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Electromagnetic joining of aluminum tubes on polyurethane cores
Journal of Materials Processing Technology, 34 (1992) 341-348 Elsevier
341
ElectromagneticJoining of AluminumTubes on PolyurethaneCores W.S.Hwang a, N.H.Kim b, H.S.Sohn b, J.S.Leeb aDepartment of Metallurgical Engineering, Sung Hwa University, Chunan, Chungnam, Korea bMaterials Processing and Application Lab., Agency for Defense Development, P.O.Box 35 Yuseong, Daejeon, Korea
Abstract In the case of metal to polymer electr~)magnetic joining processes, aluminum alloy tubes and polyurethane cores are selected and studied in order to estimate the joining strength and to determine the process variables. By increasing the discharged energy and the number of discharges, the joining strength is improved due to the residual radial strain increment of the polyurethane core. On the other hand, the increase of the other process variables such as the joined length, the thickness of tube, and the clearance between the tube and core, is weakening the joining strength. And it is noted that the joining strength is mainly dependent on the residual radial strain of the polyurethane core. The governing equation which can estimate the joining strength is proposed under considering the elastic recovery of the polyurethane core and the radial shrinkage of the core by pulling it axially. And it is found that the estimated values agree well with experimental results.
I.
INTRODUCTION
The electromagnetic joining process, applying an intense and transient magnetic field to form metals, has been widely used in automotive, aerospace, electrical and ordance industry as the joining and assembling methods of axisymmetric parts [1]. There had been several investigations [2-4] to improve the joining strength by considering the effects of process variables and varying the joining conditions. However, these studies were mainly focused on metal to metal or metal tubes to ceramic rods. Recently, there has been in great demand for metal to polymer joining for underwater sonars, ocean structures, chemical plants and etc. Here, it is noted that this electromagnetic joining can be accomplished as long as outer tubes are good electrical conductors regardless of the kinds of cores. In the present paper, aluminum alloy tubes and high toughness polyurethane rubber cores are chosen for the study of electromagnetic impulsive metal to polymer joining. The influences of various geometrical and process factors are examined experimentally and discussed in detail. Furthermore, an equation which can estimate the joining strength is
0924-0136192/$05.00 © 1992 Elsevier SciencePublishersB.V. All rights reserved.
342
proposed under considering the elastic recovery of the polyurethane core and the raidal shrinkage of the core caused by pulling it axially. 2.
30INING
STRENGTH
UNDER
THE
CONSIDERATION
OF SHRINKAGE
EFFECT
The strength of electormagnetic joining is sustained by frictional force due to contact pressure between two members, and the contact pressure is related to the residual strain after joining [3]. In the joining of thin tube and round bar (core), the tube is governed only by the state of circumferential stress o0 when the wall thickness is thin. On the other hand, the core is layed under the state of plane stress following such a relation as o0 = or between circumferential and radial stresses, and no axial stress Oz exists and e0 = ~r in the core if the shape of core is made of cylindrical rod and both ends of the core are open. When the core behaves elastically like metals and the joining strength is estimated only by an axial tensile load, its residual strain (ero) contact stress (qo) relationship in the core and the joining strength may be expressed respectively as qo(1-v) ero
(1)
=
Fo
=
2nrl~qo
(2)
But polyurethane is regarded as an elastic material with a non-linear relationship over the wide range of elastic region, then from the tension tests, the relationship of axial stress and strain is presumed by o~ = a~z b
(3)
where a and b are constants. And, when polyurethane core is subject to lateral pressure, Or in the core may be experssed, from eqs(1) and (3), as ~ro
b
Finally, from eqs(2) and (4), the joining strength is given by ~ro
Fo =
2nrlfla t~_~,
]b
(S)
On estimating the joining strength, however, the radial shrinkage of the core by an axial tensile load is to be considered since the applied load causes the radial strain of polyurethane core to reduce. From the relationship of ez = -er/V for an uni-axial tension, and using eq(3), the reduced radial strain ere by an axial tensile load is expressed as ere
=v [ vz]l/b , ~
(6)
343
If qf is a reduced radial stress due to ere, qe = a {erf/(1-p)} b is satisfied from eq(4). Thus, when an external tensile load is axially applied to the core, the resultant contact stress q is represented by a
q : qo - qf -
-
-
(l-p) b
(~ro b - £rf b )
(7)
The separation of joining specimen occurs when the value of external tensile load equals to that of frictional force by the resultant contact pressure q. Hence (8)
2nrl~q = nr2oz
Thus, the joining strength can be obtained from eqs.(6), (7) and (8) 21~a
F = nr2
(9)
£ro b
(l-P)
b • r
+ 21~v b
where r = ri exp(-~ro) and ri is the initial radius of the core. T h e r e f o r e , only from the measured strain (ero) of the polyurethane core after joining, the real joining strength can be calculated even with the consideration of c o r e ' s shrinkage.
