titanium “shape-memory” bimetallic strips fabricated by explosive cladding and rolling

Materials Science and Engineering, A 188 (1994) 267-275

267

Macroscopic and microscopic phenomena of nickel/titanium "shape-memory" bimetallic strips fabricated by explosive cladding and rolling A. G. M a m a l i s ~, A . S z a l a y b, N. M. V a x e v a n i d i s " a n d D. I. P a n t e l i s ~ ~Department of Mechanical Engineering, National Technical University of Athens, Athens (Greece) bMetalltech Ltd, Budapest (Hungary) (Received October 18. 1993; in revised form December 3(I, 1993)

Abstract In the present paper we report the fabrication of titanium/nickel bimetallic components by explosive cladding and rolling. The effect of the cladding and rolling parameters on the macrostructural and microstructural properties of the fabricated bimetallics is evaluated in terms of "surface integrity" by using standard metallographic methods, microhardness testing, optical and scanning electron microscopy, and energy-dispersive spectrometry. Interest is also directed towards the detailed characterization of the intermetallic compounds formed at the interface since certain Ti-Ni phases exhibit the so-called ~shape-memory" properties, desirable for electrical and electronic applications.

1. Introduction

Explosive cladding is the process of forming a bond by explosively impacting a metallic mass onto another under controlled conditions. It is used to joint directly a wide variety of similar or dissimilar metals that cannot be joined by any other welding or bonding technique [1-31. Followed usually by forming, i.e. hot or cold rolling, punching and/or extrusion is applied in modern technology for the fabrication of composite metal laminates, replacing solid materials because of an urgent need for improved component performance under severe working conditions [1, 4, 5]. The fabrication of bimetallic components consisting of AI/Cu and Ag-CdC/Cu plates by explosive cladding and subsequent cold rolling has been investigated and the experimental results presented in refs. 6 and 7 respectively. In ref. 8, emphasis was directed towards the influence of rolling on the soundness of the fabricated bimetallics; a theoretical approach of the rolling of multilayered solids was also proposed. The present work is a follow-up of the research regarding the fabrication of multilayered composites by explosive cladding and cold forming outlined above; it is focused on an attempt to manufacture sound "memory" nickel/titanium materials by explosive cladding and cold rolling for electrical and electronic applications. The effect of the cladding and rolling parameters on the sound fabrication of the components and on the 0921-5093/94/$7.00 SSI)I 0921-5093(93)09520-7

macrostructural and microstructural properties of the fabricated bimetallics is evaluated in terms of ~'surface integrity" [9] by using standard metallographic methods, microhardness testing, optical and scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Interest was also directed towards the detailed characterization of the interface and the possible reaction and interdiffusion between nickel and titanium at this zone, since the possibly formed equatomic TiNi phase exhibits the so-called '~shapememory" effect [10].

2. Experimental details

The parallel arrangement, described in detail in refs. 3 and 6, was used for the experimental set-up of the small plate explosive cladding, schematically shown in Fig. 1. The parent plate consisted of AT8186 a-titanium containing 0.05 wt.% C and 1 wt.% Fe with an average density of 4.49 g cm ~. The flyer plate was made from a nickel alloy containing 0.5 wt.°/,, Cu, 0.5 wt.% Fe, 0.8 wt.% Mn, 1.5 wt.% Si and 0.8 wt.% C; its density was 8.34 g cm ~. The initial thickness of each plate was 2 mm, whilst their measured mean surface roughness values R~, were 0.53 /~m and 0.66 /~m for the parent and the flyer plate respectively. A driver plate of 1 mm thickness fabricated from mild steel was also used. © 1994

Elsevier Sequoia. All rights reserved

268

A. G. Mamalis et al. /

Macroscopic and microscopic phenomena of nickel/titanium "shape-memory" bimetallic strips

geometry of the collapse of the flyer p l a t e (detail A) IB J v stand-off

distance

I~1

[ - ~ . .. 1iF - d e t o n a t o r buffer ~i.: ::;." .#x'pfrs!'Ve(P'o~i't)j .ii-_~.-,~ / l l y e r plote(Ni) ,~_._~='''1 I " " I=d.~.~ _T[[~_Iparent p l a t e ( r i ) ' . . . . . . . . . A ~.~ onvi I

