Resistance welding

Resistance welding is a welding process for electrically conductive materials based on the Joule current heat of an electrical current flowing through the connection point .

The connection partners are heated until the welding temperature is reached and welded at the point of contact under the effect of a force through solidification of the melt , through diffusion or also in the solid phase.

Process variants for resistance welding

Classification of welding processes according to DIN 1910-100 with serial numbers according to

All variants of the resistance welding process use the heat generated by the local current flow through the parts to be connected. The heat deposited on the workpiece can be calculated using the following formula:

${\displaystyle W_{\text{s}}=I_{\text{s}}^{2}R_{\text{s}}t_{\text{s}}}$

With

Welding energy , ${\displaystyle W_{\text{s}}}$

Welding current , ${\displaystyle I_{\text{s}}}$

Resistance at the welding point , ${\displaystyle R_{\text{s}}}$

Welding time . ${\displaystyle t_{s}}$

They differ in the type of power supply and the physical connection mechanism used .

	DIN 1910 / ISO 857	EN ISO 4063
Resistance welding	R.	2
Resistance spot welding	RP	21st
Roller seam welding	RR	22nd
Resistance projection welding	RB	23
Flash butt welding	RA	24
Press butt welding	RPS	25th
Resistance stud welding	RBO	782

The DIN 1910-100 divides the metal welding and thus the resistance welding process in pressing and welding processes. Ordinary numbers are assigned for the welding processes in accordance with EN ISO 4063. The picture shows examples of the subdivision with the order numbers for some process variants of resistance welding:

• Resistance pressure welding: generally without the addition of a filler material, but with a contact force on the welding partner:
  • Resistance spot welding
  • Resistance projection welding
  • Resistance stud welding
  • Resistance roller seam welding
  • Resistance butt welding
  • Flash butt welding
  • Capacitor pulse welding (both spot and projection welding)
• Resistance fusion welding : without pressing force, additional material possible:
  • Chamber welding

Types of currents in resistance welding

Alternating current

Classic resistance welding technology uses alternating current (AC) with a frequency of 50 Hz (or 60 Hz). This can be easily generated from the grid using appropriately powerful transformers . This type of current is suitable for most applications, but the current and welding time are controlled by phase control with a thyristor controller, which is why the time and current control increments are a minimum of 10 ms (or 8.3 ms), which is often too long for precise control.

Direct current

Direct current (DC) is also used to ensure that energy is introduced quickly and evenly . However, its production is more complex and therefore more expensive. A distinction is made between DC sources:

1. Single phase rectifier (rare, poor quality direct current)
2. Frequency converter (hardly common today, can generate DC pulses or low frequency depending on the setting → then AC)
3. Three-phase rectifier machines, with a three-phase welding transformer and three-phase rectifier
4. Inverter systems ( MFDC = medium frequency systems, usually with a DC link and 1000–4000 Hz clock frequency of the inverter, with higher frequencies, up to 20 kHz - then referred to as HFDC = high frequency systems) and rectification ( DC ) at the output of the corresponding transformer.

Inverters used in MFDC and HFDC are usually supplied with three-phase alternating current (e.g. 400 V). The voltage is rectified and IGBTs generate a square wave voltage of 1 to 20 kHz from it. This voltage (generally 400 V) is then fed into a suitable transformer. Due to the higher frequency, it is lighter and more efficient than a line-frequency transformer. Rectification must be carried out on the output side, as otherwise the inductance of the power supplies to the welding electrodes would limit the current too much.

Capacitor discharge

The direct current supply for resistance welding can also be made by capacitor discharge. Charged capacitor banks give off their energy in pulses. However, circuits for reversing the polarity of the current direction have also been developed which enable alternating current welding.

Resistance spot and projection welding

Resistance spot welding

→ Main article : Resistance spot welding

Resistance spot welding - schematic diagram

Resistance spot welding (short form: spot welding, RP, EN ISO 4063: Process 21) is a resistance pressure welding process for welding sheet metal of various dimensions and materials.