3.
EXPERIMENT
3 - l . Workpieces The workpieces consist of aluminum 6061 tubes (outer one) having various lengths (1 = 2 0 ~ 4 0 m m ) and wall thicknesses (t = 0 . 8 ~ 2 . 0 m m ) , and high toughness polyurethane rubber rods (inner cores) with different diameters ( d = 1 5 . 8 ~ 1 8 . 6 m m ) as shown in Fig. l(a). R2O
Polyurethane
~'~/----
~i Field shaper
Aluminum tube dl= d2 -
Insulator
. . . . . . <....... "~,J " l,-.-.-.-.,. . . . . . . . ]~1~ I , I r
15.8 ~ 18.6 mm 16.0 ~- 18.8 mm, 1=20---40 mm (a)
Aluminum tube (b)
Figure 1. Dimension of workpieces (a), and setup (b) for the electromagnetic joining experiment.
344
3-2. Experimental setup and procedure
Magneform M/C (Maximum charged energy 8KJ) is used for generating transient and impulsive magnetic pressure to compress aluminum alloy tubes. To concentrate its pressure on a certain part, the field shaper inside compression coil is used as a forming coil, which is made of aluminum 2024-T6. In the electromagnetic joining experiment for tube compression, the workpieces are mounted into the field shaper as shown in Fig. l(b) and compressed by 2 ~ 8 K J electromagnetically discharged energy. In order to estimate the joining strength, tension tests have been carried out with 3mm/min ram speed.
4. R E S U L T S A N D D I S C U S S I O N
4-1. Effects of process and configuration parameters 4-1-1. Effect of discharged energy
Fig.2 shows, in accordance with the variations of discharged energy, the radial strain of the inner core. It is also shown that the strain increases as the level of discharged energy elevates. And Fig.3 shows such a result that the joining strength is obtained when the separation of joinned structure occurs by a simple tension test of specimens as shown in Fig. l(b). As shown in the case of core's radial strain, the joining strength is inclined to increase as the level of discharged energy goes up.
From these results, it is conceivable that the magnetic pressure imposing on outer tube is increasing as the level of discharged energy is going up, and then, it causes the radial strain increase of outer tube and inner core to result in the improvement of autofrettaging force on the joined part.
0.04
0.8C r.~
0.03
-~ o.oz
[]
0.6C
o
0.4(]
;~
[]
lad
0.01
0.2G
[]
[]
o o.oo o
.
~.
"
Discharged
~.
.
~
energy
-
10
E/r~
Figure 2. Effect of discharged energy on radial strain of core.
O.OC
~.
'
Discharge
,i
'
~
energyd
'
h
E /
'
10
ILl
Figure 3. Effect of discharged energy on joining strength.
345
4-1-2. Effects of outer tube's length and thickness
Fig.4 shows the relationship between the length of the tube and its radial strain after joining, and illustrates the decrease in the radial strain as the length of outer tube increases. Fig.5 shows the relationship between the length of outer tube and the joining strength, which presents the decreasing trend of joining strength as its length increases. And, the measured results of magnetic flux density between field shaper and outer tube represent such a trend that, in accordance with the increase in the length of outer tube, the magnetic pressure imposed on the tube decreases.
1.00
0.15
.~ O.lO
0
,~1
0.80
D 0
0
o
0 o
0 'O
o
0.05
w
O.C~
o
°'°°1~
2'o
~5
3'o
Joined length
3~
~
4~
1 / mm
Figure 4. Effect of joined length on radial strain of core. (discharge number 3)
o.~
~o
2~
~o
Joined length
~
~o
4.5
1 /mm
Figure 5. Effect of joined length on joining strength.
