Spot welding

Last updated
A spot welder Spot welder.miller.triddle.jpg
A spot welder

Spot welding (or resistance spot welding [1] ) is a type of electric resistance welding used to weld various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance to electric current.

Contents

The process uses two shaped copper alloy electrodes to concentrate welding current into a small "spot" and to simultaneously clamp the sheets together. Work-pieces are held together under pressure exerted by electrodes. Typically the sheets are in the 0.5 to 3 mm (0.020 to 0.118 in) thickness range. Forcing a large current through the spot will melt the metal and form the weld. The attractive feature of spot welding is that a large amount of energy can be delivered to the spot in a very short time (approximately 10–100 milliseconds). [2] This permits the welding to occur without excessive heating of the remainder of the sheet.

The amount of heat (energy) delivered to the spot is determined by the resistance between the electrodes and the magnitude and duration of the current. [3] The amount of energy is chosen to match the sheet's material properties, its thickness, and type of electrodes. Applying too little energy will not melt the metal or will make a poor weld. Applying too much energy will melt too much metal, eject molten material, and make a hole rather than a weld. [4] Another feature of spot welding is that the energy delivered to the spot can be controlled to produce reliable welds.

Process and equipment

A spot welding robot Robotworx-spot-welding-robot.jpg
A spot welding robot

Spot welding involves three stages; the first of which involves the electrodes being brought to the surface of the metal and applying a slight amount of pressure. The current from the electrodes is then applied briefly after which the current is removed but the electrodes remain in place for the material to cool. Weld times range from 0.01 sec to 0.63 sec depending on the thickness of the metal, the electrode force and the diameter of the electrodes themselves.[ citation needed ]

The equipment used in the spot welding process consists of tool holders and electrodes. The tool holders function as a mechanism to hold the electrodes firmly in place and also support optional water hoses that cool the electrodes during welding. Tool holding methods include a paddle-type, light duty, universal, and regular offset. The electrodes generally are made of a low resistance alloy, usually copper, and are designed in many different shapes and sizes depending on the application needed.

The two materials being welded together are known as the workpieces and must conduct electricity. The width of the workpieces is limited by the throat length of the welding apparatus and ranges typically from 5 to 50 inches (13 to 130 cm). Workpiece thickness can range from 0.008 to 1.25 inches (0.20 to 32 mm). [5]

After the current is removed from the workpiece, it is cooled via the coolant holes in the center of the electrodes. Both water and a brine solution may be used as coolants in spot welding mechanisms.

In the case of resistance spot welding, there are two main parts of the tooling system, the features of which fundamentally influence the whole process: the gun and its type, and the size and shape of the electrode. In such application, where the gun layout should be as rigid as possible due to the high applying forces (e.g. welding of thick materials), the C-type gun is widely used. As well as the high resulting rigidity, this arrangement leads to a high tooling flexibility, as the motion of the electrodes is collinear. Unlike the C-type, the so-called X-type arrangement provides less rigidity, although the reachable workspace is far larger than with the C-type, thus this layout is very common, where thin and flat objects are being processed (e.g. manufacturing of floor pan or roof panel). However, it offers less flexibility in terms of tooling, because the paths of the moving electrodes are not collinear (like the tips of a scissor), so a dome-shaped electrode tip should be used.

Electrodes used in spot welding can vary greatly with different applications. Each tool style has a different purpose. Radius style electrodes are used for high heat applications, electrodes with a truncated tip for high pressure, eccentric electrodes for welding corners, offset eccentric tips for reaching into corners and small spaces, and finally offset truncated for reaching into the workpiece itself.

Characteristics

The spot welding process tends to harden the material, causing it to warp. This reduces the material's fatigue strength, and may stretch the material as well as anneal it. The physical effects of spot welding include internal cracking, surface cracks and a bad appearance. The crack around the weld nugget will be extended under an external load or fatigue to produce a different type of failure. [6] [7] The chemical properties affected include the metal's internal resistance and its corrosive properties.

