Evaluation of frictional conditions for various tool materials and lubricants using the injection-upsetting method

ELSEVIER

Journal of Materials Processing Tcchnology 53 (1995) 726 735

Journal of Materials Processing Technology

Evaluation of frictional conditions for various tool materials and lubricants using the injection-upsetting method T. N i s h i m u r a * , T. Sato, Y. T a d a Department (~[Mechanical Engineering, Tiw Uni~ersio, 0/ Tokushima, Minami/osa~jima-cho, l?)ku.shima 770, Japan Received 5 April 1994

Industrial Summary The tribological characteristics of some tool materials, including coated tools and ceramics and lubricants, were investigated using the injection-upsetting test that is combined with backward extrusion. This testing method can evaluate the frictional shear factor without measuring the flow stress of the work material and/or the forging load. Cold-, warm- and hot-forging was carried out in order to investigate the effects of strain rate and test temperature on the frictional shear factor for several tool materials and lubricants. Evident differences in the frictional shear factor are observed for different tool materials used under identical lubrication condition. It is confirmed that the injection-upsetting method can estimate sensitively the frictional conditions. Oil- and water-based graphite lubricants have a good performance at both room and elevated temperature. For MoS2 and the dry condition, the frictional shear factor increases with strain rate and temperature and is inversely proportional to the weight of applied J F lubricant. For all of the lubrication conditions except for the use of J F (a waterbased BN lubricant) adhesion was not observed at the lower tool surface. Kevwords: Injection upsetting; Backward extrusion; Friction; Surface-treated tool; Ccramic

tool; Forging

1. Introduction

In order to improve forming-tool life, attention has been paid to ceramics and surface coatings by CVD and PVD processes because of their superior wear resistance and anti-galling properties. The tribological characteristics of surface-treated steels have been investigated by many researchers and their effectiveness confirmed [1 6]. In the case of a coated extrusion die, it is suggested that CrN coating shows better wear resistance than TiCN or TiAlN coatings [7]. Although the durability of surface-treated tools for actual working has been examined, there have been few * Corresponding author. 0924-0136/95/$09.50 ~', 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 7 5 9 - T

T. Nishimura et al. / Journal of Materials Processing Technology. 53 (1995) 726 735

727

studies of the fundamental lubrication properties. In order to use coated and ceramic tools for metal working processes such as cold- and hot- forging, the lubrication characteristics need to be investigated under actual forming conditions. The ringcompression test is generally conducted in order to estimate the frictional conditions, but, the result of this test cannot be applied immediately to industrial forgings in which the geometries of the products are complicated. Therefore, many testing methods such as the spike, the bucket and the can-extrusion test have been proposed for the evaluation of lubrication conditions [8 11]. In these test methods, the flow stress of the work material at the working temperature and strain rate and/or the forging load must be measured to evaluate the frictional conditions, slight differences in the flow stress obtained by measurement causing distinctly different estimates of the frictional factor. An injection-upsetting method that can assess the frictional conditions under near industrial forging conditions was proposed by Nishimura et al. [12]. This method can evaluate the frictional conditions by the simple measurement of the diameter of a forged flange without need of measurement of the flow stress, the latter varying with the working temperature and the strain rate. In this study, the tribological characteristics of a variety of tool materials, including coated tools, ceramics, a cemented carbide and a conventional tool steel, were investigated using the injection-upsetting method. Cold forging was carried out in order to examine the effect of strain rate on the frictional shear factor of the tool materials. Warm- and hot- forging were performed also to investigate the effect of the test temperature. In addition, the influence of the quantity of lubricant applied on the frictional shear factor was examined.

2. Experimental procedure Seven kinds of tool materials, as shown in Tables 1 and 2, were utilized in the experiments. Surface-treated tools, i.e. with TiCN, TiAIN and CrN coatings, were prepared by CVD and PVD processes on the heat-treated and polished SKD62 alloy tool steel. The surfaces of cemented carbide, ceramics and SKD62 tool steel were finished by grinding (surface roughness Ra ~< 0.5 ~tm except for SiA1ON). Five lubrication conditions were explored in the experiments, as shown in Table 3. Dag is an oil-based graphite lubricant whilst JF is a water-based BN lubricant. Deltaforge is a water-based graphite lubricant including glass and is recommended for the hot Table 1 Coated tools used for injection upsetting Tool material

Substrate

Coating method

Coating thickness (~tm)

