An investigation of atomic diffusion at the interface of explosion welded CuNi couples

Materials Science and Engineering, 20 ( 1975 ) 251--260

@ Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

An Investigation of Atomic Diffusion at the Interface of Explosion Welded Cu--Ni Couples

M.D. NAGARKAR and S.H. CARPENTER Department o f Metallurgy and Materials Science, University o f Denver, Denver, Colorado 80210 (U.S.A.)

(Received in revised form March 26, 1975)

SUMMARY The effects of the explosion welding process on the kinetics of diffusion at the explosion weld interface of Cu - Ni couples were investigated by electron probe microanalysis and optical and transmission electron microscopy. Commercially roll-bonded composites of Cu Ni were explosion welded to each other and subjected to diffusion annealing between 500°C and 975°C for ten hours. Diffusion measured at the explosion weld interface was compared with that measured in commercial roll-bonded composites, subjected to similar annealing. Extensive diffusion was observed in the explosion welded samples as compared with the roll-bonded composites. The enhancement of diffusion is attributed to a thermally stable defect structure produced as a result of the intense plastic deformation and metal flow at the explosion weld interface.

INTRODUCTION Explosion welding is basically a solid-phase welding process, in which explosives are used to accelerate the parts to be joined into a high velocity oblique collision. During the collision a jet or spray of metal is formed at the apex of the collision, which is forced outward from the colliding parts at a very high velocity. In the explosion welding process the jet is the mechanism which produces a break-up and effacement of the surface films. The explosion welding process can be essentially considered as a two-step process: first, the jet breaks up and cleans the inhibiting surface layers and, second, the high pressure from the explosion

forces the clean metal surfaces into such intimate contact that interatomic forces are established across the interface effecting the weld. During the past decade the explosion welding process and its application have been the subject of numerous investigations. Several excellent review articles on the process can be found in the recent literature [1 - 4]. Explosion welding has now evolved to the point where it is now accepted as a practical and useful metals-joining technology. One of the main advantages associated with explosion welding is the ability to weld and/or clad dissimilar metal combinations. It has been shown t h a t even metals which are completely immiscible with each other can be successfully joined to one another in an explosion welding operation [5]. A review paper has listed well over 200 different metal combinations which have been successfully explosion welded [2]. The welding operation is usually judged successful if the metal parts bond to each other and the desired joint strength is obtained. However, to adequately insure the integrity of the explosion weld in a service application one must consider how the weld will react under prolonged service conditions. The explosion weld is normally characterized by a wavy profile along the interface. The major portion of the bond interface appears to be void of any melt or diffusion with an extremely sharp transition from one metal to the other. If service conditions of the weld should include being subject to elevated temperatures, then there should be thermally induced diffusion reactions across the interface. A preliminary result has indicated t h a t the diffusion across an explosion weld interface is significantly enhanced over that measured in

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conventional diffusion experiments [6]. This result may be due to the unique nature of the explosion welding process which involves extremely high pressures, high temperature of short duration and plastic deformation to the point of fluid flow. The purpose of this paper is to present results of an investigation of atomic diffusion across explosion welded interfaces. The results of this investigation as discussed below show that indeed there is a significant enhancement of the diffusion over that measured in conventional diffusion couples.

EXPERIMENTAL

In an effort to make this investigation as practical and meaningful as possible it was decided to investigate the diffusion reactions in both commercially roll-bonded and explosion welded metal couples of identical materials. Texas Instruments of Attleboro, Mass. kindly supplied roll-bonded composites of Cu - Ni. The

Cu - Ni roll-bonded composite consisted of a 50-mil layer of OFHC copper and a 50-mil layer of 201 grade nickel. To obtain explosion welds of identical metals, coupons were cut from the roll-bonded stock and were then explosion welded to each other. The final sample consisted of a four-layered sample similar to the one shown in Fig. 1. The test coupons contained three interfaces as labeled on Fig. 1: (1) a roll-bonded interface in the coupon used as the flyer plate; (2) the explosion weld interface; (3) a roll-bonded interface in the coupon used as the base plate. The diffusion across all three interfaces was investigated and compared with that measured in a plain roll-bonded sample. Test welds were made using the parallel plate arrangement with a stand-off equal to the thickness of the flyer plate. Coupons were fabricated by both welding the Cu onto the Ni and welding the Ni onto the Cu. Explosive loadings of 1.24 g/cm 2 and 1.86 g/cm 2 of Du Pont Red Cross Extra (40%) d y n a m i t e were

Ni

Interface i

Cu

Interface 2

i

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Interface 3

Cu

Fig. 1. P h o t o m i c r o g r a p h o f a n e x p l o s i o n w e l d e d s a m p l e f a b r i c a t e d f r o m r o l l - b o n d e d c o m p o s i t e m a t e r i a l . I n t e r f a c e i - Roll Bond. Interface 2 - Explosion Weld. Interface 3 - Roll Bond. 30X