Welding times are often very short, which can cause problems with the electrodes—they cannot move fast enough to keep the material clamped. Welding controllers will use a double pulse to get around this problem. During the first pulse, the electrode contact may not be able to make a good weld. 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 large magnetic field, and the electric current and magnetic field interact with each other to produce a large magnetic force field too, which drives the melted metal to move very fast at a velocity up to 0.5 m/s. As such, the heat energy distribution in spot welding could be dramatically changed by the fast motion of the melted metal. [8] [9] [10] The fast motion in spot welding can be observed with high-speed photography. [11]

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. [12] 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 much 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 to 22 volt range. [13]

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 consistent weld. The equipment may seek to control different variables during the weld, such as current, voltage, power, or energy.

Welder sizes range from 5 to 500 kVA. [14] Micro spot welders, used in a variety of industries, can go down to 1.5 kVA or less for precision welding needs.

It is common for a spray of molten metal droplets (sparks) to be ejected from the area of the weld during the process.

Resistance spot welding generates no bright arc, so UV protection is not required. OSHA requires transparent face shields or goggles for splatter protection, but does not require any filter lens. [15]

Applications

BMW plant in Leipzig, Germany: Spot welding of BMW 3 series car bodies with KUKA industrial robots BMW Leipzig MEDIA 050719 Download Karosseriebau max.jpg
BMW plant in Leipzig, Germany: Spot welding of BMW 3 series car bodies with KUKA industrial robots

Spot welding is typically used when welding particular types of sheet metal, welded wire mesh or wire mesh. 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. Aluminium alloys can be spot welded, but their much higher thermal conductivity and electrical conductivity requires higher welding currents. 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 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, 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, as might happen if conventional soldering were done.

Good design practice must always allow for adequate accessibility. Connecting surfaces should be free of contaminants such as scale, oil, and dirt, to ensure quality welds. Metal thickness is generally not a factor in determining good welds.

Modifications

Projection welding is a modification of spot welding in which the weld is localized 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 threaded machine parts to metal plate. It is 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. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Welding</span> Fabrication or sculptural process for joining materials

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.

<span class="mw-page-title-main">Shielded metal arc welding</span> Manual arc welding process

Shielded metal arc welding (SMAW), also known as manual metal arc welding, flux shielded arc welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld.

<span class="mw-page-title-main">Submerged arc welding</span> Joining metals using electricity, beneath a granulated flux material

Submerged arc welding (SAW) is a common arc welding process. The first SAW patent was taken out in 1935. The process requires a continuously fed consumable solid or tubular electrode. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.

<span class="mw-page-title-main">Arc welding</span> Process used to fuse metal by using heat from an electrical arc

Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick ("electrode") and the base material to melt the metals at the point of contact. Arc welding power supplies can deliver either direct (DC) or alternating (AC) current to the work, while consumable or non-consumable electrodes are used.

<span class="mw-page-title-main">Laser cutting</span> Technology that uses a laser to cut materials

Laser cutting is a technology that uses a laser to vaporize materials, resulting in a cut edge. While typically used for industrial manufacturing applications, it is now used by schools, small businesses, architecture, and hobbyists. Laser cutting works by directing the output of a high-power laser most commonly through optics. The laser optics and CNC are used to direct the laser beam to the material. A commercial laser for cutting materials uses a motion control system to follow a CNC or G-code of the pattern to be cut onto the material. The focused laser beam is directed at the material, which then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high-quality surface finish.

<span class="mw-page-title-main">Induction heating</span> Process of heating an electrically conducting object by electromagnetic induction

Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an inductor that creates an electromagnetic field within the coil to heat up and possibly melt steel, copper, brass, graphite, gold, silver, aluminum, or carbide.

<span class="mw-page-title-main">Electron-beam welding</span> Use of electrons to join metal parts via melting

Electron-beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed under vacuum conditions to prevent dissipation of the electron beam.

<span class="mw-page-title-main">Plastic welding</span> Welding of semi-finished plastic materials

Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.

