The capacitor discharge welding (CDW) process is an autogenous, high-energy density, rapid solidification joining process. It is considered as an electrical resistance welding process since the heat source is the joule effect induced by a capacitive discharge at the welding contact zone. It is an ideal process for joining small parts incorporating dissimilar metals. Potential applications include welding of solderless electrical contacts, welding of thermocouple wire-to-wire or wire-to-plate structure and capacitor discharge (CD) stud welding. Due to the very short time of performing the capacitor discharge; a huge magnetic field is erected around the welded parts, and consequently a huge amount of eddy current is developed. The objective of the present study is to analytically investigate the role of generated eddy current in the CDW process when used in welding two thermocouple wires. A mathematical model is developed to evaluate the intensity of the generated eddy current when the capacitor is being discharged through the two metal wires. This study is counted as the first trial to consider the effect of eddy currents in the capacitor discharge welding process, which may be beneficial in computer simulation work.
Capacitor Discharge Welding (CD Welding) is the fastest form of resistance welding, it utilizes capacitors to deliver the power to the parts. Capacitors are charged with large amounts of energy. Then, the energy is rapidly released into the parts within a few milliseconds. It delivers a laser welded-like joint, meaning that the metallurgy of the steel retains virtually the same characteristics as it did before the CD weld takes place. Like the laser weld, a CD weld delivers a joint that substantially limits surface deformations and spatter. The CD welding process also doesn’t come with the expense of laser welding equipment and the accompanying housing required to make such an operation safe .
This procedure has proven invaluable especially in vehicle construction, sheet metal forming, dissimilar metals, decorative metal designs and thermocouple wire-to-wire or wire-to-plate welds. Capacitive discharge welding has many advantages. Weld nugget formation takes place during the first few milli-seconds. Capacitive discharge welders allow extremely fast energy release with large peak currents. More of the energy goes into weld formation and less into heating surrounding material. The heat affected zone, where the properties of the metal have been changed from rapid heating and cooling, is localized to a small area around the weld spot. The quick discharge rate of CD welders also allows electrically and thermally conductive materials, such as copper and aluminum, to be welded. Capacitive welders deliver repeatable welds even during line voltage fluctuations.
Spot welding relies on the principle of metal resistivity to heat and fuse metal. A large current is passed through the work piece. Energy is dissipated due to the metal resistance in the form of heat which melts and fuses weld materials. Due to the fast rate of energy delivery, the eddy currents are likely to be generated in the parts to be welded in un-negligible amounts. The eddy currents are named so because the current looks like eddies or whirlpools. When a conductor is placed in a time-changing magnetic field, the induced current in the conductor is termed as Eddy currents. Eddy current is usually generated in a huge amount at a relatively small time. A lot of literature was carried out to study the CDW process variables to optimize the performance of the process. Moritz Meiners and Rainer Hauenstein , in their work presented an
in-situ qualitative, indirect current distribution measurement
system. They use different current sensors to erect their online
measuring system during the process. Nigel Scotchmer  in
his review paper discussed the rise of CDW applications and
advantages. Max-Martin Ketzel, et al.  mentioned that in the
CDW joining occurs without a welding nugget, but with metal
vaporization and linked activation of the surfaces.
A modelling study using ultra high-speed photography by R.
D. Wilson, et al.  was carried out on the capacitor discharge
welding. Their detailed photographic analyses revealed that
material is continuously ejected as plasma from the weld area
due to induced magnetic forces, rather than having the liquid
metal squeezed out of the weld upon contact. S. Chiozzi et al the
fatigue lifetime of CDW welded bars of Inconel 718 and TiAl6V4
superalloys . Numerous computer simulation work had been
conducted to the CDW process; [7-11] as examples; none of them
were found to deal with eddy currents generated in the welded
parts. Based on the foregoing review, it would be clear that; among
the considerable plentiful literatures carried out on the capacitor
discharge welding process, no study was documented about the
role of the eddy current in the process. Therefore, the present
study is considered to be the first trial to numerically estimate the
amount of the eddy current generated during the CDW process.