253

used. The 1.24 g/cm 2 loading is typical of that used in a standard operation for welding Cu and Ni. The 1.86 g/cm 2 loading is somewhat in excess of what is needed for a satisfactory weld. Calculations of the pressure developed at the collision point using the Gurney formulation [7,8] yield 40 kbar and 55 kbar for the 1.24 g/cm 2 and 1.86 g/cm 2 loadings, respectively. After fabrication, the welds were sectioned into small coupons which were heat treated at 500°C, 750°C, 900°C and 975°C for 10 hours. The heat treating was carried out in a vacuum of better than 5 × 10 -6 Torr. The annealed samples were sectioned, mounted and polished prior to measuring the diffusion zone width using an electron microprobe. Microprobe traverses were made perpendicular to the interface with at least 5 traverses across each interface for each set of experimental conditions. On account of the wavy nature of the explosion weld interface traverses were made across both the trough and crest of the wave.

RESULTS

Diffusion coefficients were calculated from the electron microprobe data using a computer program to apply the Boltzmann - Mantano analysis. The results obtained for a sample welded with 1.24 g/cm 2 of dynamite are shown in Fig. 2. Data show the diffusion coefficient of copper at 50 atomic per cent copper. Also included in Fig. 2 are results from the plain roll-bonded sample and results of conventional diffusion experiments from the literature [9]. The straight line fit was obtained by the least squares method. Similar data from welds with different experimental conditions and at different concentrations are shown in Figs. 3 and 4. From these data plots the frequency factors and activation energies for the diffusion process were calculated. The results are summarized in Table 1. From a careful examination of the diffusion data the following conclusions can be made:

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1. The diffusion across the explosion weld interface is significantly enhanced over that measured for the other interfaces. This is a result of the lower activation energy. 2. The diffusion measured at the wave troughs in the explosion weld interface is consistently greater than that measured at the wave crests. 3. The diffusion across the roll-bonded inter faces, i.e., the plain roll-bond interface, the roll-bond interface in the flyer plate and the roll-bond interface in the base plate, is approximately of the same magnitude. However, there are slight but consistent differences which show diffusion across the roll-bond in the base plate highest, the diffusion in the plain roll-bond lowest and the diffusion in the roll-bond in the flyer plate intermediate. The results stated in number two (2) above are interesting. To avoid confusion it is necessary to define what is meant by the wave crest and trough. The trough of the wave is defined as that region of the wave where there is m a x i m u m penetration of flyer plate material into the base plate. Likewise the crest of the wave is defined as that portion of the wave where there is m a x i m u m penetration of base plate material into the flyer plate. If care is taken to always view the wavy interface with the flyer plate on top, then according to the above definition, the crests will be the upper portions of the wave and the troughs the lower portions. The data of Table 1 indicate that diffusion at the trough is consistently

TABLE 1 A c t i v a t i o n energies a n d f r e q u e n c y f u n c t i o n s Explosive l o a d i n g (g/cm 2 )

C o p p e r - nickel interface

F r e q u e n c y factor, D O (cm2/sec)

Activation energy (kcal/mole)

None None

Plain r o l l - b o n d L i t e r a t u r e ( r e f e r e n c e 9)

1.89 X 10 - 1 + 22% 7.24 X 10 - 1

53 61

-+ 3.5%

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1.70 1.1 2.01 4.33

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greater than t h a t measured at the crest, even if the flyer and base plate materials are interchanged, i.e., Cu - to - Ni or Ni - to - Cu. This fact suggests that significantly different conditions might exist at the troughs compared with those at the crests during the wave formation, resulting in a difference in defect structure. To examine this possibility in more detail it would be necessary to investigate the defect structure over a complete wave length. Such an investigation is extremely difficult and is an extension of the work reported in this paper. Care should be exercised in generalizing the results of greater diffusion at the troughs to other systems. While the results are consistent for nickel and copper, two metals with fairly similar physical properties, no data of this type exist for explosion welds of two metals with widely differing physical properties. It was suspected that the measured enhanced diffusion observed at the explosion weld interface could be an. artifact of the interface geometry, i.e., comparing the wavy profile of

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the explosion weld with the planar roll-bonded interface. To investigate this possibility an explosion weld was fabricated with a planar interface. This was accomplished using a symmetric angle configuration giving an explosion weld interface as shown in Fig. 5. Test specimens were cut, annealed and the diffusion measured in an identical manner to that used on the original specimens. Diffusion constants, activation energies and frequency factors were then measured for the planar interface and compared with the data generated from the wavy interface welds. A comparison of the data showed diffusion across the planar explosion weld interface to be just slightly less than that measured for the wavy explosion weld interface and significantly enhanced over t h a t measured for the roll bonded specimens. The results clearly demonstrated that any geometrical effects are small and that the enhancement in diffusion is a result of the explosion welding operation.

