friction coefficients in the range m = 0.05–0.07.Whether the coating is lubricated with MoDX or oil has nosignificant influence.

3.2. Double cup extrusion test

A double cup extrusion test is carried out with high reduction tostress the coating. The container and lower punch is keptstationary. In case of zero friction along the container wall theupper and lower cup will develop equally, i.e. hu/hl = 1, whereasincreased friction will enhance upwards flow implying larger cupheight ratio. The test principle is illustrated in Fig. 5. The diameterof the container and billet is D0 = 27 mm and the reduction r = (DP/D0)2 = 69%, where Dp is the punch diameter. The shape of the punchnose is chosen according to the recommendations of ICFG [12]. Theheight to diameter ratio of the billet is h0/D0 = 1. Punches as well ascontainer core are made in AISI M3:2 PM, hardened and temperedto 62 HRC. They are polished to a surface roughness Ra = 0.1 mm.

Experiments are performed in the same pure silver as used forthe ring test with three different lubrication systems: the newporous layer lubricated with (i) MoDX paste or (ii) oil or (iii) MoDXadded directly on a clean surface. The relative punch travel z/h0 isaimed at 20, 40 and 60%. In Fig. 6, the experimental data is plottedtogether with a set of calibration curves for the workpiece materialcalculated by Deform1 2D. Some scatter is observed in theexperimental results, but it is evident that the coated specimenslubricated with the mineral oil create low friction conditions(m � 0.05) whereas no effect of the coating can be identified whenlubricating with MoDX.

The promising results obtained in testing the new conversioncoating on silver specimens encouraged the authors to carry ontesting the coating on conventional cold forging steel comparingwith the conventional phosphate coating lubricated with soap asreference.

The steel is a common unalloyed C-steel, Ma8, with the stress–strain curve determined by uni-axial compression test to:

Fig. 5. Principle of double cup extrusion test.

Fig. 6. Double cup extrusion tests of silver.

Fig. 9. Cup height ratio vs. punch stroke for a double cup extrusion test for six

different lubrication systems.

M. Arentoft et al. / CIRP Annals - Manufacturing Technology 58 (2009) 243–246 245

s = 640e0.21 [MPa]. The test design is similar to the one for the silverspecimens, i.e. a reduction r = (DP/D0)2 = 69%, a height to diameterratio of the slug h0/D0 = 1 and a container diameter D0 = 27 mm.The relative punch stroke is aimed at z/h0 = 20, 40 and 60%.

Six different lubrication systems are tested: coated and non-coated with MoDX grease or mineral oil, conventional phosphatingor the new coating both lubricated with soap.

In Fig. 7, the inner wall of the upper cup is shown for themaximum stroke obtained with various lubrications. As seen,severe scoring due to galling appears for the non-coated parts,whereas no scoring appears when applying conventional phos-phate coating plus soap or the new coating plus any of the threelubricants. Roughness measurements are carried out in circumfer-ential direction on the inner cup wall with ls = 2.5 mm andlc = 0.8 mm to quantify the amount of scoring as shown in Fig. 8. Adistance of 4 mm along the periphery is measured at threedifferent positions, i.e. 19, 20 and 21 mm from the top of the uppercup. The Ra value is calculated as the average value of the threemeasurements and can be used as an indicator for the level ofgalling, but scoring is clearly visible from the roughness plot.Smooth surfaces without any sign of galling are obtained for thecoated surfaces.

Due to severe galling, the friction test is not carried out on thenon-coated parts at larger punch strokes. In Fig. 9, the cup heightratio hu/hl as a function of the relative punch stroke z/h0, is shownfor all six lubrication systems. Some scatter can be identified due to

Fig. 7. Double cup extrusion tests of unalloyed, low C-steel.

Fig. 8. Roughness measurements on double cup test specimens.

the limited number of available test samples. It is, however, quiteclear, that the Sn-coated and soap lubricated specimens are at thesame level as the conventional phosphated and soap lubricatedones. The Sn-coating lubricated with MoDX or oil show somewhathigher cup height ratio indicating larger friction even though theobtained surfaces are smoother than the soap lubricated ones asshown in Fig. 8.

3.3. Single cup extrusion, galling test

To stress the lubrication system, a single cup extrusion withhigh reduction r = (DP/D0)2 = 69% and large relative punch stroke z/h0 = 0.75 is carried out with the same steel Ma8 as used for thedouble cup extrusion tests. The height to diameter ratio of the slugis kept at h0/D0 = 1. To avoid any influence from previous testing,the punch is polished between each experiment.

Four different lubrication systems are tested: Sn-coatinglubricated with (i) MoDX grease, (ii) mineral oil or (iii) soap and(iv) conventional phosphating and soap lubrication.