<span class="mw-page-title-main">Plasma cutting</span> Process

Plasma cutting is a process that cuts through electrically conductive materials by means of an accelerated jet of hot plasma. Typical materials cut with a plasma torch include steel, stainless steel, aluminum, brass and copper, although other conductive metals may be cut as well. Plasma cutting is often used in fabrication shops, automotive repair and restoration, industrial construction, and salvage and scrapping operations. Due to the high speed and precision cuts combined with low cost, plasma cutting sees widespread use from large-scale industrial computer numerical control (CNC) applications down to small hobbyist shops.

<span class="mw-page-title-main">Electric arc</span> Electrical breakdown of a gas that results in an ongoing electrical discharge

An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".

<span class="mw-page-title-main">Gas tungsten arc welding</span> Welding process

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas. A filler metal is normally used, though some welds, known as 'autogenous welds', or 'fusion welds' do not require it. When helium is used, this is known as heliarc welding. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. TIG welding is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing stronger, higher-quality welds. However, TIG welding is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

<span class="mw-page-title-main">Plasma arc welding</span> Welding process

Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode and the workpiece. The key difference from GTAW is that in PAW, the electrode is positioned within the body of the torch, so the plasma arc is separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities and a temperature approaching 28,000 °C (50,000 °F) or higher.

Electric resistance welding (ERW) is a welding process where metal parts in contact are permanently joined by heating them with an electric current, melting the metal at the joint. Electric resistance welding is widely used, for example, in manufacture of steel pipe and in assembly of bodies for automobiles. The electric current can be supplied to electrodes that also apply clamping pressure, or may be induced by an external magnetic field. The electric resistance welding process can be further classified by the geometry of the weld and the method of applying pressure to the joint: spot welding, seam welding, flash welding, projection welding, for example. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electric current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance as an electric current is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials.

<span class="mw-page-title-main">Welding power supply</span>

A welding power supply is a device that provides or modulates an electric current to perform arc welding. There are multiple arc welding processes ranging from Shielded Metal Arc Welding (SMAW) to inert shielding gas like Gas metal arc welding (GMAW) or Gas tungsten arc welding (GTAW). Welding power supplies primarily serve as devices that allow a welder to exercise control over whether current is alternating current (AC) or direct current (DC), as well as the amount of current and voltage.

<span class="mw-page-title-main">Electroslag welding</span> Single Pass welding

Electroslag welding(ESW) is a highly productive, single pass welding process for thick (greater than 25 mm up to about 300 mm) materials in a vertical or close to vertical position. (ESW) is similar to electrogas welding, but the main difference is the arc starts in a different location. An electric arc is initially struck by wire that is fed into the desired weld location and then flux is added. Additional flux is added until the molten slag, reaching the tip of the electrode, extinguishes the arc. The wire is then continuously fed through a consumable guide tube (can oscillate if desired) into the surfaces of the metal workpieces and the filler metal are then melted using the electrical resistance of the molten slag to cause coalescence. The wire and tube then move up along the workpiece while a copper retaining shoe that was put into place before starting (can be water-cooled if desired) is used to keep the weld between the plates that are being welded. Electroslag welding is used mainly to join low carbon steel plates and/or sections that are very thick. It can also be used on structural steel if certain precautions are observed, and for large cross-section aluminium busbars. This process uses a direct current (DC) voltage usually ranging from about 600 A and 40-50 V, higher currents are needed for thicker materials. Because the arc is extinguished, this is not an arc process.

Electrogas welding (EGW) is a continuous vertical position arc welding process developed in 1961, in which an arc is struck between a consumable electrode and the workpiece. A shielding gas is sometimes used, but pressure is not applied. A major difference between EGW and its cousin electroslag welding is that the arc in EGW is not extinguished, instead remains struck throughout the welding process. It is used to make square-groove welds for butt and t-joints, especially in the shipbuilding industry and in the construction of storage tanks.

<span class="mw-page-title-main">Butt welding</span> Welding of metal pieces placed end-to-end (butt joint)

Butt welding is when two pieces of metal are placed end-to-end without overlap and then welded along the joint. Importantly, in a butt joint, the surfaces of the workpieces being joined are on the same plane and the weld metal remains within the planes of the surfaces.