TABLE 1. Parameters for the present series of experiments Dimensions of the flyer plate (Ni) Dimensions of the parent plate (Ti) Dimensions of the driver plate (steel) Initial set-up angle a Collision angle/3 Stand-off distance Thickness he of the explosive Welding impact ratio R Flyer plate velocity vr

250 mm x 60 mm x 2 mm 256 mm × 70 mm × 2 mm 256 mm x 62 mm x 1 mm 0o 7.8° 2 mm 16 0.752 516ms -~

Fig. 1. Experimental set-up for explosive cladding.

Paxit 4 (78% ammonium nitrate, 20% trinitrotoluene and 2% dinitrotoluene) was used for the present series of experiments. Paxit 4 is a high energy explosive with a detonation velocity of 3800 m s- ~ at an initial density of 1 g cm-3; its detonation characteristics are not dependent on charge thickness. The operational parameters used and the corresponding collision and cladding variables for the present series of experiments, calculated from the detonation velocity and the characteristics of the plates according to standard procedures [1], are summarized in Table 1. After explosive cladding, strips of dimensions 70 mm × 30 mm cut from the clad bimetallic plates were cold rolled in a succession of passes in order to examine the influence of rolling parameters on the mechanical properties and the structural integrity of the bimetallic plates. Rolling of the rectangular strips was performed on an experimental two-high rolling mill, properly instrumented for roll force and torque measurements, at a constant speed of 4 m in- i between two steel rolls of 200 mm diameter and 100 mm barrel length; see ref. 8 for details. Two series of rolling passes were performed, the first in "dry" conditions and the second lubricated; in the latter case, soap emulsion was used as lubricant. The rolling direction was opposite to the cladding one. Rolling variables, force and torque measurements and surface roughness values for all rolled bimetallic strips are listed in Table 2. To evaluate the mechanical properties of the clad and rolled bimetallics the upsetting test was used. Upsetting tests were performed on an SMG hydraulic press connected with a data acquisition system. Details for the preparation of the test specimens and the testing procedure have been given in refs. 6-8. The flow stress-strain curves obtained for the clad composite and for the strips rolled with various thickness reductions as well as the relevant stress-strain curves of the initial test materials compressed under the same conditions are presented in Fig. 2. From these curves the mean yield stress was estimated to be 375 N mm -2,

247 N m m - 2 and 291 N m m - 2 for the Ni/Ti clad composite and the Ni and Ti initial materials respectively. In order to examine the weld quality and the microstructural changes at the interface after cladding and rolling, the fabricated bimetallics were sectioned in planes parallel to the detonation direction and transverse to it, and standard metallographic preparation and etching procedures were applied. The microstructural changes of the clad and rolled samples were observed on a metallographic microscope (Union Optical Co) and on a scanning electron microscope (JEOL) equipped with a system for EDS analysis whilst microhardness testing was performed on a Leitz microscope equipped with a Vickers indenter under a testing load of 1 N. The microhardness values obtained were the average of five indentations. A Talysurf (Taylor Hobson) recorder was used to measure the centre-line average surface roughness Ra; the cut-off length was selected at 0.8 mm whilst the roughness values were the average of ten measurements per specimen.

3. Results and discussion

3.1. Explosive cladding In general, under selected cladding conditions, the explosively cladded plates are expected to be adequately bonded over the entire area of the bimetallic composite except for small areas near their edges [7, 11]. Such a characteristic behaviour was verified during the present experimental work. At the inner edge of the bimetallic plate, an unbonded area was identified, the length of which varied from 3 to 5 mm; it was followed by a zone of waveless bonding of approximately the same length. Then, as the detonation wave proceeded towards the cladding direction, transition from smooth to turbulent flow occurred, resulting in the change in the initial straight interface to a "sea-wave'-like pattern with only traces of intermetallic compounds (Fig. 3(a); see also ref. 6). Note that such a waveform is often

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena of nickel/titanium "shape-memo 0''' bimetallic strips

269

TABLE 2. Rolling variables, force and torque measurements and surface roughness values Specimen

Thickness of bimetallic

Thickness reduction

Roll force per pass F R (kN)

Roll torque per pass 7"~ (N m)

Surface roughness

Conditions

h,

hi

ri

rr

RNi

R'n

(mm)

(mm)

(%)

(%)

(/~m)

(/2m)

TN0

4.00

.

