New wear tests of tool materials for metal forming

Wear, 100 (1984)

119

- 128

119

NEW WEAR TESTS OF TOOL MATERIALS MORIYA

OYANE,

Department

SUSUMU

SHIMA,

of Mechanical Engineering,

YOSHIHIRO

FOR METAL GOT0

FORMING

and TOSHIHIDE

Faculty of Engineering,

NAKAYAMA

Kyoto Uniuersity, Kyoto

(Japan) Summary Tribological phenomena, i.e. friction, tool wear and material pick-up to the tool, in metal forming processes are influenced by various factors. Therefore, in friction tests in the laboratory, these factors should be varied widely and independently and they should be measured easily. A new test method which has these features is described; the test parameters are the tool pressure, sliding speed, enlargement of the workpiece surface, length of contact between the tool and workpiece, sliding distance, contact angle and viscosity of the lubricant. Some results of the tests on tool wear are presented and discussed.

1. Introduction Tool wear in metal forming is affected by many factors such as the tool pressure and the sliding speed. To investigate the effects of these factors, a number of test methods have been proposed previously [ 1 - 51. It is impossible, however, to draw from the test results a general understanding of the effects of these factors on tool wear, because in the test methods some of the factors vary dependently and within limited ranges. The present authors have already developed some test methods in which a wedge-shaped or flat tool is pressed against a cylindrical workpiece rotated on a lathe. They obtained from the test results some criteria for the coefficient of friction and for sticking of the workpiece material; the criteria are expressed as functions of the operating parameters [ 6 - 91. The features of the test methods are as follows. (1) Each parameter can be varied widely and independently. (2) The apparatus required for the test is easily available. (3) Each factor can be measured easily. The main aim of the present paper is to present an experimental study of tool wear by the test methods. 2. Principles

of the test methods

and definitions

Figure 1 shows one of the test methods, this test a thin-walled cylinder of a workpiece 0043-1648/84/$3.00

of some parameters which is designated test I. In material is rotated on a lathe.

0 Elsevier Sequoia/Printed

in The Netherlands

120

Section

B

Fig. 1. Illustration of the testing method in test I: an end surface of a thin-walled cylinder is flattened by a tool.

A cutting tool and a forming tool are fed along the axis of the cylinder at a constant speed. The end surface of the cylinder is shaped by the cutting tool to a cylindrical wedge of angle 28, and this is flattened successively by the forming tool with plastic deformation only. The process of simultaneous cutting and flattening gives rise to long distance sliding of the workpiece material on the forming tool. For example, if the diameter is 200 mm, the length of the cylinder is 1000 mm and the feed speed is 0.5 mm rev-‘, the sliding distance reaches 1.25 km. The second test method, illustrated in Fig. 2, is designated test II. A wedge-shaped tool of angle 28 is indented such that a spiral groove forms at the cylindrical surface of the workpiece without chip formation. The tool may have either a knife edge [lo] or a flat edge as shown in Fig. 2. The third test method is a tool wear test in the drawing of a metal sheet as shown in Fig. 3. In the above-mentioned three tests, the surfaces of the tool and the workpiece are measured by a surface roughness meter. The tool wear is defined as the maximum depth W from the initial tool surface and the tool pressure p as the mean pressure of the tool on the workpiece. The value of p

(a)

(b)

Fig. 2. (a) Illustration of the testing method in test II involving indentation with (b) a wedge-shaped tool. Contact

length

!tion

Fig. 3. Illustrations of tool wear and the measurement positions in test III.