<span class="mw-page-title-main">Fusion welding</span> Welding processes which rely on melting to join materials

Fusion welding is a generic term for welding processes that rely on melting to join materials of similar compositions and melting points. Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone is created in the material.

<span class="mw-page-title-main">Gas metal arc welding</span> Industrial welding process

Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) and metal active gas (MAG) is a welding process in which an electric arc forms between a consumable MIG wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to fuse. Along with the wire electrode, a shielding gas feeds through the welding gun, which shields the process from atmospheric contamination.

Sensors for arc welding are devices which – as a part of a fully mechanised welding equipment – are capable to acquire information about position and, if possible, about the geometry of the intended weld at the workpiece and to provide respective data in a suitable form for the control of the weld torch position and, if possible, for the arc welding process parameters.

References

  1. ↑ Larry F. Jeffus (2002). Welding: Principles and Applications. Cengage Learning. p. 694. ISBN   9781401810467 . Retrieved April 18, 2014.
  2. ↑ robot-welding.com Archived January 17, 2010, at the Wayback Machine
  3. ↑ Joule effect, see Joule's laws
  4. ↑ US Patent 4456810, Adaptive Schedule Selective Weld Control, June 1984. "The weld process is stopped ... before the melt exceeds the electrode diameter. Otherwise, an impressive but totally undesired shower of sparks and hot metal will issue from the weld spot."
  5. ↑ Robert H. Todd; Dell K. Allen; Leo Alting (1994). Manufacturing Processes Reference Guide. Industrial Press. ISBN   0831130490.
  6. ↑ Al-Mukhtar, A. M. (2015). Spot Weldabaility Principles and Considerations Welding procedure and parameters Quality of the welds (1. Aufl ed.). Saarbrücken. ISBN   978-3-8381-5129-8. OCLC   934173073.{{cite book}}: CS1 maint: location missing publisher (link)
  7. ↑ Al-Mukhtar, A. M. (2016). "Review of Resistance Spot Welding Sheets: Processes and Failure Mode". Advanced Engineering Forum. 17: 31–57. doi:10.4028/www.scientific.net/AEF.17.31. ISSN   2234-991X. S2CID   137975371.
  8. ↑ YB Li, ZQ Lin, SJ Hu, and GL Chen, "Numerical Analysis of Magnetic Fluid Dynamics Behaviors During Resistance Spot Welding", J. Appl. Phys., 2007, 101(5), 053506
  9. ↑ YB Li, ZQ Lin, Q Shen and XM Lai,Numerical Analysis of Transport Phenomena in Resistance Spot Welding Process, Transactions of the ASME, Journal of Manufacturing Science and Engineering, 2011, 133(3), 031019-1-8
  10. ↑ YB Li, ZY Wei, YT Li, Q Shen, ZQ Lin, Effects of cone angle of truncated electrode on heat and mass transfer in resistance spot welding, International Journal of Heat and Mass Transfer, 2013, 65(10), 400-408
  11. ↑ A. Cunningham, M. L. Begeman, "A Fundamental Study of Project Welding Using High Speed Photography Computer", Welding Journal, 1965, Vol. 44, 381s-384s
  12. ↑ 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, .
  13. ↑ S. R. Deb; S. Deb (2010). Robotics Technology and Flexible Automation. Tata McGraw-Hill Education. p. 491. ISBN   9780070077911 . Retrieved April 18, 2014.
  14. ↑ George F. Schrader; Ahmad K. Elshennawy (2000). Manufacturing Processes and Materials. SME. p. 311. ISBN   9780872635173 . Retrieved April 18, 2014.
  15. ↑ OSHA (March 26, 2012). "Welding, Cutting, and Brazing 1910.252(b)(2)(i)(C)". Occupational Safety and Health Standards. United States Department of Labor. Retrieved October 8, 2018.
  16. ↑ Kugler, A. N. (1977). Fundamentals of Welding. International Correspondence Schools. LCCN   77360317.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.