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Joining of copper tube to polyurethane tube by electromagnetic pulse forming
Journal of Materials Processing Technology, 37 (1993) 83-93
83
Elsevier
Joining of copper tube to polyurethane tube by electromagnetic pulse forming W.S. H w a n g Department of Metallurgical Engineering, Sung Hwa University, Chunan, Chungnam, South Korea J.S. L e e , N . H . K i m a n d H.S. S o h n
Materials Processing and Application Laboratory, Agency for Defense Development, P.O. Box 35, Yuseong, Daejeon, South Korea
Industrial S u m m a r y The electromagnetic joining process of a structural body for underwater applications, which is composed of three members (copper tube/polyurethane tube/aluminum core), is studied experimentally and theoretically in order to estimate the strength of the joint and to investigate the effect of process variables on the strength. From the experiments it has been found that the strength of the joint is governed by the magnitude of the residual radial strain of the polyurethane tube. As the number of discharges and the level of discharged energy increase, the strength normally increases, but it decreases in the case of uneven deformation in which large wrinkles occur. The joint strength degrades rapidly when an axial load is applied to the polyurethane tube because the contact pressure between the polyurethane tube and the aluminum core is reduced due to the effect of the radial shrinkage of the polyurethane. Equations which can predict the joint strength are proposed: it is found that the calculated values agree well with the experimental results.
1. I n t r o d u c t i o n In accordance with the development of many advanced materials such as new metals, ceramics and polymers, there has been a great need of bonding methods with high strength and smooth configuration. Amongst them, the e l e c t r o m a g n e t i c j o i n i n g p r o c e s s is k n o w n t o a s u i t a b l e m e t h o d for t h e j o i n i n g of dissimilar materials.
Correspondence to: Dr. J.S. Lee, Materials Processing and Application Laboratory, Agency for Defense Development, P.O. Box 35, Yuseong, Daejeon, South Korea. 0924-0136/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
84
W.S. Hwang et al./Etectromagnetic pulse forming
Most of the reported investigations into the field of electromagnetic joining have been concerned mainly with the improvement and prediction of the strength of the joint. Murata et al. [1] have analyzed the mechanism of the metal/metal joining process based on experiment and have derived an equation for the axial pull-out strength, reporting that the calculated values agreed well with the experimental results. Sano and co-workers [2] have shown the possibility of joining metal tube to ceramic rod electromagnetically and have investigated the influences of process variables, including heat treatment, for joining. However, there remained some problems, in that the fracture occurred easily in the ceramic rod due to its weakness against impact and as the real contact area was narrow. On the other hand, for assembling parts of underwater structures, the joining process of metal to polymer is in great demand. Thus, considering some different features of polymeric materials from metals, Lee et al. [3,4] have investigated the electromagnetic joining process of metal tube on polymer core, both theoretically and experimentally. In the present paper, the electromagnetic joining process of a structural body for underwater usage which is composed of three members (copper tube/polyurethane tube/aluminum core) is studied in order to estimate t h e joint strength. The influence of various process variables on the joint strength is examined experimentally. Equations which can predict the joint strength are proposed, based on consideration of the elastic recovery of the polyurethane tube and the radial shrinkage of the tube resulting from pulling it axially.
2. Analysis
2.1. Deformation of a polyurethane tube subject to internal and external pressures Consider a polyurethane tube which is subjected uniformly to internal and external pressures Pi and Po respectively, where the deformation is symmetrical about the Z axis. This symmetry indicates that all shear stresses and corresponding strains inside the tube vanish. For the analysis of the deformation of the tube, the following assumptions are employed: (i) az is ignored (az = 0); (ii) the tube is considered as thick-walled; (iii) polyurethane is considered as a linearly-elastic material. Also, the aluminum core is assumed to be rigid. The radial displacement [5] of the polyurethane tube is expressed as
1--v (a2oPi-b2oPo)r ~ -l- -+~ -vUr=--~-×- bEo_ao2
2 2 (Pi-Po)aobo (bEo_a2o)r
× -
(1)
where ao and bo are respectively the inner and outer radii and E and v are Young's modulus and Poisson's ratio of the polyurethane, respectively.