Accordingly, the area of joined part increases as the length of outer tube becomes longer, however, it seems that due to the reduction of magnetic pressure, the radial strain of inner core decreases. Thus, in the electromagnetic joining process, it can be concluded that the length of joined part should be determined by considering two major factors such as the joined area and the resultant magnetic pressure on the tube. Fig.6 and 7 show the results of strain of the core and the joining strength with outer tube's thicknesses from 0.8mm to 2ram. The strain of the core is decreasing as the thickness of outer tube is increasing, and the joining strength like the case of strain is also inclined to be decreasing in accordance with its thickness increase. The thickness of outer tube should be determined by considering the relationship between the plastic strains of outer tubes and its repressive strength against the elastic recovery of inner core. In this category of experiment, the maximum joining strength has been obtained from the case of the thinnest outer tube (0.8mm). This result shows that the higher joining strength can be obtained sufficiently from the thin tube since the polymer material of inner core like polyurethane has conspicuously lower value of resilient stress than the metals do.
346
4-1-3. Effect of joining clearance The variations of radial strains in the inner core are shown in Fig.8 where the clearance between tube and core is limited from 0 . 1 m m to 1.0mm, and the joining strength is also shown in Fig.9. As the clearance is increasing, the deformation amount of outer tube is increasing but the one of inner core is decreasing. This is because the joining proceeds after outer tube has been freely compressed up to the dimension of clearance as given. It is shown that the joining strength is increasing for the smaller size of clearances as for the both cases of metal to metal joining [3] and metal to ceramic one [4].
0.15
1.00
;,'LL,';~ o
0.80 0.10
o
A
o o
[]
"~ o.0s
0
A
A
0 .~
o
o.o%.6
o A
~
0.60 El
No.
1.0 1.~1 Thickness
1.'8 t
/
0.40
0.28,
8.8
1.6
t).
Thicl~ness
nun
Figure 6. Effect of tube thickness on radial strain of core. (discharged energy 8KJ, discharge number 3, joined length 30mm)
2.z
Figure 7. Effect of tube thickness on joining strength.
~
0.15
t.~ t / mm
1.00
0.80
.5
OlO
"d -~
0.60
Cl
13
0
0.05
e~
o
[]
0-000.'0
.~
0,40
O ~
0.28 '
[]
' o.'~
' o.:,..
Clearance
o
'o.6
13
' o.'8
' t.~
d / mm
Figure 8. Effect of initial clearance on the radial strain of core. (discharged energy 8KJ,discharge number 1, joined length 30mm)
' o.~
' o.~.
Clearance
' o.6
'o.~ ' d / ram
1.'o
Figure 9. Effect of initial clearances on joining strength.
347
4-1-4. Effect of discharge numbers Fig. 10 shows the relationship between discharge numbers and the radial strain of inner core, and Fig. 11 does the relationship between discharge numbers and joining strength. In accordance with the increase of discharge numbers, the deformation continues to increase. However, the joining strength shows its maximum value at 7 times of discharges and is inclined to be decreasing at more than that. It is thought that after more than 7 times of discharges, the irregular deformation of outer tubes occurs more increasingly to reduce the total contact area of joining part and thus, its joining strength too.
0.25
1.20 f 1.00
0.20
[] 0
0
0.80 ta 0.15 0
0
0
0.10
0.40
"m cl
0
I~ 0.05
"
o
0.20 0.00~
0.00 0 Discharge number
N
Figure 10. Effect of discharge number on radial strain of core. (discharged energy 8KJ, joined length 30mm)
Discharge number
N
Figure 11. Effect of discharge number on joining strength.