The tests are carried out on a 2.5MN press with a speed of app.50 mm/s. Fig. 10 shows the formed parts. The specimen Sn-coatedand lubricated with MoDX grease show severe scoring marks allover the inner cup wall, whereas specimens with Sn-coatinglubricated with oil or with soap and phosphate coating plus soapall result in smooth surfaces. This is further clarified in Fig. 11,showing roughness profiles of the inner cup wall in circumferentialdirection. The roughness is measured over a distance of 4 mmalong the periphery at three different positions, i.e. 19, 20 and21 mm from the bottom of the cup. It is seen that the Sn-coated andMoDX lubricated specimen has significantly higher roughness thanthe other specimens, which all lie on a rather constant level ofRa � 1.03 mm.

4. Discussion

The ring compression test is due to low temperature, surfaceexpansion and sliding length considered a light deformation

Fig. 10. Single cup extrusion.

Fig. 11. Roughness measurements, single cup extrusion.

M. Arentoft et al. / CIRP Annals - Manufacturing Technology 58 (2009) 243–246246

process implying that the requirements to the lubrication system islimited. At higher reductions the experimental results show thatthe coated surfaces perform better than the non-coated. Whetherthe lubricant is MoDX or mineral oil seems to have no influence,indicating that the coating is able to trap both lubricants andestablish the crucial lubrication reservoirs.

As regards the double cup extrusion tests the basic processparameters: Normal pressure, sliding length and surface expansionare on a level, where a well functioning lubrication system is needed.Lubrication with MoDX, with or without Sn-coating is not able toestablish low friction conditions in forming of the silver and steelMa8 billets, whereas the Sn-coated parts lubricated with mineral oilperform satisfactory on both materials. This indicates, that the lowviscosity mineral oil penetrates more efficiently into the porousstructure of the Sn-coating and succeeds to provide the lubricantrequired for the extrusion process, Fig. 2C. The high viscosity of theMoDX grease does not allow for the same degree of absorption in theporosities of the coating implying lubricant film break down at anearly stage of the process, Fig. 2D. Lubrication of the phosphatecoating and the new Sn-coating with soap is applied by dipping in awarm, aqueous Na-stearate bath with very low viscosity. It istherefore expected, that the soap will be absorbed in a way similar tothe oil. At the highest punch stroke of z/h0 = 60%, the soap lubricatedspecimens give lower cup height ratio and thereby lower frictionthan the oil lubricated parts, Fig. 7. This may be explained by thelower viscosity of oil than soap at room temperature, whereexcessive oil will be squeezed out of the tool/workpiece interfaceduring the initial forming stage, whereas the soap stays as a solidlayer sticking to the billet ensuring good lubrication of the workpieceasperities and complete filling of the lubricant pockets.

As stated earlier, low friction is important in order to achievelow forming forces and thereby improve tool life, but the ability ofthe lubrication system to impede galling is more important sincegalling leads to costly interruptions of the production due torequired repolishing of the tools. The new Sn-coating lubricatedwith MoDX, which performed acceptably although with somewhatincreased friction in double cup extrusion, resulted in filmbreakdown and galling in single cup extrusion with 75% punchstroke and a reduction of 69%, whereas the Sn-coated billetslubricated with oil or soap and the conventional phosphate coatinglubricated with soap all showed excellent performance with nosign of galling. This is probably due to the larger viscosity asillustrated in previous paragraph and in Fig. 2C and D.

5. Conclusion

A new lubricant carrier has been developed for solvinglubrication problems in severe cold forging processes avoidinghazardous environmental impact. It is an electrochemicallydeposited coating of two insoluble metals, of which one issubsequently selectively etched in diluted acid to create a porouslayer acting as carrier of the lubricant. The system has been testedin conventional ring compression tests and double cup extrusiontest. Its performance to impede galling has furthermore beentested by severe stressing in single cup extrusion.

When lubricating with a low viscosity mineral oil, which canpenetrate into the porous coating, promising results are demon-strated with regards to significantly lower friction as well asincreased resistance towards galling compared to non-coatedsurfaces. When the coating is applied to a common cold forgingsteel and lubricated with soap, the friction and resistance togalling is at the same level or even better than the conventionalphosphate coating. An advantage of the new coating is theexpected improved environmental impact compared to thephosphating process.

Acknowledgments

The authors would like to thank the Danish Ministry of Science,Technology and Innovation for supporting the work by theInnovation Consortium ‘‘Mikrometal’’ no.: 66334. Furthermorethe assistance by M.Sc. N.A. Paldan, Dr. S. Lassen and Dr. I.Mizushima is appreciated.

References

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[9] Arentoft M, Bay N, Tang PT, Jensen JD, Paldan NA, Mizushima I, Eriksen RS(2008) Deposited Micro Porous Layer as Lubricant Carrier in metal Forming.ICTP 2008, Gyeongju, Korea, . pp. 211–216.

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[12] ICFG DS 5/71: Punches and Mandrels for Cold Extr. of Steel. The Int. ColdForging Group, 1971.