Resistance spot welding is used to join sheet metal in bodywork and vehicle construction and generally in sheet metal manufacturing. It is also used to weld a wide variety of metals and metal combinations in the electrical industry and electronics manufacturing, e.g. B. in the production of capacitors , contact sets for relays and circuit breakers or connections of coils and motor windings. With certain restrictions, otherwise non-weldable materials can also be connected to one another.

The principle is that the metal parts to be welded are supplied with electricity via electrodes under the action of force. Due to the resistance heating, the connection partners are heated until the required welding temperature is reached. The parts are point-welded at their point of contact between the electrodes under the action of an electrode force by solidification of the melt, by diffusion or in the solid phase. Within a relatively short period of time, a high level of energy in the form of Joule electricity is concentrated on a small area of ​​the work piece, whereby the permanent connection is created when high pressure is applied.

Resistance projection welding

→ Main article : Resistance projection welding

Resistance projection welding: (1) start of welding, (2) after welding

Resistance projection welding (abbreviated form: projection welding, RB, EN ISO 4063: Process 23) is a resistance pressure welding process for welding sheet metal to one another or with welding nuts and wires of various dimensions and materials.

In contrast to resistance spot welding , with resistance projection welding the current density required for welding is not generated by the electrodes, but by the shape of the component. In resistance projection welding, the electrodes only serve to supply power and introduce force. The basic structure of projection welding machines corresponds to that of resistance spot welding devices. Variants of resistance projection welding include cross wire welding, in which wire meshes are welded together, and ring edge welding.

Capacitor pulse welding

Principle of capacitor discharge welding (projection welding)

The capacitor pulse welding , CD welding (Engl. Capacitor discharge or KE-welding (capacitor discharge welding) and referred to) in special application and percussion welding, different from the conventional resistance welding by the type of energy production. It is mainly used for projection or butt welding. The capacitor pulse welding must not be confused with the arc stud welding , which is also based on capacitor discharge , which is an arc welding process, or with welding with electromagnetic pulse technology .

A pulse-shaped direct current is used for welding, which is created by the discharge of a capacitor. The energy stored in the capacitor can be calculated as:

${\displaystyle W={\frac {1}{2}}CU_{0}^{2}}$

With

• W ... energy
• C… capacitor capacity
• U ₀ … charging voltage of the capacitor.

The energy is transferred to the workpiece from charged capacitors via a welding transformer. The charging current can be several orders of magnitude smaller than the later discharging current, so that a pulse load on the power grid and possibly an overload can be avoided. There are no high current peaks in the power line because the capacitor is charged relatively slowly during the welding breaks (1–2 s, depending on the design). The network load is low. The aging of the capacitors should be taken into account, which leads to a loss of capacity after a few years.

Current curve and heating during projection welding with capacitor discharge

Equivalent circuit diagram for capacitor discharge welding

The welding process has two process steps:

• Charging the capacitor
• welding

The capacitor is charged via the charging voltage transformer before welding. The energy to be stored for the welding process is determined with the charging voltage U _L for a given capacity C. After charging, the welding machine is electrically disconnected from the charge transformer.

For the welding, the electrode force F is first applied. If the primary circuit between the capacitor and welding transformer is now closed, the current I _s (t) flows in the secondary circuit through the contact surface of the parts to be joined , while the electrode force F remains constant.

From an electrical point of view, the arrangement of the electrical assemblies is a damped electrical oscillating circuit with inductive coupling in which the electrical energy stored in the capacitor is converted into heat at the welding point in a relatively short time. With projection welding, the greatest heating takes place at the projection tip, since this is where the greatest resistance is present. The efficiency of KES welding is high, as there is hardly any energy dissipation through heat conduction. Heat affected zones are kept to a minimum.