121

depends on the wedge angle back tension applied to the as the mean frictional stress the forming tool. The areal strain or the the most influential factors and it is defined as follows:

28 in tests I and II and on the reduction and the sheet in test III. The frictional stress 7 is defined at the contact zone between the workpiece and enlargement of the workpiece surface is one of in tool wear as well as in friction and sticking,

A -A0 ,$=--_Ao where A is the area of the flattened portion of the end surface in test I, the area of the groove in test II or the surface area of the drawn strip in test III, and A0 is the area of the corresponding zone before plastic deformation. As it is impossible to predict this zone in tests I and II, two parallel helical lines were scratched at the workpiece surface beforehand and the distances between the two lines after and before the test were substituted as the values of A and Ao. The contact length h between the tool and the workpiece in the sliding direction is shown, for example, in Fig. 3 for test III. 3. Test conditions The test materials for the workpieces are commercially pure aluminium, copper and low carbon steel, and the test materials for the tools are high speed tool steel, alloy tool steel, carbon tool steel, Cr-Mo steel and tungsten carbide. The designations of the test materials and the definitions of the parameters are listed in Table 1 and Appendix A. The types of materials and the values of the parameters are given in each figure showing the test result. The lubricants used and their designations are listed in Table 2. TABLE 1 Test materials for workpiece and tool Material

Designation

Commercially pure Al Commercially pure Cu 0.1% C steel

Aluminium Copper SlOcY

High speed tool steel Alloy tool steel

SKHS, SKH4a SKDl la SKS3a SKD4a SKSa SCM22a WC

C tool steel Cr-Mo steel Tungsten carbide aJIS.

122 TABLE 2 Lubricants used Lubricant

Designation

Oil of rape seed Spindle oil number 60 Mineral oil

OR Sp 60 HVISO HVI-16OB HVI-650

Bonderite

Bonderlube

treatment

BB treatment

4. Test results 4.1. Ai~mini~m and copper Most of the tests for ‘aluminium and copper were carried out with tests I and II. When aluminium is used as the workpiece material, it sticks to the entire tool surface except near the exit region, where the friction is most severe and the lubricant is thinnest. Tool wear occurs only in this region. Typical test results are shown in Fig. 4, where the tool wear W, the maximum roughness R,,, of the deformed workpiece surface and frictional

(a)

Sliding

distance

1 [ km 1

Fig. 4. Change in frictional stress, tool wear and maximum surface roughness of workpiece with sliding distance (aluminium with OR): (a) less severe conditions (p = 190 MPa; g = 7%; h = 0.32 mm); (b) more severe conditions (p = 190 MPa; 4 = 35%; h = 0.8 mm).

123

Cc)

0

0.5

1.0

1.5

Sliding

2.0

distance

2.5

3.0

1 [ km ]

Sliding

distance

I [ km ]

Fig. 5. Change in amount of tool wear with sliding distance: (a) effect of tool pressurep (5‘ = 10%; h = 0.4 mm); (b) effect of contact length h (p = 150 MPa; g = 40%); (c) effect of enlargement f of the workpiece surface (p = 190 MPa; h = 1.5 mm). Fig. 6. Change in tool l= 35%; h = 0.8 mm).

wear with

sliding

distance

(aluminium

with

OR; p = 190 MPa;

stress T are plotted against the sliding distance 1. The lubrication conditions are more severe in Fig. 4(b) than in Fig. 4(a) because of the larger values of [ and of h. Abrupt increases in W, R,,, and r are seen in Fig. 4(b) at 1 = 1.5 km where severe sticking occurs, while gradual increases in W and r and are observed in Fig. 4(a). These phenomena an insignificant change in R,,, can be found only when the long sliding test is carried out with test I. The dependence of the tool wear on p, [ and h is seen from the W-l diagrams in Fig. 5. Figure 5(a) shows the results for three different pressures for constant [ and h; W is obviously larger for higherp. Figures 5(b) and 5(c) show the corresponding results for various values of h and f respectively; W is not sensitive to these two factors. The wear of five tool materials is shown in Fig. 6. The best material is seen to be SKH4. When copper is used as the workpiece material, it sticks to the tool surface very slightly and tool wear occurs almost everywhere at the tool surface. The tools are worn more severely by copper than by aluminium as shown in Fig. 7. Various tool materials were tested for copper with test II and the test results are given in Fig. 8. It is seen that SKH9 is a very good tool material.

