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Intermetallic phases in cold-welded AlCu joints
Materials Science and Engineering, A159 (1992) 231-236
231
Intermetallic phases in cold-welded A1-Cu joints M. K o b e r n a
Institute of Technology and Reliability of Structures, Veleslavinova 1l, 301 14 Plze~ (Czechoslovakia)
J. Fiala Central Research Institute Skoda, Tylova 46, 316 O0 Plze~ (Czechoslovakia) (Received October 7, 1991 ; in revised form July 20, 1992)
Abstract The behaviour of cold-welded AI-Cu joints during annealing was studied. The deterioration of the cohesion strength of such joints was found to be caused by precipitation of definite intermetallic phases at the interface. Estimates of density changes associated with phase transformation allowed quantitative comparison of the harmfulness of different phases. The present study sets and substantiates the temperature limit of 200 °C for the safe exploitation of cold-welded A1-Cu joints in electrical machines. This is the temperature above which intermetallic phases appear and mechanical properties deteriorate by more than 10% compared with the initial state.
I. Introduction
Cold welding is a specific process of making joints of metallic materials without introducing thermal energy and without melting the joint area [1-4]. Joints are created under certain conditions owing to cohesive interactions between the nearest surface atoms (metallic bond creation). The range of these atomic forces at metal surfaces is very short, 0.3 nm at most [5]. For joining to occur, it is necessary to put both welded metal surfaces at the distance of cohesive force interaction, and energy must be supplied to build up largeangle boundaries which accommodate the shape that the joint area adopts [6]. The energy necessary for the joint creation is introduced by pressing. Therefore it is possible to weld some metals in this way which cannot be welded by fusion or diffusion processes because their melting points are too different or because specific intermetallic phases are formed in the molten state (or at elevated temperatures) which are detrimental to the cohesion of the joint. Brittle intermetallic phases formed at elevated temperatures are the cause of the mechanical property degradation of A1-Cu joints [7-9]. As the deterioration of the mechanical properties of the joint may be very severe, it is necessary to know the temperature and time limits of the intended application, and the rate of mechanical property deterioration to determine the operating range of the joints. For example, electrical current flowing through an alumin0921-5093/92/$5.00
ium transformer winding with a cold-welded copper head produces heating (due to current heat losses) which increases the temperature of the AI-Cu joint so that a layer of brittle intermetallic phases is created and the joint fractures. The same concern exists for the windings of alternators, dynamos and motors. The degradation is induced by a brittle layer created in the weld zone. Experimentally it has been determined that the limiting temperature is about 200 °C; below this temperature, no process of brittle layer creation exists. This temperature is also the limit for extensive recrystallization in both pure aluminium and pure copper. Recrystallization induces changes in mechanical properties too [10, 11], but no attempt has been made to study recrystallization of the matrix in the present paper which is focused mainly on the details of precipitation processes at the interface. Our microscopic examinations show only partial recovery of the weld zone together with fine-grained recrystallization of the primary materials without significant influence on the corresponding mechanical properties. The recrystallization processes depend strongly on the chemical composition (given in Table 1) and the initial condition of the materials and their structure (Fig. 1 shows plastic deformation induced by the process of cold welding). In previous papers, only the intermetallic phase 0 (A12Cu) has been reported in A1-Cu joints [7-9]. Contrary to this, we have found several other intermetallic phases, the formation of which proved to be © 1992 - Elsevier Sequoia. All fights reserved
M. Koberna, J. Fiala / Intermetallicphasesin coM-weldedAI-Cujoints
232
TABLE 1. Compositions of materials Material
Impurities
Amount (wt.%)
AI
Si Fe Cu Zn S A1 Ag Bi Fe Mn Ni Pb Sb Sn Zn P Si As
0.11 0.28 0.01 0.04 0.002 0.001 0.002 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.004 0.003 0.01 0.001
Cu
Fig. 2. AI-Cu specimen before deflashing.
PLANE OF RUPTURE
(a) WELDING ZONE
=
22,5 65
(b) Fig. 3. Two groups of specimens prepared for evaluation (dimensions in millimetres): (a) by cutting along the longitudinal axis; (b) by breaking across the weldingzone.
deformation was defined as
D=S-S°xlO0 Fig. 1. Plastic deformed zone in a cold-welded AI-Cu joint (etched with HNO3).(Original magnification, 100 x.)
temperature and time dependent. In the present paper, the results of our examination of the transient zone between copper and aluminium, where the different intermetallic phases appear, are described. Further, the influence of these created phases on the mechanical properties of cold-welded joints is discussed.