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256 DISCUSSION

The primary result of this investigation is the sig0ificant enhancement of the diffusion measured across an explosion welded interface compared with that measured across a conventional roll-bonded interface. It is to this primary result that discussion will be directed. In the present paper we will not a t t e m p t to discuss the differences observed in diffusion at the trough and crest or the differences observed in the roll-bonded interfaces. As mentioned earlier, during the explosion welding process extremely high pressures are generated causing intense plastic deformation, to the point of hydrodynamic flow of the mating surfaces. The intense plastic deformation in turn causes high temperatures at the interface which are rapidly quenched. The intense metal flow at high strain rates should produce both a high dislocation density and high concentration of vacancies as well as other lattice defects. Transmission electron microscopy investigation o f explosion weld interfaces has shown dislocation densities of the order of 1011 dislocations per square centimeter along the interface [10]. The existence of an intense defect structure along the explosion weld interface could help explain the enhanced diffusion due to the essential role defects play in atomic diffusion processes in crystalline solids. One would not expect this in the present investigation owing to the temperature (750 - 975°C) at which the diffusion was measured. These temperatures are in excess of the recovery temperature of both metals and one would expect a completely recovered structure along the interface. A recent investigation by Wittman [11] has, however, provided evidence that a thermally stable defect structure is produced at the weld interface in the explosion welding operation. In his study of the effects of heat-treatment on the mechanical properties of explosion welded 6061 A1 alloy, Wittman found that the high rate deformation via explosion welding was more effective in extending the mechanical properties than was conventional deformation following post-welding heat-treatments. Comparison was made following isochronal anneals up to 400°C, b e t w e e n the weld tensile strength of explosion welded alloy and the ultimate strength of cold-reduced alloy. Both the materials possessed the same initial hard-

ness and strength prior to annealing. It was found that the explosion welded specimen retained a higher level of strength than did the cold-rolled material, the effect being more pronounced when the material was heat-treated to higher temperatures of anneal. The retention of explosion weld strength even following anneals at high temperatures was attributed to hardening b y likely interaction of precipitated solute atoms and thermally stable dislocation networks. To examine if such a thermally stable defect structure was also present in the annealed copper - nickel explosion welded specimen used in this study, transmission electron microscopy was carried o u t on thin foils prepared from the weld interface area. By using the ion-bombardment technique, it was possible to obtain thin foils from regions at the weld interface. Roll-bonded samples were also thinned and examined to give a meaningful comparison. Figure 6a is a transmission electron micrograph of a section in the copper side of the copper - nickel explosion welded specimen in the as-welded condition and prior to annealing. The area shown in the micrograph is a b o u t 10

Fig. 6. T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s o f c o p p e r . (a) C o p p e r - n i c k e l - e x p l o s i o n w e l d e d s p e c i m e n in asw e l d e d c o n d i t i o n . A r e a s h o w n is w i t h i n d i f f u s i o n z o n e a n d 0 . 0 1 9 m m f r o m t h e weld i n t e r f a c e .

257

microns from the weld interface. A very dense network of dislocations can be seen. The dislocations appear in dense tangles and do n o t seem to be arranged in any definite cellular pattern. Similar dislocation densities have been observed by others [10] in explosion welded interfaces. They estimate the density to be as high as 1011 dislocations cm -2. Figure 6b also characterizes a specimen in the as-welded condition, but depicts the substructure away from the weld interface. The area is approximately 1 millimeter away from the weld interface and is at the copper - nickel roll-bond interface of the cladder plate. The substructure reveals what appears to be mechanical twins in a matrix containing dislocations, similar to those observed by other workers. After subjecting the explosion welded couple to a diffusion anneal of 500°C for ten hours, the changes in substructure of an area 25 microns from the interface can be seen in Fig. 6c. The dislocation density is still very high. The microstructure of the plain roll-bonded specimen after the 500°C anneal in Fig. 6d is, however, totally different from that found in an explosion

Fig. 6(b) C o p p e r - nickel e x p l o s i o n w e l d e d s p e c i m e n in as-welded c o n d i t i o n . A r e a s h o w n is 1 m m a w a y f r o m t h e i n t e r f a c e a n d close t o t h e c l a d d e r - p l a t e rollb o n d interface.

Fig. 6(c) C o p p e r - nickel e x p l o s i o n w e l d e d s p e c i m e n after a n n e a l i n g at 5 0 0 ° C for 10 hours. A r e a s h o w n is 0 . 0 2 5 m m f r o m t h e nickel side of t h e i n t e r f a c e .

