Growth rate of intermetallic compounds in Al/Cu bimetal produced by cold roll welding process

LJournal of Alloys and Compounds 319 (2001) 233–241www.elsevier.com/ locate / jallcom

Growth rate of intermetallic compounds in Al /Cu bimetal produced bycold roll welding process

a b , a*M. Abbasi , A. Karimi Taheri , M.T. SalehiaMetallurgical Engineering Department, Iran University of Science and Technology, Narmak, Tehran, Iran

bMaterial Science and Engineering Department, Sharif University of Technology, Azadi Ave., P.O. Box 11365-9466, Tehran, Iran

Received 25 September 2000; accepted 4 December 2000

Abstract

The aim of this article is to study the growth rate of intermetallic compounds at the interface of cold roll bonded Al /Cu bimetal at2508C and compare the results with a similar study performed on friction welding of Al to Cu. Samples of tri-layered Cu–Al–Cucomposite were produced by the cold roll welding process and heat treated at constant temperature of 2508C for 1 to 1000 h. Thethickness, morphology and composition of the intermetallic compound at the interface of Cu and Al were studied by optical and scanningelectron microscope and EDX analyzer. The presence of various intermetallic compounds (Cu Al, Cu Al , CuAl and CuAl ) was3 4 3 2

detected and the priority of formation of each compound was studied. The bond strength and electrical resistivity of different sampleswere measured by peeling test and a high precision micro-ohmmeter, respectively. The variations of resistivity and bond strength versusthickness of intermetallic compound were plotted. It was observed that the strength and electrical conductivity are greatly reduced byincreasing the thickness of intermetallic compounds. Moreover, the growth rate of intermetallic compounds in roll welded bimetalcomposite is lower compared with that reported for similar friction welded specimens. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Aluminum; Copper; Roll welding; Intermetallic growth; Bond strength; Bimetal resistivity

1. Introduction Cu are incompatible metals because they have a highaffinity to each other at temperatures greater than 1208C

Metallurgically welded Al to Cu is widely used as a and produce brittle, low strength and high electricaltransition piece in high direct-current bus systems to resistance intermetallics on their interface [2,3]. Thus,transmit the electricity [1]. Such a bimetal transition piece fusion-welding processes are not applicable for welding ofis shown in Fig. 1 and acts as a connecting bridge between Al to Cu and solid state welding processes such asthe copper terminal and the aluminum bus bar conductor. explosion, friction and cold roll welding have been consid-

2Each bimetal with 20 cm connecting surface can conduct ered as the qualified welding processes of these metals.2a high electrical current up to 2000 amp (100 amp/cm ) Although, according to the reports of previous authors,

[1]. If a copper terminal and aluminum bus are connected during both the friction and roll welding processes, theto each other directly, without an Al /Cu bimetal transition formation of intermetallic compounds are inhibited [1–6],piece, the service life of the electrical connection is about 1 however, the interface of solid state welded Al /Cu isyear, but by using Al /Cu bimetal the service life is susceptible to the nucleation and growth of intermetallicincreased to more than 10 years [1,2]. In the manufacturing compounds at temperatures greater than 1208C [2–4,6].process of a high quality transition piece, the electrical This is a thermally activated process and by increasing theresistance of the Al /Cu bimetal should be kept as low as temperature the nucleation and growth of compounds arepossible to minimize the wasting of electrical energy and accelerated. These compounds have a nonmetallic coval-prevent the increase of temperature. ence bond and therefore are brittle and have high electrical

From the point of view of the welding process, Al and resistance.In both processes, there is a nonequilibrium state or

metastable condition at the weld interface. This metastablecondition provides a driving force for nucleation and*Corresponding author.

E-mail address: [email protected] (A. Karimi Taheri). growth of intermetallic compounds and the amount of

0925-8388/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 01 )00872-6

234 M. Abbasi et al. / Journal of Alloys and Compounds 319 (2001) 233 –241

Fig. 1. Cu/Al bimetal transition piece.

driving force depends on the history and mechanism of the 3400 MPa and a threshold reduction between 40 to 80bond formation. percent has been reported [4,6,9,11,12]. This threshold