2. The weld zone analysis
Experimental specimens were made by means of cold butt welding of the above-mentioned materials in air. The plastic deformation used was 250%. This
(%)
(1)
So where S is the specimen cross-section after deformation and SOthe primary cross-section. The oxide layers present on the welded surfaces were extruded during the plastic deformation into a flash and removed. The welded specimen before deflashing is shown in Fig. 2. Structural studies were performed on samples prepared by cutting along the longitudinal axis or by breaking across the welding zone (Fig. 3). The structural studies included light microscopy, electron probe microanalysis and X-ray diffraction. The X-ray diffraction involved qualitative and semiquantitative phase analysis in the Bragg-Brentano semifocusing arrangement with photoregistration, a 114.6 mm camera diameter, a cobalt anode, a fl filter and a 30 ° focusing angle. As the aim was to determine the behaviour of A1-Cu joints in the temperature range from 20 to
M. Koberna, J. Fiala
/
Intermetallic phases in cold-welded AI-Cu joints
650 °C (near the melting point), specimens were analysed in the unannealed state, and after heating at various temperatures, up to 650 °C. The microstructure of the joint in the unannealed state is shown in Fig. 4(a) while Fig. 4(b) presents the appearance of the joint annealed at 180 °C for 6 5 h. Contrary to the unannealed state, a diffusion band is clearly apparent in the central part of the annealed joint between fine-grained recrystallized copper on the right-hand side and aluminium on the left-hand side. Figures 5(a) and 5(b) shows the concentration profiles in unannealed and annealed (at 500 °C for 1 h) weld zones respectively. The diffusion zone in the annealed sample (Fig. 5(b)) is several tens of microns wide and steps in the concentration curve (scan) are perceivable. These steps correspond to separated phases. The curves were obtained by means of linear qualitative analysis (wave dispersion spectrometry) along the longitudinal axis with 1/zm step measurement intervals. The two linear scans represent terminal cases: the scan without steps (without separated phases) in the unannealed state and the other scan with
233
marked steps (with clearly separated phases) after annealing at 500 °C. We have found that the transition point, when steps begin to appear on the scan and indicate the first appearance of the intermetaUic phases inside the joint, corresponds to the annealing temperature of 200 °C. This is the temperature at which a qualitative change occurs in the structure of the AI-Cu joint. To make clear what is the consequence of these structural changes on mechanical properties of the joint, we have performed measurements of the tensile strength and impact strength vs. annealing time for various temperatures. From the results of these measurements, which are summarized in Fig. 6, we see that it is exactly the annealing temperature of 200 °C which leads to a drop of the mechanical parameters by some 10%. This may be considered to be the least significant deterioration recognized, indicating that the temperature above which the joint should not be used is equal to the temperature at which intermetallic phases begin to appear, i.e. to 200 °C.
3. Discussion ....... ~
~= k ¸
From electron microanalysis, it can be seen that a diffusion zone with a width of about 12/~m already
0
(b) Fig. 4. AI-Cu weld zones: (b) unannealed (unetched); (b) annealed at 180 °C for 65 h (etched with 0.5% HF). (Original magnification, 100 x .)
16
32 4.8 64 DISTANCE [)Jrn]
0
16
32 48 64 DISTANCE [ p r o ]
(a) (b) Fig. 5. Concentration profiles in the A1-Cu weld zones: (a) unannealed; (b) annealed at 500 °C for 1 h.
234
M. Koberna, J. Fiala
/
lntermetallic phases in cold-welded AI-Cu joints
exists before annealing. This is much greater than the diffusion width calculated according to the simplified formula [12]
atoms into the aluminium lattice, we have [10]
Q I ]'/2 x =[Dotexp(- lj
Q=141930
(2)
where D o is the diffusion coefficient, t the welding time, Q the activation energy, R the gas constant and T the absolute temperature. For the diffusion of copper lOOt
180eC ~.........,_.._----~ ----'z~c
--
"-- l o ~
80 220°C
6o
g
0
(a)
"%
/
20
0
100
200
.....
z.O0
300 ,~og time
Imlnl
1201 ~ ~ ~ . _ ~ 100
i~
500
•
18o~c 2000C
_
10%
"
220°C
'~
80
60
Z
'---..