0.66

0.53

TNR3 TNR3 TNR3 TNR3

4.02 3.97 3.92 3.72

3.97 3.92 3.72 3.29

1.2 1.3 5.1 11.3

1.2 2.5 7.5 18.0

20 38 67 11)8

206 540 1300 2227

0.66 0.65 0.57 0.53

0.52 0.52 0.49 0.47

Lubricated Lubricated Lubricated Lubricated

TNR2 TNR2 TNR2

3.87 3.84 3.67

3.84 3.67 3.42

0.8 4.4 5.2

0.8 5.2 10.0

18 80 94

----

0.66 0.60 0.54

0.53 0.50 0.46

"Dry" "'Dry" "Dry"

.

.

.

.

800 r

r.t=18 %

V zI

400 ~

."~ t""~rr~O% ,

~

200

"'//

o

I

I

0.2 Strain

I

0.4

I

I

0-6

Fig. 2. Compression stress-strain curves for the initial materials, the clad bimetallic plate and rolled bimetallic strips with various thickness reductions.

encountered in explosive cladding of metals with dissimilar densities and is indicative of a weld made at medium to lower limits of collision energy [1, 12]. Towards the middle of the plate, lengthwise, a poor asymmetrical wave formation with flattened vortices and molten pockets was identified (see Fig. 3(b)). From this point and thereafter, entrapped molten pockets tend to form a thin, almost continuous intermetallic layer, whilst towards the distal end of the plate this transition zone had an increased thickness and was also characterized by the presence of cracks and solidification defects, i.e. cooling cavities (see Fig. 3(c)). However, a clear characteristic defect such as unbonding was not observed in the case of Ni/Ti-clad bimetals; compare also with the opposite phenomenon which occurred in A1/Cu and A g - C d O / C u clad composites [6, 7]. In general, an overall length of about 180-200 mm was considered to be of a sound cladding and

bonding integrity from a macroscopic and microscopic point of view, despite the fact that a clear, fully developed wavy bond usually related to the most desirable bonding properties [11] was not apparent for the present case (see, however, ref. 12). In order to evaluate possible phase changes of the initial materials and to identify the formation of intermetallic compounds at the interface, X-ray analysis was performed on transverse cross-sections of the clad composite. A typical phase diagram obtained is shown in Fig. 4; peaks corresponding only to Ni and a-Ti phases are apparent, indicating that intermetallic Ti-Ni compounds, if present, are below the detection threshold of the X-ray analyser. Therefore, for the detailed metallographic characterization of the transition zone at the interface, SEM supported by semiquantitative EDS was applied. In general, from the possible reaction and diffusion between Ti and Ni elements at the interface zone, three different intermetallic compounds, namely TiNi3, TiNi and Ti2Ni, may be formed [13]. The existence of these phases was verified during the present investigation. A micrograph of an interfacial structure characterized by flattened vortices, molten pockets and discontinuous intermetallic layer is presented in Fig. 5(a) (compare this also with Fig. 3). The EDS diagrams corresponding to the interrupted intermetallics at the interface (point A in Fig. 5(a)) and to the molten island entrapped inside the nickel plate (point B in the same figure) are shown in Figs. 5(b) and 5(c) respectively. From these plots and taking also into account the phase diagram of Ti-Ni system [13], it is revealed that the interlayer (point A in Fig. 5(a)) consists of approximately 92% TieNi and 8% a-Ti whilst the entrapped intermetallic ribbon at the nickel side of the clad composite consists of equal percentages of TiNi~ and TiNi phases. Furthermore, the entrapped island inside the titanium plate (point C in Fig. 5(a)) was found to be of

