Experimental investigation of aluminium–copper wire crimping with electromagnetic process: Its advantages over conventional process

Journal of Manufacturing Processes 26 (2017) 57–66

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Technical Paper

Experimental investigation of aluminium–copper wire crimping with electromagnetic process: Its advantages over conventional process Ashish Kumar Rajak ∗ , Sachin D. Kore Department of Mechanical Engineering, Indian Institute of Technology, Guwahati 781 039, India

a r t i c l e

i n f o

Article history: Received 9 July 2016 Received in revised form 24 December 2016 Accepted 20 January 2017 Keywords: Electromagnetic (EM) Wire crimping Aluminium Copper Connector terminals

a b s t r a c t Crimping of lugs to wire strands is a crucial part in electrical power transmission process. Improper design increases the resistance of current flow through terminals and causes problems like power loss, spark and heating in the joint. Which affect many industries like shipping, automobile, aerospace, satellite and communication where cable connections are used in large quantities. Electricity distribution boards faces similar problems during generation due to improper connection. To overcome these problems in conventional crimping process a new novel technique for effective wire crimping has been proposed in this paper. A strong pulsed electromagnetic (EM) field is used in wire crimping of aluminium connector terminal with copper wire. Experiments are carried out at different discharge voltages to demonstrates the feasibility of EM wire crimping process and to study the properties. With developed method, positive results were found from EM crimped samples. The gap between aluminium connector terminal and copper wire was reduced by 70% than conventional process. The electrical resistance of EM process was decreased by 34% than conventional one. Its pullout strength is 978 N higher. The surface finish is improved as mean roughness value of 0.8 ␮m. The hardness was reached up to 47 HV0.1 . X- ray diffraction (XRD) technique was used to find the residual stresses in EM and conventional crimped samples. Thermal behavior is improved over conventional crimped process. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

1. Introduction Reduction of weight with the help of cables and cable systems in a car to further reduce the car weight besides using lightweight car body materials will be a worthy challenge. Since, in a hybrid power car, cables and cable systems can reduce the total weight of the car upto 15 to 45 kg. The average length of copper cables used in light weight cars is approximately 1650 m [1]. To make light weight construction one can make use of aluminum lug instead of existing copper lug in contact sites. This will help in reduction of weight and it is an economical method without compromising the power loss. Crimping is a process where cable is stripped and the strands of the wire are placed into a common metal terminal. The terminal is then compressed around the wire stands to ensure good electrical contact between the terminal and the wire. Crimping of dissimilar materials in order to achieve a durable joint is a difficult process and important challenges for electricity boards, electrical industries, shipping, automobiles, aviation, satellite and communication.

∗ Corresponding author. E-mail addresses: [email protected] (A.K. Rajak), [email protected] (S.D. Kore).

Crimped connector terminals under various applications are exposed to various types vibrations, temperature gradients and different electrical environment. Study carried out on electrical problems shows that more than 60% problems occurs in contact area of connectors than any other technical problem [2]. The fatigue and fracture failure, wear tear, increase in stress and energy losses are caused on the terminal connectors when exposed to most of the vibrations. Some important sources of vibrations in a vehicle are mostly due to powertrain consisting of engine, differential section and gear box. This severely affects the connector resistance stability and durability [3]. The compression using conventional crimping tool deteriorates the material due to relaxation or partly releasing. This results in increase in resistivity and high amount of losses in wiring system and battery power system [4,5]. An inhomogeneous stress distribution is found in materials thereby increasing the critical notch stresses, which reduces the transferable load [6]. Therefore, there is need to develop a new technique to make contact joints between aluminum and copper in wiring system that can give us a required strength with minimum power loss, more uniform with no radial nor longitudinal misalignment so that it can be beneficial for industries where cable connections are used in large numbers.

http://dx.doi.org/10.1016/j.jmapro.2017.01.009 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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Fig. 1. Equivalent circuit diagram for EM wire crimping process.

