The Role of Contact Friction and Rheology in the Deformation at Plastometric Tests of Rheologically Complex Materials
Vpliv kontaktnega trenja in reologije pri plastometričnih preizkusih reološko kompleksnih materialov
G. G. Shlomchack*, Dnepropetrovsk Metallurgical Institute, Ukraine
I. Mamužič, Metalurški fakultet, Sisak, Croatia
F. Vodopivec, Institute of metals and technologies, Ljubljana, Slovenia
The deformation anomalies of higher orders at plastometric test are studied on easily deformabie iead alloys-modeis of different rheologicai compiexity by original testmg methods. It is ascertained that the development of deformation anomalies depends upon the degree of rheologicai complexity of the material. Simple strain anomalies are due to the inadequate conditions of contact friction, vvhile those of the higher orders results mainly from microrelief of the sample butts. Recommendations for the obtention of homogeneity of deformation at plastometric tests are given. The tests shovv that the deformation of rheologically complex metals develops according to laws basically different of the contemporary notions of the mechanics of plastic deformation. Key words: plastic deformation, rheology, deformation anomalies, contact friction, homogeneous deformation
Opredeljene so deformacijske anomalije višjega reda pri plastometričnih preizkusih svinčevih spojin z različno reološko kompleksnostjo. Razvoj anomalij je odvisen od reologije materiala. Enostavne anomalije so posledica neustreznega stičnega trenja, anomalije višjega reda pa so predvsem odvisne od mikroreliefa stične površine. Priporočeni so ukrepi za doseganje homogene deformacije pri plastometričnih preizkusih. Ti kažejo, da deformacija reološko kompleksnih materialov poteka po zakonih, ki se razlikujejo od sodobnega razumevanja mehanizma plastične deformacije. Ključne besede: plastična deformacija, reologija, deformacijske anomalije, kontaktno trenje, homogena deformacija.
1. Introduction
The development of new technologies of pressure shaping depends on the knowledge of rheologicai properties of the plas-ticallv deformed material. The dependence betvveen the resis-tance to deformation tr and the deformation rate e at different strain rates <r and tenipcratures T is a family of curves a-e, it is specific for eacli steel and allov and it is a rheologicai passport of the material. The most reliable are rheologicai curves obtained bv compression tests on cam plastometer (fig. 1) at constant strain rates (1). The condition <r = const is ensured by the cam profile which is given bv the logarithmic lavv
i ( \
k 8 -5-= exp —. x ,
K - A/7 ^ v J
with h , - initial height of the cvlindrical specimen 4; Ah - absolute cogging of the specimen ali over its height; n - circumferential speed of the drum 1: x - length of the cam profiled part 2.
■ I)i. st. Georg GEORGUEVIf SHLOMCHACK. Dnepropemivsk \1ctallun:it-;il Institute. I kraine
The ensurance of the specimen deformation homogeneity (2). thus the elimination of barrel formation. twisting. shift, bending and other deformation anomalies is the basic require-ment in plastometric tests.
It is generallv accepted that contact friction forces onlv in-fluence the deformation of the specimen during its plastic compression. The present investigations refute this point of view and show experimentally that the development of deformation anomalies depends to a great extent upon the rheologicai complexity of the deformed metal.
2. Rheology of the investigated material
Non-strengthenable materials of the first rheologicai class and monotonously strengthenable materials of the second rheologicai class are regarded as rheologicai simple (3). Rheologicai complex materials differ from them by presence of extrema on the <r-e curves.
In this work steels and alloys with one maximum on ir-e curves are examined which belong to the most widespead third rheologicai class (4) of metallic materials with the characteris-
4
5
Figure 1: Sclieme of cam plastometer: 1 - drum; 2 - profiled cani;
3 - plunger; 4 - specimen; 5 - force-measuring element. Slika I: Shema tlačnega (cam) plastometra: I - valj. 2 - profiliran nastavek. 3 - bat, 4 - preizkušanee. 5 - merilnik sile.
t i c deformation < r, at the maximum of rcsistance to deformation <rm.lx (5). The smaller is the valuc crx vvithin the limits of the rhe-ological class. the higher is the degree of the material rheologi-cal complcxity. In fig. 2 rheological curves of carbon steel (a) vvith a characterislic dcgrcc of deformation <r, = 0.4...0.6 (6). high-speed allov steel (B) vvith <tx = 0.3...0.4 (7). and as unique for it degree of rhcological complexity (C) a zirconium allov (S) are shovvn. The characterislic degree of deformation of the cirkonium 2.5% Nb allov is onlv <r, = 0.03...0.05 and its rheological curves could be better qualified as "curves of unstrengthening" than as "curves of strengthening". The deformation of such materials occurs according to lavvs vvhich are different in principlc of the modern conception of the mechan-ics of material plastic deformation. The close studv of the dcvclopment of deformation anomalies of higher order (4) of rheologically complex materials vvould probablv promote the development of the theorv and technology of metal pressure shaping.
