Capacitor Discharge Welding is a welding process that has been succesfuly used for the past 50 years, but in spite of its many advantages such as rapidity, minimum heat affected zone, and ecological features, it is not a very “popular” process.

Fig.1: Electrical scheme of Capacitor Discharge Welding CDW.

GREAT POWER TO WELDING
CDW welding process is performed with very high current values applied in a short application time. This process is so rapid that it does not alter the microstructure of the metal.
With the CDW process it is possible to achieve a rapid solidification with a
solidification speed of 103 K/s. with an achievement of good grain refinement and reduced segregation. The will have therefore high technological qualities.
CDW allows welding different metals that have a small solubility when melted, without using any bonding metal.
CDW welding has a small thermically affected area and consequently a thin microstructural transition layer.
CDW is a quick cost effective technology with several commercial, ecological, metallurgical, and quality advantages.
This welding process is easily automated allowing good production rates and high quality standards with an integrated control system.
Thanks to the rapidity of this welding process there is no need for any protective environment such as inert gas or others.
First attempts of CDW welding were done in the ‘50s but this process was soon abandoned due to the limits of electronics available in those years. In the 80’s this technology was taken up again and introduced as “stud welding” and it was used for welding nuts, bolts and nails on plates. CDW welding is nowadays finding a wider diffusion thanks to the new electrical and electronics available and is now applied in more and more fields such as welding metal sheets, hydraulic valves and head pipe welding.
Thanks to the new available technologies for the components the welded area can be up to 500 mm2. Focusweld in collaboration with the Polytechnic of Turin is doing a lot of research in the ZTA (Thermically Altered Zone) and in the micro structural characteristics of welded parts.


CDW WELDING MACHINE

Fig. 2a
CDW is a welding process that is performed by means of a high intensity current flow produced with the discharge process of a capacitor battery.
In fig 1 we can see the five main parts of this machine:
- Supply section, it is a high voltage output transformer.
- Energy bank: it’s a capacitor battery that gradually accumulates energy to be then freed by a current impulse,
- Power switch, it is the component that frees the high energy current.
- Welding section. In this section there is yet again a transformer that will
lower the tension and amplify the current.
- Control Section. Every parameter involved in this process is controlled, from
discharge time to peak current and upsetting force.
High power supplies are not needed for CDW welding. This process has a relatively low energy consumption because the “energy Bank” is filled in a relatively long period () 3-5-sec) while the machine is loading / unloading the parts to be welded. The charging process is specially designed so to avoid any interference to the energy supply.
The welding process is fully automated and the operator loads the components and starts the machine. The components will then be brought in contact with the correct upsetting force. After this the high current welds the components and the machine can unload the welded parts and to load the new components.
Welding time can vary from 10 to 50 ms and at the end of the process the unloaded welded parts are practically cold and will not be distorted due to thermic dilatation. In comparison to standard resistance welding machines the power absorption is relatively low. A CDW machine with a peak energy output of 12kWs with a capability of 30 cycles per minute will absorb 20kVA.
A comparable resistance welding machine will require a mains supply of approx. 500kVA.

Fig 2b

WELDING PARAMETERS

The parameters to be selected on the CDW machine are:
- Energy: on standard machines this parameter will vary from 7 to 60kJ;
- Upsetting force
- Number of discharges: it is possible to apply more than one discharge
perwelding process so as to allow the material to settle better.
- Transformer step-up ratio. This parameter will influence the discharge time
and the peak current.
Once the step-up ratio has been set, the mean parameters to be selected are energy and upsetting force. These two parameters have to be kept proportional to the volume of material to be melted during the welding process.
Usually every mm of material requires 600 Ws and 200 N of upsetting force. These are gross indications and have to be selected according to the materials to be welded, the surface extension, and the shape of the components. Before starting mass production it is necessary to do some pilot productions in order to choose the ideal power to upsetting ratio.
fig 2 CDW machine for welding 4 pipe s simultaniously
Fig. 2.a Welding electrodes

