5.4. Resistance Welding

Resistance welding is a group of pressure welding processes wherein coalescence is produced by the heat obtained from resistance of the work to the flow of electric current in welding circuit and by the application of pressure. There is no any external heat source. Heat is developed in the parts to be welded and pressure is applied by the welding machine through electrodes. No fluxes or filler metals are used.

Current for resistance welding is usually supplied through welding transformers, which transform the high-voltage, low-amperage power supply to usable high amperage at low-voltage.

Pressure, or more properly, the electrode force, is supplied either by air or oil pressure through a cylinder, mechanically by cams, manually by foot or hand levers through linkages or some other means.

5.4.1 Heating Fundamentals in Resistance Welding

Any current flow in an electrical conductor creates heat. The amount of heat generated depends on three factors:

- the current intensity;

- the resistance of the conductor;

- the time of current flow;

(5.6)

where Q is heat generated in Joules;

I is current in Amps;

R is resistance of the work in Ohms;

t is time of current flow in seconds.

The formula shows that the heat generated is directly proportional to the resistance and square welding current. The total heat generated is partly used to make the weld and partly lost to the surrounding metal, mainly, by thermal conductivity.

Figure 5.16 represents spot welding. In making a weld, the current passes from one electrode through the base metal to the other electrode. During this flow it encounters seven separate resistance zones as shown in figure 5.16.

Points 1 and 7. The electrical resistance of the electrode material (copper, or bronze) is of low value.

Points 2and 6 are corresponding to the contact resistance between the electrode and the base metal. The magnitude of this resistance depends on the surface condition of the base metal and electrode, the size and contour of the electrode face and the electrode force P. This is a point of high heat generation, but due to thermal conductivity of the electrode material and the fact, that it is usually water cooled the base metal does not reach the fusion temperature during the current passage.

Points 3 and 5. The total resistance of the base metal itself is low, but it is higher than electrode material resistance.

Point 4. This is a point of highest resistance and therefore of the greatest heat generation. Due to hot spots 2 and 6, the heat generated at this interface is not readily lost to the colder electrodes. That effects formation of the weld interface contact.

Fig. 5.16. Scheme of spot welding: 1…7 – resistance and temperature zones; 8 – electrode;

A, B – parts to be welded; T – electrical transformer; P – compressing forces

The following types of resistance welding are distinguished: spot, seam, projection, flash, upset and percussion welding.

5.4.2 Spot Welding

Spot welding is a resistance welding wherein coalescence is produced by heat obtained from resistance to the flow of electric current through the work parts held together under pressure of electrodes. The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. In the simple single spot weld shown in figure 5.17 the passage of current and electrode force application must be through the electrodes 2, the overlapped work pieces 1 and the weld.

The are four definite stages of time in the spot-welding cycle (Fig.5.18):

- squeeze time is the time between the first application of the electrode force P and welding current turn-on;

- weld time is the time, during which welding current flows:

- time, during which the electrode force either is still applied or is increased (for better deformation of weld) after the welding circuit has being deenergized;

- off time is the time when the electrodes are off the work.

Fig. 5.17. Spot welding: 1 – work pieces; 2 – electrodes; 3 – transformer; P – pressure

The thickness of welding works is from 0.5 to 12 mm. There are two main regimes of welding:

- soft (easy):

j=I/F=80...160 A/mm² (F-area of a weld);

P=15...40MPa;

t=0.5...3.0 sec;

- rigid:

j=120...360 A/mm²;

P=40...150MPa;

t=0.001...0.010 sec.

Fig. 5.18. Spot welding cycle: I-current; P-pressure; t-time

5.4.3. Seam Welding

Seam welding (Fig.5.19, 5.20) is a resistance-welding process wherein coalescence is produced by the heat obtained from resistance to the flow of electric current through the work parts held together under pressure by circular electrodes. The resulting weld in seam welding is series of overlapping spot welds (practically continuous or persistent) made progressively along a joint by rotating circular electrodes with automatic current cut-off.

Fig. 5.19. Seam welding process: 1-works; 2-electrodes; 3-transformer

Fig. 5.20. Seam welding cycles: I – current; P – compressing force;

S-distance of works’ movement; t – time

Seam welding has much in common with spot welding. Welds may be single or multiple, that is a single seam or more parallel seams may be produced simultaneously. Welds may be direct, or indirect, similar to spot welding.

Seam welding is used usually when persistent leak-free weld is required.

5.4.4. Projection Welding

Projection welding is schematically very similar to spot welding, but has some differences:

- a work undergoes stamping before welding for formation of projections (ledges);

- electrodes have large sizes and welding machine has high capacity; Types of projection welds are shown in figure 5.21.

After welding the upsetting is produced to smooth down the work surface. Main advantage of the process is high productivity. Main disadvantage is high welding machine capacity.

Fig. 5.21. Examples of projection welds

5.4.5 Flash welding

Flash welding (Fig. 5.22) is a resistance-welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from resistance to the flow of electric current between the two surfaces, and by the application of pressure after heating is substantially completed. Flashing and upsetting are accompanied by expulsion of metal from the joint.

Fig. 5.22. Scheme of flash welding (a) and welding cycle (b):

1 – immovable plate; 2 – clamp (electrode); 3 – work; 4 – movable plate; 5 – transformer;

6 – contact (weld); I – current; S – replacement of movable plate; P – pressure

Flash welding is done by placing one of two work parts in the jaws of the machine. As the parts are brought together into very light contact, a voltage of sufficient magnitude is applied to form a flashing action between the parts. Flashing continues as the parts advance until the work pieces reach a forging temperature (sometimes a melting point). The weld is completed by the application of sufficient forging pressure and the interruption of current.

5.4.6 Upset welding

Upset welding is a resistance-welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces or progressively along a joint by the heat obtained from resistance to the flow of electric current through the area of contact of those surfaces. Pressure is applied before heating is started and maintained throughout the heating period (Fig. 5.23).

Fig. 5.23. Upset welding cycle

In upset welding the parts are brought into solid contact and current takes the path through the contact area until a sufficiently high temperature is generated to allow the forging of a weld. The heat is generated mainly by the contact resistance between the two pieces. The difference between upset welding and flash welding is that no flashing from the abutting surfaces occurs at any time and the heat is developed slowly by the resistance between the two parts.

5.4.7 Percussion Welding

Percussion welding (capacitor energy-storage, electrostatic percussive welding) is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from an arc produced by a rapid discharge of stored electrical energy with pressure percussively applied during or immediately following the electrical discharge.

There are two main variations of this process:

- transformer welding (with transformer 5)

- transformerless welding (without transformer 5) (Fig. 5.24).

Fig. 5.24. Percussion welding: 1 – rectifier; 2, 5 – transformer; 3 – capacitor;

4 – switch; 6 – parts to be welded

Transformer 2 is the main transformer. It reduces voltage, for instance, from 16 kV to 220V; transformer 5 is reducing welding transformer.

When switch 4 is on the left position the charging of the capacitor 3 takes a place. When the switch is in the right position the discharge of the capacitor 3 and welding of works 6 takes a place.

The method allows:

- to remove overloads in electrical circuit (network), which is peculiar to resistance welding;

- to produce a welding by definite dose of electrical energy and eliminate overheat of welding parts.