Roller seam welding
The resistance seam welding (short form: Seam welding RR, ranking number 22 on the EN ISO 4063) is a resistance seam welding, one of the resistance pressure welding process and from the resistance spot welding was derived. 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.
Areas of application of the process
The different variants of roller seam welding are used in the sheet metal processing industry, in the construction of small containers, in car body and wagon construction, and in the manufacture of pipes or packaging material. In sheet metal production, coils are endlessly connected in rolling and coating lines. In the packaging industry, barrels and tinplate packaging for food, beverages, chemical products and aerosol cans are joined using roller seam pinch welding. In the automotive industry, blanks ( tailored blanks ) or silencer pots are joined using this process for body production . The household appliance industry uses the process for welding heater housings, tubs and drums for washing machines, tumble dryers and dishwasher housings. Possible applications for the variant of butt weld welding are the joining of metal sheets for the production of large-area shaped sheet metal parts, such as roofs and side walls of rail vehicles and buses. This variant of roller seam welding is also used in small container production, for fire extinguishing containers and galvanized sheet steel drums.
Principle and connection formation
As with resistance spot welding , the contact area under the electrodes is heated by Joule current heating. In most cases, the electrodes are a pair of rollers which, like the point electrodes, only touch the workpiece over a small area, so that the current flows from roller to roller through a limited cross-section of the workpiece and creates a spot weld. The rollers rotate, move the workpiece further as a result of the contact pressure and do not have to be lifted to get to the location of a new welding point. Compared to seam welding with spot electrodes, roller seam welding has the advantage of much less electrode wear and a higher welding speed. Tight seams can also be produced.
Welding parameters
The heat required to generate a weld results from Joule's law:
With
- ... welding energy in the unit of time ,
- ... welding current as a function of time,
- ... resistance at the welding point as a function of time,
- ... differential of time.
The resistance depends on the acting roller force, the roller profile shape, the surface properties of the workpieces and the material properties. The welding time is determined by the pulse duration of the switched-on current and the time the heat energy acts on the unit of volume of the material by the roller speed. The following adjustable welding parameters result:
- the welding current and the type of current,
- the pulse duration,
- the roller speed (welding speed),
- Roll profile;
- Electrode force.
The welding current can be switched on continuously as a continuous current or periodically according to a current contact program.
If an alternating current remains switched on continuously over the entire length of the seam, this is referred to as continuous alternating current . Each half-wave creates a spot weld. The point spacing depends on the welding speed and the welding current frequency. With a mains frequency of 50 Hz and a welding speed of 6 m / min, one welding point is created per millimeter and thus a tight seam.
When using a current cycle program, the welding current flow is periodically interrupted for adjustable current pauses t p after the selectable current time t s has elapsed . In this way, sealing seams or interrupted seams can be welded. The point spacing is determined by the current break time and the welding speed.
The rotation of the rollers can be interrupted. One speaks of step seam welding. In order to weld materials that are only conditionally suitable for welding, such as non-ferrous metals with high demands on the seam quality, current and power programs can also be used in addition to the current cycle program. So z. B. the stronger welding current is followed by a weaker reheating current, during the duration of which the pressing force is increased to improve the welding quality.
Process variants
Sheets can be overlapped or butt-connected by roller seam welding.
The lap seam is most often used for reasons of effort. The remaining thickening of the cross-section in the seam area, the deflection of the force flow during the transmission of force from one workpiece part to the other and the remaining gap in the plane of contact between the two sheets have a disadvantageous effect. The latter has a notch effect and crevice corrosion.
A second variant is pinch seam welding. The metal sheets to be connected are arranged with little overlap. Under the effect of the current and the electrode force, a pressure-tight connection is created between the sheet metal ends with a smaller seam thickness. The sheet metal edges to be welded can be chamfered in order to keep the seam transitions as flat as possible. An intermediate wire electrode can be used to join steel sheets with metallic coatings in the sheet thickness range up to 0.5 mm.
Another variant of roller seam welding is foil seam welding. A foil strip of the same type is automatically fed via guide devices between the roller electrodes and the top and bottom sides of the workpiece. The task of the foil strip is to concentrate the current and to reduce the heat extraction by the cooled roller electrodes.