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Joining processes

Capacitor discharge welding (CDW), welding with medium frequency direct current (MFDC) and alternating current (AC) are conductive resistance pressure welding processes. The substance-to-substance bond between the joined components arises due to the simultaneous application of force and heat. The heat required for this arises inside the joining components due to an electric current flow, which leads to resistance heating of the welding zone. A sufficiently high current concentration is attained with projection welding through a joining partner component with a hump-like contour. In contrast, with spot welding the necessary current concentration is realised through the geometry of the electrode tip.

Capacitor discharge welding (CD welding) takes place with a very high, short-term surge current (up to 1 000 kA), which is supplied via a transformed capacitor discharge. This is a special form of projection welding, although it is also individually applied as resistance spot welding. With CD welding, the top and bottom electrodes press against the joining components with high contact forces (from 10 kN and significantly higher). Through the ignition of a thyristor, the charge of a capacitor bank is fed via a pulse transformer and on the secondary side via electrodes into the joining parts. The welding current heats the materials partially until close to their melting point, so that the parts are substance-to-substance bonded after cooling.

Significant advantages of CD welding are not only the short process time, but also the low electrical power input required, the high degree of efficiency and the low thermal load on the workpieces and the tools. The short-term and local heat application, which results in negligibly low heat dissipation into the basic material, facilitates the welding of mixed combinations of various materials, welding in thermally sensitive areas and the welding of greater thickness and cross-sectional variations. Furthermore, CD welding also permits the joining of materials with high electrical and thermal conductivity, such as aluminium, copper and brass. The reproducible discharge of the welding capacitor delivers high process reliability.

You can find further information on CD welding here.

MF welding technology is employed in a multitude of applications in both spot and projection welding. With medium frequency welding, an inverter chops the current from mains frequency to 1000 Hz with the aid of power semiconductors and then supplies this to the welding transformers. On their secondary side, the current is commutated through water-cooled diode packages. With spot welding, this lies within a range of around 5-12kA.

Advantages:
The current course progression in MF welding is characterised in particular by a rapid current increase and an even curve progression.

  • 1000 Hz clocking of the current facilitates short welding cycles
  • Good controllability of the welding current
  • Possible to use U/I controllers

Disadvantages:
The disadvantages of MF welding lie in the required mains connection output and the influence of the fluctuating contact resistances on the welding results:

  • High connection output required
  • Limit to the peak current
  • Dependence of the thermal energy introduced on the welding current
  • High influence of fluctuating contact resistances on the welding results

Welding with this type of energy supply is the most common and simple form of resistance welding and is primarily used in spot welding. For this purpose, single-phase mains voltage of 400V is connected to a welding transformer and transformed by this to around 5-9V. The mains current is converted with a similar conversion ratio into the welding current required. With spot welding, this lies within a range of around 5-12kA.

Advantages:
The significant advantages lie in the equipment required. Specifically, this comprises:

  • Simple, economical control
  • Space-saving installation for thyristor power parts
  • Robust, reliable hardware
  • Easy to operate, minimal training required

Disadvantages:
The disadvantages lie in particular in process reliability and efficiency:

  • Fine control is not possible due to the low clock frequency
  • Low current density and therefore long welding times
  • High thermal losses and therefore loading of the electrodes and components
  • Low efficiency
  • Asymmetrical loading of the supply network