4-2. Relationship between polyurethane's strain and joining strength
As the above, a computational procedure to estimate the joining strength is proposed by assuming that it heavily depend on polyurethane's residual elastic strain. Since polyurethane has a wide range of elastic region and much smaller young's modulus in comparision with metals. Also, the elastic shrinkage of polyurethane core is considered when the core is axially pulled by a simple tension. It reduces the joining strength due to the reduction of autofrettaging force in the compressive tube. Fig. 12, compares the computed joining strengths with the experimental ones in accordance with polyurethane's radial strains (e~o) after the completion of joining. In this figure, the dotted line shows such a case that the reduction of autofrettage force under the external tension is not considered, i.e. the computed results from Eq.(5). And the real line shows the computed results from Eq.(9) in which this reduced effect is considered. As shown in Fig. 12, the computed results from Eq.(5) show significantly higer value than the experimental ones do but the computational results for Eq.(9) considering the reduction of autofrettage force are in good agreement with the experimental ones. These results show that the joining strength can be estimated from
348 the above mentioned computational procedure by measuring the residual radial strains of polyurethane core after the completion of joining. 2.50
/ / / /
/
2.00
,/
ooooo
eq- (9) e q . 15). experLmenE
.
! !
1.50
I / / /
-~
[]
/
1.00
0
I I
o ° []
0
/
oo
0.50 • F.,,,I
0
°
60 0.6z 0.64 0.68 0.68 o io o lz 014 Radial 8train
Figure 12. Comparision of joining strength between the computed and experimental results
5. C O N C L U S I O N S
In this paper, its joinability by electromagnetic forming process has been diagnosed and studied by presenting and analyzing certain problems which are arising. And also, the joining technique has been invetigated through experiments by considering the process and configuration parameters which influence the joining strength. For estimating the joining strength, a computational procedure has been proposed and its values has been compared with the experimental results. Finally, it can be concluded that the process variables chosen for this electromagnetic joining of aluminum tubes on polyurethane cores represent the characteristics of this process and estimated values agree well with the experimental results.
6. REFERENCES 1 D.F.Brower, Metals Handbook, ASM, Metals Park, Vol.4 (1969) 256. 2 L.G.Kuncl, R.B.Hagemann and J.Kotora, ANL Report 7343, (1967). 3 M.Murata, H.Negishi and H.Suzuki, J.Japan Soc. for Teehnol. of Plas., Vol.25, No.283 (1984) 702. 4 T.Sano, M.Takahashi, Y.Murakoshi, M.Terasaki and K.Matsuno, J.Japan Soc. for Technol. of Plas., Vol.28, No.322 (1987) 1192.
341
ElectromagneticJoining of AluminumTubes on PolyurethaneCores W.S.Hwang a, N.H.Kim b, H.S.Sohn b, J.S.Leeb aDepartment of Metallurgical Engineering, Sung Hwa University, Chunan, Chungnam, Korea bMaterials Processing and Application Lab., Agency for Defense Development, P.O.Box 35 Yuseong, Daejeon, Korea
Abstract In the case of metal to polymer electr~)magnetic joining processes, aluminum alloy tubes and polyurethane cores are selected and studied in order to estimate the joining strength and to determine the process variables. By increasing the discharged energy and the number of discharges, the joining strength is improved due to the residual radial strain increment of the polyurethane core. On the other hand, the increase of the other process variables such as the joined length, the thickness of tube, and the clearance between the tube and core, is weakening the joining strength. And it is noted that the joining strength is mainly dependent on the residual radial strain of the polyurethane core. The governing equation which can estimate the joining strength is proposed under considering the elastic recovery of the polyurethane core and the radial shrinkage of the core by pulling it axially. And it is found that the estimated values agree well with experimental results.
I.
INTRODUCTION
The electromagnetic joining process, applying an intense and transient magnetic field to form metals, has been widely used in automotive, aerospace, electrical and ordance industry as the joining and assembling methods of axisymmetric parts [1]. There had been several investigations [2-4] to improve the joining strength by considering the effects of process variables and varying the joining conditions. However, these studies were mainly focused on metal to metal or metal tubes to ceramic rods. Recently, there has been in great demand for metal to polymer joining for underwater sonars, ocean structures, chemical plants and etc. Here, it is noted that this electromagnetic joining can be accomplished as long as outer tubes are good electrical conductors regardless of the kinds of cores. In the present paper, aluminum alloy tubes and high toughness polyurethane rubber cores are chosen for the study of electromagnetic impulsive metal to polymer joining. The influences of various geometrical and process factors are examined experimentally and discussed in detail. Furthermore, an equation which can estimate the joining strength is
0924-0136192/$05.00 © 1992 Elsevier SciencePublishersB.V. All rights reserved.
