LECTURE TWENTY SEVEN Resistance Spot Welding Process SPOT

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LECTURE TWENTY SEVEN Resistance Spot Welding Process

LECTURE TWENTY SEVEN Resistance Spot Welding Process

SPOT WELDING Spot welding is the best-known resistance welding method. It is used for

SPOT WELDING Spot welding is the best-known resistance welding method. It is used for joining thin sheet materials (up to 3 + 3 mm) by overlap joints, and is widely used, e. g. in the automotive industry as in figure (27. 1). An ordinary private car can have up to 5 000 spot-welded joints. The high current, in combination with a rapid heating time, means that thermal energy input is efficiently used: very little is conducted away to the surrounding metal.

Spot welding therefore has several advantages over other methods of welding sheet metal, such

Spot welding therefore has several advantages over other methods of welding sheet metal, such as: 1. Little deformation of the workpiece, 2. Very high rate of production 3. Easy to automate therefore suitable for mass production. 4. Low energy requirement and little pollution 5. Fast 6. No filler materials required. 7. Little special training required. Two electrodes clamp the two sheets of metal together with a considerable force, while passing a high current through the metal. Thermal energy is produced as the current passes the electrical contact resistance between the two sheets, as given by Equation (26. 1): H = I 2. R. t

Figure 27. 1 the principle of spot welding.

Figure 27. 1 the principle of spot welding.

The total resistance between the electrodes (see Figure 27. 1) is made up of:

The total resistance between the electrodes (see Figure 27. 1) is made up of: 2 rl +2 r 2+r Where rl = contact resistance between each electrode and the workpiece r 2 = the resistance through the metal of each of the pieces to be joined r 3 = the contact resistance between the two pieces of metal. As welding starts, the contact resistances are very high. The initial passage of current breaks through the surface layers. Most of the heat formed at the contact between the electrodes and the workpiece is conducted away through the water-cooled electrodes. However, this is not the case with the heat developed in the contact resistance between the two workpiece sheets, so the temperature here rises until the melting temperature of the metal is reached, while the surfaces continue to be pressed together by the clamping force, so that a weld nugget forms in the contact area.

The electrodes need to be of a material with a high hardness, low electrical

The electrodes need to be of a material with a high hardness, low electrical resistance and high thermal conductivity. Cooling is decisive for their life. Wear and tear, together with deformation, increases the effective contact size of the electrodes, which reduces the current density and, in due course, the strength of the welds produced. An electrode normally has a life of about 5 000 -10 000 welds: when welding galvanised steel, this life is reduced to about 500 -2 000 welds.

IMPORTANT PARAMETERS OF SPOT WELDING The spot welding process includes a number of variables

IMPORTANT PARAMETERS OF SPOT WELDING The spot welding process includes a number of variables that can be adjusted in order to achieve optimum welding performance: The welding current is the current that flows through the workpiece. Of all the parameters, it is that has the greatest effect on strength and quality of the weld, as the amount of heat produced is proportional to the square of the welding current. The clamping force is the force with which the electrodes press the sheets together (k. N) Welding time is the time for which current flows through the workpiece, and is measured in cycles. Hold time is the time from when the current is interrupted until the clamping force can be released. The electrode area determines the size of the area through which the welding current passes, i. e. the current density.

EQUIPMENT OF SPOT WELDING Because of its wide spread industrial use , various machines

EQUIPMENT OF SPOT WELDING Because of its wide spread industrial use , various machines and methods are available to perform spot-welding operations. The equipment includes rocker-arm and press-type spot-welding machines, and portable spot-welding guns. Rocker-arm spot welders, shown in Figure 27. 2 , have a stationary lower electrode and a movable upper electrode that can be raised and lowered for loading and unloading the work. The upper electrode is mounted on a rockerarm (hence the name)whose movement is controlled by a foot pedal operated by the worker. Modern machines can be programmed to control force and current during the weld cycle.

Press-type spot welders are intended for larger work. The upper electrode has a straightline

Press-type spot welders are intended for larger work. The upper electrode has a straightline motion provided by a vertical press that is pneumatically or hydraulically powered. The press action permits larger forces to be applied, and the controls usually permit programming of complex weld cycles. The previous two machine types are both stationary spot welders , in which the work is brought to the machine. For large, heavy work it is difficult to move and position the part into stationary machines. For these cases , portable spot-welding guns are available in various sizes and configurations. These devices consist of two opposing electrodes contained in a pincer mechanism. Each unit is lightweight so that it can be held and manipulated by a human worker or an industrial robot. The gun is connected to its own power and control source by means of flexible electrical cables and air hoses. Water cooling for the electrodes , if needed, can also be provided through a water hose. Portable spot-welding guns are widely used in automobile final assembly plants to spot weld car bodies . Some of these guns are operated by people , but industrial robots have become the preferred technology

Figure 27. 2

Figure 27. 2