As the temperature rises, the heat resistance of the material decreases and the hump deformation increases. The increase in temperature is increased because the specific electrical resistance of the weld metal increases with the temperature. When the melting temperature is reached, welding occurs. The increasing hump deformation leads to a growing electrical contact area of ​​the parts to be joined, and thus to a falling resistance and a falling temperature. This tendency is reinforced by the increase in the specific heat capacity with increasing temperature. Thermal conduction and falling discharge current cause the welded parts to cool down quickly.

During the discharge there is a very steep increase in current and short welding times. The shortest welding time is around 1 ms. The welding process is completed as soon as the capacitors are discharged. The welding time is calculated from the start of the current flow and until the current drops to a value of 50% of the peak current. The short welding time causes a local energy concentration in a small heat-affected zone in the component. This enables the safe welding of high-strength steels and the most varied of material combinations, including highly conductive welding partners of various dimensions.

Welding using a high-current pulse is characterized by two parameters:

• the welding force,
• the welding energy

The welding parameters are matched to the parts to be welded and set accordingly. The maximum energy and the maximum welding current of the machines are determined by the size of the machine and are independent of the mains connection.

With KE welding, workpieces of different material thicknesses made of different materials can be welded (e.g. steel with brass ). Alloyed stainless steels, high-carbon or hardened workpieces can also be connected with the process.

Welding equipment

→ Main article : Welding equipment for resistance welding

Components of a welding device for resistance spot and projection welding

The welding equipment for resistance welding includes a mechanical machine body and an electrical system consisting of a power unit and a controller.

Welding equipment

Welding devices can be stationary welding machines or welding guns of various sizes and designs. Stationary welding machines are used for spot, projection and roller seam welding or butt welding in workshop production. The respective welding process determines the type and the electrical and mechanical requirements for the machines EN ISO 669. Welding guns are mobile welding devices for spot welding that are guided to the welding location by hand or by manipulators - such as industrial robots .

Controls and power units

The power unit is a device for the electrical supply of an adapted welding transformer, which generates a high welding current with a low welding voltage from the high mains voltage with a comparatively low mains current. The force and the welding current are set by the control according to a time sequence specified by the welding task. The power sources can be designed very differently depending on the type of current and application conditions. In spot, projection and roller seam welding, the electrode force and the welding current must be set by a controller according to a time sequence specified by the welding task. The power unit and an adapted welding transformer generate a high welding current with a low welding voltage from the high mains voltage with a comparatively low mains current. Welding control, power unit and, if necessary, transformer are often combined in one housing, but can also be designed as independent components.

Mains frequency power sources

Thyristors are used as AC power controllers in mains frequency power sources . This allows the voltage to be changed continuously via an ignition point delay so that only part of each voltage half-wave reaches the welding transformer. This phase control makes the rms value of the secondary voltage variable and the welding current adjustable. The current flow is controlled by antiparallel connection of the thyristors. After the current has passed zero, further current flow is blocked until the corresponding thyristor receives an ignition pulse. From this point on, the consumer is supplied with energy until the next zero crossing. The later the respective thyristor is ignited, the lower the average power.

DC power sources

Inverter for medium frequency welding (box in the top of the control cabinet); the transformer is external.

There are different versions of direct current sources (see), of which only the inverter current source (medium frequency = MF, high frequency = HF) is mentioned here. With inverter power sources, which have become more and more popular in recent years, the welding transformer is fed with an alternating voltage (1 kHz to 4 kHz (MF) or up to 20 kHz (HF)) generated by an inverter. The three-phase alternating voltage is first converted into a direct voltage and then into a single-phase square-wave alternating voltage. This is fed into the welding transformer on the primary side. The current intensity is set by changing the pulse width , which changes the rms value of the voltage and thus the current intensity in the secondary circuit. Rectification is carried out immediately after the transformer in order to avoid inductive voltage drops in the cable and / or clamp.

The significantly higher control speed is advantageous (with 1 kHz inverters, the response time of the power controller is 0.5 ms compared to 10 ms with 50 Hz welding current controllers). Another technological advantage of inverter welding is the lower mass of the transformers, so that it is possible to integrate them into the welding guns. a. a small secondary circuit causes low inductive resistance and no heavy secondary cables have to be manipulated.