W.S. Hwang et al./ Electromagnetic pulse forming
85
The inner diameter of the tube is initially made smaller (in its unstressed state) than the diameter of the core. The tube is then assembled on the core prior to the electromagnetic joining, after which the polyurethane tube is further compressed by the dynamic shrinking onto it of a copper tube by means of magnetic pressure. The assembled structure and the free-body diagram of the polyurethane tube at this stage are shown in Fig. 1, where Pi and Po are internal and external pressures, respectively. 2.2. Analysis of the strength of the joint Using eqn. (1), the internal pressure Pi and external pressure Po are derived from the displacement boundary conditions on the inner and outer surfaces of the polyurethane tube. The inner radius of the tube is equal to the radius (a¢) of the aluminum core, whilst the outer radius (b2) of the tube is measured after joining. The joint strength at the separation of the joint structure is estimated by pulling it axially: this strength is known to be governed by the frictional forces due to the contact pressures. First, for type B (Fig. 2 (d)), its joint strength FB is determined by the smaller of the two frictional forces arising from contact pressures Pi and Po on the opposite surfaces of the polyurethane tube. By comparison of two frictional forces, the joint strength for type B is determined by FB = F~= 2xac lttPi
(2)
where l is the joint length, ac is the radius of the aluminum core and p is the friction coefficient between the aluminum core and the polyurethane tube.
PO
$
4--
2ac ]
j
2 b1 2
c.I
(a)
!
t 2ac
I
I
2b2
I
(b)
Fig. 1. (a) Initial assembled structure; (b) the free-body diagram of the polyurethane tube after joining.
86
W.S. Hwang et al./Electromagnetic pulse forming
However, on estimating the joint strength of type A (Fig. 2(b)), FA, the thinning effect of the polyurethane tube should be considered. For this type of joint structure, axial stress arises in the polyurethane tube upon pulling it, so that thinning of the polyurethane occurs. Thus, the reduced radial strain ~,p due to the applied axial stress az is given by ~,,,
=
-
(;3)
v(az/E)
Hence, its reduced radial stress due to e,p is expressed as E
E
OrP--l~-v2(,Srp+V,~Op)~-~(l+l:)erp--
v I__Va.
(4)
As a,p is uniform throughout the wall thickness, the resultant contact pressure P with internal contact pressure Pi before axial loading is represented by P = Pi-
1--V
6z
(5)
Separation of the joint specimen occurs when the external axial load is equal to the frictional force due to the resultant pressure P. The joint strength for type A is given by (6)
Fn = 2n ac lit P = n(b 2 - a~ ) o.
TYPE
~ ~ ]
Tensile load o
Aluminum
A
T
core
Copper tube
+1.1 ,, tl.o Polyurethane tube
N
t 1
I
(a)
(b)
Fig. 2. Basic dimensions and configurations of specimens before and after joining in: (a, b) type A; (c, d) type B.
W.S. Hwangetal./Electromagneticpulse~rming TYPE
87
B
Tensile load Al core uminum
F~-~
~'=[
T
i ~ 53.0- I
Poylurethane~~tube '~l~'[ .. ¢ 61.1 .. tl.0 IT
II
Copper tube
(c)
(d)
Fig. 2. (Continued). where bE is the outer radius of the polyurethane tube after joining. Finally, FA is obtained from eqns. (5) and (6),
FA =n(b~-a2¢ ) 2aclttPi 2a¢ lp 1 v +(b~-a~) ) - 1
(7)
3. E x p e r i m e n t s
3.1. Specimens The materials used for these experiments are high-toughness polyurethane rubber tubes, annealed pure copper tubes and pure aluminum cores. The mechanical properties of the polyurethane and the copper are listed in Table 1. The basic dimensions and configurations of type A and B specimens, before and after joining, are shown in Fig. 2. Each specimen is designed for a tension test so as to be able to estimate its joint strength subsequent to electromagnetic joining. There are two kinds of joint specimens which are prepared separately for each operational mode of external forces; the type A specimen in Fig. 2 (b) is made such that an axial load is applied to the polyurethane tube and the type B specimen in Fig. 2 (d) is made for no load to be applied to the tube.