Table 1 Geometry set-up and working conditions. Workpiece

Circuit condition

in the overdamped condition [12]. The total energy that is stored within the capacitor bank is given by:

Outer diameter of terminal connector Inner diameter of terminal connector Length of terminal connector Al conductivity Density

12 mm 10 mm 27 mm 35 × 106 S/m 2.70 g/cm3

Capacitor bank Maximum voltage Circuit inductance Circuit resistance

90 ␮F 15 kV 20 ␮H 23 m

In comparison to other widely used joining techniques, like conventional mechanical crimping, electromagnetic crimping shows interesting characteristics which results in uniform forming pressure distribution [6,7]. The advantage of EM process including no contact, low mould cost, no lubrication and less spring back makes it more suitable for materials that are difficult to form [8]. The earliest applications of electromagnetic crimping originate from the electrical industry. Examples are swaging of copper tubes to coaxial cables and joining of metal fittings to ceramic insulators [9]. In 1996 Belvy et al. [10] found that the electrical resistance of EM crimped cable connectors is up to 50% lower than those of produced by mechanical crimping. Since then no further published research paper has been found in this field. Although a large number of studies present EM forming as having a tremendous application potential [8,11], but no research has been carried out in the field of EM wire crimping process. In this paper EM wire crimping is carried out with different parameters and analyzed with conventional crimping process. The intension of the present work is to develop a new method for wire crimping that can be an alternative option for existing conventional wire crimping method. The aim of this experiment is to give advantages in uniform lug deformation, minimum electrical resistance, high pullout strength, good surface finish, increased hardness number and improved thermal property.

Ec (t) =

A high energy system that can discharge its energy within a very short period of time is required for Electromagnetic (EM) crimping process. This is achieved using a high voltage capacitor bank which is connected in series with the inductor (coil) as shown in Fig. 1. The circuit parameters (the total inductance, capacitance and resistance of the circuit) should be selected such that, the RLC circuit operates

(1)

where Ec (t)is the total energy stored in the capacitor bank (Joules) and C is the equivalent capacitance of the capacitor bank (farads) and E(t) is the charge voltage (volts). The stored energy of the capacitors is suddenly discharged by closing the high current switch between the charged capacitors and the inductor. This generates a current I(t), which is a damped sinusoidal oscillation [14]. The current generates an electromagnetic field that can be found according to the first of Maxwell’s Equations:  ×H  = J ∇

(2)

 is the electric field. In Eq. (2), where J is the current density and H  × represents the curl operator. According to the second Maxwell’s ∇ Equations, a current is induced on the workpiece due to the electromagnetic field caused by the high frequency current on the coil. The magnetic flux density can be given as:  ×B =0 ∇

(3) →

 is the magnetic flux density and B  = H.  Also, ∇ is the where B curl operator. The induced current due to the flux density flows in reverse direction to the coil current according to Lenz’s law. The induced current generates Lorentz force on the workpiece which may be expressed as:  F = J × B

(4)

For a particular number of turns (N), length of the coil (l), the developed magnetic field increases with the increase in the discharge coil current as illustrated in equation below. B=

NI l

(5)

For lesser skin depth magnetic pressure (P) is calculated by the simplified equation as:

 1 2 Hgap (t) 2

p (t) = 2. Theoretical background: electromagnetic crimping process

1 CV (t)2 2

(6)

Which is deduced from the damped coil current from the sinusoidal current. But when skin depth is considered magnetic pressure (P) is calculated using magnetic field for various discharge voltage using the equation illustrated below:



P= ı=

B2 2



1



f

1−e

− 2t ı

 (7)

(8)

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Fig. 2. Different types of experimental coil used for the optimization process. (a) Coil 1, (b) Coil 2, (c) Coil 3 and (d) Coil 4.

The mentioned magnetic pressure distribution depends on the skin depth as could be seen from Eq. (7). Discharge voltage (V) is given by,



I V= k

C L

(9)

where (K) is the reversal factor, C and L are the machine capacitance and inductance in RLC circuit. 3. Experimental work 3.1. Optimization of EM wire crimping coil The EM wire crimping process utilizes a helical coil of copper material in order to concentrate the magnetic field. For carrying out the experiments, dimensions of workpiece and EM specifications are included in Table 1. It is very important to predict the optimal parameters of the crimping coil to control the quality of wire crimp joints. Optimization of parameters like pitch, coil length, number of turns and diameter of cross-section of copper wire used are included in Table 2. Different coils used for optimization of the EM wire crimping process are shown in Fig. 2. High voltage insulation sleeve of 0.4 mm and 0.5 mm over the copper wires diameter of 4 mm and 5 mm was used for manufacturing of coil to maintain required gap between to turns for optimization process. Experiments were conducted using four different coil geometries keeping all other process paramters constant as shown in Fig. 3. The radial deformation was found to be maximum for coil

Fig. 3. Change in radial deformation on variation of discharge energy for different types of helical coil.