Power sources for capacitor pulse welding

In principle, these are direct current sources that are used for capacitor pulse welding. The principle is that a capacitor battery is first charged and then suddenly discharged via the welding transformer and the welding point for welding. There is a high input of energy at the beginning of the welding up to a peak value and the subsequent decay of the current.

The power sources consist of a component for direct current generation, a capacitor battery to store electrical energy, a thyristor switch and a special transformer.

Spot weld quality and quality assurance for spot and projection welding

→ Main article : spot weld quality

Shunt

Decisive for the spot weld quality are, among many other factors beyond the welding control input welding parameters , by the type and thickness of the components to be welded, the number of which fit to each other and are dependent on their surface (coated, galvanized, zinc-plated ).

The cooling of the spot welding electrode , the temperature of the cooling water and its flow rate also play a role .

When arranging the welding points on the component, the possibility of a shunt should be taken into account. Part of the current can become ineffective. The current not only flows through the weld nugget to be melted, but also past it. A typical example are shunt effects caused by weld spots that have already been completed (Fig. Shunt 1) and 2)). The current flowing through the welding points i is . The shunt effect can be kept negligibly small by a suitable choice of the welding point spacings. If this is not possible, the applied current must be increased accordingly so that the required minimum spot weld diameter is achieved. Shunt also occurs with one-sided resistance spot welding (Fig. Shunt 3)). With this type of welding, the influence of the shunt can be reduced by means of a welding arrangement “component with low wall thickness - component with greater wall thickness - lower copper”. ${\displaystyle I_{i}={\frac {U}{R_{i}}}}$

The wear of the electrode must also be taken into account when carrying out the process: As the number of welds increases, the electrode is worn, which increases its cross-section due to thermal and mechanical influences, increases its roughness and changes its metallurgical surface properties through "alloying". This reduces the current density and changes the contact resistance between the electrodes and the sheet metal surface, which leads to a change in the thermal energy conversion. This behavior must be taken into account when parameterizing the control (stepper function). The electrode caps may have to be reworked in order to retain the original cross-section and restore the surface condition. This usually happens after about 300 weld spots have been set, but it depends very much on the thickness of the material to be welded, its surface and the thermal load. The right choice of electrode material can reduce wear .

Depending on the materials to be connected, their suitability for welding and the quality of the connection depend on the electrode materials selected . Special electrode material combinations have proven to be effective for small-part and micro-welding.

Weld points are subject to specific quality assurance measures adapted to the conditions of use of the components to be welded. These include the methods of material testing and online process monitoring . Material testing methods can be carried out destructively or non-destructively. The destructive tests include the chisel test as a workshop test, various forms of tensile test, roll-off or torsion tests, metallurgical cuts. Dynamic material test methods such as fatigue tests are also common. The non-destructive testing methods used are ultrasonic testing or active thermography . When monitoring the welding process , some physical process variables that are essential for quality, such as current, voltage, resistance, electrode force, electrode travel, are recorded during the welding process and evaluated with regard to the expected weld point quality. The latter methods are much cheaper, but can also produce false positive results depending on the method and its application.

Resistance roller seam welding

Roller seam welding (lap seam) - schematic diagram

→ Main article : Roller seam welding

Resistance roller seam welding (short form: roller seam welding RR, order number 22 according to EN ISO 4063) is resistance seam welding that is one of the resistance pressure welding processes and was derived from resistance spot welding . The force and current required for welding are transmitted through pairs of roller electrodes , or a roller and a mandrel, or a roller and a flat electrode. The electrodes, designed as rollers, press the sheets together and conduct the welding current in a concentrated manner into the workpieces. Correspondingly equipped stationary welding machines are used for this.

Resistance butt welding

The resistance butt welding methods are also based on the principle of resistance heating.

