The DVS bulletin sheet 2911 “Capacitor discharge welding – basics, processes and technology” describes the main features of CD welding and defines general terms and variables:

Component check Comparison of the attained actual position in relation to the target position after the application of the moving electrode to the joining parts and completion of the force generation prior to welding. If the actual/target difference lies outside the adjustable limit value, an error message is generated and welding is prevented.
Yield path
Path of the moving electrode during the current flow and the hold time (until electrode standstill). This value can be monitored.
Electrode force FS/kN Contact pressure with which the electrodes press on the parts to be joined.
Charging voltage U0/V The voltage with which the capacitors were changed before welding. The energy fed to the transformer is determined by the capacitance and the charging voltage.
Hold time
Directly follows the current flow time; during the hold time the weld point cools under the contact pressure.
Welding energy

The energy stored in the capacitors before welding, which is calculated per E = 0.5 C U02.

Setting path
Path of the electrode from the default position (electrodes open) to application to the joining parts. At the end of the setting process, the component check takes place.
Peak current IP/kA Maximum value over the time progression of the weld current, which is to be characterised as exponential surge current (one half sine wave, which is superimposed by a subsiding exponential function). Incorrectly also referred to as “welding current”.

Through the technical further development, primarily through the use of the multi-pulse technology MCS and the analysis of the operating behaviour (including the machine measurement), further terms have been coined by Kapkon:

Impaired coupling of the transformer; the main inductance of the transformer is so low that the magnetising current cannot be ignored, which leads to a deviating translation ratio of the peak currents in comparison to the translation ratio of the transformer.
Attenuation key values Generic term for the three characteristic variables angular eigenfrequency ωe, decay constant δ and angular resonance frequency ω0, which characterise the behaviour of the function circuit of the CD machine.
Sweep-through charging voltage UD0/V is what a capacitor receives if it is charged without a freewheel circuit in the circuitry by the main current with opposite charge as the initial charging voltage. Differentiation is made between the sweep-through charging voltage and the back charging voltage URück.
Electrical basic variables Generic term for the variables resistance, inductance and capacitance, which determine the time progression of the process variables current and capacitor voltage.
End current ISt/kA 5 % of the peak current (ISt = 0.05 IP) on the descending path of the current curve, which determines the end of the current time tI.
Discharge time point tUn Time point at which a capacitor that feeds the main current is discharged (uC = 0).
Fixed coupling of the transformer; due to the large main inductance of the transformer, the magnetising current can be ignored; the translation ratio of the peak currents and that of the transformer concur.
Function circuit as part of the power circuit of a CD machine, encompasses the primary and secondary circuit, as well as the transformer.
Joining energy EF Energy fed directly to the joining point.
gKE “Clocked capacitor discharge”; the discharge of a capacitor does not take place continuously via a switching thyristor, but rather in a clocked manner via a power semiconductor that can be switched off (IGBT).
Half-value current Ih Half value of the peak current (Ih = 0.05 IP) on the descending path of the current curve, which determines the end of the welding time th.
Main phase (HP) Phase einer KE-Schweißung, in der der Hauptstrom den Kondensator entlädt.
Commutation (KV) Phase of CD welding, in which the main current from one semiconductor valve (the thyristor) changes to another (the freewheeling diode).
KShW “Short-term welding with high heat flux density” is the process designation that consolidates resistance pressure welding and metal vapour pressure welding. The process is characterised by a high energy density, which is fed to the joining point within a very short time.
CP “combined pulse technology”; technical system that uses multiple capacitors with difference capacitances for one CD welding process.
Metal vapour
pressure welding
Process whereby the current density in the joining point must be sufficiently large that metal vapour forms, which activates the surfaces of the joining partners. A large force presses these together to inter-atomic distance, whereby a bond is created through atomic attraction. A partial melt (focus) does not (generally) occur.
Nominal basic variables Electrical basic variables pertaining to nominal conditions.
Back charging
voltage URück/V
is what a capacitor receives if it is charged by the subsiding partial current of the thyristor branch during commutation with opposite charge as the initial charging voltage. Differentiation is made here between the back charging voltage and sweep-through charging voltage UD0.
Key parameters A group of individual parameters – current increase time tP, peak current IP, welding time th, current flow time tI, capacitor discharge time tUn, current-time integral Qit, specific joining energy ϵF and effective current Ieff – which characterise the characteristic operating points of the CD welding process in its electrical behaviour. For a current progression without freewheel, it is necessary to add the current flow duration of the first half period t.
Welding start t0/ms the time point at which the welding process begins. According to DVS bulletin sheet 2911, this moment is marked by an exceeding of the trigger threshold (≤ 5 % of the measuring range end value) or by a specially defined parameter (e.g. the first time the transformer secondary voltage of 4 V is exceeded).
joining energy ϵF
The joining energy pertaining to a resistance of 1 mΩ. Additionally, ϵ50 is defined as the joining energy pertaining to a resistance of 50 μΩ.
Peak current ratio IDP/IP as a relationship of the peak currents of the second and first half-wave of damped oscillation without equal directionality.
Current run-on (NL) Phase of CD welding, in which the main current is fed from the freewheeling branch.
Thermal coupling for multiphase welding. The temperature of the joining point only drops until the welding process is continued with a new pulse. For this purpose, the current must not fall below the lower limit of i > 0.3 IP.
Typifying of CD machines; representation of the electrical type in the form Q1itvIP; whereby the two type parameters current-time integral and current increase speed characterise the welding behaviour of the machine. Types can also be determined for multi-capacitor welding.
Re-charge ratio VUU Ratio of the sweep-through charging voltage to the initial charging voltage of a capacitor with operation without the freewheeling branch.
“Virtual inductance” A clocked IGBT as a circuit element in the function circuit, which can be used via different duty cycles like variable inductance.
Resistance pressure welding Process whereby the joining point is conductively melted and the connection between the partners is established through the solidified melt (referred to as focus). During the welding process, the joining parts are pressed together under great force.