~o
•-
500Oc 20
0
100
200
300
400
500
(b) ...... ~ng t,me Im~nl Fig. 6. Mechanical testing results as a function of temperature: (a) tensile strength vs. annealing time; (b) impact strength vs. annealing time.
D0=2.0
(cm2s -l) (J)
and for the diffusion of aluminium atoms into the copper lattice we have [13] D0=7.2 x 10 -3
Q=163290
(cm2s -l )
(J)
So, the computed diffusion width would amount to 5 × 10 -6 #m if we suppose that the real welding time is about 10 s and the temperature of the weld zone reaches 80 °C [14]. The existing broad diffusion zone can be explained as a consequence of an energy increase and its fluctuation in the weld zone during the strong plastic deformation used for the butt welding ("cold diffusion" [15]). This transition layer represents a solid solution a, with low aluminium content (5 wt.%). The width of this layer grows with increasing temperature and time without significantly influencing the mechanical properties of the joint until a temperature of 200 °C is reached. At this temperature, intermetallic phases begin to precipitate in the solid solution as is revealed by steps appearing on the concentration profile measured across the interface with an electron probe microanalyser. At the same time, deterioration of mechanical properties starts. Further increases in the annealing temperature lead to the development of a number of intermetallic phases. By X-ray diffraction analysis, we have found that the layer consists of a mixture of a, fl and 7 phases after annealing at 500 °C for 1 h. The phases are defined in Table 2. The relative amounts of the phases vary, and the quantity of y phase decreases towards the copper end of the couple; the fl phase prevails but in the end it also disappears being substituted by the a phase. The fracture of specimens annealed at 500 °C for 8 h went through the intermetallic layer, and the fracture surface showed two different structures. The central part of the surface consisted of a mixture of a,
TABLE 2. Phases found in the examined AI-Cu joints Phase
a /3 )' 0 ~c
Composition
(Cu-)10at.%Al Cu3AI CugAI4 CuAI2 (Al-)l.2at.%Cu
Density (kgm -3)
Structure type Strukturbericht symbol
Pearson symbol
A1 A2 D83 C16 A1
cF4 ci2 cP52 tI12 cF4
8310 7340 6890 4390 2770
M. Koberna, J. Fiala
/
lntermetallic phases in cold-welded Al-Cu joints
fl and 7 phases and the boundary was created by a mixture of 0 phase and aluminium r phase. The specimens annealed at a temperature of 650 °C for 1 min contained only a thick 0 phase layer (Fig. 7), which was determined by means of X-ray diffraction. This layer was separated out like a lamellar eutectic (Fig. 8). The aluminium side showed a high diffusion porosity at the same time (Fig. 9). Phase transformations can develop in a diffusion joint when it is made or heated. Such phase transformations may cause stresses in the joint associated with the corresponding changes in density. In general, the density changes brought about by phase transformations amount to
(3) where the subscripts S and F represent the initial state and the state after reaction respectively and where u i is
235
the volume proportion of the ith phase and /9i the density of the ith phase. For the special case when two terminal solid solutions with densities Pl and/92 and a volume fraction ratio of v/( 1 - v) react to produce an intermetallic phase with the density p, we have (4)
R =p-p2+v(p2-pl)
The larger the density change R, the greater are the stresses introduced by such a reaction and the greater are the effects connected with the creation of the corresponding intermetallic phase. Under optimum circumstances, the value of R approaches zero: R =p-p2
+ v(lOz-pl)---'O
(5)
The criterion (5) allows quantitative comparison of the individual phases which may appear in a diffusion joint during heating. In Table 3, the values of R calculated from eqn. (4) are given for the intermetallic phases fl, ), and 0 [16-18] (see Table 2), which we found in the structure of the Cu-Al joints studied in the present work. It is seen that the 0 phase is the least favourable as it causes large tensile stresses (R < 0) which endanger the diffusion joint. The 7 phase also brings about large tensile stresses but much less than the 0 phase (about 30%). The creation of the fl phase produces almost no stresses at all.