Fasilv delormed lead alloys-models of different rheological complcsitv vvere developed for the modelling nevv processes of metal pressure shaping at the Ukrainian Metallurgical Academv. Fig. 3 shovvs some o-e curves for a technically pure lead (99.98% Pb, curve A) and alloys vvith 99.9% Pb (curve B), 99.4% Pb (curvc C) and 99.0% Pb (curve D) obtained by cam plastometric tests in compression at the temperature of 15 C and strain rate <r=0.3 s '. If pure lead is qualified as a second order rhcological monotonously strengthenable material ("A") then, depending on the content of additions. the alloys have a maxi-mum on curves <r-e. vvhich shifts to a smaller deformation dcgrcc. from (Txb = 0.63 to trxi, = 0.23. vvhich complicates their rhe-ology.
It should be noted that earlier the rheological (but not defor-mational) complexity of lead vvas observed either at high temperatures (9) or by static loading (10) vvhat made it unsuitable for the use in phvsical quantitative modelling of the pressure shaping processes based on the similaritv theorv. 580
<5, Mfla
120 60 0
240
180
120
120

1
I I I I
I
i_L
£x= 0,4.0.6
	"-T" X 1 o-l^ i 1 \	
	1 l 1 1 1 1 1 l	
Y	! 1 — I | 1 1 j 1	0,5
	1 ' 1 1 1 1 ! 1 1 1 1	
£x=0,3.0,4
40
0
/1 \ lf\ \ ' 1 X > 1 \			
0 iV			310~2
11 11 11			STO"
1 1 1 1 11			310
!! Sr0,03. 0,05			
0,2
0.4
Figure 2: s-s curves of different rheological complevitv: .1 - steel vvith 0.19% C; 0.04% Si: 0.86% M11: 0.022% P; 0.029'/, S at 900°C (61; b - high speed steel vvith 0.88% C: 0.39% Si: 0.23% Mn: 0.03% P: 0.011% S: 3.3% Cr: 6.39% W; 4.72% Mo; 2.23% V at 1100°C (7): c - zirconium allov vvith 2.5% niobium at 775 C. (Ni ali at the indicated
values ol" strain rate in s-1. Slika 2: s-s krivulje / različno stopnjo reološke kompleksnosti: a - jeklo z 0.19% C; 0.04% Si; 0.86% Mn; 0.022% P in 0.029% S pri 900°C (6): b-hitrorez.no jeklo z 0.88% C: 0.39% Si; 0.23% Mn; 0.03% P; 0.011% S: 3.3% Cr; 6.39% W; 4.72% Mo in 2.23% V pri 1100 C (7); c - cirkonijcva zlitina z 2.5% Nb pri 775°C. (Si vse pri označeni deformacijski hitrosti v s-1.
3. Specimens and methods of testing
After vacuum melting and chemical checking the ingots of lead allovs of 50 mm diameter and 10 mm high vvere pressed into rods of diameter of 5.9 111111 and cut into initial blanks of I I.5...1 l.S mm of lentgh. I11 fig. 4 press mould details used for the calibration of specimens and the simultaneous indentation of a regular microrelief on their butts in form of concentrical trian-
0 O-1 0,2 0,3 0,4 0,5 0,6 €
Figure 3: Rheologieai curves of technically pure lead SI (<W.98<3f Pb, "A" curvet and allo> s of 99.9% ("B" curve). 99.4% ("C" curve) and 99.0% lead ("D" curve). Slika 3: Reološka krivulja za tehnično čisti svinec SI (99.98% krivulja A) in zlitine z 99.99! (krivulja B). 99.4% (krivulja C) in 99.0% svinca (krivulja D).
barrel-shaped. the number of layers is inereased, if it becomes concave then their number is diminished.
Figure 5: Container for plastometrie tests by compression: I - upper die; 2 - body; 3 - specimen; 4 - lovver die.