4, where maximum radial deformation of 3.4 mm was obtained for 9.6 kV discharge voltage. While for coil 1, coil 2, coil 3, maximum radial deformation was found to be 3 mm, 3.1 mm and 2.9 mm for the same discharge voltage. It was found that the coil 4 produced the maximum deformation with uniformity along the axis. Hence, coil 4 which provided

Fig. 4. Five turn solenoidal coil used for carrying out experiments. (a) side view of the coil (b) sectional view of coil assembly consisting of Al lug and Cu strands.

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Table 2 Variation done in coil geometry and pitch for the optimization of coil. Coil Crossection Dimater (mm)

No. of turns

Gap (mm)

Standoff distance (mm)

Coil length (mm)

Inner diameter of coil (mm)

Maximum deformation (mm)

Remarks

Coil 1 Coil 2 Coil 3

4 4 5

4 5 4

1 0.8 1

0.5 0.6 0.6

20 24 25

13 13 13

3.0 3.1 2.9

Coil 4

5

5

0.8

0.5

29

13

3.4

Non uniform Non uniform Less deformation and non uniform Uniformity and maximum deformation

Variation of current with time was obtained from Cathode Ray Oscilloscope which is shown in Fig. 7. The current wave form measures different parameters such as amplitude, frequency, rise time and time interval. Frequency measured was calculated to be 20 kHz which remained constant throughout the experiments and value of current was found to be 127 kA for discharge energy of 2.8 kJ. 3.3. Conventional crimping

Fig. 5. Variation in discharge energy for different discharge voltage.

the best results was used to carry out further experiments and compared to conventional crimped samples. After the optimization of coil, experiments were carried out at various discharge energy to study the influence of EM high strain rate deformation process on wire crimping applications on various parameters which are discussed in results and discussion section.

3.2. Experimental work carried out on optimized coil A five turn solenoidal copper coil 4 was used to carry out the experiments. The side view of coil with fixtures is shown in Fig. 4(a) and cross-section view of assembled aluminum lug and copper wire strands inside the copper coil as shown in Fig. 4(b). Experiments were carried out at different voltages from 8.1 kV to 9.6 kV for crimping the samples. The standoff distance of 0.5 mm between the coil and the terminal connectors was kept constant for different iterations of the experiment. Effective crimping of aluminium lugs over the copper wire strands was carried out between the calculated (using Eq. (1)) processed energy of 2.9 kJ to 4.1 kJ. Discharge energy for various discharge voltages is calculated and shown in Fig. 5. And crimped samples are shown in Fig. 6. With the increases in discharge voltage deformation of terminal connector kept increasing due to increase in magnetic pressure (Eqs. (5), (7), (9)). For 2.9 kJ, 3.0 kJ, 3.2 kJ, 3.4 kJ, 3.6 kJ, 3.9 kJ and 4.1 kJ of discharge energy outer diameter was found to be 11.2 mm, 11.1 mm, 10.8 mm, 10.6 mm, 9.3 mm and 8.4 mm. At discharge energy of 4.1 kJ maximum radial deformation of 3.4 mm was obtained and as per the standard [13] of crimping of 35 mm2 a deformation of 3.34 mm is required to avoid damage inside the terminal.

