Wear - Elsevier Sequoia %A,, Lausanne - Printed in the Netherlands 375

Short Communication

An observation of surface deformation in copper under high wear rate conditions

G. PASCOE

Fry’s Metals Ltd., Tandem Works, M&on Abbey, London (Ct. Britain)

(Received August 19,rg7o)

Severe wear occurs during the initial stages of friction welding when the rotating component contacts the static component of the materials to be joined. This is shown diagramatically in Fig. I. A series of trials have been carried out to measure the coef- ficient of friction of copper under the physical conditions of the initial stages of friction welding to assess the magnitude of the heating effect. This work is of interest in inves- tigations of the friction weldability of copper.

- Clamp

Fig. I. Diagram of friction welding component configuration.

Friction specimens, of dimensions shown in Fig. 2, were tested using the appara- tus illustrated in Fig. 3. A pin chuck holding the rod specimen is rotated by a a h.p. motor while being in contact with the block mounted on a torque drum. The drum is loaded by a calibrated spring to apply the load to the rod/block interface, and the whole drum assembly is located in bearings to permit the torque measurements to be made. Pertinent properties of the materials used are given in Table I. A range of test- ing speeds (expressed as distance travelled per unit time) was achieved by varying the inner dimension D, the values of which are included in Table II.

The linear speeds were calculated from a mean radius of the annular contact ring. Weighted mean radius values given in Table II were calculated by considering the friction forces on an elemental ring and integrating this value over the surface :

s Rl

znrZdr=n (RI~-R~) & R2

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376 SHORT CORIiNUNICATIOKS

0.250

-

I I-

O 0

Fig. 2. The rod/block specimen configuration, illustrating the measured torque m produced on tht block by the applied load W and speed z).

Fig. 3. Schematic layout of the apparatus.

so R, = 2 (R13--R33) w

3 (R12--22)

The values of the coefficient of friction were then determined from the measured load on the torque drum, and the weighted mean radius K,, from the conventional relationship: ,u=m/~ i?,. The results of the coefficient of friction measurements from the detailed measurements made are summarised in Table III.

The extent of rod damage during these trials is illustrated in Fig. 4. The photo- micrographs of a cross-section of a rod, Fig. 5, show an embryonic wear particle as- sociated with a heavily flowed subsurface region. Experimental measurements with

Wear, 16 (1970) 375-380

SHORT COMMUNICATIONS 377

TABLE I

PROPERTIES OF THE MATERIALS

Property Rod Block

Hardness HV L.S. 5 76.7 92.4 T.S. 128

Wt. y. oxygen 0.0378; 0.048b 0.078 Grain size (cm x 10-4) 6.25 23.8 Surface finish mean asperity height,

in. x 10-4 4 4

Spectrographic analysis (wt. %) Sb

Bi As Sn Pb Te Ni Zn co Fe

Ag Mn, Si,P

0.0005 0.0005 <0.0001 0.001

0.0003 0.0003 <O.OOI <o.oor < 0.0003 0.001

n.d. 0.0005 0.0006 0.001 0.001 <O.OOI

<0.0001 0.0002

O.OOI O.OOI

0.003 0.005

n.d. n.d.

a Harris and Hickman vacuum fusion technique. b Estimated optically.

TABLE II

SAMPLE PARAMETERS

Sample B C D

Diameter 0.062 0.094 0.161

Nominal contact area, in2 x 10-2 4.618 4.336 2.47 Weighted mean radius, in. 0.0872 0.091 0.105

Mean linear speed, in./min 1531 I 600 1844

TABLE III

RESULTS OF COEFFICIENT OF FRICTION MEASUREMENTS

Load Coefficient of friction, p (lb.) B C D

6.63 8.84

11.05

13.26

IS.47 17.68

19.89

22.IO

24.31

0.95 0.82

0.92 0.82 0.92 0.84

0.94 0.805 0.84 0.86 0.89 0.89 0.82 0.91 0.89 0.94 0.98 0.93

1.60

1.015

0.90

0.845 0.87 0.88 0.83 0.85 0.78

TABLE IV SIZE ANALYSIS OF WEAR DEBRIS

Si‘W (B.S.S. mesh)

o/O oftotal

+ 25 57.2 -25 + 44 20.5 -44 + roe 20.2

-100 2.2

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378 SHOKT COMMUNICATIOIiS

thermocouples located 0.010 in. behind the abraded surfaces were supported by calcu- lations based on the work of RYKALIK. that the maximum temperature rise at the interface was <4”C as determined by the relationship:

tg22 = KC Tz = const.

where the specific power 4- =7.44 cal and t =5 sec.

lpig. 4. Photograph of rod clamagc~ after a friction trial. (x 6)

The size range of wear debris collected after about 170 friction trials is given in Table IV. The largest proportion (51yC at + 25 mesh fraction) of this wear debris had a uniform disc shape which could be the wedge-shaped particles noted by COCKS2, who observed that during the friction of copper to copper materials the surfaces were held apart by wedge-shaped portions of displaced metal which break away and are reformed at rapid intervals. Thus there is evidence that under certain load conditions wear debris is generated which welds to, and causes deformation of, the subsurface region.

The observations appear to be paradoxical at first sight, as the friction welding process depends upon heat generation by the rubbing surfaces. However, this work not only demonstrates the extensive wear that takes place during the initial stage of friction welding, which is characterised by a period of intense vibrations, but also provides data to show that friction welding is possible from thermal considerations. Using the coefficient of friction of Cu (,u =0.88) obtained, together with the relation- ships developed by CHENG~,~, VILL” and HOLLANDER7, it can be shown that friction of copper to copper surfaces will provide an adequate heat source for friction welding. This has been supported by a subsequent experiment on a friction welding machine, which demonstrated that interface temperatures of the order of 800°C can easily be

SHORT COMMUNICATIONS 379

Fig. 5. Photomicrographs of cross-section of a rod specimen. (Upper x LOO; lower x 3’

achieved and which, as has recently been reporteds, can be utilised to weld c other materials.

The author is grateful to his supervisor Mr. R. L. SAMUEL and to KELLEY for their help and encouragement in this work.

00)

:op

Ml

‘P ler and

D 1. A.

Wear, r6 (1970) 375-380

NOMENCLATURE

SHORT COMMUNICATIONS

c Volume specific heat D Internal diameter of rod specimen K Thermal conductivity m Torque

P Coefficient of friction

42 Specific power

RI Internal Radius R2 External Radius

ww Mean radius of surface r Radius of an elemental ring T Effective temperature rise t Heating period V Rotational speed w Load

REFERENCES

I N. N. RYKALIN, A. I. FUGIN AND V. A. VASILEVA, The heating and cooling of rods butt welded by the friction process, Svar. Prciz. (U.S.S.R., H. W.R.A tram.j , (I 959) 6 I.

2 M. COCKS, Interaction of sliding metal surfaces, J. Appl. Phys., 33 (1962) 215~. 3 V. I. VILL, The friction welding of metals, A.W.S. trans., 1961. 4 C. J. CHENG, Transient temperature distribution during friction welding of two similar metals

in tubular form, Welding J., 41 (1962) 5425. 5 C. J. CHENG, Transient temperature distribution during friction welding of two dissimilar metals

in tubular form, W&i?Zg I., 42 (1963) 2335. 6 V. I. VILL, Energy distribution in the friction welding of steel bars, LVeZd. Prod. C.S.S.R., (1959)

3’. 7 M. B. HOLLANDER, Developments in friction welding, Mater. Eng. @z++., (1962) ‘4. 8 Friction welding feature, Met. Constr., 2 (1970) 125.

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