4.2. Steel All the tests for steels as workpiece materials were carried out with test III. The surface roughness of the tools was measured at three positions: A (the inlet), B (the intermediate position) and C (the exit) of the contact zone as shown in Fig. 3. Examples of the surface profiles of the tool at these positions are shown in Fig. 9. The maximum depths or the tool wear at these positions are designated WI, W2 and W3 respectively. In Fig. 10 WI, W2 and W3 as well as the friction coefficient and the roughness of the workpiece are plotted against the sliding distance. The tool wear increases proportionally to the sliding distance except in the initial stage of W,.

124 I20

I -_.-__ (heattreated) ------

iAl)

0

11

Sliding distance

Fig. 7. Comparison piece materials.

of amount

Fig. 8. Tool wear for various p = 490 - 687 MPa).

Fig, 9. Example

so

0

t r km 1

of a surface

of tool wear when tool materials

profile

100 150 200 Sliding distance

using aluminium

(copper;

no lubricant;

250 300 2 I ml

and copper

as work-

V = 0.48 - 0.55 m s-l;

of a tool (test III).

Fig. 10. Variation in coefficient of friction, tool wear and maximum surface roughness of the workpiece with sliding distance (SlOC; BB treatment; r = 15.7% - 16.4%; V = 100 mm s-1; a = 10”).

Figures 11 - 16 show the dependences of the tool wear at 1~ 30 m on the various parameters, i.e. r, V, p, a, H, and V. 5. Discussion As Fig. 4(b) shows, the tool wear, the fictions shear stress and the surface roughness increase not linearly but abruptly at I= 1.5 km. Therefore a long run wear test is required for the prediction of tool wear even under severe forming conditions which give rise to sticking, In test I the sliding distance can be as long as several kilometres.

125

L

12

I

1

1

I

I

15

20

25

30

33

Enlargement

of

surface

area

E

[%I

Fig. 11. Effects of reduction in area on tool wear, maximum workpiece and coefficient of friction (SlOC; BB treatment; 100 mm s-l; I = 30 m; Cr= 10”).

surface

roughness

Fig. 12. Effects of drawing velocity on tool wear, maximum surface roughness workpiece and coefficient of friction (SlOC; I = 30 m; (II = 10”; r = 14.0% - 16.4%).

Contact

Fig. 13. Effects of tool pressure piece and coefficient of friction ~-~;a=15”;1=30m).

of the

p = 650 - 850 MPa; V=

angle

of the

= [deg.]

on tool wear, maximum surface roughness (SlOC; BB treatment; r = 14.3% - 16.0%;

of the workV = 100 mm

Fig. 14. Effects of contact angle on tool wear, maximum surface roughness of the workpiece and coefficient of friction (SlOC; BB treatment; I = 30 m; V = 100 mm s-l; r = 13.2% - 16.4%).

126

'I

300

I 400

500

Vickers

I 600 hardness

I I 800

700 Hv

10 Viscosity

50

100

of lubricant

Y [mm’/s

at 20°C I

Fig. 15. Effects of hardness of the tool material on tool wear, maximum surface roughness of the workpiece and coefficient of friction (SlOC; BB treatment; r = 14.3% - 16.4%; V=100mms-1;Z=30m;o=10”). Fig. 16. Change in tool wear, maximum surface roughness of the workpiece and coefficient of friction with viscosity of the lubricant (SlOC; r = 13.7% - 17.2%; V = 100 mm s-r; I= 30 m; (Y = 10”): Lub. 1, Sp 60; Lub. 2, HVI60; Lub. 3, HVI-16OB; Lub. 4, 35%HVI-60-65%HVI-650.

It is clear from the test results in Figs. 5 and 12 - 16 that the tool wear depends not only on the material of the tool, the workpiece and the lubricant but also on parameters such as the tool pressure, enlargement of the workpiece surface, sliding speed and contact length. In order to obtain generalized information on tool wear, all the factors must be changed independently as in the present tests. To predict tool wear and friction in a practical metal forming process, a simulation test should be done, if possible, with equal values of the operating parameters. For this purpose, the test method must have a wide variation for each parameter. The temperature rise in the deformation zone plays an important role in lubrication in metal forming and it depends on the size and the shape of the tool and the workpiece. One of the present authors is now trying to clarify the effects of the size and the shape by an analytical and experimental study. Without accurate knowledge of the size and shape effects, it is impossible to utilize the results of a test with an apparatus of a small size for the prediction of tool wear, friction and sticking in practical processes. The present paper does not deal with the velocity of the tool. Figure 17 shows a modification of a test of the present method, where a rotating tool [ 111 is adopted instead of the fixed tool. This test method can be used as a simulation test for rolling, spinning etc.