The commonly used crimp shape for cable lugs and connectors is the hexagonal shaped crimp as this crimp profile is best suitable for copper and aluminium conductors. The advantage of a hexagonal shaped crimp is the uniformity of radial forces which are applied consistently from all directions and over a whole area during the crimping operation. Due to the uniform compression, the hexagonal shaped crimp is mainly used for medium and high voltage applications [15]. Hence, EM wire crimping process is compared with standard hexagonal shaped crimping conventional tool. Crimped samples for both processes were of same materials. Crimped samples using conventional crimping tool are shown in Fig. 8. 4. Results and discussions 4.1. Cross section analysis Comparison of the cross section of electromagnetic and conventional crimping process was done using upright optical microscope at 20x. Images of the cross section are shown Fig. 9. It was found that compression done using electromagnetic process was more effective than conventional crimping process as compression of wire strands was higher, giving denser compaction of wire strands compared to conventional crimping where large gap was observed. A gap of 50 ␮m between the copper wire strand and the aluminium lug exists, while in conventional crimping usually a gap of 174 ␮m exists between the wire strands and aluminium lug contact surface. It was also observed that, due to uniform pressure by EM process a uniform deformation of aluminium lug was obtained compared to a conventional process. 4.2. Electrical characterization It was important to know the type of circuit bridge to be used for finding low resistance value. Modifications in Wheatstone bridge was done to obtain a Kelvin bridge, which was not only suitable for measuring low value of resistance in microns but gave more precise value compared to other available circuits [16,17]. As shown in Fig. 10, it was found that EM crimped samples gave lower resistance value by 4.4 ␮ compared to conventional crimping samples. Increase in the discharge energy, resistance value kept decreasing and became constant after 4.0 kJ due to denser compaction of copper wire and a minimum gap between the contact surface. The effect of high impact velocity may result in cleaning

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Fig. 6. Electromagnetic crimped samples at various discharge energy.

of oxide layer and moisture content between the contact surfaces. Conventional crimped samples process had high value of resistance comparatively as complete compression of wire strands using a hexagonal crimp or standard crimping dies is not possible [15]. As the moisture and oxides always remain between the lug and wire due to quasi static die compression process which results in increase in the resistance. 4.3. Mechanical pull out testing

Fig. 7. Damped current graph obtained in experiments (value of current was calculated as 127 kA for 2.8 kJ energy).

Comparison of pull out load value between electromagnetic and conventional crimping was carried out. Arrangement of pull out process of a wire crimped sample is shown in Fig. 11. Standard procedure for wire pull out test was carried out where a transverse speed of 50 mm/min was maintained [18]. The length of wire beyond the crimp terminal (a = 165 mm) and gripping length (b = 15 mm) was maintained. For holding wire at one end, insulation over the wire was stripped for effective gripping to prevent any slip. A maximum pull-out load was obtained for EM crimped and conventional crimped samples. Pull out value as shown in Fig. 12, for 2.9 kJ, 3.0 kJ and 3.2 kJ of discharge energy was found to be 685 N, 750 N and 913 N. These values are below the load value of 980 N obtained for conventional crimping process. As per the crimping standard [19] minimum pullout value for 35 mm2 crimped sample is 801 N. The crimped samples obtained for 2.9 kJ and 3.0 kJ of discharge energy due to lesser deformation are below the required pullout value. As the discharge energy increases above 3.4 kJ to 4.1 kJ, value of pullout exceeds the conventional crimped sample. Maximum value obtained for 4.1 kJ was found to be 1958 N which is 978 N more than conventional crimped sample. This is due to the increase in deformation with the increase in discharge energy and uniform radial compaction as shown in Fig. 13. While in conventional crimping process only terminal coming in contact with die get crimp resulting in lesser pullout strength. 4.4. Surface roughness

Fig. 8. Samples crimped using crimping tool.

The surface roughness was measured on the outer diameter of the connector terminals which was deformed on the application of magnetic pressure and conventional tool die pressure in crimping process. It was measured using a surface profile meter. As shown in Fig. 14, an average roughness value of 0.8 ␮m on EM crimped samples while 2.4 ␮m on conventional crimped sample. This improved surface finish. Since EM crimping is a contact less process whereas conventional crimping is a die contact process where deterioration of the

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Fig. 9. Cross section images are taken under optical microscope for crimped samples. (a) compressed wire strands of conventional crimped sample (b) conventional sample (c) gap between wire strands and compressed lug of conventional sample. (d) compressed wire strands of EM crimped sample. (e) EM sample (f) gap between wire strands and compressed lug of EM sample.

Fig. 12. Comparison of pull out value between EM crimped sample and conventionally crimped sample. Fig. 10. Comparison of resistance value for EM crimped and conventionally crimped samples.

Fig. 11. Configuration of pullout test for connector terminal crimped over wires.

surface results in increase in roughness value. It was found that conventional crimping process leads to increases in surface roughness due to contact between crimping tool and terminal connector. EM crimping process being contactless crimping process uses gen-

Fig. 13. Comparison of deformation of tube of EM crimped sample and conventional crimped sample to initial sample.

erated Lorentz force leading to a smoother surface with lower value of average surface number of 0.8 ␮m.