Press butt welding

Press butt welding - schematic sketch, (1) heat input (2) force input (3) finished component

The pressure butt welding (atomic number 25 according to EN ISO 4063) is used to connect solid cross-sections, such as z. B. occur in chains and wires, on their respective blunt side.

The components are firmly clamped in clamping jaws , which serve both as electrodes for power transmission and power transmission.

The components are pressed together so that there is good contact over the entire cross-sectional area. The welding point is heated up to welding temperature (melting) by the flowing current. When the required temperature is reached over the entire cross-sectional area of ​​the weld, the current flow is interrupted and the components are firmly compressed onto one another by means of the advance movement of the electrodes.

The result of the press butt welding is characterized by a burr-free thick bead. The components to be welded must be clean at the joint for an optimal welding result.

Flash butt welding

Flash butt welding is a variation of pressure butt welding . With this welding method, there is only selective contact between the components during the heating phase. Due to the resulting high current density, the material is liquefied, evaporated or thrown away like a splash at these contact points. The components therefore partially burn off at the welding point. As a result of the ongoing formation and destruction of the contact bridges, an advance movement of an electrode including the component must also take place during the heating phase in order to maintain contact. After the required temperature has been reached, the current flow is interrupted and an axial force is suddenly applied, which compresses the components onto one another. The advantage of this method is that, as a result of the burning off, contamination at the welding point is automatically removed and a protective gas atmosphere protects the welding point through the evaporating metal. However, a burr usually forms at the compression point, which must then be removed.

Chamber welding

The chamber welding is similar to the above-mentioned resistance butt welding methods. The component ends are placed in a ceramic tube (chamber), touch each other, are melted by the flowing current and welded together with constant advance. Possible applications of this process are e.g. B. the welding of wire rope ends or cable strands.

In contrast to the other processes described here, chamber welding is counted as a resistance fusion welding process according to the standard.

Osh

Prohibited for people with pacemakers

Magnetic field warning

All resistance welding processes work with low voltages (below the maximum permissible contact voltage) and high currents. For this reason, direct contact with the parts carrying the welding current and the workpieces during welding is generally harmless.

Resistance welding can cause hazards

• Weld spatter: molten weld metal splashed out of the weld (these are not electrical sparks)
• Effect of force: Possibility of crushing hands in the area of ​​the electrodes and electrode arms
• Noise emission by placing the electrodes on the weld metal and by blowing out compressed air when operating compressed air-operated power generation systems
• Vapors from organic coatings on the weld metal. For these reasons, respiratory protection (suction devices or a respirator), eye protection (protective goggles or protective screen), hearing protection and, if necessary, hand protection are prescribed.
• After a welding inverter has been disconnected from the mains, the intermediate circuit still has a dangerous contact voltage. Depending on the type and embodiment, the discharge time can be up to several minutes.
• When opening the welding systems for capacitor pulse welding , there is a risk of a life-threatening electric shock if the capacitors are not or only partially discharged during the welding process. Unless suitable protective mechanisms have been integrated, this risk also exists when the welding machine is switched off.

In the immediate vicinity of resistance welding equipment, strong magnetic fields occur during welding. As a rule, workplaces at resistance welding facilities are marked with the prohibition sign "Prohibition for people with pacemakers" and the warning signs "Warning of magnetic field" or (incorrectly) "Warning of electromagnetic field". This is often done for precautionary reasons and is not necessarily a sign of an actual risk. Whether there is really a risk for those with active implants or whether they can continue to work must be checked on a case-by-case basis.