3.2. Experimental procedure A Magneform machine (maximum charged energy 16 kJ) is used for generating the impulsive magnetic flux to perform the electromagnetic joining experiments. A universal compression coil with an inner diameter of 101.6 mm and
88
W.S. Hwang et al./Electromagnetic pulse forming
Table 1 Mechanical properties of the copper and the polyurethane Material
copper polyurethane
Mechanical properties Young's modulus (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Poisson's ratio
85.3 24.5
178.3
235.2 15.7
40 350
0.34 0.50
height of 100 mm is used as a forming coil. This coil is of open type, allowing various field shapers to be placed inside it, wherein the field shaper is used for concentrating the magnetic pressure on the joint area. The split cylindrical field shaper, is made of A1 6061, has a height of 100 mm, and the inner diameter of protrusion inside the field shaper is 34 mm and its thickness is 32 mm. Joining experiments are carried out in the range of discharged energy 4-16 kJ and the number of discharges is from 1-15. In order to measure the strengths of the joints, tension tests were carried out. 4. R e s u l t s and d i s c u s s i o n
4.1. Deformed shape by electromagnetic joining Figure 3 indicates the distribution of the radial strain of the polyurethane tube along its axial length according to the discharged energy. Here, the radial strain represents compressive value. The strain is relatively uniform and small at a low energy level, but as the discharged energy increases, the strain increases and has higher values toward the edges. Therefore, the profile of the joint section has a convex shape as a whole. This trend is not similar to the results in metal/metal [1] and metal/ceramic [2] joining, but is similar to the case of free tube compression [6] by the electromagnetic process. This is because the core material (polyurethane) is easily deformed by the deformation of the copper tube, since polyurethane has much lower mechanical strength than t h a t of metal or ceramics. In the case of metal/metal (copper tube/aluminum core) joining at a discharged energy of 8 kJ and at the first time of discharge, the roundness of deformed copper tube is improved to 32 ~m from 64 ~m before joining. It is thought t h a t this improvement is caused by direct contact such t h a t the roundness of the copper tube approaches that of the aluminum core with its roundness of 8 ~m. The roundness of the copper tube in type A specimen joining under the same experimental conditions is measured as 340 ~Lm,which is near to the roundness (410 ~m) of the polyurethane tube before joining. Thus, it is found that the
W.S. Hwang et al./ Electromagnetic pulse forming
89
uniform and non-contact magnetic pressure on the copper tube results in the correction of the roundness of the polyurethane tube. Figure 4 shows the variation of the measured roundness of the copper tube with various discharged energies. The roundness of the polyurethane tube is improved with increase in discharged energy, and it reaches 285 pm at a discharged energy 16 kJ. It seems that the roundness of the polyurethane is improved, whilst the increased magnetic pressure makes the deformation of the copper tube more uniform, as the energy becomes higher. Figure 5 shows the relationship between the roundness of the copper tube and the number of discharges at the levels of discharged energy of 8 kJ and 16 kJ. At the level of discharged energy of 8 kJ, as the discharge number increases, the roundness of the polyurethane tends to be improved. However, after more than 10 discharges, the irregular deformation of the copper tube results in the degradation of the roundness. As the discharged energy increases to 16 kJ, the roundness becomes further improved by the effect of the increased magnetic pressure, as was described previously. 0.50 f
type B
===== 4(kJI, / :.:.:.:.-" 8(kJ}]. / , , , , , 12(k J). :===~ 16(kJ)
0.40 x
~0.20 ~0.10
°°°-zo
-~o
6
1'o
Distance from center (ram)
zo
Fig. 3. Distribution of the radial strain of the polyurethane tube according to the discharged energy (1 discharge, joint length 30 mm). 450 The initiol roundness level of P.U. 400
350
300
250 type A 200
4
fi
1'2
1'6 '~o
Discharged energy (k J)
Fig. 4. Variation of the roundness of the copper tube with various levels of discharged energy (1 discharge, joint length 30 mm).
90
W.S. Hwang et al./ Electromagnetic pulse forming 500
450
Discharged energy : 8(k J} Diecherged energy :16(kJ) The
initiol roundaess
.~
v~400 350 300
250
/
type A
2°°o' ~' ~.' 6 ' ~ ';o'1'2 Discharge number
14~-~6
Fig. 5. Relationship between the roundness of the copper tube and number of the discharges (joint length 30 mm).
4.2. Effect of the discharged energy on the joint strength Figure 6 shows the radial strain of the polyurethane tube according to the discharged energy. The strain increases as the discharged energy increases, because the magnitude of the magnetic pressure on the copper tube is dependent on the discharged energy. A comparison of the joint strengths for type A and type B at various levels of discharged energy is shown in Fig. 7. The joint strengths, as in the case of radial strains for both types, also increase as the level of energy increases, and the maximum values for each type reach 1.8 kN (type A) and 7.3 kN (type B) at the level of discharged energy 16 kJ. The reason why the joint strength becomes improved at the higher energy is because the contact pressure goes up with the increased radial strain. It is noted that the joint strength of type A is much lower than that of type B. 4.3. Effect of the number of discharges on the joint strength The variation of the radial strain of the polyurethane tube and of the joint strengths with various numbers of discharges is shown in Fig. 8 and Fig. 9, respectively. Both radial strain and joint strength increase as the number of discharges increases. However, as the number of discharges increases beyond seven, the joint strength becomes lower. This result can be explained by uneven deformation in which wrinkles occur severely with the increasing number of discharges, as indicated in Fig. 5. 4.4. Comparison of calculated values and experimental results In the previous Section 2, polyurethane was assumed to be a linearly-elastic material, and the joint strength could be regarded as a frictional resistance force caused by the contact pressure between the aluminum core and polyurethane tube. For type B the joint strength can be represented as the frictional force itself. On the other hand, in the case of type A, an axial stress exists in the polyurethane tube and thinning of polyurethane occurs when it is pulled axially. Thus, on estimating the joint strength of this structure, the
W.S. Hwang et al./Electromagnetic pulse forming
91
0,30 type B
•~ 0.20
~0.10 O
Q
D
0.00
Discharged energy (kJ)
Fig. 6. Radial strain of the polyurethane tube according to discharged energy (1 discharge, joint length 30 ram).
