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TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet
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TALAT Lecture 3702: Tribology in Cold Forming of Aluminium Sheet

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This lecture aims at gaining an appreciation the importance of friction in sheet metal drawing. It describes the mechanism of friction and lubrication; it shows the importance of surface topography; …

This lecture aims at gaining an appreciation the importance of friction in sheet metal drawing. It describes the mechanism of friction and lubrication; it shows the importance of surface topography; it aims at learning about factors improving the problem of adhesion and about methods of determining the coefficients of friction in different tribological systems.
Background in production engineering and sheet metal forming and familiarity with the subject matter covered in TALAT This lecture 3701 is assumed.

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  • 1. TALAT Lecture 3702 Tribology in Cold Forming of Aluminium Sheet 13 pages, 12 figures Basic Level prepared by K. Siegert and S. Wagner, Institut für Umformtechnik, Universität Stuttgart, and F. Ostermann, Aluminium Technologie-Service, Meckenheim Objectives: − to appreciate the importance of friction in sheet metal drawing − to describe the mechanism of friction and lubrication − to show the importance of surface topography − to learn about factors improving the problem of adhesion and about − methods of determining the coefficients of friction in different tribological systems Prerequisites: − background in production engineering and sheet metal forming − TALAT Lecture 3701 Date of Issue: 1996 © EAA – European Aluminium Association
  • 2. 3702 Tribology in Cold Forming of Aluminium Sheet Table of Contents: 3702 Tribology in Cold Forming of Aluminium Sheet .................................2 3702.01 Friction in Deep Drawing and Drawing of Car Body Parts ................... 3 3702.02 The Effects of Microtopography of the Sheet Metal Surface ................. 4 3702.03 Mechanism of Friction................................................................................ 7 3702.04 Effect of Surface Properties on Friction Behaviour ................................ 8 3702.05 The Tool Surface....................................................................................... 10 3702.06 Lubrication ................................................................................................ 11 3702.07 Literature/References ............................................................................... 12 3702.08 List of Figures............................................................................................ 13 TALAT 3702 2
  • 3. 3702.01 Friction in Deep Drawing and Drawing of Car Body Parts Apart from the formability of sheet materials the tribological conditions in the contact zones between the sheet surface and the tool surface play an important part in determining the procedural limits of the forming process. Friction in the various contact zones affects the flow of the material in the tool and is used deliberately to control the forming process. The friction zones in deep drawing and in drawing of car body parts are illustrated schematically in Figure 3702.01.01 and Figure 3702.01.02, respectively. The demands made on the friction situation in these friction zones can vary greatly depending on the type of part being drawn and on the forming procedure. In deep drawing, low friction is required under the blankholder (zone 1) and at the drawing die curvature (zone 2), in order to reduce drawing forces. At the punch edge (zone 3), friction needs to be as high as possible, so that high forces are introduced into the cup wall at the transition zone from punch to cup wall. If special areas have to be drawn out by stretch forming in the bottom of the drawn part, low friction values are desirable at the punch face (zone 4). To control material flow in the case of irregular drawn parts, such as, e.g. car body parts, higher friction may be necessary in certain parts of the blankholder, which can be achieved with locally higher surface pressure or with braking bulges (draw beads). Friction Zones in Deep Drawing FSt FN FN Blankholder 1 Punch 1 2 4 Drawing Die 3 Friction zones: 1 Drawing die and blankholder 3 Punch edges 2 Drawing die curvature 4 Punch-shank surface alu Training in Aluminium Application Technologies Friction Zones in Deep Drawing 3702.01.01 TALAT 3702 3