270

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena of nickel/titanium "shape-memory" bimetallic strips

Ni e

i ntermetal islands

! ic

I

' I00

intermetallic

layer

IJm

Fig. 3. Micrographs of the interface of the Ni/Ti explosivelyclad plates: (a) sea-wave pattern; (b) asymmetricalwave pattern with flattened vortices; (c) continuous intermetalliclayer with coolingdefects.

tl-NI

ss:

Q. Q 3 N

era:

$.~

Cu]~lI+2

Ni

32.74I

x : 2theta

~ : 25593.

I, i n e a ~ ,

l~g.Q~l)

°

Fig. 4. X-ray analysis pattern of an Ni/Ti explosively cladded plate.

pure Ti2Ni phase whilst, on the other side, intergranular weakness and cracking were observed in the nickel flyer plate near the interface (see Fig. 5(a)); this is probably due to the existence of microsegregations and inclusions of alloying elements in the initial test materials.

Interfacial patterns with an almost continuous intermetallic layer, often encountered in the middle of the cladded composite and extended lengthwise, are presented in Fig. 6(a). As far as the composition of the intermetallic layer is concerned, from the EDS diagrams presented in Figs. 6(b) and 6(c) it is concluded that the intermetallic layer consists of a fine mixture of TiNi and Ti2Ni phases. The amount of TiNi is larger on the Ti side of the interlayer (about 55% at point A) (see Figs. 6(a) and 6(b)) than on the other side of the transition zone adjacent to the Ni plate (about 35% at point B)(see Figs. 6(a) and 6(c)). Moreover at certain points of the almost flat interface, through the thickness of the intermetallic layer, only the TiNi phase was detected; see point C in Figs. 6(a) and 6(d). The presence of pure TiNi phase is important since the shapememory response of the nickel-titanium alloys is based on the equatomic TiNi compound (see also ref. 10). SEM and EDS analysis were also supported by microhardness testing in order to evaluate deformation zones and microstructural changes through the thickness of the cladded composites. A typical Vickers microhardness profile of a Ni/Ti clad plate is shown in Fig. 7; it is evident that intense stress waves developed during the processing led to a hardness increase in both

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena o[ nickel/timnium "shape-memoo'" bimetallic strips

II

2 71

Ni

i i, Ji

Ni

Ti,

,I

!:~

I! ,'FS

OOQ

= 204--~

iO

24~

(b)

'I

I~

!

Ni

(G) Ni

-.:L,.,,.

.........

,E, T i T.. i ............. ,",'",

OO~

;;! ,F-S

Ni ..... =

f02~

10

.--4~

(c) Fig. 5. (a) Scanning electron micrograph of an Ni/Ti clad plate;(b) E D S diagram corresponding to point A; (¢) E D S diagram corresponding to point B.

plates. The Vickers hardness of the Ni plate measured after cladding at 1 mm distance from interface was 225 HV, increasing towards its peak value (260 HV) measured at the nickel side of the interface. As far as the parent Ti plate is concerned, besides the overall increase in hardness, localized shock hardening at the interface occurred; values of 2 8 0 - 2 9 0 HV were measured in the region adjacent to the interface. The existence of brittle hard Ni-Ti intermetallics at this zone was revealed by EDS analysis. Note also that another shock-hardened zone developed at the surface layers, near the free end of the titanium plate (see Fig. 7). The size of this zone exceeded 400/~m and its development may be attributed to the transient stress waves produced by the explosive charge and propagating through the crosssection of the parent plate; see also the similar remarks in ref. 14. As far as the overall strength of the cladded composites is concerned, from Fig. 2 it may be concluded that a macroscopic acceptable weld was produced since the clad plate is stronger than the two components. Note, however, that in general the clad plate is

expected to be stronger than the softer of the two initial metallic components [1, 3, 5]; this was also verified during our previous investigations [6-8]. The strength increase of the resulted composite plate may be attributed to the influence of shock and work hardening and to the existence of very hard intermetallics at the interface.