8.0 o ~ u u o type B ~ z , A ~ type A
6.0
~.o
•~
2.0 &
8
1'2
Discharged energy (kJ)
16
Fig. 7. Effect of the level of discharged energy on the joint strength (i discharge, joint length 30 ram).
0.30 type A
.~0.20
"o ~0.10
0.00
t~
48 1'2 Discharged number
'
1~
Fig. 8. Radial strain of the polyurethane tube according to number of the discharges (discharged energy 8 kJ).
W.S. Hwang et al./Electromagnetic pulse forming
92 8.0
000oo t y p e ~,~.AAZ~ t y p e
,~
B
A
6.0
4.0
"~ 2.0 A
,
1
4
_
I
~
-,
l
~. . . .
8 12 Discharged number
L ....
16
Fig. 9. Effect of number of the discharges on the joint strength (discharged energy 8 kJ, joint length 30 mm).
8.0 1/
~
,' 6.0
o
~o ^^^^~ooo00
4.0
g
"~ 2.0
--
,,'
/
~00
-
Experiment:type Ex~riment:
Eq.(2):type
A
B
,~
A
0.05
0.10 0.15 Radial strain
0.20
0.25
Fig. 10. Comparison of calculated joint strengths with experimental results.
radial shrinkage of the polyurethane tube by the axial load is considered. The equations which can predict the joint strengths for both types are proposed as eqn. (2) and eqn. (7). Figure 10 represents the relationship between the radial strain of polyurethane tubes and the joint strength for both types of specimens under the various joining conditions such as discharged energy, discharge number and joint length. In this figure, the dotted line represents the values calculated by the use of eqn. (2) for type B, and the full line by the use of eqn. (7) for type A. It can be seen t h a t calculated joint strength in both cases has a nearly linear relationship with the radial strain, but t h a t the gradient of the strength and strain increments for type A is much lower, and it is noted t h a t the values calculated agree relatively well with the corresponding experimental results. This means t h a t the proposed equations based on the residual strain of the polyurethane after joining can represent the joint strengths well. Furthermore, the consideration of the reduced radial strain of the polyurethane tube is reasonable when applied to determine the joint strength of type A.
W.S. Hwang et al./Electromagnetic pulse forming
93
Since the joint strength of type A is much lower than t hat of type B, in view of the results of both calculation and experiment, it is interesting to compare the ratio of calculated strengths by the following equation. FA/.F,=l[l+(2a¢ll~r)/(b~-a~)1-1
(8)
From this equation, it is found that the joint strength decreases rapidly when an axial load is applied to the polyurethane tube because the contact pressure reduces due to the radial shrinkage of the tube. 5. C o n c l u s i o n s The electromagnetic joining process of a three-component (metal/polymer/ metal) structure has been studied both experimentally and theoretically in order to estimate the joint strength and investigate the effects of variation of the process variables on the strength. The following results are obtained: (1) The joint strength is governed by the magnitude of the residual radial strain of the polyurethane tube. (2) The joint strength increases with the number of discharges and as the level of discharged energy increases, but decreases in the case of uneven deformation in which large wrinkles occur. (3) The joint strength decreases when an axial load is applied to the polyu r e t h a n e tube, because the contact pressure between the polyurethane tube and the aluminum core reduces due to the radial shrinkage of the tube: the values calculated for both types agree well with the experimental results. (4) As the electromagnetic joining energy and the number of discharges increase, the roundness of the polyurethane tubes is improved.
References [1] M. Murata, H. Negishi and H. Suzuki, High speed joining of tube by solenoidal compression coil, J. Jpn. Soc. Technol. Plast., 25(283) (1984) 702 708. [2] T. Sano, M. Takahashi, Y. Murakoshi, M. Terasaki and K. Matsuno, Electromagnetic joining of metal tubes to ceramic rods, J. Jpn, Soc. Technol. Plast., 28(322) (1987) 1192 1198. [3] J.S. Lee, H.S. Sohn, W.S. Hwang and N.H. Kim, Analysis of joining strength in electromagnetic joining of metal to high toughness polymers, J. Korean Soc, Prec. Eng., 9(3) (1992) 110-116. [4] W.S. Hwang, N.H. Kim, H.S. Sohn and J.S. Lee, Electromagnetic joining of aluminium tubes on polyurethane cores, J. Mater. Process. Technol., 34 (1992) 341-348. [5] A.K. Ugural and S.K. Fenster, Advanced Strength and Applied Elasticity, Elsevier Applied Science, 1981. [6] T. Sano, M. Takahashi, Y. Murakoshi and K. Matsuno, Electromagnetic tube compression with a field shaper, J. Jpn. Soc. Technol. Plast., 25(283) (1984) 731-738.
