Proceedings of SerbiaTrib '13

die, where there is a layer of the lubricant which isnot yet squeezed out, there is a formation of socalled"nipple slip". During this process quasi-weldzone is steadily increased and thus coefficient offriction is increased as well. When the surface ofquasi-weld zone become equal to the entire frictionsurface, friction coefficient reaches its maximumvalue. In addition, due to strong frictionconnections, micro cracks in the subsurface layerare formed. Due to the continuous material inflowin the zone of deformation noticeable nipple isformed, which at some point, because of crackscaused by the weakening of the frictionalconnection, detach itself from the base material. Inthis way the quasi-welded zone is reduced andlubricant starts to penetrate places of brokenconnections, which reduces the friction coefficient.Chipped of metal fragments are trapped betweenthe die and the surface of sheet metal and are beingcontinuously moved towards the exit part of the die.When they came out of the zone of deformationcoefficient of friction will have a minimum value.Then, the aforementioned process continuouslyrepeats itself.If the inadequate lubricant is used coupled withgreater gripping forces, stickers are formed andcontact conditions are greatly deteriorated whichlead to the significant increase of drawing force foreach subsequent passage (Fig. 6).Sila izvlačenja, kN2 µm14121086420.25 mmFigure 5. Grooved surfaceIVIIIIIIAlMg300 10 20 30 40 50Put klizanja, mmFigure 6. Change of drawing force4. CONCLUSIONFigure 4. Aluminum glued particles on die and punchersurfacesSome lubricants, no matter the fact that theyproduce satisfactory results in steel plates, in caseof plates made of AlMg3 have very poor results.Their usage leads to intense gluing of aluminumonto tool, which is shown in Figure 4. Gluedparticles that are formed on the die during theironing can cause severe damage to the sheet metalsurface (galling) (Figure 5).In the case of ironing one of the maincharacteristic of this processes is the fact thatmultiple repeated operations lead to a gradualincrease in thickness of glued layer, which meansthat the sum of the individual resulting layerscrosses over into the continuous layer.As a result of predominance of the adhesionforces over resistance to plastic flow within gluedlayer plastic deformation of glued particles occursfollowed by their tearing off and smearing.When a sufficiently large thickness of the gluedlayers is achieved, as a result of the multiplerepeated process of plastic deformation, process ofseparating glued particle from tool material begins.This separation is done by pealing (shear) or tearingoff, and creates significant damage to the surfacelayers of tool, and therefore increases the surfaceroughness of the work piece. Such processes arecharacteristic of ironing sheet metal made ofaluminum alloys, where no adequate lubricant isapplied.268 13 th International Conference on Tribology – Serbiatrib’13