342
proposed under considering the elastic recovery of the polyurethane core and the raidal shrinkage of the core caused by pulling it axially. 2.
30INING
STRENGTH
UNDER
THE
CONSIDERATION
OF SHRINKAGE
EFFECT
The strength of electormagnetic joining is sustained by frictional force due to contact pressure between two members, and the contact pressure is related to the residual strain after joining [3]. In the joining of thin tube and round bar (core), the tube is governed only by the state of circumferential stress o0 when the wall thickness is thin. On the other hand, the core is layed under the state of plane stress following such a relation as o0 = or between circumferential and radial stresses, and no axial stress Oz exists and e0 = ~r in the core if the shape of core is made of cylindrical rod and both ends of the core are open. When the core behaves elastically like metals and the joining strength is estimated only by an axial tensile load, its residual strain (ero) contact stress (qo) relationship in the core and the joining strength may be expressed respectively as qo(1-v) ero
(1)
=
Fo
=
2nrl~qo
(2)
But polyurethane is regarded as an elastic material with a non-linear relationship over the wide range of elastic region, then from the tension tests, the relationship of axial stress and strain is presumed by o~ = a~z b
(3)
where a and b are constants. And, when polyurethane core is subject to lateral pressure, Or in the core may be experssed, from eqs(1) and (3), as ~ro
b
Finally, from eqs(2) and (4), the joining strength is given by ~ro
Fo =
2nrlfla t~_~,
]b
(S)
On estimating the joining strength, however, the radial shrinkage of the core by an axial tensile load is to be considered since the applied load causes the radial strain of polyurethane core to reduce. From the relationship of ez = -er/V for an uni-axial tension, and using eq(3), the reduced radial strain ere by an axial tensile load is expressed as ere
=v [ vz]l/b , ~
(6)
343
If qf is a reduced radial stress due to ere, qe = a {erf/(1-p)} b is satisfied from eq(4). Thus, when an external tensile load is axially applied to the core, the resultant contact stress q is represented by a
q : qo - qf -
-
-
(l-p) b
(~ro b - £rf b )
(7)
The separation of joining specimen occurs when the value of external tensile load equals to that of frictional force by the resultant contact pressure q. Hence (8)
2nrl~q = nr2oz
Thus, the joining strength can be obtained from eqs.(6), (7) and (8) 21~a
F = nr2
(9)
£ro b
(l-P)
b • r
+ 21~v b
where r = ri exp(-~ro) and ri is the initial radius of the core. T h e r e f o r e , only from the measured strain (ero) of the polyurethane core after joining, the real joining strength can be calculated even with the consideration of c o r e ' s shrinkage.
3.
EXPERIMENT
3 - l . Workpieces The workpieces consist of aluminum 6061 tubes (outer one) having various lengths (1 = 2 0 ~ 4 0 m m ) and wall thicknesses (t = 0 . 8 ~ 2 . 0 m m ) , and high toughness polyurethane rubber rods (inner cores) with different diameters ( d = 1 5 . 8 ~ 1 8 . 6 m m ) as shown in Fig. l(a). R2O
Polyurethane
~'~/----
~i Field shaper
Aluminum tube dl= d2 -
Insulator
. . . . . . <....... "~,J " l,-.-.-.-.,. . . . . . . . ]~1~ I , I r
15.8 ~ 18.6 mm 16.0 ~- 18.8 mm, 1=20---40 mm (a)
Aluminum tube (b)
Figure 1. Dimension of workpieces (a), and setup (b) for the electromagnetic joining experiment.
344
3-2. Experimental setup and procedure
Magneform M/C (Maximum charged energy 8KJ) is used for generating transient and impulsive magnetic pressure to compress aluminum alloy tubes. To concentrate its pressure on a certain part, the field shaper inside compression coil is used as a forming coil, which is made of aluminum 2024-T6. In the electromagnetic joining experiment for tube compression, the workpieces are mounted into the field shaper as shown in Fig. l(b) and compressed by 2 ~ 8 K J electromagnetically discharged energy. In order to estimate the joining strength, tension tests have been carried out with 3mm/min ram speed.