83
Elsevier
Joining of copper tube to polyurethane tube by electromagnetic pulse forming W.S. H w a n g Department of Metallurgical Engineering, Sung Hwa University, Chunan, Chungnam, South Korea J.S. L e e , N . H . K i m a n d H.S. S o h n
Materials Processing and Application Laboratory, Agency for Defense Development, P.O. Box 35, Yuseong, Daejeon, South Korea
Industrial S u m m a r y The electromagnetic joining process of a structural body for underwater applications, which is composed of three members (copper tube/polyurethane tube/aluminum core), is studied experimentally and theoretically in order to estimate the strength of the joint and to investigate the effect of process variables on the strength. From the experiments it has been found that the strength of the joint is governed by the magnitude of the residual radial strain of the polyurethane tube. As the number of discharges and the level of discharged energy increase, the strength normally increases, but it decreases in the case of uneven deformation in which large wrinkles occur. The joint strength degrades rapidly when an axial load is applied to the polyurethane tube because the contact pressure between the polyurethane tube and the aluminum core is reduced due to the effect of the radial shrinkage of the polyurethane. Equations which can predict the joint strength are proposed: it is found that the calculated values agree well with the experimental results.
1. I n t r o d u c t i o n In accordance with the development of many advanced materials such as new metals, ceramics and polymers, there has been a great need of bonding methods with high strength and smooth configuration. Amongst them, the e l e c t r o m a g n e t i c j o i n i n g p r o c e s s is k n o w n t o a s u i t a b l e m e t h o d for t h e j o i n i n g of dissimilar materials.
Correspondence to: Dr. J.S. Lee, Materials Processing and Application Laboratory, Agency for Defense Development, P.O. Box 35, Yuseong, Daejeon, South Korea. 0924-0136/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
84
W.S. Hwang et al./Etectromagnetic pulse forming
Most of the reported investigations into the field of electromagnetic joining have been concerned mainly with the improvement and prediction of the strength of the joint. Murata et al. [1] have analyzed the mechanism of the metal/metal joining process based on experiment and have derived an equation for the axial pull-out strength, reporting that the calculated values agreed well with the experimental results. Sano and co-workers [2] have shown the possibility of joining metal tube to ceramic rod electromagnetically and have investigated the influences of process variables, including heat treatment, for joining. However, there remained some problems, in that the fracture occurred easily in the ceramic rod due to its weakness against impact and as the real contact area was narrow. On the other hand, for assembling parts of underwater structures, the joining process of metal to polymer is in great demand. Thus, considering some different features of polymeric materials from metals, Lee et al. [3,4] have investigated the electromagnetic joining process of metal tube on polymer core, both theoretically and experimentally. In the present paper, the electromagnetic joining process of a structural body for underwater usage which is composed of three members (copper tube/polyurethane tube/aluminum core) is studied in order to estimate t h e joint strength. The influence of various process variables on the joint strength is examined experimentally. Equations which can predict the joint strength are proposed, based on consideration of the elastic recovery of the polyurethane tube and the radial shrinkage of the tube resulting from pulling it axially.
2. Analysis
2.1. Deformation of a polyurethane tube subject to internal and external pressures Consider a polyurethane tube which is subjected uniformly to internal and external pressures Pi and Po respectively, where the deformation is symmetrical about the Z axis. This symmetry indicates that all shear stresses and corresponding strains inside the tube vanish. For the analysis of the deformation of the tube, the following assumptions are employed: (i) az is ignored (az = 0); (ii) the tube is considered as thick-walled; (iii) polyurethane is considered as a linearly-elastic material. Also, the aluminum core is assumed to be rigid. The radial displacement [5] of the polyurethane tube is expressed as
1--v (a2oPi-b2oPo)r ~ -l- -+~ -vUr=--~-×- bEo_ao2
2 2 (Pi-Po)aobo (bEo_a2o)r
× -
(1)
where ao and bo are respectively the inner and outer radii and E and v are Young's modulus and Poisson's ratio of the polyurethane, respectively.
W.S. Hwang et al./ Electromagnetic pulse forming
85
The inner diameter of the tube is initially made smaller (in its unstressed state) than the diameter of the core. The tube is then assembled on the core prior to the electromagnetic joining, after which the polyurethane tube is further compressed by the dynamic shrinking onto it of a copper tube by means of magnetic pressure. The assembled structure and the free-body diagram of the polyurethane tube at this stage are shown in Fig. 1, where Pi and Po are internal and external pressures, respectively. 2.2. Analysis of the strength of the joint Using eqn. (1), the internal pressure Pi and external pressure Po are derived from the displacement boundary conditions on the inner and outer surfaces of the polyurethane tube. The inner radius of the tube is equal to the radius (a¢) of the aluminum core, whilst the outer radius (b2) of the tube is measured after joining. The joint strength at the separation of the joint structure is estimated by pulling it axially: this strength is known to be governed by the frictional forces due to the contact pressures. First, for type B (Fig. 2 (d)), its joint strength FB is determined by the smaller of the two frictional forces arising from contact pressures Pi and Po on the opposite surfaces of the polyurethane tube. By comparison of two frictional forces, the joint strength for type B is determined by FB = F~= 2xac lttPi
(2)
where l is the joint length, ac is the radius of the aluminum core and p is the friction coefficient between the aluminum core and the polyurethane tube.