Surface roughness Ra (/am)

TiCN coating CrN coating TiAIN coating

SKD62 SKD62 SKD62

lon plating PVD Ion plating PVD Plasma CVD

2.0 4.0 2.5

0.45 0.48 0.30

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T. Nishimura et al. /Journal q/'Materials Processing Technolog), 53 (1995) 726 735

Table 2 Tools used for injection upsetting Tool material

Surface roughness Ra (p.m)

WC ZrO2 SiA1ON SKD62

0.12 0.40 2.10 0.25

Table 3 Kinds of lubricant Lubricant

Substance

Diluation

Method of application

Dag Deltaforge JF MoS2 Dry

Oil-based graphite Water-based graphite Water-based BN

0 2 0 or 15

Brush Spray Brush or spray Aerosol spray

forging of a l u m i n i u m . MoS2 is a c o m m e r c i a l l y a e r o s o l spray. T h e l u b r i c a n t was a p p l i e d on all surfaces that w o u l d be in c o n t a c t with the d e f o r m i n g billet d u r i n g the experiment. C o m m e r c i a l l y - p u r e a l u m i n i u m was selected as the billet m a t e r i a l a n d was a n n e a l e d for 7.2 ks at 673 K to r e m o v e the residual strain. C y l i n d r i c a l billets of 15.5 m m d i a m e t e r were t u r n e d from 16 m m d i a m e t e r bars, the lengths of billets being varied from 30 to 50 mm, d e p e n d i n g on the l u b r i c a t i o n conditions. T h e testing a p p a r a t u s for the c o m b i n e d i n j e c t i o n - u p s e t t i n g process is illustrated s c h e m a t i c a l l y in Fig. 1. T h e die c a v i t y for injection upsetting is c o m p o s e d of an u p p e r a n d a lower tool m a d e from v a r i o u s tool materials. F o r facilitating tool changing, the u p p e r tool was p r o v i d e d with a slope of [~ = 15" a n d inserted as a slight loose fit into the c o n t a i n e r the latter being m a d e from S K D 6 2 alloy steel. T h e b a c k w a r d - e x t r u s i o n die, the conical surface of which is g r o o v e d to secure sticking friction, has an extrusion r a t i o of 4 a n d a semi-cone angle of 2 - 4 5 . It was desired that the u p p e r tool and c o n t a i n e r be m a d e as a body, but precise finishing a n d c o a t i n g on the c o n t a i n e r b o r e surface are difficult. In the experiments, the hollow p u n c h was forced d o w n w a r d s until the billet was e x t r u d e d b a c k w a r d s from the die, a n d then the actual c o n t a c t d i a m e t e r of the formed flange Dr, a n d the residual billet length, l, were measured. These values were n o r m a l i z e d by d i v i d i n g them by the billet radius a n d were plotted on a n o m o g r a m c a l c u l a t e d p r e v i o u s l y using an u p p e r - b o u n d analysis [12]. To o b t a i n the frictional shear factor, c o l d - f o r g i n g tests were carried o u t at r o o m t e m p e r a t u r e and hot- a n d w a r m - forging tests were p e r f o r m e d at 673 a n d 473 K, respectively. T h e strain rates was varied from 0.002 to 15 s 1. In the e l e v a t e d - t e m p e r a t u r e experiments,

T. Nishimura et al. / Journal of Materials Processing Technology 53 (1995) 726-735

729

before [ after -~ Pressure Hollow punch \ Container Backward ~'~.\

extrusion die~ Upper

. ~

P

t o ~

Spacer r i ~ _ i ~

Lowertoo~.~ '

I

°I

DF Fig. 1. Showing the test apparatus, which employs a combined radial and backwards extrusion process.

in order to prevent temperature drop, the whole testing apparatus and the lubricated billet were heated in an electric furnace up to the testing temperature and were then held for 5.4 ks, the injection upsetting test being performed subsequently on a hydraulic press. All tools were cleaned with acetone before each test, following which aluminium adhering on the coated tools was removed by immersing them in a 10% N a O H solution, in some cases emery paper being used also.

