Spot welding is a joining process whereby metal surface points are joined by heat generated from resistance to electric current. The process is also known as resistance spot welding.
How does spot welding work?
Spot welding is a type of electric resistance – a process in which contacting metal surface points are joined by the heat generated from resistance to electric current. The process uses two copper alloy electrodes to direct current into a spot on the target piece. The target is held together under pressure created by the electrodes. Forcing a large current through the spot will melt the metal, creating the join. A large amount of energy can be concentrated onto the spot in approximately 10–100 milliseconds. The extreme heat is localised, not affecting the rest of the workpiece.
The energy is determined by the resistance between the electrodes and the magnitude and duration of the current. Applying too little energy will not melt the metal or will make a low quality weld. Applying too much energy will melt too much metal, eject molten material, and pierce the work piece.
Spot welding process (stages of spot welding)
Spot welding can be broken down into several stages; initially the electrodes are brought to the surface of the metal and a slight amount of pressure is applied. The current is then applied after which the current is removed but the electrodes remain in place for the material to cool.
The tool holders simply hold the electrodes in place, supporting optional water hoses that cool the electrodes. Tool holding processes include a paddle-type, light duty and regular offset. The electrodes are made of a low resistance copper alloy and vary in size according to the application. The width of the workpieces is limited by the throat length of the apparatus and ranges typically from 13 to 130 cm. After the current is removed from the workpiece, it is cooled via the coolant holes in the centre of the electrodes.
Spot welding gun
In the case of resistance spot welding, the main parts of the tooling system are the gun, its type, and the size and shape of the electrodes. In a situation where there are locally high applied forces of thick materials, a C-type gun is used, which gives a high tooling flexibility, as the motion of the electrodes is collinear. An X-type arrangement provides less rigidity, although the reachable workspace is far larger, e.g. manufacturing a thin and flat piece such as a car floor pan. The paths of the moving electrodes are not collinear, offering less flexibility.
Types of electrodes used in spot welding
Radius style electrodes are used for high temperature applications, eccentric electrodes for welding corners, offset eccentric tips for reaching into small spaces and offset truncated for reaching into the target material itself.
Advantages and disadvantages of spot welding
The process will harden the material, causing distortion. This reduces the material’s fatigue strength, and will anneal the material. The physical effects of spot welding include internal cracking and surface cracks. This will negatively affect the chemical properties of the metal, for example, its corrosive properties.
Welding times are often very short and the electrodes cannot move fast enough to keep the material clamped. A double pulse will solve this. The first pulse will soften the metal. During the pause between the two pulses, the electrodes will come closer and make better contact.
During spot welding, the large electric current induces a magnetic field. They interact with each other to produce a large magnetic force field, which drives the melted metal to move at a velocity up to 0.5 m/s. The heat energy distribution in spot welding could be changed by the fast motion of the melted metal.
Spot welding vs arc welding
Arc welding and spot welding are fundamentally different, however, both use electricity to leverage the process. Arc welding melts the base metal and uses a filler rod. Spot welding uses the electric resistance of the base metal to heat and fuse the workpieces, via electrodes. Arc welding penetrates deeply, therefore can be used on much thicker target materials. Arc welders require an edge to create a weld, whilst spot welders work across the piece, often a large, flat, thinner material. Shielding gas is often required in arc welding.
Spot welding transformer
The principle equipment is a power supply, energy storage unit, a switch, a welding transformer and the electrodes. The energy storage element provides instantaneous power. The transformer steps down the voltage and steps up the current. The transformer reduces the current level that the switch handles. The welding electrodes are effectively the secondary circuit.
There is the resistance of secondary winding, the cables, and the welding electrodes. There is also the contact resistance between the welding electrodes and the workpiece. There is the resistance of the workpieces, and the contact resistance between the workpieces.
Initially, the contact resistances are high, so most of the initial energy is dissipated. The heat and the clamping force will soften the material and make better contact. Consequently, more electrical energy will go into the workpiece. A the temperature rises, the electrodes and the workpiece are dispersing the heat. The objective is to apply enough energy so that only a portion of material within the spot melts. The perimeter of the spot will conduct away heat and keep the perimeter at a lower temperature. The interior of the spot has less heat conducted away, so it melts first. If the welding current is applied too long, the entire spot melts, the material runs out and a hole develops.
The voltage required depends on the resistance of the material, the sheet thickness and desired size. When welding a combination like 1.0 + 1.0 mm sheet steel, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V towards the end of the process. This decrease in voltage results from the reduction in resistance caused by the workpiece melting. The open circuit voltage from the transformer is higher than this, typically in the 5 to 22 volt range.
The resistance of the weld spot changes as it flows and liquefies. Modern welding equipment can monitor and adjust the weld in real-time to ensure a consistency. The equipment controls variables such as current, voltage, power and energy.
Spot welding applications
Spot welding is typically used when welding particular types of sheet metal, welded wire mesh or wire mesh. Thick material is difficult to spot weld as the heat flows into the surrounding metal. Aluminium alloys can be spot welded, but their higher thermal conductivity and electrical conductivity requires an increased current.
The most common application of spot welding is in the welding of sheet metal, in the manufacture of cars. These spot welders are completely automated and many of the industrial robots found on assembly lines are part of this process.
Spot welding in Orthodontics
Small-scale spot welding equipment is used when resizing metal molar bands used in orthodontics.
Spot welding in batteries
One application is the spot welding of straps to nickel–cadmium, nickel–metal hydride or Lithium-ion battery cells to make batteries. The cells are joined by spot welding thin nickel straps to the battery terminals. Spot welding can keep the battery from getting too hot. Traditional soldering is at a disadvantage in this instance.
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