literature

• M. Krause: Resistance pressure welding . DVS-Verlag, Düsseldorf 1993, ISBN 3-87155-531-2 .
• A. Kirchheim, A. Lehmann, R. Staub, G. Schaffner, N. Jeck: Force measurement in resistance welding. (pdf, 11.94 MB) In: 19th DVS special conference, special edition. DVS, May 27, 2004, archived from the original on September 28, 2007 ; accessed on June 5, 2020 . 
• NN: Pocket book DVS data sheets and guidelines, resistance welding technology . DVS-Verlag, Düsseldorf 2002, ISBN 3-87155-201-1 .
• Manfred Beckert: Basics of welding technology. Welding process . Verlag Technik, Berlin 1993, ISBN 3-341-00812-8 .
• Ulrich Dilthey: Welding production processes. Volume 1 Welding and Cutting Technologies . Springer Verlag, Berlin 2003, ISBN 3-540-21673-1 .
• Hans-Jürgen Rusch: Repair welding. Vogel Buchverlag, Würzburg 2005, ISBN 3-8343-3019-1 .
• Thoralf Winkler: Magnetic field emission from resistance welding equipment . docupoint Verlag, Magdeburg 2007, ISBN 978-3-939665-09-0 .
• BGI 5011. Assessment of magnetic fields from resistance welding equipment. (pdf, 3.15 MB) Berufsgenossenschaft Energie, Textil, Elektro Medienprodukte, November 2006, accessed on June 5, 2020 .  .
• Lorenz Pfeiffer: Expertise in resistance welding . W. Girardet publishing house, Essen 1969.
• Patent US451345 : Method of Electric Welding. Published July 20, 1909 , inventor: Elihu Thomson ( pat2pdf.org ). ‌
• Patent US928701 : Uniting the Component Parts of Composite Sheet Metal Structures. Published on July 20, 1909 , inventor: Adolph F. Rietzel ( pat2pdf.org ). ‌
• Wolfgang Piersig: Historical overview of the development of metalworking: Contribution to the history of technology (8) , GRIN Verlag, 2010 ( limited preview in the Google book search).

Web links

Commons : Resistance Welding  - Collection of pictures, videos and audio files

• A. Wolf: Capacitor discharge welding. RS electronics ( PDF ).
• German Association for Welding and Allied Processes eV
• Working group AG V 3 "Resistance welding"

Individual evidence

1. ↑ ^{a b} DIN 1910-100: 2008-02: Welding and related processes - Terms - Part 100: Metal welding processes with additions to DIN EN 14610: 2005
2. ↑ ^{a b c d e f} DIN EN ISO 4063: 2011-03 Welding and related processes - list of processes and serial numbers
3. ↑ http://www.matuschek.de/html/de/gleichstrom_hochfrequenz.htm
4. ↑ Patent US6321167 : RESISTANCE-WELDING POWER SUPPLY APPARATUS. Published on 1998 , Inventors: Takashi Jochi, Mikio Watanabe. ‌
5. ↑ DVS: Measurement of spot, projection and roller seam welding. DVS bulletin 2908, 2006.
6. ↑ DIN ISO 669: 2001-06: "Resistance welding equipment - Mechanical and electrical requirements"
7. ↑ ^{a b c} DVS : "Controls and power units for resistance welding" , DVS bulletin 2904, 2010
8. ↑ Small lexicon. ( Memento from March 22, 2014 in the Internet Archive ) Harms & Wende GmbH & Co. KG.
9. ↑ GSI, IMWF: Tool life increase in resistance welding by electrode milling, RESEARCH PROJECT AiF no. 13.134 N / DVS no. 4.031
10. ↑ DVS, Electrodes for Resistance Welding, leaflet, DVS 2903
11. ↑ DVS, Resistance Welding in Electronics and Precision Engineering - Overview and Basics , Leaflet, DVS 2950
12. ↑ D. Steinmeier: Resistance Welding - Weld Monitoring Basics-1 . (PDF) microJoining Solutions - microTips ™.
13. ^ Yi Ming Zhang: Real-time weld process monitoring. (= Woodhead Publishing Series in Welding and Other Joining Technologies. No. 62) 2008, ISBN 1-84569-268-3 .
14. ↑ DVS "Resistance Roller Seam Welding - Processes and Basics" , DVS Leaflet 2906-1, 2006
15. ↑ DVS: Press butt welding of steel , DVS bulletin 2931, 2008