3. Results and discussion

3.1. Effects of the tool material and the lubrication on the frictional shear factor The relationship between the frictional shear factor and the lubrication conditions for various tool materials is shown in Fig. 2. The frictional shear factors of the tool materials in the dry condition are almost equal, except for the ceramic tools. When MoS2 is used as a lubricant, however, there are considerable differences in the frictional shear factors between the tool materials. The values for the ceramic tools and the WC tool are a little low in comparison with those for coated tools lubricated with Dag. The frictional shear factors of all coated tools and the substrate material SKD62 for all lubricants are equal, except that the TiCN-coated tool shows the lowest for JF lubricant. SiAION has the best lubrication characteristics amongst the tools used. The frictional shear factor represents an average value for three upset billets. The experimental results have good reproducibility, as the m a x i m u m range is about 0.03 for all of the lubricants. This injection-upsetting test will be useful for the evaluation of lubrication conditions because the values obtained are estimated sensitively for various tool materials under given lubricating conditions.

730

E Nishimura et al. /Journal o f Materials Processing Technology 53 (1995) 726 735

0.6

E 0.4 elf)

[] [] [] []

SKD62 TICN.,coatedSKD62 TIAIN.-e_,oatedSKD62 CrN',coatedSKD62

[] []

Zr02 SIAION

g 0.2

._o t-. U_

0.0

Dag

JF

MoS 2

Dry

Lubricant Fig. 2. Comparison of tile frictional shear factor for various tool materials at room temperature.

3.2. Effects o / t h e strain rate and the lubrication on the jHctional shear Jactor

Fig. 3 presents the plots of the frictional shear factor against the strain rate for Dag, MoS 2 and 'dry" at room temperature. The strain rate is defined as a ratio of the initial billet length to the ram speed of the hydraulic press. The curves for the TiA1N-coated tool are approximately fiat for all lubricants. For the carbide tool, the m-value for Dag drops to a half, whilst for the ZrO2 tool, the m-value for MoS2 becomes high suddenly when the strain rate exceeds 1 s 1. For 'dry" and MoS2, the m-value for most tool materials increase with strain rate, but for Dag lubricant it remains constant, irrespective of the strain rate. The increase in the frictional shear factor may be attributed to the seizure that is promoted by the high sliding velocity at the contact surface between the deforming billet and the tool. Some experimental results for different tools are shown in Fig. 4. The fi'ictional shear factor of the carbide tool remains at a lower level as compared with those for the other tools and carbide tool shows excellent lubricity, especially with Dag lubrication, this result agreeing with those shown in Fig. 2. For MoS2 lubricant, the range of scatter is from 0.2 to 0.3 and the differences between tool materials are not clear. 3.3. E~i, ct O/the test temperature and the strain rate on the.fi'ictional shear fi, ctor

The changes in the frictional shear factor as a function of the test temperature is shown in Fig. 5. In the experiments, a TiCN-coated tool was employed and the strain rate was chosen as about 0.25 s 1. The m-values for 'dry' and Deltaforge are approximately fiat, showing their independence of temperature, whilst that for MoS2 increases with the test temperature. The values for Deltaforge remain at a low level, with good performance at elevated temperatures. Since MoS2 is oxidized at elevated temperatures deterioration of the lubricating performance causes the frictional shear factor to rise. The relationships between the frictional shear factor and the strain rate at 673 K are shown in Fig. 6. The m-values of both tool materials for all lubricants increase with strain rate, except for Deltaforge. The m-value for 'dry' at increasing

72 Nishimura et al. / Journal o f Materials Processing Technology 53 (1995) 726 735

E0.8

E 0.8

,~_~0.6

~

~

.~ 0.4

0.4

E

~

731

0.6

.~0.2

0.2

(a)

LL 0.(]

.......

.001

i

......

.01

,a

.

i I,,=~

.1

........

1

Strain rate i; (s1)

i

0.0

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= i, ..,..I

.001

100

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E 0.8

~0.6

0.6

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r"

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=

........

i

o .......

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(d)

"E

u_ 0.0

.....

.01 .1 1 10 Strain rete ~ (s ~)

100

.01

.001

.I

I

Strain rate ~ (st)

Fig. 3. Relationships between the frictional shear factor and the strain rate for different tool materials at room temperature: (a) tool steel SKD62; (b) TiAIN-coated SK D62; (c) cemented carbide WC: and (d) ZrO2 ceramic( © Dag: • MoS2; • dry).

04

E 0.4

E

~

0.3

•~ 0.3

~

0.2

x:~ 0.2 ~0.1

(a)

"rU-

U_

0.0

.001

,01 .I I 10 Strain rate ~ (s -~)

100

(b)

0.0

,001

.......

