Spot welding

Spot welding is a type of resistance welding used to weld various sheet metal products. Typically the sheets are in the 0.5-3.0 mm thickness range. The process uses two shaped copper alloy electrodes to concentrate welding current into a small "spot" and to simultaneously clamp the sheets together. Forcing a large current through the spot will melt the metal and form the weld. The attractive feature of spot welding is a lot of energy can be delivered to the spot in a very short time (ten to one hundred millisecondsFact|date=March 2008). That permits the welding to occur without excessive heating to the rest of the sheet.

The amount of heat (energy) delivered to the spot is determined by the resistance between the electrodes and the amplitude and duration of the current. The amount of energy is chosen to match the sheet's material properties, its thickness, and type of electrodes. Applying too little energy won't melt the metal or will make a poor weld. Applying too much energy will melt too much metal and make a hole rather than a weld. Another attractive feature of spot welding is the energy delivered to the spot can be controlled to produce reliable welds.

Applications

Spot welding is typically used when welding particular types of sheet metal. Thicker stock is more difficult to spot weld because the heat flows into the surrounding metal more easily. Spot welding can be easily identified on many sheet metal goods, such as metal buckets. Aluminum alloys can also be spot welded. However, their much higher thermal conductivity and electrical conductivity mean that up to three times higher welding currents are needed. This requires larger, more powerful, and more expensive welding transformers.

Perhaps the most common application of spot welding is in the automobile manufacturing industry, where it is used almost universally to weld the sheet metal to form a car. Spot welders can also be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use for robots being painting).

Spot welding is also used is in the orthodontist's clinic, where small scale spot welding equipment is used when resizing metal "molar bands" used in orthodontics.

Another application is spot welding straps to nickel-cadmium or nickel-metal-hydride cells in order 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, as might happen if conventional soldering were done.

Electrical notes

The basic spot welder consists of a power supply, an energy storage unit (e.g., a capacitor bank), a switch, a welding transformer, and the welding electrodes. The energy storage element allows the welder to deliver high instantaneous power levels. If the power demands are not high, then the energy storage element isn't needed. The switch causes the stored energy to be dumped into the welding transformer. The welding transformer steps down the voltage and steps up the current. An important feature of the transformer is it reduces the current level that the switch must handle. The welding electrodes are part of the transformer's secondary circuit. There is also a control box that manages the switch and may monitor the welding electrode voltage or current.

The resistance presented to the welder is complicated. [Geoff Shannon, Advances in Resistance Welding Technology Offer Improved Weld Quality and Reliability for Battery Manufacturers, Battery Power Products & Technology, July/August 2007, Vol 11, Issue 4, www.batterypoweronline.com.] 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.

At the beginning of the weld, the contact resistances are usually high, so most of the initial energy will be dissipated there. That heat and the clamping force will soften and smooth out the material at the electrode-material interface and make better contact (that is, lower the contact resistance). Consequently, more electrical energy will go into the workpiece and the junction resistance of the two workpieces. As electrical energy is delivered to the weld and causes the temperature to rise, the electrodes and the workpiece are conducting that heat away. The goal is to apply enough energy so that a portion of material within the spot melts without having the entire spot melt. The perimeter of the spot will conduct away a lot of 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 or otherwise fails, and the "weld" becomes a hole.

The voltage needed for welding depends on the resistance of the material to be welded, the sheet thickness and desired size of the nugget. When welding a common 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 at the end of the weld. 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 5-10 V range Fact|date=August 2008, but there is a large voltage drop in the electrodes and secondary side of the transformer when the circuit is closed Fact|date=August 2008.

The resistance of the weld spot changes as it flows and and liquefies. Modern welding equipment can monitor and adjust the weld in real-time to ensure a consist weld. The equipment may seek to control different variables during the weld, such as current, voltage, power, or energy.

Projection welding is a modification of spot welding. In this process the weld is locailzed by means of raised sections, or projections, on one or both of the workpieces to be joined. heat is concentrated at the projections, which permits the welding of heavier sections or the closer spacing of welds. The projections can also serve as a means of positioning the workpieces. Projection welding is often used to weld studs, nuts, and other screw machine parts to metal plate. It's also frequently used to join crossed wires and bars. This is another high-production process, and multiple projection welds can be arranged by suitable designing and jigging. [Kugler, A. N. and Robert W. Wiswesser. (Fundamentals of Welding Part 1)]

ee also

* Signature image processing

References

External links

* [http://www.dengensha.com/ Manufacturer of Resistance Welder] Spot Welder, Projection Welder, Seam Welder, Nut Feeder, Bolt Feeder, Welding Gun.
* [http://www.millerwelds.com/pdf/Resistance.pdf Handbook for Resistance Spot Welding]
* [http://www.hobbyspotwelders.com/ hobbyspotwelders] has some information in its "Spot Welding" menu.
* [http://www.sunstoneengineering.com/ Sunstone Engineering] has a technical resources page on its website. It also has some videos showing some spot welding being done.
* [http://www.philpem.me.uk/elec/welder/ The Poor Man's Battery Tab Welder] describes a jury rigged capacitive discharge spot welder suitable for 3 mil thick Nickel ribbons.
* [http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv009.cgi?read=21825 HP Forum Archive 09] has a discussion of spot welding batteries and spot welding schedules.
* [http://www.miyachi1.com Miyachi Unitek website] has many technical resources available from its home page.
* [http://www.aws.org/wj/2005/07/032 Power Supply Designed for Small-Scale Resistance Spot Welding] discusses weld nugget quality control.

*US Patent 2077600 filed September 3, 1936, Welding by the Charge of a Condenser, G W Watson. A capacitor (aka condenser) is used to limit the energy delivered during a spot welding operation. Capacitor is charged rather than discharged. Goal is delicate work.

*US Patent 2184627 filed July 15, 1937, Dual Current Condenser Welder, G W Watson. Second condenser is discharged into the weld. "I have discovered that the weld is considerably improved if a second condenser is provided and connected in such a manner that it discharges into the weld...."

*US Patent 2428390 filed April 26, 1945, Welding System, C G Smith. Capacitive discharge with mechanical energy storage and step down transformer. Goal is high peak powers.

*US Patent 2942214 filed March 7, 1957, Long-Lived Impulse Transformer. Construction details for welding transformer.

*US Patent 3050618 filed July 13, 1961, Portable Spot Welding Tool, M N Fischer. Capacitive discharge spot welder with stepdown transformer. Goal is delicate work.

*US Patent 3211885 filed June 28, 1962, Spot Welder, M N Fischer.

*US Patent 6756558 filed June 29, 2004, High Current, Low Impedance Resistance Welding Device, T E Salzer et al. Goal is welding seams that have a resistance in the 1 to 100 micro-ohm range.