127

Rotating

tool

Fig. 17. Illustration of the test using a rotating tool.

6. Conclusions New test methods for tool materials in metal forming which can be carried out with apparatus which is widely used, such as a lathe, a dynamometer and a surface roughness meter, are proposed. The tool wear is affected by many factors such as the enlargement of the surface, the tool pressure and the contact length. References 1 H. Kudo, M. Tsubouchi,

2

3 4

9

10 11

Y. Fukuhara and Y. Ito, Determination of friction and wear characteristics of some lubricants and tool materials for cold forging with the simulation testing machine, Ann. CIRP, 28 (1979) 159. M. Oyane, S. Shima and T. Nakayama, Universal testing for workpiece material, tool material and lubricant in metal forming, Proc. Int. Symp. on Metalworking Lubrication, San Francisco, CA, American Society of Mechanical Engineers, New York, 1980, p. 13. A. W. J. Chisholm, Wear testing of die materials rubbing against aluminium and copper, Ann. CIRP, 28 (1979) 165. Y. Nakamura, H. Kawakami, T. Matsushita and Y. Sawada, A study of evaluating lubrication and die life in steel wire drawing, Preprints 30th Jpn. Natl. Conf. on the Technology of Plasticity, Nagoya, 1979, Japan Society of Technology of Plasticity, Tokyo, 1979, p. 533. A. Ohnuki, T. Kikuchi, T. Kawanami and T. Asamura, Study on work roll wear of cold strip mill, J. Jpn. Sot. Technol. Plast., 23 (1982) 990. M. Oyane, S. Shima, T. Nakayama and A. Ueda, Simulation test for material pick-up and tool wear for metal forming, J. Jpn. Sot. Technol. Plast., 22 (1981) 257. Y. Goto, S. Wakasugi and T. Kozai, A test for investigating the lubrication properties of solid lubricants in cold metal forming, J. Mech. Work. Technol., 6 (1982) 51. Y. Goto, S. Wakasugi and K. Tsuji, Effects of sliding speed, surface extension, viscosity of lubricant and tool pressure on friction and material pick-up to tool surface, J. Jpn. Sot. Technol. Plost., 24 (1983) 207. K. Yoshikawa and T. Sato, Effect of factors on adhesion in relative sliding between rolling tool and workpiece, Preprints 31st Jpn. Joint Conf. on the Technology of Plasticity, Tokyo, 1980, Japan Society of Technology of Plasticity, Tokyo, 1980, p. 259. M. Oyane, R. Yoshinaga and Y. Goto, A testing method for investigating friction and lubrication in metal working process, Proc. 18th Int. Machine Tool Design and Research Conf., London, 1977, Macmillan, London, p. 339. K. Yoshikawa and T. Sato, Evaluation of adhesion at sliding and rolling contact surface in simulation of metal forming, Proc. 1st Int. Conf. on Technology of Plasticity, Tokyo, 1984, Japan Society of Technology of Plasticity, Tokyo, 1984, p. 273.

128

Appendix h

HV 1 r

P V o!

2% 12 V t 7

A: Nomenclature

contact length between tool and workpiece (mm) Vickers hardness of tool materials sliding distance (km, m) reduction in area (‘31) mean tool pressure (MPa) sliding speed, drawing velocity (m s-l) contact angle (deg) wedge angle of the end surface of the cylinder or tool for indentation (deg) mean coefficient of friction viscosity of the lubricant at 20 “C (mm2 s-l) enlargement of the workpiece surface (‘96) mean frictional stress (MPa)