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Fig. 14. Surface roughness of the area exposed to crimping (a) EM crimped sample. (b) conventional crimped.

Fig. 15. Cross-section area for measuring Vickers hardness test (a) EM crimped sample. (b) conventional crimped.

Fig. 16. Vickers hardness value carried over the various points as shown in figure on EM and conventional crimped cross-section sample.

4.5. Hardness analysis By adjusting the parameters of the EM field compressive residual stress can be introduced along the surface to get peening effects [20,21]. This compressive residual stresses due to electromagnetic peening (EMP) keeps varying with the thickness of the workpiece. The Vickers hardness test was carried across the cross sectioned sample of EM and conventional crimped sample on different locations as shown in Fig. 15. For testing 100-g force is chosen. It was observed from Fig. 16(a), in EM crimped sample the hardness of connector terminal increases with the increase of the

discharge voltage. The hardness has a drastic rise moving away from the contact surface. A largest value of 47 HV0.1 was obtained for 4.14 kJ of discharge energy. In conventional process as shown in Fig. 16(b) maximum value of 39.6 HV0.1 is obtained. Hardness value was found to be higher by 7.4 HV0.1 for EM crimped value compared to conventional crimped sample. Increase in hardness along the thickness is due to more compressive residual stresses developed by EM process compared to conventional crimped sample [16]. To find the compressive residual stresses X-ray diffraction (XRD) technique was used [22]. Residual stress analysis is determined where only the peak shift occurs. As shown in Fig. 17 any change in

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Fig. 17. (a) XRD analysis of EM and conventional crimped sample and (b) enlarged image of 2␪ from 64 ␪ to 87 ␪ to distinguish the pick shift of EM and conventional sample exposed to X Rays.

Fig. 18. Stress calculation from the slope obtained from the peak points from XRD data for (a) EM crimped sample and (b) Conventionally crimped sample.

Fig. 19. EM crimped lug and conventional crimped lug connected to a high power consumption source to study the variation of temperature over the lug surface. Temperature reading was conducted using industrial infrared thermometer.

the lattice spacing, d, results in a corresponding shift in the diffraction angle 2␪. Measuring the change in the angular position of the diffraction peak provides the data for calculation of residual stress

of the sample lying in the plane of diffraction, which contains the incident and diffracted X-ray beams.

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Williamson Hall method [23] was used to calculate induces strain using the equation: ˇcos =

 k   d

+ 4sin



(10)

Where d is the particle size, ␭ is the wavelength of radiation, ␬ is the constant with value of 0.94, ␤ is the peak width, ␩ is strain induced due to crystal distortion and ␪ is peak position. Here residual strain was given by Y intercept and the value for EM crimped sample and conventional crimped sample was found to be 0.00184 and 0.00162. Applying Hookes law approximations residual stress was calculated: ˇcos =

 k   4sin  d

+

Y

(11)

Where ␴ is the stress of the crystal and Y is the modulus of elasticity. This equation slope represents residual stress [22,23] as shown in Fig. 18. Value of residual stresses for EM crimped sample and conventional crimped sample was calculated to be 3.97259 MPa and 3.02922 MPa. This increases in compressive residual stresses Due to the higher compressive residual stress along the thickness in EM crimping process it will improve the mechanical property compared to conventional crimped sample.

Fig. 20. Temperature reading of EM crimped lug and conventional crimped connector terminal.

lower resistance value using. Heat produces in circuit is given by Eq. (6).

4.6. Temperature measurement

Hr = I 2 Rt

In a circuit too high electrical current passage can cause overheated. This additional heat is detrimental to the integrity of the termination means, conductor insulation and even the overcurrent protective device which transfers into the devices through the terminals. Excess heat could cause integrity issues. If there were loose terminal connections, then:

Where I is the current passing through wire and R is the resistance. As rise in temperature even by 0.1 ◦ C is important in electronic, power consumption circuit as it may lead to fatal. Temperature over the EM crimped sample was found to be lower than the conventionally crimped sample due to the minimum change in resistance in the contact area and lower heat dissipation making it more attractive option to conventional crimping process.

a) The conductor might overheat and the conductor insulation might break down. This could lead to a fault; typically line to ground. Conductors of different potential were touching, the insulation of both may deteriorate and a phase-to-neutral or phase-to-phase fault might occur. b) Arcing could occur between the conductor and lug. Since a poor connection is not an overload or a short circuit, the overcurrent protective device does not operate.