3.2. Rolling Multiple-pass cold rolling was selected as the postwelding forming operation for shaping clad plates to their final dimensions. Small passes were selected in order to avoid high rolling forces and to obtain better dimensional accuracy. Note that small passes are associated with the development of compressive residual stresses [9] which in turn favours the integrity of the rolled product. Reductions in thickness up to 10% and 18% were obtained for "dry" and lubricated conditions respectively (see Table 2). Failures occurring during compression-like working processes such as rolling are predominantly caused by secondary tensile stresses [15]. In addition, problems and difficulties arising from the previous process, i.e.

A. G. Mamalis et al.

272

/

Macroscopic and microscopic phenomena of nickel/titanium "shape-memory" bimetallic strips

Ti

~: il

Ni

il

Ii I

~,

Ni ....................

JbL

. . . . . . . . . . . . .

VFS

000

A

: 2048

10 2 4 0

(b) ,T i !

,LI =

Ni

;!~

T i

,

Ti

Ni

!~

~ITi

Ni

Ni

oeo

'JF~

1024

.~,

i 10 2 ~ 0

Ti

~i T i '~ ;':L

0 00~

Ni ,iii

~!i,

Ni

/ ~ ,~, '.P-~ : 1Oa-.-2

~.~

~_4-0

(d)

(c)

Fig. 6. (a) Scanning electron micrograph of the interface of an Ni/Ti clad plate at half-length;(b) EDS diagram corresponding to point A; (c) EDS diagram corresponding to point B; (d) EDS diagram correspondingto point C.

Ni

Ti - - i n t e r f a c e

300

~o

,.A

, ~ =\, , o .. , , ~. ,o/to . %p.= \'~ o u-- o,O

: 2so c

u

:~ 200

initial microhardness I

2

I

1 Distance

I

Ni

Ti

,

o

•

rr=O

•

[] r , = 7 . 5 %

~

• I

0 across

rT=18 I

*/o % I

1 thickness

(mm)

Fig. 7. Microhardness profiles of explosively clad and rolled Ni/ Ti bimetallic plates.

the explosive cladding, associated with certain structural changes and material properties of the bimetallic plate are sometimes intensified during subsequent rolling; see, for example, the remarks in ref. 6. However, during the present investigation cladding resulted in the fabrication of bimetallic plates with sufficient bonding integrity and no major defects. Therefore, during the subsequent rolling, defects such as bond breakage, alligatoring, centre-splits and distorted end shapes were not observed. Since the elongation of the nickel layer of the bimetallic strip was greater than that of the titanium layer, the rolled strip was slightly curved with the nickel side becoming convex; this phenomenon was more profound in "dry" rolling. Note also that, in general, atitanium is only slightly deformable under cold-rolling conditions. Typical structures at the Ni-Ti boundary of the clad plates after rolling at an intermediate (rT= 7.5%) and

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena qf nickel/titanium "shape-memoo'" bimetallic strips

Ni

Ni

interface

,

(a)

(b)

y

-

Ti

Ti

flattened i ntermetallic regions

y

roiling direction 'I00

lam

Fig. 8. Micrographs showing the microstructure at the interface of Ni/Ti rolled bimetallic strips: (a) r r = 7.5%; (b) r r = 18°/,.

273

the final (r v = 18%) total thickness reductions are presented in Figs. 8(a) and 8(b) respectively. It is shown that, during rolling, the deformed interface becomes less wavy whilst Ni grains are slightly elongated and oriented parallel to rolling direction. Similarly intermetallic regions at the front and the back ends of the waves are flattened (see Fig. 8(b)). EDS analysis supported by SEM was also performed on rolled bimetallic strips in order to examine the effect of rolling on the intermetallic diffusion zone. In general, EDS patterns obtained are qualitatively similar to the ones corresponding to merely clad plates; no additional phases were detected. A typical interracial structure with deformed vortices and elongated intermetallics is presented in Fig. 9(a). The EDS diagram corresponding to the intermetallic ribbon adjacent to the Ti side of the interface (point A in Fig. 9(a)) and to the deformed vortex (point B in the same figure) are shown in Figs. 9(b) and 9(c)