PO
$
4--
2ac ]
j
2 b1 2
c.I
(a)
!
t 2ac
I
I
2b2
I
(b)
Fig. 1. (a) Initial assembled structure; (b) the free-body diagram of the polyurethane tube after joining.
86
W.S. Hwang et al./Electromagnetic pulse forming
However, on estimating the joint strength of type A (Fig. 2(b)), FA, the thinning effect of the polyurethane tube should be considered. For this type of joint structure, axial stress arises in the polyurethane tube upon pulling it, so that thinning of the polyurethane occurs. Thus, the reduced radial strain ~,p due to the applied axial stress az is given by ~,,,
=
-
(;3)
v(az/E)
Hence, its reduced radial stress due to e,p is expressed as E
E
OrP--l~-v2(,Srp+V,~Op)~-~(l+l:)erp--
v I__Va.
(4)
As a,p is uniform throughout the wall thickness, the resultant contact pressure P with internal contact pressure Pi before axial loading is represented by P = Pi-
1--V
6z
(5)
Separation of the joint specimen occurs when the external axial load is equal to the frictional force due to the resultant pressure P. The joint strength for type A is given by (6)
Fn = 2n ac lit P = n(b 2 - a~ ) o.
TYPE
~ ~ ]
Tensile load o
Aluminum
A
T
core
Copper tube
+1.1 ,, tl.o Polyurethane tube
N
t 1
I
(a)
(b)
Fig. 2. Basic dimensions and configurations of specimens before and after joining in: (a, b) type A; (c, d) type B.
W.S. Hwangetal./Electromagneticpulse~rming TYPE
87
B
Tensile load Al core uminum
F~-~
~'=[
T
i ~ 53.0- I
Poylurethane~~tube '~l~'[ .. ¢ 61.1 .. tl.0 IT
II
Copper tube
(c)
(d)
Fig. 2. (Continued). where bE is the outer radius of the polyurethane tube after joining. Finally, FA is obtained from eqns. (5) and (6),
FA =n(b~-a2¢ ) 2aclttPi 2a¢ lp 1 v +(b~-a~) ) - 1
(7)
3. E x p e r i m e n t s
3.1. Specimens The materials used for these experiments are high-toughness polyurethane rubber tubes, annealed pure copper tubes and pure aluminum cores. The mechanical properties of the polyurethane and the copper are listed in Table 1. The basic dimensions and configurations of type A and B specimens, before and after joining, are shown in Fig. 2. Each specimen is designed for a tension test so as to be able to estimate its joint strength subsequent to electromagnetic joining. There are two kinds of joint specimens which are prepared separately for each operational mode of external forces; the type A specimen in Fig. 2 (b) is made such that an axial load is applied to the polyurethane tube and the type B specimen in Fig. 2 (d) is made for no load to be applied to the tube.