5. Conclusions

To confirm the temperature, rise in the terminals, test was carried on EM crimped lug and conventional crimped lug, where both the lugs were connected to a high power consumption device as shown in Fig. 19. Test was carried out for 12 h where temperature of the terminals was measured periodically after 30 min using infrared thermometer with a precision of 0.5 ◦ C. Experiment was carried at controlled room temperature of 25 ◦ C. It was observed that temperature rise in both the terminal connector took place as shown in Fig. 20. Near room temperature the electrical resistance of the material increased linearly with rising temperature and the resistance equation is given by, R = R0 (1 + C ( T ))

(12)

Where Ro is the initial resistance of the metal, C is temperature coefficient of resistance (for Al it is 3.9 × 10−3 /◦ C) and T is the temperature change. A rise in temperature was observed continuously at an interval of 0.5 h, for EM crimped and conventional crimped sample. It was found that after 12 h, the conventionally crimped terminal connector surface temperature of 35 ◦ C was observed and for EM crimped sample it was found to be 31 ◦ C. As the resistance value is higher for conventionally crimped sample, it resulted in higher heat (Hr ) dissipation compared to EM crimped sample with

(13)

In present study comparison of electromagnetic crimping and conventional crimping of aluminium lugs to copper wire was carried out. The main summarized points are: I Deformation of aluminium lug increased with the increased discharge voltage but after 9.6 kV remains constant as further radial deformation of aluminium lug is restricted by copper strands. II With this method of EM wire crimping average gap was reduced by 70% as the conventional crimping was more up to 170 ␮m. However, the new method reduced this gap to 50 ␮m. III The resistance of EM crimped samples keep on decreasing with the increased discharge energy and after 4.0 kJ, it remained constant as 8.1 ␮, which was lower by 34% compared to conventional process. IV The pullout strength increased with discharge energy and further increased when energy is increased. A maximum of 1.958 kN had been observed for 4.14 kJ discharge energy while conventionally crimped sample maximum value was found to be 0.98 kN. V The mean surface roughness value of 0.8 ␮m was obtained in EM crimped sample while in conventional process the value is more as 2.4 ␮m. Lower value of surface roughness minimizes power loss. VI The hardness increased upto 47 HV0.1 for EM process while the conventional process it was 39 HV0.1 . While in conventional die crimping contact area exposed hardened with value lower than EM crimped. XRD analysis of compressive residual stresses which increases the hardness along the thickness of

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the crimped sample was found to be 23.7% more in EM crimped samples. VII Temperature over the EM crimped sample shows 30.5 ◦ C lower than conventionally crimped process that is 34.5 ◦ C due to the minimum change in resistance of the contact area and lower heat dissipation. This is very important for high power transmission applications. Further work will be carried out for different materials for commercial applications by numerical simulations to comprehend the mechanism in detailed. References [1] Bergmann JP, Petzoldt F, Schürer R, Schneider S. Solid-state welding of aluminum to copper—case studies. Weld World 2013;57:541–50, http://dx.doi.org/10.1007/s40194-013-0049-z. [2] Liskiewicz T, Neville A, Achanta S. Impact of corrosion on fretting damage of electrical contacts. Electr. Contacts-2006. Proc. 52nd IEEE Holm Conf. Electr. Contacts, IEEE 2006:257–62. [3] Flowers GT, Xie F, Bozack MJ, Hai X, Rickett BI, Malucci RD. A study of the physical characteristics of vibration-induced fretting corrosion. IEEE Trans Compon Packag Technol 2006;29:318–25. [4] Gissila T. Connectors and vibrations–damages in different electrical environments; 2013. [5] Weddeling C, Demir OK, Haupt P, Tekkaya AE. Analytical methodology for the process design of electromagnetic crimping. J Mater Process Technol 2015;222:163–80. [6] Schneider R, Lobl H, Großmann S, Schoenemann T, Holdis M. Langzeitverhalten von Aluminium-Kupfer-Verbindungen in der Elektroenergietechnik. Metall 2009;63:591. [7] Balanethiram VS, Daehn GS. Hyperplasticity: increased forming limits at high workpiece velocity. Scr Metall Mater 1994;30:515–20.

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