Ti miD,.

i

~00

/

i'oIling direction

(a)

v'~E

: 20a-~

iD

2a-C

(b)

Ti Ni ii

I~,

Ti

.;/T i

Ti

(c)

Ti

Ni

:~i

i

(d)

Fig. 9. (a) Scanning electron micrograph of the interface of a rolled Ni/Ti strip (r I = 18%); (b) EDS diagram corresponding to point A; (c) EDS diagram corresponding to point B; (d) EDS diagram corresponding to point C.

274

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena of nickel/titanium "shape-memory" bimetallic strips

Fig. 10. Scanningelectron micrographs of the interface of rolled Ni/Ti bimetallicstrips: (a) rr= 10%; (b) rT= 18%.

The variation in microhardness through the thickness of the rolled bimetallic strips is illustrated in Fig. 7. At the transition zone near the interface of the Ni layer the microhardness remains almost unaffected; a slight increase with increasing thickness reduction was observed at the intermetallic zone at the Ti side of the interface. However, the effect of work hardening due to rolling is more profound in the interior in both the Ti and the Ni layers towards their outer surface. A clear microhardness increase was observed in the Ni plate with increasing thickness reduction (see Fig. 7), whilst on the other side the shock-hardened zone observed near the free-end of the Ti plate spread towards the entire thickness of this plate, probably owing to the penetration of the deformation zone through the whole thickness of the strip. Furthermore, the excessive work hardening due to multiple-pass rolling results in a considerable improvement in the overall strength of the bimetallic composite; compare the stress-strain curves in Fig. 2. As far as the surface morphology of the rolled strips is concerned, it was found that the directional mechanical process of rolling does not impart any other impressive surface alterations; a decrease in surface roughness R a with increasing total thickness reduction rT was obtained; see Table 2 and also the similar remarks in refs. 6-9. The variation in roll force and torque with total thickness reduction for a number of passes is presented in Fig. 11. It is shown that both the total roll force and the total roll torque increase with increasing total thickness reduction. Moreover, rolling with lubrication leads to a considerable reduction in the roll force (see also Table 2). 4. Conclusions

respectively. From these plots and taking also into account the Ti-Ni phase diagram [13], it is evident that the intermetallic layer at the interface (point A) consists of approximately 67% Ti2Ni and 33% a-Ti. The deformed vortex consists of almost pure a-Ti whilst the elongated intermetallic ribbon entrapped inside the Ni side of the interface (point C in Fig. 9(a)) consists of equal percentages of TiNi 3 and TiNi phases; compare with the similar observations for the clad plates outlined above and see also Figs. 5(a) and 5(b). Towards the distal end of the plate the directional mechanical rolling process results in a partial delamination of the continuous intermetallic layer which is more profound at greater thickness reductions and in an almost flat interface (Fig. 10). EDS analysis revealed that in this case the elongated laminates of the intermetallic consist of almost pure TiNi phase desirable for the shape-memory response of the bimetallic (compare also with Figs. 6(a) and 6(d)).

Summarizing the main features outlined above on the influence of the explosive cladding and rolling parameters on the integrity of fabricated bimetallics, the following conclusions may be drawn. (a) The use of high energy explosives and the proper processing parameters results in explosive cladding of Ni/Ti sound bimetallics with an overall strength considerably higher than that of the initial materials. (b) Non-fully developed wavy patterns at the interface as well as the formation of intermetallic Ni/Ti phases diffused from the flyer and parent plate, were revealed using EDS analysis and optical and scanning electron metallographies. Transition zones near the interface are shock hardened with only minor defects, resulting in a sufficient bonding integrity. (c) Cold rolling performed subsequently to cladding results in the flattening of the interface and in work

A. G. Mamalis et al.

/

Macroscopic and microscopic phenomena of nickel~titanium "shape-memory'" bimetallic strips

300

+ FR_ "d r y" • FR- l u b r i c a t e d ~TR"

E

i 6

z

/

~200

Q

rr"

/

@"

," / /' ."

d o

/"/

0

///o

z

4

~

O"

•/

./

~ ~00

0

2 ~.