  • 4. Friction Zones in Drawing of Car Body Parts FSt FN FN Blankholder 1 Punch 4 1 5 3 Drawing Die 2 Friction zones: 3 Punch edges 1 Drawing die and blankholder 4 Punch-shank surface 2 Drawing die curvature 5 Punch face alu Training in Aluminium Application Technologies Friction Zones in Drawing of Car Body Parts 3702.01.02 3702.02 The Effects of Microtopography of the Sheet Metal Surface The tribological system as a whole consists of the sheet surface, the tool surface and the lubricant. The lubricant prevents abrasion and wear of the tool and workpiece surfaces, and, particularly in the case of drawn aluminium parts, prevents adhesion at the tool surface. Lubrication is, therefore, vital to the drawing of parts from bare aluminium. At the same time, it is necessary to keep the use of lubricants as low as possible, since they have to be removed after forming prior to any further operations such as joining or surface treatment. The capacity of the sheet surface to absorb lubricant and thus the precise surface microtopography of the sheet are correspondingly important. The standard rolled surface of aluminium sheet is the so-called “mill-finish“ surface with relatively low roughness coefficients. It is produced with tangentially ground rolls and thus exhibits a directional roughness, which produces different tribological behaviour parallel and transverse to the rolling direction. The topographic image of the mill-finish surface is depicted in Figure 3702.02.01. Lubricant contained in the long stretched roughness valleys will be squeezed or drained out under the force of the die pressure before a significant hydrostatic pressure can build up. Such surfaces are, therefore, rather prone to adhesion and abrasion during forming operations and exhibit directionality in the coefficient of friction and with regard to the tendency of adhesion, as will be seen later. TALAT 3702 4
  • 5. Surface Texture anisotropic isotropic stochastic isotropic deterministic Conventional Mill-Finish Spark Erosion, Laser Texturing, Blasting with Hard Particles Electron Beam Texturing Surface Preparation Technique Source: P. Fuller, Alusuisse-Lonza alu Microtopographic Structures of Al Carbody Sheet 3702.02.01 Training in Aluminium Application Technologies The anisotropy of the friction performance of the mill-finish surface can be overcome by the use of roll surfaces prepared by blasting, electro-discharge or laser texturing. The resultant isotropic structures of the surface microtopography is also shown in Figure 3702.02.01. These surface structures are characterised by closed pits which entrap the lubricant. Flattening of the rims due to contact with the tool surface builds up hydrostatic pressure in the entrapped lubricant, which helps to reduce the danger of adhesion. To model the friction behaviour and to study the various tribological effects and parameters a number of special tests have been developed, see Figure 3702.02.02. Most of these tests methods incorporate friction tests on sheet specimen strips, which are drawn between a mock die and blankholder. Blankholder pressure and drawing force can be monitored individually and the resulting friction coefficient can be measured. Friction strip tests can be performed with and without simulating the drawing over a die curvature. TALAT 3702 5