Slika 5: Container za plastometrične tlačne preizkuse: 1 - zgornja matrica. 2 - ohišje, 3 - preizkušanec, 4 - spodnja matrica.
gular juts 0.3 mm high bv the plungers 2 and 4 are shovvn. The final dimensions of the specimen vvere: diameter - 6.0 mm and height - I 1.0 mm. Before the plastometrie tests the specimens vvere annealed at 100°C during an hour and then aged during thir-ty dav s at room temperature.
Fig. 5 shovvs the Container for the compression tests of cylin-drical specimens. The initial adjustment of the specimen 3 and the plungers 1 and 4 in the container 2 is obtained by means of a simple centring device. Polished vvorking surfaces of plungers and profiled butts of the specimens are covered vvith layers of viscous lubricant Litol 42 and separated by thin rubber. polyeth-v lene orpolvurethane foils. The number of layers of lubricant en-suring the homogeneitv of deformation vvas determined experi-mentallv. If in the process of compression the specimen becomes
Figure 4: Press mould for pressing and calibration of specimens for plastometrie tests: 1 - body; 2 - upper die; 3 - calibrating matrix: 4 - lovver die.
Slika 4: Orodje za stiskanje in kalibriranje preizkušancev za plastometrične preizkuse: 1 - ohišje. 2 - zgornja matrica. 3 - kalibracijska matrica, 4 - spodnja matrica.
4. The influence of contact frietion on the growth of deformation
In čase vvhen the radial displacement of metal on the vvorking surface of the plungers is hampered, the butts of the specimens are formed vvith the contact of it lateral surface and the plunger. Fig. 6b shovvs a pressed pure lead specimen covered vvith chalk on the initial contact area. The light eircle on the butt is the unstrained initial surface, dark circie is a pari of the contact area lifted from the lateral surface of the specimen. Also the initial form and the shape after a true homogeneous deformation of the specimen during plastometrie tests as a result of the proper lubrication of the contact area are shovvn in fig. 6.
In fig. 7 specimens of pure lead after sagging at plastometer vvith strain rate ir = 0.3 s 1 by identical parameters, bul different conditions of frietion on the contact are shovvn.The specimen 2 vvas pressed vvith dry contact surfaces, and a barrel shape vvas obtained. The specimen 3 vvas deformed homogeneously as a result of the optimal seleetion of the lubricant. The concave shape of the specimen 4 vvhich vvas obtained by inereasing the number of layers of lubricant to three vvith tvvo intermediate rubber separa-tors. In this čase the frietion force veetor changed to the opposite sign and the frietion became active in promoting the strong radial displacement of metal adjacent to the contact and the concave lateral surface of the specimen vvas obtained.

Figure 6: Heterogeneous (b) and homogeneous (c) deformation of pure lead samples SI (a - initial form of the sample, d - butt relief of the sample).
Slika 6: Heterogena (b) in homogena (c) deformacija preizkušancev i/ čistega svinca SI (a - začetna oblika preizkušanca, d - reliefna osnovni ploskvi valjastega preizkušanca).
D m ■ H
12	3	4
Aetivation of lubrication
M
B	C	D
Growth of rheology complexity
Figure 7: Strain development in pure lead samples by increase of lubrication of butts (2-4) and lead allovs specimens from fig. 3 by
different degree of rheological complexity (A-D). Slika 7: Razvoj dclormacije v preizkušancih i/, čistega svinca pr i povečanem H mazanju stičnih ploskev (2-4) in preizkušancev iz svinčevih zlitin s slike 3 pri različnih stopnjah reološke kompleksnosti (A-D).
Fvidentlv. by plastometric tests of rheologically simple ma-terials the significance of lubricant is very substantial. With modification of the conditions of friction on the contact surfaces it is possible to regulate the development of deformation and achieve its homogeneitv.
5. The role of metal rheological comple\it> in development of deformation
Let tis novv present some results of plastometric tests of rheological^ complex alloys at the same conditions (e=0.6; a=0.3 s 1) vvithout lubrication.