3.2. Experimental procedure A Magneform machine (maximum charged energy 16 kJ) is used for generating the impulsive magnetic flux to perform the electromagnetic joining experiments. A universal compression coil with an inner diameter of 101.6 mm and
88
W.S. Hwang et al./Electromagnetic pulse forming
Table 1 Mechanical properties of the copper and the polyurethane Material
copper polyurethane
Mechanical properties Young's modulus (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Poisson's ratio
85.3 24.5
178.3
235.2 15.7
40 350
0.34 0.50
height of 100 mm is used as a forming coil. This coil is of open type, allowing various field shapers to be placed inside it, wherein the field shaper is used for concentrating the magnetic pressure on the joint area. The split cylindrical field shaper, is made of A1 6061, has a height of 100 mm, and the inner diameter of protrusion inside the field shaper is 34 mm and its thickness is 32 mm. Joining experiments are carried out in the range of discharged energy 4-16 kJ and the number of discharges is from 1-15. In order to measure the strengths of the joints, tension tests were carried out. 4. R e s u l t s and d i s c u s s i o n
4.1. Deformed shape by electromagnetic joining Figure 3 indicates the distribution of the radial strain of the polyurethane tube along its axial length according to the discharged energy. Here, the radial strain represents compressive value. The strain is relatively uniform and small at a low energy level, but as the discharged energy increases, the strain increases and has higher values toward the edges. Therefore, the profile of the joint section has a convex shape as a whole. This trend is not similar to the results in metal/metal [1] and metal/ceramic [2] joining, but is similar to the case of free tube compression [6] by the electromagnetic process. This is because the core material (polyurethane) is easily deformed by the deformation of the copper tube, since polyurethane has much lower mechanical strength than t h a t of metal or ceramics. In the case of metal/metal (copper tube/aluminum core) joining at a discharged energy of 8 kJ and at the first time of discharge, the roundness of deformed copper tube is improved to 32 ~m from 64 ~m before joining. It is thought t h a t this improvement is caused by direct contact such t h a t the roundness of the copper tube approaches that of the aluminum core with its roundness of 8 ~m. The roundness of the copper tube in type A specimen joining under the same experimental conditions is measured as 340 ~Lm,which is near to the roundness (410 ~m) of the polyurethane tube before joining. Thus, it is found that the
W.S. Hwang et al./ Electromagnetic pulse forming
89
uniform and non-contact magnetic pressure on the copper tube results in the correction of the roundness of the polyurethane tube. Figure 4 shows the variation of the measured roundness of the copper tube with various discharged energies. The roundness of the polyurethane tube is improved with increase in discharged energy, and it reaches 285 pm at a discharged energy 16 kJ. It seems that the roundness of the polyurethane is improved, whilst the increased magnetic pressure makes the deformation of the copper tube more uniform, as the energy becomes higher. Figure 5 shows the relationship between the roundness of the copper tube and the number of discharges at the levels of discharged energy of 8 kJ and 16 kJ. At the level of discharged energy of 8 kJ, as the discharge number increases, the roundness of the polyurethane tends to be improved. However, after more than 10 discharges, the irregular deformation of the copper tube results in the degradation of the roundness. As the discharged energy increases to 16 kJ, the roundness becomes further improved by the effect of the increased magnetic pressure, as was described previously. 0.50 f
type B
===== 4(kJI, / :.:.:.:.-" 8(kJ}]. / , , , , , 12(k J). :===~ 16(kJ)
0.40 x
~0.20 ~0.10
°°°-zo
-~o
6
1'o
Distance from center (ram)
zo
Fig. 3. Distribution of the radial strain of the polyurethane tube according to the discharged energy (1 discharge, joint length 30 mm). 450 The initiol roundness level of P.U. 400
350
300
250 type A 200
4
fi
1'2
1'6 '~o
Discharged energy (k J)
Fig. 4. Variation of the roundness of the copper tube with various levels of discharged energy (1 discharge, joint length 30 mm).
90
W.S. Hwang et al./ Electromagnetic pulse forming 500
450
Discharged energy : 8(k J} Diecherged energy :16(kJ) The
initiol roundaess
.~
v~400 350 300
250
/
type A
2°°o' ~' ~.' 6 ' ~ ';o'1'2 Discharge number
14~-~6
Fig. 5. Relationship between the roundness of the copper tube and number of the discharges (joint length 30 mm).
4.2. Effect of the discharged energy on the joint strength Figure 6 shows the radial strain of the polyurethane tube according to the discharged energy. The strain increases as the discharged energy increases, because the magnitude of the magnetic pressure on the copper tube is dependent on the discharged energy. A comparison of the joint strengths for type A and type B at various levels of discharged energy is shown in Fig. 7. The joint strengths, as in the case of radial strains for both types, also increase as the level of energy increases, and the maximum values for each type reach 1.8 kN (type A) and 7.3 kN (type B) at the level of discharged energy 16 kJ. The reason why the joint strength becomes improved at the higher energy is because the contact pressure goes up with the increased radial strain. It is noted that the joint strength of type A is much lower than that of type B. 4.3. Effect of the number of discharges on the joint strength The variation of the radial strain of the polyurethane tube and of the joint strengths with various numbers of discharges is shown in Fig. 8 and Fig. 9, respectively. Both radial strain and joint strength increase as the number of discharges increases. However, as the number of discharges increases beyond seven, the joint strength becomes lower. This result can be explained by uneven deformation in which wrinkles occur severely with the increasing number of discharges, as indicated in Fig. 5. 4.4. Comparison of calculated values and experimental results In the previous Section 2, polyurethane was assumed to be a linearly-elastic material, and the joint strength could be regarded as a frictional resistance force caused by the contact pressure between the aluminum core and polyurethane tube. For type B the joint strength can be represented as the frictional force itself. On the other hand, in the case of type A, an axial stress exists in the polyurethane tube and thinning of polyurethane occurs when it is pulled axially. Thus, on estimating the joint strength of this structure, the
W.S. Hwang et al./Electromagnetic pulse forming
91
0,30 type B
•~ 0.20
~0.10 O
Q
D
0.00
Discharged energy (kJ)
Fig. 6. Radial strain of the polyurethane tube according to discharged energy (1 discharge, joint length 30 ram).