Fig. 7. Thick ruptured 0 phase layer (650 °C for 1 min) (unetched). (Original magnification, 50 x .)
Fig. 9. Diffusion porosity on the aluminium side of the joint (unetched). (Original magnification, 100 ×.) TABLE 3. Density changes R brought about by precipitation of individual phases in the A1-Cu system
Fig. 8. Separated thick 0 phase layer (unetched). (Original magnification, 410 × .)
Phase
R (kgm -3)
fl 7 0
-34 -122 -382
236
M. Koberna, J. Fiala
/
Intermetallic phases in cold-welded AI-Cu joints
4. Conclusion
References
The influence of time and temperature on the formation of intermetallic layers was examined. It has been found that increasing temperature and time leads to the creation of less desirable types of intermetallic compound, which endanger the joint reliability by inducing the loss of structural coherence and even causing rupture of interface bonds. The abovementioned processes lead to the deterioration of the mechanical properties of the welded joint. The examination of A I - C u welded joint behaviom proved that increasing the temperature above 200 °C, leads to the formation of intermetallic phases and leads to fast joint degradation. As a result, heating above 200 °C must be avoided.
1 R. E Tylecote, Brit. Weld. J., 1 (1954) 117. 2 H. A. Mohamed and J. Washburn, Weld. J., Res. Suppl., 54 (1975) 302s. 3 N. Bay, Weld. J., Res. Suppl., 62(1983) 137s. 4 N. Bay, Met. Constr., 17(1986)369. 5 Ch. Kittel, Introduction to Solid State Physics, Wiley, New York, 1976. 6 J.M. Marcinkowski, Phys. Status Solidi A, 73 (1982) 409. 7 D.R. Milner and G. W. Rowe, Metall. Rev., 7(1962) 433. 8 E Shiickher, Metall., 28(1974) 481. 9 K. J. B. McEwan and D. R. Milner, Brit. Weld. J., 8 (1962) 406. 10 K. Dies, Kupfer und Kupferlegierungen in der Technik, Springer, Berlin, 1967. 11 D. Altenpohl, Aluminium und Aluminiumlegierungen, Springer, Berlin, 1965. 12 G. F. Carter, Principles of Physical and Chemical Metallurgy, American Society for Metals, Metals Park, OH, 1979. 13 A. G. Guy, Introduction to Materials Science, McGraw-Hill, New York, 1958. 14 W. Pohl, Ph.D. Thesis, University of Stuttgart, 1972. 15 M. Koberna, Ph.D. Thesis, West Bohemia University, Pilsen, 1990. 16 M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 17 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 1, Pergamon, Oxford, 1958; A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967. 18 P. Viliars and L. D. Calvert, Pearson's Handbook of Crystallographic Data for Intermetallic Phases, Vols. 1, 2 and 3, American Society for Metals, Metals Park, OH, 1986.
Acknowledgments This work was carried out with the support of the technical staff of The Central Research Institute Skoda, Plzen, and especially of its Metallography Section. The invaluable help of a referee, which substantially improved the paper and made its publication possible, is greatly acknowledged.
231
Intermetallic phases in cold-welded A1-Cu joints M. K o b e r n a
Institute of Technology and Reliability of Structures, Veleslavinova 1l, 301 14 Plze~ (Czechoslovakia)
J. Fiala Central Research Institute Skoda, Tylova 46, 316 O0 Plze~ (Czechoslovakia) (Received October 7, 1991 ; in revised form July 20, 1992)
Abstract The behaviour of cold-welded AI-Cu joints during annealing was studied. The deterioration of the cohesion strength of such joints was found to be caused by precipitation of definite intermetallic phases at the interface. Estimates of density changes associated with phase transformation allowed quantitative comparison of the harmfulness of different phases. The present study sets and substantiates the temperature limit of 200 °C for the safe exploitation of cold-welded A1-Cu joints in electrical machines. This is the temperature above which intermetallic phases appear and mechanical properties deteriorate by more than 10% compared with the initial state.