Fig. 6(d) C o p p e r - nickel r o l l - b o n d e d s p e c i m e n a f t e r a n n e a l i n g at 5 0 0 ° C for 10 h o u r s .

258

welded specimen. There is evidence of recrystallization and the dislocation density is low. Average subgrain size is a b o u t 0.25 micron and the boundaries seem well-defined. Figure 6e of the explosion welded specimen shows a significant change in the substructure after the 750°C anneal. The dislocation density is still high in some of the grains but annealing has resulted in some recovery and recrystallization. The dislocations are rearranged in subgrain boundaries. I n comparison, in Figure 6f the plain roll-bonded material exhibits a larger subgrain size of a b o u t 0.5 micron, with a greater percentage of recrystallized grains. The difference in the substructure at the explosion welded interface and the roll-bonded material is even more significant at 900°C and 975°C, as seen in Figs. 6g - 6j. It is unusual to find the presence of a large number of dislocations even after annealing to such high temperatures. Dislocations appear to be trapped in sub-boundary walls, The roll-bonded specimen, however, presents a well-annealed structure with a low dislocation density. The results shown in Fig. 6 clearly indicate a defect structure of dislocations along the

Fig. 6(e) C o p p e r - nickel explosion welded s p e c i m e n after annealing at 7 5 0 ° C for 10 hours. A r e a s h o w n is 0.020 m m f r o m the nickel side o f the interface.

Fig. 6(f) C o p p e r - nickel roll-bonded s p e c i m e n after annealing at 7 5 0 ° C for 10 hours.

Fig. 6(g) C o p p e r - nickel explosion welded s p e c i m e n after annealing at 9 0 0 ° C for 10 hours. Area s h o w n is 0.025 m m f r o m the nickel side of the interface.

259

Fig. 6(h) Copper - nickel roll-bonded s p e c i m e n after annealing at 9 0 0 ° C for 10 hours.

Fig. 6(j) C o p p e r - nickel roll-bonded specimen after annealing at 9 7 5 ° C for 10 hours.

sub-boundary walls at the explosion welded interface, even after annealing to 975°C. The presence of the dense dislocation tangles along the sub-boundaries is believed to be the most likely cause of the enhanced diffusion. Accelerated diffusion along subboundaries is well d o c u m e n t e d in the literature [ 12,13 ].

CONCLUSIONS

Fig. 6(i) Copper - nickel explosion welded s p e c i m e n after annealing at 9 7 5 ° C for 10 hours. Area s h o w n is 0.018 m m f r o m the nickel side of the interface.

It is believed that the following conclusions can be drawn from the present investigation: 1. There is a significant enhancement of the high temperature atomic diffusion across an explosion weld interface of Cu - Ni compared with that observed in roll-bonded couples of identical materials. 2. Transmission electron microscopy of the explosion weld interface shows a large number of dislocations trapped at sub-grain boundaries along the explosion weld interface after annealing at high temperatures. By comparison, the roll-bonded sample exhibits a well annealed structure with a low dislocation density after identical anneals.

260

3. It is believed that the thermally stable defect structure along the explosion weld interface is a result of the unique conditions caused by the explosion welding process. It is also believed that the existence and stability of the defect structure at high temperatures are the major causes of the enhanced diffusion.

REFERENCES 1 B. Crossland and J.D. Williams, Met. Rev., 15 (1970) 79. 2 U.D. Linse, R.H. Wittman and R.J. Carlson, Defense Metals Information Center, Memo No. 225, 1970. 3 S.H. Carpenter and R.H. Wittman, Univ. Denver Res. Inst. Internal Rept. on the Theory and Application of Explosion Welding, 1972.

4 A.A. Ezra, Principles and Practice of Explosive Metalworking, Industrial Newspapers Ltd., London, 1973, p. 173. 5 H.E. Otto and S.H. Carpenter, Welding J., 51 (1972) 467. 6 L. Trueb, Met. Trans., 2 (1971) 145. 7 R.W. Gurney, Rept. No. 405, Ballistic Res. Lab., Aberdeen Proving Ground, MA, 1943. 8 A.A. Ezra, Principles and Practice of Explosive Metalworking, Industrial Newspapers Ltd., London, 1973, Chap. 10. 9 M.S. Anand, J.P. Murarka and R.P. Agarwala, J. Appl. Phys., 36 (1965) 3860. 10 L. Trueb, Trans AIME, 242 (1968) 1057. 11 R.H.Wittman, Metallurgical Effects at High Strain Rates, AIME, New York, 1973, p. 669. 12 A.L. Ruoff and R.W. Balluffi, J. Appl. Phys., 34 (1963) 1848. 13 G. Love and P.G. Shewmon, Acta Met., 11 (1963) 899.