SIMULATION OF FRICTION AND LUBRICATION IN ...

SIMULATION OF FRICTION AND LUBRICATION IN COLD FORGING

N. Bay and B. G. Hansen Institute of Manufacturing Engineering Technical University of Denmark 2800 Lyngby,

Denmark

SUMMARY Basic process parameters governing friction and lubrication in cold forging are surface enlargement, sliding length and normal pressure. The simulative tests in literature are discussed. Most tests are not fit to simulate the conditions in cold forging very well due to too small surface enlargement or pressure, or uncontrolled, varying process parameters. A new test controlling these parameters is suggested. In this test measurement of friction and lubricant film breakdown is performed under variable surface enlargement and sliding length. Friction and lubrication conditions in cold forging of aluminium with various lubricants and tool materials (cold working steel, HSS, CVD (TiN/TiC) coated steel) have been investigated. Zinc stearate turns out to give the lowest friction whereas soap phosphate coating provides the strongest resistance to film breakdown, tearing and cold welding of workpiece to tool surface. INTRODUCTION The tribological conditions in cold forging processes are in several ways extreme compared to other tribo-systems like journal bearings. The normal pressure can reach values as high as 3000 N/mm 2 compared to a maximum of 20N/mm 2 in bearings. Plastic deformation of the workpiece involves surface enlargement up to 10 times the original surface area. The sliding velocity between tool and workpiece surface is small compared to most bearing systems thereby often excluding the possibility of hydrodynamic lubrication. These conditions give rise to severe demands to the lubricant film which under the high pressure and surface expansion must be able to cover the new formed surface area everywhere thereby establishing solid film or boundary lubrication. The main parameters governing the tribological conditions in cold forging are the following: Specimen:

Material Geometry Surface topography

Tool:

Material Geometry Surface topography

Lubricant:

Composition Amount (film thickness) Viscosity Compressibility

It is most often impracticable if not impossible to test this large spectrum of parameters in the real process itself. Especially the process conditions are normally very difficult to change freely since the parameters in a given process are interrelated. In order to obtain test results of more general value the application of simulative tests is useful. SIMULATIVE TESTS In literature several simulative tests for metal forming tribology have been suggested, see f.i. SCHEY (1). A necesssity when simulating these processes is that the test should be performed at high pressure. The examples mentioned are therefore confined to such tests. The pin-an-disk test using a hemispherical end of the pin has been tried by several investigators, Fig. 1a-b, (2-4). PETERSON and LING (5) used a variant of the test by compressing a thin sheet of the specimen material between a flat anvil and a punch, Fig. 1c. The pin-an-disk test may be characterized by tool material meeting new workpiece material or workpiece material meeting new tool material. a)

b)

c)

Process conditions: Normal pressure Surface enlargement Sliding length Normal velocity and relative sliding velocity between tool and workpiece (before and during forging) Machine:

Fig. 1.

Stiffness (vibrations) 55

Pin-an-disk tests.

The twist compression test is a contrast to this where a ring or a stud of specimen material is pressed and slid by rotation over the same tool surface. The test is developed in several versions, (6-11), which are shown in Fig. 2, after SCHEY (1) a)

b)

tool a)

c)

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b)

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cross section of workpiece

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Fig. 2.

Methods of twist compression tests after D..l...

Interface pressure in the twist compression test can be increased by enclosing the workpiece in a container. Fig. 3a shows the principle used in ref. (12-14). WANHEIM (15) uses a better design, Fig. 3b, where the disk formed specimen is compressed between flat anvils and is radially extruded against an outer die ring which is rotated, thereby giving a uniform pressure and sliding length in the whole friction contact area (the cylindrical surface of the specimen) .

a)

Fig. 3.

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Twist compression tests at high normal pressures.

The third group of tests to be mentioned is here named the ridge plough tests, Fig. 4. There are two variants of these tests. In Fig. 4a after KUDO et al. (16), and Fig.4b after OYANE et al. (17), local plastic deformation of the smooth specimen surface is obtained with a wedge tool. The tool geometry is designed so that ploughing without chipping is performed. In Fig. 4c from GOTO et al. (18), a plane tool is sliding over a wedge formed specimen surface thereby obtaining local surface expansion by plastic deformation of the wedge. Normal pressure in these tests will be about twice the flow stress, but the surface expansion is rather small.

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Fig. 4.

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tool

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Ridge plough tests.

The tests mentioned until now all have a common weakness namely the rather small plastic deformation or surface expansion possible to obtain. A group of tests which account for larger plastic deformation combined with superimposed relative sliding in the tool/workpiece interface are the "compression with simultaneous sliding tests", after SCHEY (1). Fig. 5a-d shows variants reported in literature, PAWLOW et al. (19), SAGA et al. (20), NITTEL (21), EL-MAGD (22), and GRABENER (23). Due to the large plastic deformatIOn these tests are more suitable for evaluating solid film lubricants and tendency to pick-up. However, the pressure distribution on the tool/workpiece interface is far from uniform and friction measurements can therefore only be qualitative. A number of tests are based on strip drawing (24-35). SCHEY (1) gives a good description of the variants. A final group of tests to be pointed out are the special upsetting tests where friction can be determined by geometric measurements only. Two tests are developed to an operative level. The ring compression test was first proposed by

I (36) and KUDO (37) and later deed by MALE and COCKCROFT (38) and al others, ref. (39-44). The upng of rectangular slabs between overhanging anvils was proposed LL (45), and later developed by ER and WANHEIM (46). aJ

TEST EQUIPMENT A new simulative test eliminating some of the above-mentioned objections has been developed by BAY (49). The test is a new variant of the twist compression tests. Fig. 6 shows a schematic outline of the test. The cylindrical workpiece is compressed between two overhanging anvils. After reaching the desired surface enlargement the workpiece is rotated at constant load by rotating the lower punch. Rotation of the workpiece is ensured by radial ridges in the lower anvil surface. The ridges will transfer the necessary torque without giving large friction during compression, since they are radially oriented. The flat upper punch is divided into an outer ring formed punch which is stationary and an inner punch which can rotate freely with respect to the outer punch. This is established by mounting the inner punch on roller bearings in the housing (not shown).

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