I. Introduction
Cold welding is a specific process of making joints of metallic materials without introducing thermal energy and without melting the joint area [1-4]. Joints are created under certain conditions owing to cohesive interactions between the nearest surface atoms (metallic bond creation). The range of these atomic forces at metal surfaces is very short, 0.3 nm at most [5]. For joining to occur, it is necessary to put both welded metal surfaces at the distance of cohesive force interaction, and energy must be supplied to build up largeangle boundaries which accommodate the shape that the joint area adopts [6]. The energy necessary for the joint creation is introduced by pressing. Therefore it is possible to weld some metals in this way which cannot be welded by fusion or diffusion processes because their melting points are too different or because specific intermetallic phases are formed in the molten state (or at elevated temperatures) which are detrimental to the cohesion of the joint. Brittle intermetallic phases formed at elevated temperatures are the cause of the mechanical property degradation of A1-Cu joints [7-9]. As the deterioration of the mechanical properties of the joint may be very severe, it is necessary to know the temperature and time limits of the intended application, and the rate of mechanical property deterioration to determine the operating range of the joints. For example, electrical current flowing through an alumin0921-5093/92/$5.00
ium transformer winding with a cold-welded copper head produces heating (due to current heat losses) which increases the temperature of the AI-Cu joint so that a layer of brittle intermetallic phases is created and the joint fractures. The same concern exists for the windings of alternators, dynamos and motors. The degradation is induced by a brittle layer created in the weld zone. Experimentally it has been determined that the limiting temperature is about 200 °C; below this temperature, no process of brittle layer creation exists. This temperature is also the limit for extensive recrystallization in both pure aluminium and pure copper. Recrystallization induces changes in mechanical properties too [10, 11], but no attempt has been made to study recrystallization of the matrix in the present paper which is focused mainly on the details of precipitation processes at the interface. Our microscopic examinations show only partial recovery of the weld zone together with fine-grained recrystallization of the primary materials without significant influence on the corresponding mechanical properties. The recrystallization processes depend strongly on the chemical composition (given in Table 1) and the initial condition of the materials and their structure (Fig. 1 shows plastic deformation induced by the process of cold welding). In previous papers, only the intermetallic phase 0 (A12Cu) has been reported in A1-Cu joints [7-9]. Contrary to this, we have found several other intermetallic phases, the formation of which proved to be © 1992 - Elsevier Sequoia. All fights reserved
M. Koberna, J. Fiala / Intermetallicphasesin coM-weldedAI-Cujoints
232
TABLE 1. Compositions of materials Material
Impurities
Amount (wt.%)
AI
Si Fe Cu Zn S A1 Ag Bi Fe Mn Ni Pb Sb Sn Zn P Si As
0.11 0.28 0.01 0.04 0.002 0.001 0.002 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.004 0.003 0.01 0.001
Cu
Fig. 2. AI-Cu specimen before deflashing.
PLANE OF RUPTURE
(a) WELDING ZONE
=
22,5 65
(b) Fig. 3. Two groups of specimens prepared for evaluation (dimensions in millimetres): (a) by cutting along the longitudinal axis; (b) by breaking across the weldingzone.
deformation was defined as
D=S-S°xlO0 Fig. 1. Plastic deformed zone in a cold-welded AI-Cu joint (etched with HNO3).(Original magnification, 100 x.)
temperature and time dependent. In the present paper, the results of our examination of the transient zone between copper and aluminium, where the different intermetallic phases appear, are described. Further, the influence of these created phases on the mechanical properties of cold-welded joints is discussed.