  • 6. Measuring Friction with Strip Drawing Tests FN FN FG FZ FZ FN FZ Strip drawing without bending Strip drawing with bending FZ Drawing force FG Reaction force FN Blankholder force Source: Wojtowicz / Littlewood and Wallace / Woska alu Training in Aluminium Application Technologies Strip Drawing Tests 3702.02.02 Figure 3702.02.03 shows two friction strip testing methods without bending actions. In the test set-up (a) the strip is drawn between two stationary dies, measuring an average of the friction coefficients on both sides of the strip. As sheet qualities become available, which have different surface treatments on either side of the sheet, it may be necessary to determine the friction behaviour separately for both sides of the sheet. The test set-up (b) employs a tool sled, which is moved on frictionless air bearings (cf. Institut für Umformtechnik, Universität Stuttgart). Forces and Friction Coefficients in Strip Drawing Friction Tests a) double-sided friction b) single-sided friction 2 FN 2 FN 1 1 FR1=FN µ1 FR=FN µ1 µ1 FZ=FR1+FR2 µ1 vZ vZ µ2 FR2=FN µ2 3 2 FN 2 FR=FZ=FN (µ1+µ2) FR=FN µ1 1 Sheet strips 2 Gripping (drawing) jaws 3 Drawing sledge (air bearings) Source: IfU - Stuttgart alu Forces and Friction Coefficients in Training in Aluminium Application Technologies Strip Drawing Tests 3702.02.03 TALAT 3702 6
  • 7. 3702.03 Mechanism of Friction The tribological conditions in sheet metal forming operations are characterised by rather low relative velocities between work piece and die surfaces, by generally low pressures in the macroscopic contact area between die and sheet metal and by relatively large areas of macroscopic contact between die and work piece surfaces. Under these conditions liquid or pasty lubricants can be employed to reduce the frictional forces between dry metal surfaces. The low relative velocities do not provide conditions for general hydrodynamic lubrication. On a microscopic scale, however, there are zones between the die and work piece surfaces separated by a thin layer of lubricant and zones of direct metallic contact. The magnitude of metallic contact depends on a number of factors, among which the surface roughness and its microtopological structure as well as the amount of lubricant are the most important ones. When the die surface meets the generally rougher and softer surface of the sheet metal the area of direct metallic contact is at the first instant relatively small and restricted to a few roughness peaks. Due to the high specific local pressure the peaks are flattened and the die surface sinks deeper into the sheet surface. The lubricant enclosed in the roughness valleys builds up a hydrostatic pressure and transmits the die pressure onto the sheet metal surface. At the same time the excess lubricant is driven out of the valleys, forced between the flattened roughness peaks and forms a thin boundary film made up of tribo-chemical reaction products and reacting substances (e.g., metallic soaps, E., P. additives among others). This situation is depicted in Figure 3702.03.01. Mechanism of Friction FN tool τ V PHS kƒ τ Vrel Adhesion PHD (bridging due to cold welding) Hydrodynamic Hydrostatic Abrasion particle pressure build-up pressure Levelling processes (elastic, plastic deformations) (fluid friction) build-up Lubricant film made up of tribo-chemical reaction products and reacting substances (e.g. metallic soaps, E. P. additives among others) (µ liquid + µ interface + µ solid) = µ total = ƒ (t,s,v,....) Source: IfU - Stuttgart alu Mechanism of Friction 3702.03.01 Training in Aluminium Application Technologies TALAT 3702 7
  • 8. The relative motion during the drawing operation builds up shear stresses in the flattened peak zones. If the boundary layer ruptures due to motion or high die pressure the metallic surfaces get into direct metallic contact. The result will be adhesion due to local pressure welding. These metallic bridges rupture during further motion, partly sticking to the die surface and partly breaking loose as abraded particles. At this stage lubrication has broken down. 3702.04 Effect of Surface Properties on Friction Behaviour The friction and adhesion behaviour of aluminium car body sheet materials have been determined using the strip friction test method without bending. For standard mill-finish surfaces the results are shown in Figure 3702.04.01. (The tests were carried out using a drawing speed of 2 mm/sec, a drawing distance of 100 mm and lubricant Oest Platinol V711/80). The effect of the directionality of the surface roughness becomes apparent with respect to the point of seizure and to the friction coefficient. 