Spot welding electrode

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Spot welding electrodes transmit the welding current and the electrode force to the workpieces during resistance spot and projection welding . A process derived from spot welding is roller seam welding , which uses rotating rollers as electrodes. Electrodes are the direct welding tool that transfers the current from the power source to the workpiece, concentrates the current flow in the parts to be joined and fixes the parts to be welded. They consist of conductive materials and must be adapted to the specific welding task.

Shape and structure of resistance spot welding electrodes

Construction of a two-part electrode for resistance spot welding according to
Electrode shapes for welding small parts
Electrode caps according to EN ISO 5821

The shape and the geometrical dimensions of the electrodes influence the heat balance of the welding process, the accessibility of the welding device to the workpiece and the behavior of the penetration into the material during the heating process. The material resistance of the electrodes should be kept as small as possible. Their cross-sections should be as large as possible, their lengths short and their specific electrical resistances low, because the following applies to the material resistances:

With

... electrode length,
... specific resistance ,
... electrode cross-section .

The electrodes can be constructed in one or two parts. Two-part electrodes consist of a cooled electrode shaft and attached electrode caps of different shapes. The choice of cap shape depends on the welding area and the process reliability ( process capability and control ), and ultimately the weldability of the component. The EN ISO 5821 and DVS leaflet 2903 differ seven cap forms. Notwithstanding this, a wide variety of special forms are used in practice. One-piece electrodes of various shapes are used in precision engineering and electrical engineering.

Shape and structure of roller seam electrodes

Electrode rolls with a spherical and flat profile

The roller electrodes have the additional task of transporting the workpiece. In almost all cases the electrodes consist of a pair of rollers. For straight seams, e.g. longitudinal seams of hollow bodies, one of the two roller electrodes is replaced by a cylindrical or profiled mandrel.

The current density is determined by the width of the electrode contact area. The roll diameter must be adapted to the structural conditions of the workpiece. Depending on the accessibility to the welding point, roller welding electrodes for seam welding are straight or with a one-sided or double-sided edge. The roller profile is adapted to the material to be welded and can be spherical or flat. Flat profiles are used for welding bare steel sheets. For metal-coated steel sheets and light metals, spherical electrodes are used.

The electrodes are often designed as rings that are clamped in a water-cooled flange.

Electrode materials

The choice of electrode material also influences the achievable welding area and process reliability ( process capability and control ).

Desired material properties are high electrical conductivity (often given in% IACS - International Annealed Copper Standard - electrical conductivity compared to copper) to reduce electrical power losses. When used on low-conductivity parts to be joined, a high thermal conductivity is desired and for highly-conductive parts to be joined, a low thermal conductivity is desired. Electrodes should have a high degree of hardness at room and welding temperature in order to retain their shape and avoid microcracks. The tendency to alloy on the workpiece or electrode should be low.

The materials are designated in EN ISO 5182 according to material group, type and number:

  • Group A - copper and copper alloys (three material types)
  • Group B - Sintered Materials (six material types)
  • Group C - Dispersion-Hardened Copper Alloys (DSC) (two types of materials)

Electrode materials according to AWS-RWMA are in international use. A distinction is made between two material groups (copper materials, sintered materials) with different material classes, which correspond to the materials according to EN ISO 5182.

The suitability of materials for resistance spot welding depends to a large extent on the choice of favorable electrode materials. The DVS data sheet 2903 contains a table: "Welding suitability and selection of electrodes according to the material of the metals to be joined" .

Electrode wear and tool life

During their use, electrodes are subject to wear due to thermal, mechanical and chemical stress. The electrode surfaces increase due to the constant impact and pressing of the electrodes on the workpieces. In addition, especially with coated materials, there are alloys on the electrode surface. Due to the larger area, the current density is lower and the contact resistance is higher due to the alloys. In spot and projection welding, electrode wear is described by the term tool life . The tool life is according to EN ISO 8166:

"The electrode life is [...] defined for all materials as the number of welds that meet the specified welding quality before reworking of the electrode contact surface is necessary. An electrode has reached its service life when the welds made with it have a point diameter (determined in the peeling test) below 3 * √t (with three out of five successive welds in a test piece, where t is the sheet metal thickness in mm) [...]. "

The amount of electrode in use is influenced by various factors. The shape, mass, material and tendency of the electrode cap to alloy are essential. During welding, the cooling and the choice of welding parameters affect the tool life that can be achieved. This also depends on the properties of the welded material, its thickness and coating. The attachment and repositioning behavior of the welding equipment also plays a role. The electrode life can be increased significantly by regular mechanical processing of the electrode surfaces by milling or forming.

Electrode cooling

The electrodes in resistance spot welding are generally cooled from the inside by water. The water flow should be at least 4 l / min, but higher flow rates are recommended when welding coated steel. The cooling water supply pipe should be arranged so that the water acts on the back of the electrode. The inlet water temperature should be less than 20 ° C and the outlet temperature should not exceed 30 ° C.

Roller electrodes for roller seam welding must also be adequately cooled, especially when steel sheets with metallic coatings as well as aluminum and magnesium alloys are welded, as these tend to form an alloy with the material of the roller electrode. In the case of large electrode diameters, the shaft (picture "Electrode rolls with a spherical and flat profile") is used for cooling, and in the case of small electrodes, a water bath is used. In the former case, the cooling water is usually supplied to the cavity of the electrode or the flange of the electrode shaft.

Individual evidence

  1. ↑ a b DVS Resistance Roller Seam Welding - Process and Fundamentals , DVS Leaflet 2901-1, 2006
  2. ↑ DIN EN 28430-2: 1992: Electrode holder - Part 2: Morse taper fastening
  3. ↑ a b MiyachiUnitek: Fundamentals of Small Parts Resistance Welding
  4. ↑ a b DIN EN ISO 5821: 2009: Resistance welding - spot welding electrode caps
  5. ↑ DIN ISO 669: 2001-06: Resistance welding equipment - Mechanical and electrical requirements
  6. ↑ DVS : Resistance spot welding of steels up to 3 mm thick , DVS-Merkblatt 2902-4, 2001
  7. ↑ a b Electrodes for resistance welding , leaflet DVS 2903, 1998
  8. ↑ a b DVS Resistance Roller Seam Welding - Process and Fundamentals , DVS Leaflet 2906-1, 2006
  9. ↑ DVS: Resistance welding in electronics and precision engineering - overview and fundamentals, DVS bulletin 2950
  10. ↑ a b DIN EN ISO 5182: 2009-08: Resistance welding - materials for electrodes and auxiliary equipment
  11. ↑ Resistance Welding Manufacturing Alliance (RWMA)
  12. ↑ DIN - German Institute for Standardization: Procedure for evaluating the service life of spot welding electrodes with a constant machine setting EN ISO 8166
  13. ↑ Tool life increase in resistance welding through electrode milling . (PDF) GSI, IMWF; Research project AiF no. 13.134 N / DVS no. 4.031
  14. ↑ DIN EN ISO 14327: 2004: Resistance welding. Procedure for determining the weld area diagram for resistance spot, projection and roll seam welding