WELDING CURRENT

Fig.3
In CDW welding the peak current will be up to 500.000 A and this value will be reached in a very short time (from 10 to 25 ms). When the peak current is reached the metal will be fully plasticized and there will be an increase of the contact area and a reduction of electrical resistance.
This impulsive current will have the following effects:
- The transmitted energy will be displaced in the contact zone, in this area
the metal will be plasticized due to the Joule effect
- The relatively small energy involved will create a very resistant weld and a
very small thermically altered zone that will not modify the mechanical
properties of the metal.
- The electrodes will not reach high temperatures and have a long life
expectancy.
- The heat affected zones are confined to the contact zone so no thermic
dilatations will be detected in other parts of the components.
It is possible to weld coated metals without de-coating.
- Process costs are lower than conventional resistance welding.
CDW welding will have a lower energy absorption, reduced electrode wear
and due to the small thermic dilatations no stress relieving heat treatment
is necessary.
Fig. 3Current-time graph in the CDW process
Next Figures 4x and 200x micrographs

The welded metal distribution appears well deposed all over the welded line, the continuous variation of the grain size points out a good thermic gradient in the ZTA without any micro-defect formation in the transition area. Fig.5 Electrode for welding radiator heads ELECTRODES The electrodes used in CDW are very simple to manufacture and usually do not require any cooling. The CDW electrodes are made of copper alloys and have a life time of approximately 10,000- 20,000 welding cycles. The shape and dimension of the electrodes are proportional to the welded parts that they will hold and will be designed in order to reduce wear and extend life. Usually the electrodes are granted for 10,00 cycles and can be retooled to extend their life.

Fig.5

CDW thermic cycle
The flow of the electric current will encounter the resistance of the material and therefore it will generate heat that will be used in the welding process. The thermic effects on the welded parts is a relative to the current impulse. The current parameters are a consequence of the other parameters that have been set and can be read on the machine while it is working. The machine will illustrate the current-time graph and it will be therefore possible to read the peak current, the “current area”, and the compression of the parts during the welding process.
In fig 3 there is the current-time graph of a generic welding process. On the horizontal axis it is possible to read the discharge time that will also be the heating time. The descending part of the function will be proportional to the ZTA. A very important parameter of this process is the electrical resistance of the metal and the variation of the mechanical proprieties with temperature.

CDW Features

Fig.6
The CDW process involves very quick cooling and solidification process, and these aspects of the process will be studied with 3D and 2D heat transmission models. Symmetric shapes such as those involved in pipe welding will grant an even current distribution on the weld area and therefore a perfect tightness of the weld is granted.
In fig 6 there is a schematic illustration of a CDW process developed for pipes. Essential to validate the potentials of this technology are the profile of the parts in the contact zones. A regular and continuous contact zone must be granted. In order to speed up plasticization of the material it is necessary to obtain on one of the two parts some igniters. These igniters will concentrate the current and therefore the generated heat will be localized in these areas.
Fig 6 One of the two pipes is shaped so as to concentrate the current .
Positoning of the electrodes
Fig.7 CDW welding process:
a) electrode positioning
b) uphold force application
c) current discharge
d) plasticization of the material and weld line formation
The igniter can have several shapes and dimensions depending on material, material surface and on the thermic effect to be achieved in the welding area. If the welded part is a metal sheet the igniters will be spherical caps formed during stamping process.
The process is described in picture 7. The electrodes are brought in contact with the components with an uphold force necessary to grant a good electrical contact; as soon as the electrodes are positioned the capacitor battery is fully charged and the circuit is open. In phase b) the two parts to be welded are brought in contact with the uphold force necessary to bring in contact the plasticized metal during the discharge of the current (fig 7.c)). In this phase the current passes through the electrodes to the components encountering minimum resistance, and so with minimum heat generation, then the electric impulse goes through the igniters. In the igniters the electric current will encounter great resistance, thus generating the heat necessary to plasticize the metal in the welding area. The uphold force is necessary so that the two parts penetrate to form the welded joint. As soon as the igniter is completely melted the resistance drops drastically and the current impulse fades away rapidly, and the melted metal soon solidifies.

COMPARISON BETWEEN CDW and RESISTANCE WELDING
Among the many advantages in using the CDW is the possibility to weld coated metal parts, esthetical parts, and to weld more than one item at a time, and is possible to weld components with a welded joint up to 500 mm2.In tables 1-2 it is possible to have a schematic comparison between the two technologies.

Resistance welding
- The welding current is generated through a generator directly connected to
the electric supplies.
- The welding current is related to the electric supplies.
For high energy welds it is necessary to have a suitable electric supply:
a) bigger transformers
b) cables suitable for higher Amp
c) higher power required from the electric supplies.

CDW
Welding current is generated with a transformer fed by a capacitor battery Welding quality is not so affected by tension fluctuations of the supplies
High voltage supplies require less power, its function is to charge the capacitor battery in a relatively long time
Current absorption is lower and more steady.

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