4. R E S U L T S A N D D I S C U S S I O N
4-1. Effects of process and configuration parameters 4-1-1. Effect of discharged energy
Fig.2 shows, in accordance with the variations of discharged energy, the radial strain of the inner core. It is also shown that the strain increases as the level of discharged energy elevates. And Fig.3 shows such a result that the joining strength is obtained when the separation of joinned structure occurs by a simple tension test of specimens as shown in Fig. l(b). As shown in the case of core's radial strain, the joining strength is inclined to increase as the level of discharged energy goes up.
From these results, it is conceivable that the magnetic pressure imposing on outer tube is increasing as the level of discharged energy is going up, and then, it causes the radial strain increase of outer tube and inner core to result in the improvement of autofrettaging force on the joined part.
0.04
0.8C r.~
0.03
-~ o.oz
[]
0.6C
o
0.4(]
;~
[]
lad
0.01
0.2G
[]
[]
o o.oo o
.
~.
"
Discharged
~.
.
~
energy
-
10
E/r~
Figure 2. Effect of discharged energy on radial strain of core.
O.OC
~.
'
Discharge
,i
'
~
energyd
'
h
E /
'
10
ILl
Figure 3. Effect of discharged energy on joining strength.
345
4-1-2. Effects of outer tube's length and thickness
Fig.4 shows the relationship between the length of the tube and its radial strain after joining, and illustrates the decrease in the radial strain as the length of outer tube increases. Fig.5 shows the relationship between the length of outer tube and the joining strength, which presents the decreasing trend of joining strength as its length increases. And, the measured results of magnetic flux density between field shaper and outer tube represent such a trend that, in accordance with the increase in the length of outer tube, the magnetic pressure imposed on the tube decreases.
1.00
0.15
.~ O.lO
0
,~1
0.80
D 0
0
o
0 o
0 'O
o
0.05
w
O.C~
o
°'°°1~
2'o
~5
3'o
Joined length
3~
~
4~
1 / mm
Figure 4. Effect of joined length on radial strain of core. (discharge number 3)
o.~
~o
2~
~o
Joined length
~
~o
4.5
1 /mm
Figure 5. Effect of joined length on joining strength.
Accordingly, the area of joined part increases as the length of outer tube becomes longer, however, it seems that due to the reduction of magnetic pressure, the radial strain of inner core decreases. Thus, in the electromagnetic joining process, it can be concluded that the length of joined part should be determined by considering two major factors such as the joined area and the resultant magnetic pressure on the tube. Fig.6 and 7 show the results of strain of the core and the joining strength with outer tube's thicknesses from 0.8mm to 2ram. The strain of the core is decreasing as the thickness of outer tube is increasing, and the joining strength like the case of strain is also inclined to be decreasing in accordance with its thickness increase. The thickness of outer tube should be determined by considering the relationship between the plastic strains of outer tubes and its repressive strength against the elastic recovery of inner core. In this category of experiment, the maximum joining strength has been obtained from the case of the thinnest outer tube (0.8mm). This result shows that the higher joining strength can be obtained sufficiently from the thin tube since the polymer material of inner core like polyurethane has conspicuously lower value of resilient stress than the metals do.
346
4-1-3. Effect of joining clearance The variations of radial strains in the inner core are shown in Fig.8 where the clearance between tube and core is limited from 0 . 1 m m to 1.0mm, and the joining strength is also shown in Fig.9. As the clearance is increasing, the deformation amount of outer tube is increasing but the one of inner core is decreasing. This is because the joining proceeds after outer tube has been freely compressed up to the dimension of clearance as given. It is shown that the joining strength is increasing for the smaller size of clearances as for the both cases of metal to metal joining [3] and metal to ceramic one [4].
0.15
1.00
;,'LL,';~ o
0.80 0.10
o
A
o o
[]
"~ o.0s
0
A
A
0 .~
o
o.o%.6
o A
~
0.60 El
No.
1.0 1.~1 Thickness
1.'8 t
/
0.40
0.28,
8.8
1.6
t).
