UTILISATION OF MICROPHOSPHATED SHEETS IN AUTOMOTIVE INDUSTRY. Pavel Solfronk a Jiří Sobotka a Pavel Doubek a Michaela Kolnerová a

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1 UTILISATION OF MICROPHOSPHATED SHEETS IN AUTOMOTIVE INDUSTRY Pavel Solfronk a Jiří Sobotka a Pavel Doubek a Michaela Kolnerová a a) Technical University of Liberec, Faculty of Mechanical Engineering, Department of Engineering Technology Section of Metal Forming and plastics, Hálkova 6, Liberec 1, CR, pavel.solfronk@tul.cz Abstract Paper deals with problems about processing of steel sheets with protective layers based on Zinc. There are compared tribological properties of sheets with Zinc protective layer, combination of zinc + phosphate layer and combination of zinc + microphosphate layer. Tribological properties for individual tested materials are evaluated on account of friction coefficient progression measured under continuously increasing pressure during testing. 1. INTRODUCTION One of the most important criteria for using of product in the market-place is its successful design. Is already evidence that in keen competition has a shape of car body an outstanding influence on car s sales ability. On this account there is still increase in costingness about size and shape complexity for sheets mouldings. An effort to increase car s body corrosion resistance which is first of all crucial about car s service life leads sheets processors to use sheets with protective layers. Practically most widespread types are protective layers on zinc base. Formative heftiness in combination with used material with protective zinc coating however bears definite complication at processing of these types of sheets. One of the biggest problem of zinc-coated sheets is their galling (so-called stick-slip" effect). The most widespread surface treatment which inhibits this unpleasant effect is subsequent phosphate coating of zinc layer on sheet. Materials with phosphated surface layer then embody excellent tribological properties and in light of forming they are identified for formative best-more exacting mouldings. For resulting used manufacturing operation as is welding and especially painting however they represent a considerable problem. Endeavour of sheets producers is consequently to find balancing properties of steel sheets in light of all technological operation at production car. Of late years get to the foreground of interest materials whose utility characteristics conform with to all requirements of sheets processors. These materials are marked like materials with microphospated surface layer. 2. PRINCIPLE OF EXPERIMEN AND USED MATERIALS 2.1 Principle of test For tribological testing is at department of engineering technology TU in Liberec used tribological machinery SOKOL EVO II. Experiments are carried out with the help of strip drawing test between jaws made from testing material. One jaw is fixed and the second is possible to control by means of hydraulic system which enables to make out requisite contact pressure during actual tribological test. Regulation of hydraulic unit allows pursue tests at constant pressure or at continuously increasing pressure. Sheet sample is drawing under 1

2 constant speed among jaws of testing preparation. Sliding is speed is possible to change to the extent of v = 1 till 400 mm s -1. Measured length after which are investigated tribological conditions is selecting always with reference to used sliding speed. Construction of preparation allows simulating conditions in the area of drawing edge (variant 1A) and in the holder area (variant 1B). Equipment for tribological testing and principle of test is perceptible from fig.1. p Var.1B F t Var. 1A p Fig. 1. Equipment for tribological testing and principle of test The result of measuring is graph of force feed dependence which serves as groundwork for evaluation of tribological properties of system tested lubricants tested sheet tool. With regard to that materials with protective zinc layer are sensitive to occurrence of so-called galling, was at calculation of friction coefficient also took into account also double amplitude of measured drawing force F t. In relation to calculation of friction coefficient was for measuring chosen only variant 1B. Instantaneous friction coefficient is then calculated from simple equation (1). µ Ft = 2 p S, (1) where p is instantaneous applied contact pressure and S is contact area. For experiment was chosen the feed speed of sheet sample 1 mm s -1. Length of measured distance was 500 mm. Conditions of experiment were chosen so that during testing was continuously increased pressure from initial value 10 MPa until 75 MPa. 2.2 Used materials For tribological experiments were used 3 sheets variants with different protective surface treatment. Producer of these materials is Voest Alpine Eurostahl GmbH. Based material subsequently used for different surface treatment is deep-drawing steel of grade DX 54 (marked according EN ). Were used these types of surface treatments: Based material DX 54 + electroplated zinc layer (zinc quantity 75 g m -2 ) material further marked like EG Based material DX 54 + electroplated zinc layer (zinc quantity 75 g m -2 ) + phosphate layer - material further marked like EG + PH Based material DX 54 + electroplated zinc layer (zinc quantity 75 g m -2 ) + microphosphate layer - material further marked like EG + µph 2

3 Individual tested materials were characterized with the help of recording sheet s surface from scanning electron microscope and by means of roughness measuring. Level of surface layer for single materials is shown in fig. 2. Values of roughness for tested materials are written in table 1 where is arithmetical mean from 5 metering noted. a) b) c) Fig. 2. Photographs of tested materials (a-eg, b-eg + µph, c-eg + PH) Table 1. Measured values of roughness for individual tested materials Measured value EG EG + µph EG + PH Maximal high of profile Rz [µm] 6,74 7,85 6,51 Mean arithmetical value of roughness Ra [µm] 1,36 1,25 1,14 Number of juts RPc [cm -1 ]

4 3. MEASURED RESULTS In fig. 3 till fig. 5 are shown recorded progressions values of drawing force F t. During entry increased magnitude of contact pressure from10 MPa until 75 MPa. Fig.3. Course entry of drawing force for material EG + µph Fig.4. Course entry of drawing force for material EG 4

5 Fig.5. Course entry of drawing force for material EG + PH In fig. 6 till fig. 8 are shown calculated courses of friction coefficient depending upon magnitude of contact pressure. Fig.6. Course of fiction coefficient for material EG + µph 5

6 Fig.7. Course of fiction coefficient for material EG Fig.8. Course of fiction coefficient for material EG + PH 6

7 4. CONCLUSION From tribological test is perceptible that at using sheets with zinc protective layer leads to considerable galling of sheet s surface thereby also to its damage. Friction coefficient is oscillating in relatively wide zone of values and is proportional to size of double amplitude drawing force (galling size). At used material with protective zinc layer with subsequent phosphate coating or more precisely microphospated is size of double amplitude drawing force minimum and there virtually isn t galling of surface. The best results from tribological view were achievement near sheet with phosphated surface layer. Microphosphated protective coating is however with its tribological properties practically comparable with this layer. With respect to that for following operations used in automotive industry as are welding, bonding and painting is microphosphated layer preferable, I advise use of these microphospated coatings. These protective layers are virtually full-value compensation for till now used sheets with phosphated layers. Acknowledgement This contribution was supported by MSM LITERATURA [1] Suchánek, J.: Tribologické charakteristiky povrchových vrstev. Sborník konference 16. dny tepelného zpracování s mezinárodní účastí, Brno, , str [2] Technické listy materiálů fy Voest Alpine Eurostahl GmbH 7