Cold roll welding processes (Fig. 2) take place at room reduction is greatly depending on surface treatment andtemperature [4,6,7], and evidence of no solidified and cast the magnitudes of rolling parameters such as diameter,structure at the interface has indicated that no liquid or rigidity and surface roughness of roll, preliminarymelted phase is formed and therefore a direct bond thickness of each layer before rolling and geometry ofemerges in the solid state [8,9]. The external pressure, deformation zone.mechanical work and plastic deformation applied on theinterface zone supply the activation energy for cold roll The friction welding process, Fig. 3, is also a solid statewelding [4,7]. The main steps reported for welding of Al welding process but the welding mechanism of this differsto Cu by the cold rolling process are as follows [4,6,9–11]: greatly from that of cold roll welding, since during friction

welding the interface temperature is highly elevated and a1. The surface treatment of both Al and Cu to remove transient viscous phase is formed. The transient viscous

surface contamination. phase is forced out of the bonding zone by pressure and2. Aligning and fixing the aluminum and copper strips in a the clean faces of parent metals in solid state are brought

pack. together to produce the weld.3. Cold rolling of pack (Fig. 2), to produce a metallic In this article the growth rate of intermetallic com-

continuity between Cu and Al at the interface. For roll pounds at the interface of cold roll bonded Al /Cu bimetalwelding of Al to Cu, a roll pressure between 1000 to at 2508C is studied and compared with the results of a

Fig. 2. Cold roll welding of Al /Cu pack.

M. Abbasi et al. / Journal of Alloys and Compounds 319 (2001) 233 –241 235

with a thickness of 0.3 mm. Horizontal speed of workpieces under the brush, outside diameter and rotationalspeed of brush were 50 cm/min, 17.5 cm and 650 rpmrespectively.

After the surface preparation a fixed pack of aluminumand copper was made in order that the pack pass uniformlyand adjustably through the roller gap. The rolling of thepack is shown in Fig. 2. It has been reported that the oxidelayer is formed very rapidly on the surface of thealuminum and copper and acts as a barrier to the formationof a metallurgical bond [9–11]. Therefore, the intervaltime between the surface preparation and rolling was keptless than 2 min to avoid the formation of a thick andcontinuous oxide layer on the surfaces of the strips.

The last step for making the tri-layer bimetal was toFig. 3. Friction welding of Al to Cu.

cold-weld the two metals by the rolling process. In thisstep by applying 72% reduction in total thickness of thesimilar study on friction welding of Al to Cu presented inpack a sound metallurgical bond between Cu strip and AlRef. [2].plate was achieved. Roll diameter, surface roughness ofrolls (R ) and roll speed were 70 cm, 35 mm and 400z

mm/min respectively.2. Materials and experimentsThe produced bimetal samples were heat treated at

constant temperature of 2508C for 1 to 1000 h withIn this investigation the electrical grade of copper stripstemperature deviation of 628C. Then the interfaces of Al(UNS No. C11000 as annealed) and aluminum platesand Cu were studied by optical and electron microscopes(UNS A91050-H12) were roll welded at room temperatureand EDX analysis to detect the formation, thickness and(20–258C). The composition and properties of the Alcomposition of intermetallic compounds. Thicknesses ofplates and Cu strips are summarized in Table 1.intermetallic layers were measured by the computer con-To produce a satisfactory metallurgical bond by cold rolltrolled image analyzer of the electron microscope with awelding, it was essential to remove the contaminationprecision of 0.1 micron.layers on the surfaces of two metals to be joined [4,6,9–

For evaluating the bond strength of samples a 10 kN11]. These layers are composed of oxides, absorbed ionstensile testing machine was used and the mean peeling(ions of sulfur, phosphor and oxygen), grease, humidityforce of welded layers was measured. All samples wereand dust particles. The following treatments were per-tested by a crosshead speed of 1 cm/min. The repeatabilityformed to remove the contamination layers:of tests and the sensitivity of the load cell were 0.5 N and0.1 N, respectively. The peeling force was measured using• Detergent washing, degreasing of surfaces, and waterthe following relation:rinsing.

• Final cleaning of surfaces by a powerful solvent mean tearing force(propanon: CH .CO.CH ). ]]]]]peeling force 53 3 length of bond• Scratch brushing for removal of oxide film on both (1)

F̄surfaces of the aluminum plate and copper strip. The ]F 5 N/cmb Lbristles of the brush were made of stainless steel wire b

Table 1Specification of aluminum and copper materials used in this investigation

Cu and Al Chemical Temper Hardness Yield Resistivity2grade composition HV3Kgf point V mm /m

2(wt%) M/mm

Cu-UNS C11000 99.95% Cu Annealed 45 120 0.0165Max 0.04% Oxygen

Al-UNS A91050 99.5% Al H12 40 90 0.0276Max 0.25% SiMax 0.4% FeMax 0.05% Mg


The electrical resistance of Al /Cu tri-layer samples wasmeasured using a high precision micro-ohmmeter withaccuracy of 0.01 micro-ohm. The micro-ohmmeter waspassing a certain current (I) parallel to layers of the sampleand measured the potential difference between two pointsof the sample with defined distance (L). The micropro-cessor of the micro-ohmmeter was calculating automatical-ly the resistance (R) of the length (L) of the bimetal bydividing the difference of potential to the passing current,i.e.;

DV]R 5 (2)I

The other dimensions of the sample (width and thick-ness, 15 mm34.8 mm) were measured by a micrometer of0.01 mm accuracy and then the resistivity (r) wascalculated by the resistance (R), length (L) and thicknesscross section area (S) of the sample and the followingrelation:

R.S]r 5 (3)L

3. Results and discussion

3.1. The growth rate of intermetallic compounds

The gradual growth of intermetallic layers with increas-ing annealing time is observed in the micrographs pre-sented in Fig. 4. The width of the intermetallic compoundwas increased by extending the annealing time. Thevariation of intermetallic width (X) versus annealing time(t), at constant temperature of 2508C is summarized inTable 2 and is plotted in Fig. 5.