8.0 o ~ u u o type B ~ z , A ~ type A
6.0
~.o
•~
2.0 &
8
1'2
Discharged energy (kJ)
16
Fig. 7. Effect of the level of discharged energy on the joint strength (i discharge, joint length 30 ram).
0.30 type A
.~0.20
"o ~0.10
0.00
t~
48 1'2 Discharged number
'
1~
Fig. 8. Radial strain of the polyurethane tube according to number of the discharges (discharged energy 8 kJ).
W.S. Hwang et al./Electromagnetic pulse forming
92 8.0
000oo t y p e ~,~.AAZ~ t y p e
,~
B
A
6.0
4.0
"~ 2.0 A
,
1
4
_
I
~
-,
l
~. . . .
8 12 Discharged number
L ....
16
Fig. 9. Effect of number of the discharges on the joint strength (discharged energy 8 kJ, joint length 30 mm).
8.0 1/
~
,' 6.0
o
~o ^^^^~ooo00
4.0
g
"~ 2.0
--
,,'
/
~00
-
Experiment:type Ex~riment:
Eq.(2):type
A
B
,~
A
0.05
0.10 0.15 Radial strain
0.20
0.25
Fig. 10. Comparison of calculated joint strengths with experimental results.
radial shrinkage of the polyurethane tube by the axial load is considered. The equations which can predict the joint strengths for both types are proposed as eqn. (2) and eqn. (7). Figure 10 represents the relationship between the radial strain of polyurethane tubes and the joint strength for both types of specimens under the various joining conditions such as discharged energy, discharge number and joint length. In this figure, the dotted line represents the values calculated by the use of eqn. (2) for type B, and the full line by the use of eqn. (7) for type A. It can be seen t h a t calculated joint strength in both cases has a nearly linear relationship with the radial strain, but t h a t the gradient of the strength and strain increments for type A is much lower, and it is noted t h a t the values calculated agree relatively well with the corresponding experimental results. This means t h a t the proposed equations based on the residual strain of the polyurethane after joining can represent the joint strengths well. Furthermore, the consideration of the reduced radial strain of the polyurethane tube is reasonable when applied to determine the joint strength of type A.
W.S. Hwang et al./Electromagnetic pulse forming
93
Since the joint strength of type A is much lower than t hat of type B, in view of the results of both calculation and experiment, it is interesting to compare the ratio of calculated strengths by the following equation. FA/.F,=l[l+(2a¢ll~r)/(b~-a~)1-1
(8)
From this equation, it is found that the joint strength decreases rapidly when an axial load is applied to the polyurethane tube because the contact pressure reduces due to the radial shrinkage of the tube. 5. C o n c l u s i o n s The electromagnetic joining process of a three-component (metal/polymer/ metal) structure has been studied both experimentally and theoretically in order to estimate the joint strength and investigate the effects of variation of the process variables on the strength. The following results are obtained: (1) The joint strength is governed by the magnitude of the residual radial strain of the polyurethane tube. (2) The joint strength increases with the number of discharges and as the level of discharged energy increases, but decreases in the case of uneven deformation in which large wrinkles occur. (3) The joint strength decreases when an axial load is applied to the polyu r e t h a n e tube, because the contact pressure between the polyurethane tube and the aluminum core reduces due to the radial shrinkage of the tube: the values calculated for both types agree well with the experimental results. (4) As the electromagnetic joining energy and the number of discharges increase, the roundness of the polyurethane tubes is improved.
References [1] M. Murata, H. Negishi and H. Suzuki, High speed joining of tube by solenoidal compression coil, J. Jpn. Soc. Technol. Plast., 25(283) (1984) 702 708. [2] T. Sano, M. Takahashi, Y. Murakoshi, M. Terasaki and K. Matsuno, Electromagnetic joining of metal tubes to ceramic rods, J. Jpn, Soc. Technol. Plast., 28(322) (1987) 1192 1198. [3] J.S. Lee, H.S. Sohn, W.S. Hwang and N.H. Kim, Analysis of joining strength in electromagnetic joining of metal to high toughness polymers, J. Korean Soc, Prec. Eng., 9(3) (1992) 110-116. [4] W.S. Hwang, N.H. Kim, H.S. Sohn and J.S. Lee, Electromagnetic joining of aluminium tubes on polyurethane cores, J. Mater. Process. Technol., 34 (1992) 341-348. [5] A.K. Ugural and S.K. Fenster, Advanced Strength and Applied Elasticity, Elsevier Applied Science, 1981. [6] T. Sano, M. Takahashi, Y. Murakoshi and K. Matsuno, Electromagnetic tube compression with a field shaper, J. Jpn. Soc. Technol. Plast., 25(283) (1984) 731-738.