2000 Drawing Force, FZ [N] 1500 AlMg0,4Si1,2 Strip Drawing Test AlMg2,5 without Bending AlMg5 v z = 2mm/s 1000 Lubricant: Oest Platinol V711/80 500 Adhesion 0 0 2 4 6 8 Blankholder Pressure P [N/mm²] N Source: E. Mössle, 1983 Friction Behaviour of Al Carbody Sheet alu in Strip Drawing Tests 3702.04.01 Training in Aluminium Application Technologies From Figure 3702.04.02 it is evident that the isotropic surfaces, such as „Lasertex“ or „EDT“, perform much better with regard to seizing and galling. Good results are also achieved with roll surfaces prepared by blasting. (The tool material was cast iron GG25CrMo, drawing speed 100 mm/sec, drawing distance 370 mm and lubricant mineral oil M100). TALAT 3702 8
  • 9. Figure 3702.04.03 exhibits the friction and adhesion behaviour of aluminium car body sheet with surface treatments prepared by coil coating. While CrVI-conversion coatings have little beneficial influence on adhesion when compared with the bare metal surface, the organic coatings prove superior in this respect. µ = 0,10 2000 AlMg0,4Si1,2-T4 Lasertex Lubricant: M100 Vz = 100 mm/s Drawing Force, FZ [N] 1500 Tool: X155CrVMo121 1000 µ = 0,05 EDT 500 Mill-Finish 0 0 4 8 12 16 Blankholder Pressure P [N/mm²] N Source: S. Wagner, IfU, Universität Stuttgart, 1994 alu Effects of Microtopographic Structure 3702.04.02 Training in Aluminium Application Technologies on Friction Properties 3000 µ = 0,10 2500 O n- nc Strip Drawing Friction Test M zi g5 ona without Bending M Al ith B Drawing Force, F [N] w Lubricant: M100 Mill-Finish 2000 4 ,2-T Vz = 100 mm/s Z Si1 g 0,4 azinc AlM Bon Tool: 1.2379 1500 with µ = 0,04 1000 Chip-Resist 500 0 0 8 16 24 32 40 48 Blankholder Pressure PN [N/mm²] Source: S. Wagner, IfU, Universität Stuttgart, 1994 alu Friction Properties of Coil Coated Aluminium 3702.04.03 Training in Aluminium Application Technologies TALAT 3702 9
  • 10. 3702.05 The Tool Surface The second friction partner in the tribological system, the tool surface also makes an important contribution to the tribological situation. Basically the same tool materials are used for drawing aluminium body parts as for manufacturing steel bodies, for example cast iron GG26, GG25CrMo and tool steel inserts for drawing edges, drawing beads and cutting edges. To prevent the occurrence of adhesion, the roughness of the tool surface in critical contact zones should meet the following requirements Rz ≤ 1 µm λp ≥ 0,46 where Rz = average peak-to-valley depth λp = degree of profile emptiness = Rp/Rt Rp = peak to mean line height Rt = peak to valley height The adhesion tendency and the friction behaviour can be particularly effectively influenced by surface treatment of the tool, as shown in Figure 3702.05.01. The type of surface treatment which is particularly suitable for a specific case depends on the technical and economical parameters, such as the type of tool material and the size of the tool. 3000 Adhesion Paste Strip Drawing Test boronised without Bending 2500 AlMg0.4Si1.2-T4 Ionitrided vz = 2mm/s Drawing Force, FZ [N] (GG26) 2000 Lubricant: TiC coated Oest Platinol V711/80 1500 Hardened (X165CrMoW12) Hard chrome Grey cast iron 1000 plated (GG26) 500 Bath nitrided (Tenifer) 0 0 2 4 6 8 Blankholder Pressure P [N/mm²] N Source: E. Mössle, 1983 Effects of Tool Surface Treatments on Friction alu and Adhesion of Aluminium Carbody Sheet 3702.05.01 Training in Aluminium Application Technologies TALAT 3702 10