From the inserted plot in Fig. 5, the relation betweenannealing time and the square of the intermetallic widthcan be presented by a linear function as below:

Fig. 4. The gradual growth of intermetallic layers with increasing2X 5 K . t annealing time.

Intermetallic width: X cm(4)2Proportional constant (growth rate): K cm /s

compounds is larger than that of the cold roll weldingAnnealing time: t sprocess.

2 214 2K is the slope of X versus t that is 6.3310 cm /s forannealing time below 200 h. This constant for friction 3.2. The intermetallic growth sequencewelding and for annealing time below 200 h is reported as

2131.6310 [2]. Therefore, it can be concluded that the Electron microscopy observations of Al /Cu samples atintermetallic growth rate constant for the roll welding the interface exhibit five layers between Al and Cu,process is lower than that for friction welding. This including four major intermetallic compounds and andifference in growth rate may be due to the difference in intermediate layer of saturated solid solution, Fig. 6. Thesewelding mechanism. There is neither heat input nor a layers were qualitatively and quantitatively analyzed by thesignificant rise in the temperature in cold roll welding, but energy dispersive X-ray (EDX) analyzer of an electronin friction welding a large amount of heat is produced at microscope on a line at points A, B, C . . . , L, M to specifythe interface and raises the interface temperature. Thus, the composition of each layer and compound. The resultsone can ascertain that in the friction welding process the of this analysis are presented in Table 3. The compositiondriving force for the formation and growth of intermetallic of five detected layers were Cu Al, Cu Al , saturated solid3 4 3


Table 2Variation of intermetallic width and conductivity with annealing time at 2508C

Times of Intermetallic Resistance Relative Bimetalannealing layer width of bimetal resistance conductivityat 2508C (mm) sample change, r 1/r

2(h) (mV) (%) (m/Vmm )

0 (referencesample) 0 35.88 0 37.61

1 0 35.88 0 37.615 0.3 36.01 0.67 37.47

25 0.5 36.14 0.72 37.3450 1 36.62 2.06 36.84

100 1.5 37.13 3.48 36.34200 2.2 37.42 4.29 36.06250 3.6 37.60 4.79 35.88400 8.4 38.68 7.80 35.69600 11 39.01 8.72 35.46800 13 39.20 9.25 34.96

1000 15.6 39.61 10.39 34.13

solution (9–13% Al in Cu), CuAl, and CuAl . Aluminum might be caused by impurities, which are present accord-2

line scan across the intermetallic width and layers also ing to Tables 1 and 3. Among the impurities the presenceconfirms the above results as shown in Fig. 7. The of Si and Fe are detected with a higher concentration in thesummarized results of the mentioned analysis is a solid saturation layer. The presence of Si and Fe mayschematic figure of the intermetallic compounds and solid inhibit the growth of the intermetallic compounds andsolution layer presented in Fig. 8. induces the formation of an intermediate layer of solid

For a binary system of Al–Cu and a diffusion-controlled saturation layer with low amount of Al.process the composition has to change from low to highAl-content without a decrease in between. Therefore, it is 3.3. Resistance and conductivity of the bimetal samplesnot expected that the solid solution layer, with a lowamount of Al content, be situated between the Al-richer The results of change in resistance of the samples withcompounds (Cu Al , CuAl). The fact that this is found increasing intermetallic thickness are presented in Table 24 3

Fig. 5. Variation of intermetallic width (x) with annealing time at 2508C.


Fig. 6. EDX analysis is performed on a line at points A, B, C, . . . , L, M Fig. 7. Al-line scan across the intermetallic width and layers.to specify the composition of each layer and compound.

samples was their resistivity. The theoretical values ofand plotted in Fig. 9. The relation of relative change in resistivity of bimetal for parallel current are calculatedresistance with increasing the intermetallic thickness may from the following equation [13]:be defined by the following relation: y y1 Al Cu

]] ] ]5 1r r r#· Al Cur 5 (R 2 R ) /R 5 C. Xf r r

2Relative change in resistance: r where: resistivity of aluminum is, r 50.0276 V.mm /m;Al2Resistance before annealing: R resistivity of copper is, r 50.0165 V.mm /m; theoretical(5)r Cu