2. The weld zone analysis
Experimental specimens were made by means of cold butt welding of the above-mentioned materials in air. The plastic deformation used was 250%. This
(%)
(1)
So where S is the specimen cross-section after deformation and SOthe primary cross-section. The oxide layers present on the welded surfaces were extruded during the plastic deformation into a flash and removed. The welded specimen before deflashing is shown in Fig. 2. Structural studies were performed on samples prepared by cutting along the longitudinal axis or by breaking across the welding zone (Fig. 3). The structural studies included light microscopy, electron probe microanalysis and X-ray diffraction. The X-ray diffraction involved qualitative and semiquantitative phase analysis in the Bragg-Brentano semifocusing arrangement with photoregistration, a 114.6 mm camera diameter, a cobalt anode, a fl filter and a 30 ° focusing angle. As the aim was to determine the behaviour of A1-Cu joints in the temperature range from 20 to
M. Koberna, J. Fiala
/
Intermetallic phases in cold-welded AI-Cu joints
650 °C (near the melting point), specimens were analysed in the unannealed state, and after heating at various temperatures, up to 650 °C. The microstructure of the joint in the unannealed state is shown in Fig. 4(a) while Fig. 4(b) presents the appearance of the joint annealed at 180 °C for 6 5 h. Contrary to the unannealed state, a diffusion band is clearly apparent in the central part of the annealed joint between fine-grained recrystallized copper on the right-hand side and aluminium on the left-hand side. Figures 5(a) and 5(b) shows the concentration profiles in unannealed and annealed (at 500 °C for 1 h) weld zones respectively. The diffusion zone in the annealed sample (Fig. 5(b)) is several tens of microns wide and steps in the concentration curve (scan) are perceivable. These steps correspond to separated phases. The curves were obtained by means of linear qualitative analysis (wave dispersion spectrometry) along the longitudinal axis with 1/zm step measurement intervals. The two linear scans represent terminal cases: the scan without steps (without separated phases) in the unannealed state and the other scan with
233
marked steps (with clearly separated phases) after annealing at 500 °C. We have found that the transition point, when steps begin to appear on the scan and indicate the first appearance of the intermetaUic phases inside the joint, corresponds to the annealing temperature of 200 °C. This is the temperature at which a qualitative change occurs in the structure of the AI-Cu joint. To make clear what is the consequence of these structural changes on mechanical properties of the joint, we have performed measurements of the tensile strength and impact strength vs. annealing time for various temperatures. From the results of these measurements, which are summarized in Fig. 6, we see that it is exactly the annealing temperature of 200 °C which leads to a drop of the mechanical parameters by some 10%. This may be considered to be the least significant deterioration recognized, indicating that the temperature above which the joint should not be used is equal to the temperature at which intermetallic phases begin to appear, i.e. to 200 °C.
3. Discussion ....... ~
~= k ¸
From electron microanalysis, it can be seen that a diffusion zone with a width of about 12/~m already
0
(b) Fig. 4. AI-Cu weld zones: (b) unannealed (unetched); (b) annealed at 180 °C for 65 h (etched with 0.5% HF). (Original magnification, 100 x .)
16
32 4.8 64 DISTANCE [)Jrn]
0
16
32 48 64 DISTANCE [ p r o ]
(a) (b) Fig. 5. Concentration profiles in the A1-Cu weld zones: (a) unannealed; (b) annealed at 500 °C for 1 h.
234
M. Koberna, J. Fiala
/
lntermetallic phases in cold-welded AI-Cu joints
exists before annealing. This is much greater than the diffusion width calculated according to the simplified formula [12]
atoms into the aluminium lattice, we have [10]
Q I ]'/2 x =[Dotexp(- lj
Q=141930
(2)
where D o is the diffusion coefficient, t the welding time, Q the activation energy, R the gas constant and T the absolute temperature. For the diffusion of copper lOOt
180eC ~.........,_.._----~ ----'z~c
--
"-- l o ~
80 220°C
6o
g
0
(a)
"%
/
20
0
100
200
.....
z.O0
300 ,~og time
Imlnl
1201 ~ ~ ~ . _ ~ 100
i~
500
•
18o~c 2000C
_
10%
"
220°C
'~
80
60
Z
'---..
~o
•-
500Oc 20
0
100
200
300
400
500
(b) ...... ~ng t,me Im~nl Fig. 6. Mechanical testing results as a function of temperature: (a) tensile strength vs. annealing time; (b) impact strength vs. annealing time.