J

01

........

i

........

i

........

i

10 Strain rate ~ (s -1) .1

1

. . . . .

100

Fig. 4. Changes in the frictional shear factor as a function of the strain rate at room temperatare, for lubricant: (a) Dag; and (b) MoS2 ( © SKD62; A - TiAIN-coated SKD62; • WC; [] ZrO2).

temperature is not substantially greater than that for room temperature and it is greater than that for MoS2. Fig. 7 represents the experimental results for the various tool materials. The frictional shear factors of the TiCN-coated tool and the Z r O 2 a r e higher than that for the conventional tool steel SKD62. Consequently, it appears that

732

T. Nishimura et al. /Journal (?./'Materials Processing Technolog3' 53 (1995) 726 735

E08 J

f

~if) 0.4 t'-

~-~_-------[3

00.2 U_ 0.0 20O

I

I

400

600

800

Temperature (K) Fig. 5. Change in the frictional shear factor as a function of the forging temperature for TiCN-coated SKD62.(-E2 Deltaforge; • MoS2; • dry).

E 0.8

E o.8

~

~o.6

0.6

X

~LOo . 4 r J (-

o0.2 rj "E LL

0.0 .01

~j 0.2 P E]""-~-"~t3

0-(a) ........

4El

(b) i

........

i

.......

0.0 . . . . . . . . . . . . . . . . . ' ...... .01 ,1 1 10 Strain rate ~ (s-1)

.1 1 Strain rate ~ (s4)

Fig. 6. Relationships between the frictional shear factor and the strain rate for various tool materials at 673 K: (a) tool steel SKD62: (b) TiCN-coated SKD62 [] Deltaforge: • MoS2: -O dry).

E

0.8

80.6 ~0.4 ¢/3

~0.2 "rLL

0.0 .01

.1

1

10

Strain rate ~ (s1)

Fig. 7. Result for different tool materials under conditions of A TiCN-coated SKD62: [] ZrO2).

MoS

2

lubrication at 673 K ( O

SKD62:

T. Nishimura et al. / Journal o f Materials Processing Technology 53 (1995) 726 735

733

the coated and the ceramic tools do not improve the lubrication characteristics at elevated temperature.

3.4. Effect of the weight of lubricant applied on the frictional shear factor In order to investigate the effect of the weight of lubricant applied on the frictional shear factor in injection upsetting, the TiCN coated tool and JF lubricant were employed, the JF lubricant being diluted to provide a ratio of lubricant to water of 1 : 15. The upper and lower tools and the billet were pre-heated for 3.6 ks in a furnace at 473 K. Subsequently, the tool surface and the billet were lubricated by spraying, after cooling the billet being upset at constant strain rate (0.1 s-l). The weight of lubricant was controlled by changing the spray time. Change in the frictional shear factor as a function of the weight of lubricant per unit area is shown in Fig. 8. The m-value is inversely proportional to the weight of lubricant, but the influence of a weight of lubricant above 2 g/m 2 is insignificant: this result is similar to that of a ring-compression test in a previous study [13]. Thus, the present method using injection-upsetting can distinguish slight differences in lubrication.

3.5. Se&ure characteristic For most of the lubrication conditions including dry, adhesion was not observed at the lower tool surface and at the lower surface of flange a lustre was obtained. However, the adhesion on the surface of the coated tool surface found for JF lubricant only took place at a point slightly distant from the bore of the tool, the profiles of the coated and ceramic tool surfaces before and after testing being shown in Fig. 9. The adhesion on the TiA1N coated tool is more severe than that on the TiCN-and CrN-coated tools, which may be attributable to the high frictional shear factor of the TiA1N-coated tool under JF lubricant in comparison with those for others

0.6

I

i

i

i

0.4

om 0 . 2

8 .D

I.L •

0.0 0

i

1

.

i

2

i

I

3

i

I

4

5

Weight of applied lubricant (g.m.2) per unit area Fig. 8. Change in the frictional shear factor as a function of the weight of lubricant (JF) applied per unit area, for TiCN-coated SKD62.