2resistivity of bimetal is, r 50.0246 V.mm /m; volumeResistance after annealing: R #·ffraction of aluminum is, y 50.82; volume fraction ofAlProportional constant: Ccopper is, y 50.18.Cu

The proportional constant C is calculated as 0.44 from In the absence of intermetallic compounds the theoret-the slope of the plot. In friction welding, this constant has ical value of bimetal conductivity (inverse of resistivity) is

2been evaluated as 0.48 [2] being nearly close to that of the equal to 40.65 m/V.mm , while the maximum actual2cold roll welding process. measured value is equal to 37.61 m/V.mm . This devia-

Another interesting property of the produced bimetal tion from theory may be attributed to the inherent resistiv-

Table 3EDX quantitative analyses at points A, B, C, . . . , L, M to specify the composition of each intermetallic compound and their conductivity

Analysis Atomic Atomic Atomic percent Nearest intermetallic Conductivity ofapoint percent percent of impurity compound intermetallics

2of Cu of Al m/Vmm [Ref.]Si Fe

bA 99.87 ,0.08 * * – –B 99.76 ,0.08 * * – –C 92.72 7.19 * * – –D 70.38 29.61 * * Cu Al(z ) 8.1 [14]3 2

E 54.59 45.41 * * Cu Al 8.04 [14]4 3

F 89.50 9.35 0.11 0.22 Saturated solid solution (a) no referenceG 88.50 11.35 0.13 0.22 Saturated solid solution (a) no referenceH 86.37 13.02 0.11 0.23 Saturated solid solution (a) no referenceI 47.63 51.3 * * CuAl (h ) 8.77 [14]2

J 33.50 66.45 * * CuAl (u ) 12.5 [14]2

K 3.41 96.8 0.13 0.24 – –L 1.07 98.32 0.10 0.22 – –M 0.86 98.45 0.11 0.24 – –

a The atomic percent of the other main impurity (Mg, Mn) was less than 0.03.b The atomic percent of Si and Fe was less than 0.03.


Fig. 8. Schematic diagram of intermetallic compounds and solid solution layer detected by EDX analyzing.

ity of the bond that is not considered in the theoretical ing the intermetallic width may be represented by acalculation. function as below:

The results of change in the conductivity of samples3 21 /r 5 aX 1 bX 1 cX 1 d#·with increasing intermetallic thickness are plotted in Fig.

10. Referring to this figure, conductivity decreases highlywith increasing the intermetallic width. This decrease of One can obtain the constants a, b, c and d from theconductivity is due to the formation of intermetallic layers experimental results. It is possible to prepare a calibratedof low conductivity values presented in Table 3 [14]. The plot such as Fig. 10 for each manufacturing condition toconductivity of intermetallic compounds is about one inspect the intermetallic growth and electrical quality ofseventh of the conductivity of copper or one fifth of the bond. By a resistivity test one can determine to which partconductivity of aluminum, indicating that due to the of the plot the condition of the sample corresponds andformation of these compounds the total conductivity predict the progress of intermetallic growth. A qualityreduces highly. Decreasing the conductivity with increas- acceptance criterion may be established as ‘‘a transition

Fig. 9. Variation of resistance change with intermetallic width. Fig. 10. Variation of conductivity with intermetallic width.


2508C is noticeably smaller compared with that of thefriction welding process.

2. For cold roll welded Al /Cu bimetal the relation be-tween annealing time and the square of intermetallic

2width (X ) at a constant temperature of 2508C can bepresented by a linear equation.

3. Priority of formation of intermetallic phases in cold rollwelding of electrical grade of aluminum and coppersamples is such that the intermetallic compounds Cu3

Al, Cu Al , saturated solid solution of Al in Cu, CuAl,4 3

and CuAl are observed successively.2

4. For the binary system of Al–Cu and a diffusion-con-trolled process the composition has to change from lowto high Al-content without a decrease in between. Thefact that a decrease of Al in between Al-richer layers isfound might be caused by presence of impurities suchas Si and Fe.

5. The relation between conductivity and intermetallicwidth can be presented by a three-order function asbelow:

3 21 /r 5 aX 1 bX 1 cX 1 d#·

6. Using such a relationship, it is possible to determine thethickness of the intermetallic layer by a nondestructiveresistivity test.

7. There is a critical width of intermetallic compound atwhich the strength of bond is sharply reduced and a

Fig. 11. Variation of peeling force with intermetallic width. ductile to brittle behavior is observed. For the cold rollwelding and experiments of this research the criticalwidth is about 2.5 microns.

joint has an accepted quality if its conductivity valueplaced within the first part of the plot where the thicknessof intermetallic layers is below 2.5 micron’’.

References

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Growth rate of intermetallic compounds in Al/Cu bimetal produced by cold roll welding process

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