  • 11. 3702.06 Lubrication The third tribological partner is the lubricant. Successful drawing of aluminium body parts depends decisively on the choice of a lubricant and its application to the sheet blank and the tool. Greasing of the sheet blank is generally performed by roll application today. In particular cases it may be necessary to use pressure lubrication in specific areas of the tool itself. There is a large selection of lubricants available today. One important factor in the choice of the lubricant is the dynamic viscosity. Figure 3702.06.01 shows the influence of the dynamic viscosity on the friction coefficient as tested with unalloyed mineral oil M10, M100 and M300, the numbers corresponding roughly with the dynamic viscosity η (in 10-3 Ns/m2) at room temperature. The higher the viscosity the lower is the friction coefficient. Effect of Lubricant 1600 µ = 0,20 Viscosity 1400 Mill-Finish on Friction in µ = 0,16 Strip Drawing Drawing Force, FZ [N] 1200 Tests 1000 µ = 0,12 Sheet alloy: AlMg0,4Si1,2-T4 800 µ = 0,08 Tool material: 600 X 210 Cr 12 400 Drawing velocity: µ = 0,04 vZ = 100 mm/s 200 Lubricant: Mineral Oil 0 M 10 0 1 2 3 4 5 6 7 8 M100 Blankholder Pressure PN [N/mm²] M300 Source: IfU, Universität Stuttgart alu Friction Behaviour and Lubricant Viscosity 3702.06.01 Training in Aluminium Application Technologies It is important also to realize that the viscosity of lubricants can vary significantly with temperature. Figure 3702.06.02 shows the pressure and temperature dependence of an alloyed lubricant (Oest Al2N). The lubricant exhibits a steep reduction in dynamic viscosity with temperature increasing slightly above room temperature. Temperature rises have been measured at the drawing die radius in the order of 10 to 15 °C. In critical areas this temperature rise and the resulting decrease in dynamic viscosity must be taken into account. TALAT 3702 11
  • 12. Dynamic viscosity behaviour of a compounded lubricant dynamical viscosity η in mPa.s 60 40 20 0 C RT ° 40 in 60 80 T 100 ure 150 rat pe 200 m 1 50 100 150 200 Te Pressure p in bar Source: IfU - Stuttgart alu Temperature and Pressure Dependence of the Training in Aluminium Application Technologies Dynamic Viscosity of Lubricants (Oest AL2N) 3702.06.02 3702.07 Literature/References Mössle, E., The effect of the sheet surface in drwaing of sheet parts made of aluminium alloys (in German), Report No. 72, Institut für Umformtechnik, Universität Stuttgart, 1983, Springer-Verlag Balbach, R.: Optimierung der Oberflächenmikrogeometrie von Aluminiumfeinblech für den Karosseriebau, Report no. 97, Institut für Umformtechnik, University Stuttgart, 1988, Springer-Verlag Woska, R.: Einfluß ausgewählter Oberflächenschichten auf das Reib- und Verschleißverhalten beim Tiefziehen, Diss. TH Darmstadt, 1982 Siegert,K., Thoms,V.: Anforderungen an den Schmierstoff bei der Blechumformung in Karosseriewerken, in: Blechbearbeitung 1986, VDI report no. 614, Dusseldorf, VDI- Verlag, 1986 Siegert, K and Thoms, V. The use of lubricants to influence friction during forming of body sheet (in German) ALUMINIUM, 1987, vol. 63, p 401-406 Lange, K.: (Editor) Umformtechnik - Handbuch für Industrie und Wissenschaft, vol. 3: Blechbearbeitung, 2nd edition, Chapter 4 Tribologie der Blechumformung, Springer- Verlag, Berlin 1990 Ostermann, F.: Principles of drawing aluminium body parts, in F. Ostermann (Editor) Aluminium Materials Technology for Automobile Construction, english edition by Roy Woodward, Mechanical Engineering Publications Ltd., London 1993 TALAT 3702 12
  • 13. 3702.08 List of Figures Figure No. Figure Title (Overhead) 3702.01.01 Friction Zones in Deep Drawing 3702.01.02 Friction Zones in Drawing of Car Body Parts 3702.02.01 Microtopographic Structuresof Al Carbody Sheet 3702.02.02 Strip Drawing Tests 3702.02.03 Forces and Friction Coefficients in Strip Drawing Tests 3702.03.01 Mechanism of Friction 3702.04.01 Friction Behaviour of Al Carbody Sheet in Strip Drawing Tests 3702.04.02 Effects of Microtopographic Structure on Friction Properties 3702.04.03 Friction Properties of Coil Coated Aluminium 3702.05.01 Effects of Tool Surface Treatments on Friction and Adhesion of Al Carbody Sheet 3702.06.01 Friction Behaviour and Lubricant Viscosity 3702.06.02 Temperature and Pressure Dependence of the Dynamic Viscosity of Lubricants (Oest Al2N) TALAT 3702 13

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