Fig. 7 (A-D) shovvs samples of the tested alloys by order of rheological complexity: A - pure lead, B - "B" alloy, C - "C" allov, D - "D" alloy. As in the previous series of tests the specimen of pure lead (A) is barrel-shaped. The shape of the specimen of allov "B" vvith a maximum (amaxh) on the rheological curve ;it it^ = 0.63 (see Fig. 3) is also barrel shaped. Since only a deformation belovv (r = 0.63 vvas achieved the rheological complexity of the alloy did not come to evidence and the specimen vvas deformed according to the simple strengthenable material of the second class. The specimen of "D" allov inspite of the sagging vvithout lubrication assumed a quite different shape, a strongly marked concavity of the lateral surface. The rheological curve of the allov "D" (third rheological class in Fig. 3) shovvs that if the deformation is inereased above the characteristic value. the resistance to deformation is decreased sharplv from <:rmax = 95 MPa at o\=0.23 to 60 MPa at e = 0.6. Simple calculations shovv that bc-fore the deformation h;ts embraced ali the specimen volume the triangular juts of it butts did de form to the extent of 6 = 0.4...0.6. This startcd a deformation vvith strengthening of the metal in the bulk volume of specimen and unstrengthening of metal in lavers adjaccnt to the contact. The resistance to deformation of these lavers in the very initial stage is smaller and tliev flovv in radial direetion vvith higher speed than the inner lavers forming the specimen with concave lateral surface. It should bc noted, that in spite of the small depth of the butt relief (0.3 mm) the volume of this unstrengthenable metal is sufficient lo initiate the deformation of metal more distant from the contact.
The deformation anomalies in the process of sagging of the alloy "C" (fig. 7C| is of special interest. The characteristic degree of deformation of the allov "C" is achieved by cr, = 0.43. Up to this moment the adjaccnt to contact volumes of metal, on ac-count of the greater deformation of the butt microrelief. are strengthened more intcnsivelv than those in the depth and the sample becomes barrel-shaped. Bv further sagging the deformation of metal lavers adjacent to the contact above <r, start to un-strengthen and in the last stage rush intcnsivelv in radial direetion outstripping the lav ers in the depth. The specimen acquires a specific shape: concavitv near the contact vvith the tool and bar-rel-shaping during the final stage of the process. It is concluded. therefore. that the deformation homogeneitv must be achieved during ali the test.
It is possible to eliminate deformation anomalies of higher orders at plastometric tests of rheological complex allovs by means of greater friction on the contact specimen-tool vvith the deposition of chalk. Also it can be achieved by seleeting the proper specimen butts asperitv.
Conclusions
1.The	development of deformation anomalies at plastometric tests depends substantiallv on the degree of metal rheological complexity. Deformation anomalies are caused by inadequate conditions of contact friction vvhile deformation anomalies of higher orders result from the primary deformation of specimen microrelief on the contact vvith the tool.
2.	The homogeneitv ,of deformation at plastometric tests of rheological^ complex metal s can be achieved by the proper roughness of the specimen surfaces in contact vvith the tool. Friction at this plače either prevents or promotes the radial dis-placement of the deformed metal.
3.	Bv plastometric tests of metals and allovs it is necessarv to control the deformation of the specimen during the vvhole process in order to reveal and eliminate intermediate deformation anomalies.
References
1 Poluhin P. I.. Gun (i. Ya. Galkin A. M.. Resistance to plastic deformation of metals and allovs. M. Metalhirgia. 1983. 352
; Trefilov V. Y.. Moisevev V. F.. Pechkovskv E. P.. Gornava M. D.. Vasiliev A. D., Strain hardening and failure of polverv stalline metals, Naukova dnmka. Kiev. 1989. 256. ' Shlomchack G. G.. Deformation features of rheologieall) comples metals, Deposited in UkrNIINTI 13.08.91. N I167.-Uk91. Kiev. 11. 1 Shlomchack G. G., Reological classes of materials. Internationa! con-ferenee: "Materials for Building" Works. Dnepropetrovsk, 1993. 69-70.
5 Shvvartsbart Y. S., Resistance to deformation of steels and allovs at continuous hot rolling. lzvestia AS USSR. "Menih". 1980, 1. 87-91. " Suzuki II., Report of Unst. of huluslrial Science the L'niversity of
Tokvo. IS. 1968. 3. 139-240. 7 Lehmann G.. Tiets A.. Nene /lune. 24. 1979. 9. 325-327. s The llol Deformation of Austenite (edited by J. B. Ballance) Al ME
New-York: 1977. 631. " Bailev f. Singer A.. ./. of Inst. of metals. 92. 1963-64. 5. 288-289; 12.404-408. "
"'Thomson E.. Young t'., Kohovashv. Se. Meehanics of plastic deformation at menil workint>. Transi, from English. M.. Mashinostroenie. 1969. 504.