Thicl~ness
nun
Figure 6. Effect of tube thickness on radial strain of core. (discharged energy 8KJ, discharge number 3, joined length 30mm)
2.z
Figure 7. Effect of tube thickness on joining strength.
~
0.15
t.~ t / mm
1.00
0.80
.5
OlO
"d -~
0.60
Cl
13
0
0.05
e~
o
[]
0-000.'0
.~
0,40
O ~
0.28 '
[]
' o.'~
' o.:,..
Clearance
o
'o.6
13
' o.'8
' t.~
d / mm
Figure 8. Effect of initial clearance on the radial strain of core. (discharged energy 8KJ,discharge number 1, joined length 30mm)
' o.~
' o.~.
Clearance
' o.6
'o.~ ' d / ram
1.'o
Figure 9. Effect of initial clearances on joining strength.
347
4-1-4. Effect of discharge numbers Fig. 10 shows the relationship between discharge numbers and the radial strain of inner core, and Fig. 11 does the relationship between discharge numbers and joining strength. In accordance with the increase of discharge numbers, the deformation continues to increase. However, the joining strength shows its maximum value at 7 times of discharges and is inclined to be decreasing at more than that. It is thought that after more than 7 times of discharges, the irregular deformation of outer tubes occurs more increasingly to reduce the total contact area of joining part and thus, its joining strength too.
0.25
1.20 f 1.00
0.20
[] 0
0
0.80 ta 0.15 0
0
0
0.10
0.40
"m cl
0
I~ 0.05
"
o
0.20 0.00~
0.00 0 Discharge number
N
Figure 10. Effect of discharge number on radial strain of core. (discharged energy 8KJ, joined length 30mm)
Discharge number
N
Figure 11. Effect of discharge number on joining strength.
4-2. Relationship between polyurethane's strain and joining strength
As the above, a computational procedure to estimate the joining strength is proposed by assuming that it heavily depend on polyurethane's residual elastic strain. Since polyurethane has a wide range of elastic region and much smaller young's modulus in comparision with metals. Also, the elastic shrinkage of polyurethane core is considered when the core is axially pulled by a simple tension. It reduces the joining strength due to the reduction of autofrettaging force in the compressive tube. Fig. 12, compares the computed joining strengths with the experimental ones in accordance with polyurethane's radial strains (e~o) after the completion of joining. In this figure, the dotted line shows such a case that the reduction of autofrettage force under the external tension is not considered, i.e. the computed results from Eq.(5). And the real line shows the computed results from Eq.(9) in which this reduced effect is considered. As shown in Fig. 12, the computed results from Eq.(5) show significantly higer value than the experimental ones do but the computational results for Eq.(9) considering the reduction of autofrettage force are in good agreement with the experimental ones. These results show that the joining strength can be estimated from
348 the above mentioned computational procedure by measuring the residual radial strains of polyurethane core after the completion of joining. 2.50
/ / / /
/
2.00
,/
ooooo
eq- (9) e q . 15). experLmenE
.
! !
1.50
I / / /
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Figure 12. Comparision of joining strength between the computed and experimental results
5. C O N C L U S I O N S
In this paper, its joinability by electromagnetic forming process has been diagnosed and studied by presenting and analyzing certain problems which are arising. And also, the joining technique has been invetigated through experiments by considering the process and configuration parameters which influence the joining strength. For estimating the joining strength, a computational procedure has been proposed and its values has been compared with the experimental results. Finally, it can be concluded that the process variables chosen for this electromagnetic joining of aluminum tubes on polyurethane cores represent the characteristics of this process and estimated values agree well with the experimental results.
6. REFERENCES 1 D.F.Brower, Metals Handbook, ASM, Metals Park, Vol.4 (1969) 256. 2 L.G.Kuncl, R.B.Hagemann and J.Kotora, ANL Report 7343, (1967). 3 M.Murata, H.Negishi and H.Suzuki, J.Japan Soc. for Teehnol. of Plas., Vol.25, No.283 (1984) 702. 4 T.Sano, M.Takahashi, Y.Murakoshi, M.Terasaki and K.Matsuno, J.Japan Soc. for Technol. of Plas., Vol.28, No.322 (1987) 1192.