734

E Nishimura el al. ,' Journal of Materials Processing Technology 53 (1995) 726 735

Profiles of coaled tool and ceramic tool surface Tool material

before testing

after lesting

0.2mm

TiCNcoated SKD62

TiAIN coated SKD62

CrNcoaled SKD62

SKD62

• '~ " 4 '

ZrO2

SiAION

Fig. 9. Surface profiles of coated tools and ceramic tools beR)re and after testing (in the circumfercnlial direction: lubricant JF).

tool-lubricant combinations. In injection upsetting, only the lubricant trapped between the tool and flange interfaces acts effectively, because most of the lubricant film applied to the tool surface is scraped offby the rim of the growing flange. The cause of the adhesion seems to be thinning and consumption of the lubricant film. Adhesion was not observed in the case of ceramic tools, since substantial lubricant is retained in the troughs on the rough tool surface before testing. As the frictional shear factor has nearly the same value for Dag and JF, the diameter of the deformed flange is also of the same value. It is known that Dag has a better anti-adhesion performance than that of JF. For the Mry' condition, the absence of adhesion is reasonable because the area of new surface becomes smaller with increase in the frictional shear factor. The present test method is not adequate for estimating the anti-seizure property of the tool materials• Therefore, it is desirable that the seizure characteristics be evaluated for a constant flange geometry that has a new-surface generation that is as large as possible.

T. Nishimura et al. / Journal of Materials Processing Technology 53 (1995) 726-735

735

4. Conclusions The lubrication characteristics of several tool materials and lubricants were investigated using injection upsetting combined with backward extrusion, the experiments being performed at various strain rates and temperatures. The results obtained are as follows: 1. Differences in the frictional shear factor were obtained for the different tool materials under conditions of MOSE condition, SiA1ON displaying the best lubrication characteristics amongst the tool materials used. 2. Oil-based graphite lubricant (Dag) and water-based graphite lubricant (Deltaforge) have a good performance at room temperature and elevated temperature, respectively. 3. For M o S 2 and 'dry' conditions, the frictional shear factor increases with strain rate and temperature, whilst it remains almost constant for Dag and Deltaforge conditions. 4. The frictional shear factor is inversely proportional to the weight of lubricant, but the influence of the weight of lubricant above 2 g/m z is insignificant. 5. For all of the lubrication conditions except for JF, adhesion was not observed at the lower tool surface. 6. The present test method using injection upsetting can detect sensitively for the slight difference in lubrication, and is suitable for evaluation of the frictional conditions.

References [1] J.K. Dennis and E.A.A.G. Mahmoud, Wear resistance of surface-treated hot forging dies, Tribology Int., 21 (1) (1987) 10-17. [2] K. Osakada and F. Murayama, A method for evaluating tool materials against seizure in metal forming, J. Jap. Soc. Technol. Plast., 24 (265) (1983) 195 200. [3] T. Arai, Tool materials and surface treatment, J. Mat. Proc. Tech., 35 (1992) 515 528. [4] T. Arai and Y. Tuchiya, in: Proc. 1986 Jap. Spring Conf. Teehnol. Plast., (1986) pp. 143-146. [5] A. Azushima and T. Hasegawa, in: Proc. 1990 Jap. Spring Conf. Technol. Plast., Vol. 1, 1990 pp. 181 184. [6] T. Arai and Y. Tuchiya, Evaluation of wear and galling resistance of surface treated die steels, Adv. Technol. Plast., 1 (1984) 225-230. [7] F. Sato and M. Abe, Wear resistance improvement of aluminium hot extrusion dies, Sumitomo Light Metal Technical Reports, (April 1993) pp. 54 59. [8] S. Isogawa and A. Kimura, Proposal of an evaluating method on lubrication, Ann. CIRP, 41 (1) (1992) 263-266. [9] G. Ahen, A. Vedhanayagam, E. Kropp and T. Altan, A method for evaluating friction using a backward extrusion-type forging, J. Mat. Proc. Tech., 33 (1992) 109 123. [10] A. Buschhausen, K. Weinmann, J.Y. Lee and T. Altan, Evaluation of lubrication and friction in cold forging using a double backward-extrusion process, J. Mat. Proc. Tech., 33 (1992) 95 108. [11] N. Bay, S. Lassen and K.B. Pedersen, Lubricant limits in backward can extrusion at low reduction, Ann. CIRP, 40 (1) (1991) 239-242. [12] T. Nishimura, T. Sato, Kyin Hoke and Y. Tada, Evaluating method of lubrication using injection upsetting, (submitted t o J. Mat. Proc. Tech.) [13] M. Tukuda and Y. Takada, Alutopia., 5 (1979) 72 77.