Spot welding parameters
Welding parameters for resistance welding are called the setting values for a resistance welding process , which can be set on the welding devices or can be changed in a controlled manner during the welding process . For resistance spot welding these are the sizes:
- Welding current I s ,
- Welding time t s ,
- Electrode force F E .
These parameters are defined in the DVS 2904 data sheet and must be set in such a way that the required weld quality is achieved.
Welding parameters for welding sheet steel up to 3 mm
Welding area
Welding parameters are set so that they are within the welding current range (WCR ).
- Definition: welding current range that allows production without spatter during spot welding with a spot diameter equal to or greater than a predetermined value with a constant welding time or constant electrode force.
In the case of setting values outside the secured welding range, the required quality is usually not achieved. There are weld joint defects.
The determination of the welding area diagrams is specified in EN ISO 14327. With their help, the possible welding conditions are determined, i. H. with constant electrode force the permissible welding currents and welding times and with constant welding time the permissible welding currents and electrode forces which allow welding between the adhesive weld and the spatter boundary. In the past, welding areas were determined in complex welding tests. Today, FEM simulation software systems are available for this, with the help of which welding areas can be calculated. These may then have to be verified through targeted tests.
The welding area is of decisive importance for process capability and control under given technological conditions. The narrower the welding area, the more sensitive a technology is to technological deviations and the greater the susceptibility to errors. The high-strength steel materials used in lightweight body-in-white construction have different welding areas depending on the surface properties and the choice of welding parameters. The electrode shape selected also plays an important role and influences the process reliability of resistance spot welding of these materials.
The occurrence of spatter does not only depend on the electrode force, the welding time and the welding current, but a number of other factors also influence it. In the case of current variations between the sticking and spatter limit, when approaching the spatter limit, initially only sporadic spraying is observed, which gradually changes to steady spraying as the current increases further.
Setting the welding parameters
The welding parameters welding current, welding time and electrode force can be combined in very different variants depending on the welding task, the control system, the power unit and the power generation . During the welding process, these values can be kept constant or changed in a controlling or regulating manner. For welding thick sheets or steels that are sensitive to hardening, it can make sense to weld with variable current and in several current pulses (multi-pulse welding). The use of “adaptive” or “intelligent” controllers always leads to parameter changes during the welding process.
Simple rules of thumb can be used for single-pulse welding with constant welding parameters on bare sheet steel:
- I s = 9.5 * √t [kA]
- t s = 8 * t [Per] or 20 ms
- F E = 2000 * t [N]
with t as the sheet thickness of the thinner joining partner.
The DVS in his leaflet specifies 2902-4 guideline for setting welding parameters. Suppliers of welding machines or automobile manufacturers have each proposed their own guide values. The regulation MBN 10382 makes tabular specifications that refer to sheets of quality DC04 with a good fit. A minimum point diameter of d p = 4 * √t min is aimed for. The values given assume a comparison sheet thickness t V out. In the case of unequal sheet thicknesses, it is assumed to be t V = 0.8 * t min + 0.2 * t 2 (with t min as the sheet thickness of the thinner sheet). The welding time is specified with t s = 200 * t v [ms], with a two-pulse welding with a pause time of 40 ms being carried out from a comparative sheet thickness t v > 1.3 mm. For coated sheets and higher material strengths and poor fit, electrode force surcharges must be taken into account:
- Galvanized sheets with or without organic coating + 10%
- higher strength materials:
- H240 LA (D) + 10%
- H320 LA (D) + 15%
- H400 LA (D) + 20%
- Materials with a yield point over 400 MPa + 20%.
The comparison of the parameter settings in the adjacent pictures is only apparently contradictory. Due to the specification of higher electrode force according to MBN 10382 compared to the values of DVS 2902-4, welding must be carried out with a higher welding current and increasing current time, since the resistance between the sheets and thus the heating are lower. If a larger spot diameter is required, higher welding currents and longer welding times are required. The point diameter and the setting parameters are closely related, which is made clear by the graphic "Relationship between the setting parameters".
Predefined values for electrode force, welding current, welding time and pause time can only be guide values that have to be adapted for the respective joining task. It must be taken into account that a change in the electrode force also requires a change in the welding current strength and possibly the welding time and the welding current setting range is reduced with lower electrode forces, the tendency to spatter increases and the process reliability decreases. This can be partially compensated for by longer welding times. Under given welding conditions, the tendency to spatter increases sharply above a certain welding current. Splashes tend to lead to great variance in quality and a smaller lens diameter.
The electrodes wear out as the number of welds increases. The electrode surfaces increase as a result of the electrodes being hit and pressed onto the workpieces and alloys are formed on the electrode surface. This results in a larger electrode working area, a lower current density and a changed contact resistance. The point diameter falls. An almost constant spot diameter can be achieved by increasing the welding current in a controlled manner using a stepper control.
Individual evidence
- ↑ DVS : Controls and power units for resistance welding. (= DVS data sheet. 2904). 2010.
- ↑ a b EN ISO 14327: Resistance welding. Procedure for determining the weld area diagram for resistance spot, projection and roll seam welding ; EN ISO 14327: 2004.
- ↑ a b c G. Weber, K. Momeni, S. Göklü: Welding areas of high-strength steel sheets for lightweight body-in-white - influence of welding parameters in resistance spot welding. In: welding and cutting. 55, issue 10, 2003.
- ↑ a b G. Weber, M. Rethmeier, S. Göklü: Influence of the Type of Electrode Caps on the Welding Current Ranges and the Process Reliability in Resistance Spot Welding. (= IIW Document. No. III-1484-08). 2008.
- ↑ a b c T. Dupuy, E. Groleau: Welding range scatter assessment. (= IIW Document. No. III-1582-10).
- ↑ a b c d M. Krause: Resistance pressure welding . DVS- Verlag, Düsseldorf 1993, ISBN 3-87155-531-2 .
- ↑ a b c d e DVS : Resistance spot welding of steels - up to 3 mm thick. (= DVS data sheet. 2902-4). 2001.
- ↑ a b c d Mercedes-Benz: MBN 10 382: Resistance spot welding of steel up to 3 mm sheet thickness. , 2005.