D0=2.0
(cm2s -l) (J)
and for the diffusion of aluminium atoms into the copper lattice we have [13] D0=7.2 x 10 -3
Q=163290
(cm2s -l )
(J)
So, the computed diffusion width would amount to 5 × 10 -6 #m if we suppose that the real welding time is about 10 s and the temperature of the weld zone reaches 80 °C [14]. The existing broad diffusion zone can be explained as a consequence of an energy increase and its fluctuation in the weld zone during the strong plastic deformation used for the butt welding ("cold diffusion" [15]). This transition layer represents a solid solution a, with low aluminium content (5 wt.%). The width of this layer grows with increasing temperature and time without significantly influencing the mechanical properties of the joint until a temperature of 200 °C is reached. At this temperature, intermetallic phases begin to precipitate in the solid solution as is revealed by steps appearing on the concentration profile measured across the interface with an electron probe microanalyser. At the same time, deterioration of mechanical properties starts. Further increases in the annealing temperature lead to the development of a number of intermetallic phases. By X-ray diffraction analysis, we have found that the layer consists of a mixture of a, fl and 7 phases after annealing at 500 °C for 1 h. The phases are defined in Table 2. The relative amounts of the phases vary, and the quantity of y phase decreases towards the copper end of the couple; the fl phase prevails but in the end it also disappears being substituted by the a phase. The fracture of specimens annealed at 500 °C for 8 h went through the intermetallic layer, and the fracture surface showed two different structures. The central part of the surface consisted of a mixture of a,
TABLE 2. Phases found in the examined AI-Cu joints Phase
a /3 )' 0 ~c
Composition
(Cu-)10at.%Al Cu3AI CugAI4 CuAI2 (Al-)l.2at.%Cu
Density (kgm -3)
Structure type Strukturbericht symbol
Pearson symbol
A1 A2 D83 C16 A1
cF4 ci2 cP52 tI12 cF4
8310 7340 6890 4390 2770
M. Koberna, J. Fiala
/
lntermetallic phases in cold-welded Al-Cu joints
fl and 7 phases and the boundary was created by a mixture of 0 phase and aluminium r phase. The specimens annealed at a temperature of 650 °C for 1 min contained only a thick 0 phase layer (Fig. 7), which was determined by means of X-ray diffraction. This layer was separated out like a lamellar eutectic (Fig. 8). The aluminium side showed a high diffusion porosity at the same time (Fig. 9). Phase transformations can develop in a diffusion joint when it is made or heated. Such phase transformations may cause stresses in the joint associated with the corresponding changes in density. In general, the density changes brought about by phase transformations amount to
(3) where the subscripts S and F represent the initial state and the state after reaction respectively and where u i is
235
the volume proportion of the ith phase and /9i the density of the ith phase. For the special case when two terminal solid solutions with densities Pl and/92 and a volume fraction ratio of v/( 1 - v) react to produce an intermetallic phase with the density p, we have (4)
R =p-p2+v(p2-pl)
The larger the density change R, the greater are the stresses introduced by such a reaction and the greater are the effects connected with the creation of the corresponding intermetallic phase. Under optimum circumstances, the value of R approaches zero: R =p-p2
+ v(lOz-pl)---'O
(5)
The criterion (5) allows quantitative comparison of the individual phases which may appear in a diffusion joint during heating. In Table 3, the values of R calculated from eqn. (4) are given for the intermetallic phases fl, ), and 0 [16-18] (see Table 2), which we found in the structure of the Cu-Al joints studied in the present work. It is seen that the 0 phase is the least favourable as it causes large tensile stresses (R < 0) which endanger the diffusion joint. The 7 phase also brings about large tensile stresses but much less than the 0 phase (about 30%). The creation of the fl phase produces almost no stresses at all.
Fig. 7. Thick ruptured 0 phase layer (650 °C for 1 min) (unetched). (Original magnification, 50 x .)
Fig. 9. Diffusion porosity on the aluminium side of the joint (unetched). (Original magnification, 100 ×.) TABLE 3. Density changes R brought about by precipitation of individual phases in the A1-Cu system
Fig. 8. Separated thick 0 phase layer (unetched). (Original magnification, 410 × .)
Phase
R (kgm -3)
fl 7 0
-34 -122 -382
236
M. Koberna, J. Fiala
/
Intermetallic phases in cold-welded AI-Cu joints
4. Conclusion
References
The influence of time and temperature on the formation of intermetallic layers was examined. It has been found that increasing temperature and time leads to the creation of less desirable types of intermetallic compound, which endanger the joint reliability by inducing the loss of structural coherence and even causing rupture of interface bonds. The abovementioned processes lead to the deterioration of the mechanical properties of the welded joint. The examination of A I - C u welded joint behaviom proved that increasing the temperature above 200 °C, leads to the formation of intermetallic phases and leads to fast joint degradation. As a result, heating above 200 °C must be avoided.
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Acknowledgments This work was carried out with the support of the technical staff of The Central Research Institute Skoda, Plzen, and especially of its Metallography Section. The invaluable help of a referee, which substantially improved the paper and made its publication possible, is greatly acknowledged.