-6

,o

-6 5

]T R of 3 ra p a s s

0

I

0 Totat

10 20 thickness reduction r T (*/o)

Fig. 11. Variation in roll force and torque with total thickness reduction of rolled Ni/Ti bimetallic strips for a number of passes.

hardening of the c o m p o s i t e plate without any metallurgical changes a n d / o r defects. (d) A s far as the formation of a continuous intermetallic layer of TiNi phase with " s h a p e - m e m o r y " properties is concerned, this phase was only detected at certain regions of the interface together with other N i - T i intermetallic c o m p o u n d s .

References 1 B. Crossland, Explosive Welding of Metals and its Applications, Clarendon, Oxford, 1982. 2 V. D. Linse and N. S. Lalwaney, Explosive welding, J. Met., 36 (5)(1984) 62. 3 R. A. Patterson, Explosive bonding, Welding." Theory and Practice, Elsevier, Amsterdam, 1990, p. 265. 4 R. Hardwick and F. Weld, Some more recent advances in cladding technology, Proc. 9th Int. Conf. on High Energy Rate Forming, Acad. of Sci. USSR, Novosibirsk, 1986, p. 270. 5 N. V. Naumovich, A. 1. Yaderich and N. M. Ghrigrinova, Explosion welding: parameters, structures, properties and

275

applications of bimetals, Shock Waves for lndustrial Applications, Noyes, Park Ridge, NJ, 1988, p. 270. 6 A. G. Mamalis, N. M. Vaxevanidis, A. Szalay and J. Prohaszka, Fabrication of aluminium/copper bimetallics by explosive cladding and rolling, J. Mater. Process. Technol., 42 (1994). 7 A. G. Mamalis, J. Prohaszka, N. M. Vaxevanidis and A. Szalay, On the manufacturing of Ag-CdO/Cu bimetallics by explosive cladding and rolling, Int. J. Mater. Prod. Technol. (1994) in press. 8 A. G. Mamalis, N. M. Vaxevanidis and D. I. Pantelis, On the rolling of bimetallic explosively cladded plates, Proc. 4th Int. Conf. on the Technology of Plasticity, Forging and Stamping Institute of Chinese Society of Metals, Beijing, 1993, p. 874. 9 A. G. Mamalis, N. M. Vaxevanidis and A. P. Karafillis, Surface Integrity and Formabifity of Steel Sheet, VDI, Dtisseldorf, 1990. 10 K. Otsuka and K. Shimizu, Pseudoelasticity and shape memory effects in alloys, Int. Metall. Rev., 31 (3)(1986) 93. 11 E V. Vaidyanathan and A. R. Ramanathan, Design for quality explosive welding, J. Mater. Process. TechnoL, 32 (1992)439. 12 A. Szecket, O. T. Inal, D. J. Vigueras and J. Rocco, A wavy versus straight interface in the explosive cladding of aluminum to steel, J. Vac. Sci. Technol. A, 3 (6) (1985) 2588. 13 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 14 S. A. Meguid, On the explosive hardening of one end of a metallic block, Int. J. Mech. Sci., 18 (1976) 351. 15 A. G. Mamalis and W. Johnson, Defects in the processing of metals and composites, in M. Predeleanou (ed.), Computational Methods for Predicting Material Processing Defects, Elsevier, Amsterdam, 1987, p. 231.

Appendix A: Nomenclature FR he hi h. ri rx R Ra RN~ Rxi TR Vp a fl

roll force thickness of the explosive thickness of the bimetallic after the ith pass thickness of the bimetallic before rolling thickness reduction ratio of the bimetallic after the ith pass total thickness reduction ratio of the bimetallic welding impact ratio mean surface roughness surface roughness of the nickel layer surface roughness of the titanium layer roll torque velocity of the flyer plate initial set-up inclination angle collision angle