This study concerns the welding of two tips of thermocouple
wires to each other using the CDW process. The two wires have
the same diameter row and common contact line of length Zo, see
Figure 1. The aim of this work is to get a general mathematical
formula to estimate the value of the generated eddy current
through the wires during the welding process, which can be
benefitted in computer simulation studies. Figure 2 shows the
application of capacitor discharge circuit through the two wires
tips. Charge will cross through the line of contact AB from one
wire to another without applying a pressure force on the joint.
Charge Q is enough to generate power Em to melt a considerable
part of each of the two wires along the line of contact.
Capacitor power given by the equation
Where EC is the power exerted by the capacitor; C is the
capacitor capacity; V is the capacitor voltage; is considered equal
to the power dissipated in melting the joint Em given by:
Where R is the contact resistance between the two wires; is the instantaneous total current flow in the capacitor circuit at a time (t); and T is the total time of the capacitor discharge, Figure 3. Total current equals to the summation of the momentum capacitor current ,and the value of the generated eddy current Ie , as given by the equation:
The voltage on the capacitor
= voltage on capacitor C; at time t = 0.
To = time constant = RC (4)
R = Total resistance of the two wires in the region of
Zo : interference height where contact resistance between the
two wires, Figure 4.
ro : radius of wire, Figure 4.
Eddy Current Analysis
Generated magnetic field lines = H
Ien = enclosed current
Figure 5 shows a representative sketch of the generated
magnetic field lines, it is clear from the figure that the magnetic
lines are formed in the shape of closed lines.
μ : The magnetic constant, the conductor permeability.
This estimation is based on the Biot-Savart law that describes
the magnetic flux density propagation B in the distance r caused
by an electric current ic . The qualitative magnetic flux density
propagation is handled as the primary indicator of the welding
Noting that; the eddy current ; is in the same direction of the
capacitor current , and is concentrates at the center of contact
length of the two conductors. The eddy current can be expressed
another way according to the value of height Z as follows:
Thus, the required energy is a function of time and
dimensions. Some of the generated heat energy will dissipate
into the surrounding medium (air), while the majority, which
is concentrated at the interface between the two wires will
contribute to the welding process.
Among the previous literatures carried out on the CDW
process no one documented dealing with the generated eddy
current through his work, accordingly it was the aim of the
present research to develop the equations to evaluate the eddy
current in CDW process as applied on two thermocouple wires
junction. However, through the present study the eddy current
values are found greater than the original capacitor current, and
consequently the generated power. Therefore, it is advised in
general for every study of electric source welding process not to
neglect the generated eddy current. Through this work the major
equations of estimating eddy current concerns CDW process were
formulated, and it can be used in further computer simulation
study of the process. The generated heat energy at free surfaces
will be dissipated to atmosphere by radiation; while the energy
will be concentrated at the interface between the two wires.
Dan Davis (2018) Capacitor discharge resistance welding emerges as an important projection welding option. The Fabricator (Issue August 21): 54-55.
Moritz Meiners and Rainer Hauenstein (2020) Current Distribution Monitoring in Capacitor Discharge welding. 25th IEEE International Conference on Emerging Technologies and Factory Automation (ETFA), Vienna, Austria pp. 447-453.
Nigel Scotchmer (2015) The Current Rise in the Use of Capacitor Discharge Welding. Welding Journal 94(2): 32-36.
M Abu-Aesh (2008) Numerical study of thermal effects of electric-discharge through thermocouple wires using finite differences approach. Proceedings of Third IMS International Conference on Applications of Traditional and high-performance materials in harsh environment, American University of Sharjah, UAE, pp. 226-246.
Johannes Koal, Martin Baumgarten, Stefan Heilmann, Jörg Zschetzsche, Uwe Füssel (2020) Performing an Indirect Coupled Numerical Simulation for Capacitor Discharge Welding of Aluminium Components. Multidisciplinary Digital Publishing Institute MDPI, Processes Journal 8(11): 1330.