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Manufacturing Processes > Joining processes > Resistance welding

 

Resistance welding

 

Process description

Covers a range of welding processes that use the resistance to electrical current between two materials to generate sufficient heat for fusion. A number of processes use a timed or continuous passage of electric current at the contacting surfaces of the two parts to be joined to generate heat locally, fusing them together and creating the weld with the addition of pressure, provided by current supplying electrodes or platens.

Materials

  • Low carbon steels commonly, however, almost any material combination can be welded using conventional resistance welding techniques. Not recommended for cast iron, low melting point metals and high carbon steels.
  • Electroslag Welding (ESW) is used to weld carbon and low alloy steels typically. Nickel, copper and stainless steel less common.
Resistance Welding

Process variations

  • Resistance Spot Welding (RSW): uses two water-cooled copper alloy electrodes of various shapes to form a joint on lapped sheet-metal. Can be manual portable (gun), single or multi-spot semiautomatic, automatic floor standing (rocker arm or press) or robot mounted as an end effector.
  • Resistance Seam Welding (RSEW): uses two driven copper alloy wheels. Current is supplied in rapid pulses creating a series of overlapping spot welds which is pressure tight. Usually floor standing equipment, either circular, longitudinal or universal types.
  • Resistance Projection Welding (RPW): a component and sheet-metal are clamped between current carrying platens. Localized welding takes place at the projections on the component(s) at the contact area. Usually floor standing equipment, either single or multi-projection press type.
  • Upset resistance welding: electrical resistance between two abutting surfaces and additional pressure used to create butt welds on small pipe assemblies, rings and strips.
  • Percussion resistance welding: rapid discharge of electrical current and then percussion pressure for welding rods or tubes to sheet-metal. Flash Welding (FW): parts are accurately aligned at their ends and clamped by the electrodes. The current is applied and the ends brought together removing the high spots at the contact area deoxidizing the joint (known as flashing). Second part is the application of pressure effectively forging the weld.
  • ESW: the joint is effectively ‘cast’ between joint edges between a gap of about 20 to 50 mm. An electric arc is used initially to heat a flux within water-cooled copper molding shoes spanning the joint area. Resistance between the consumable electrode and the base material is then used to generate the heat for fusion. The weld pool is shielded by the molten flux as welding progresses up the joint.
  • A variant of ESW is Electrogas Welding (EGW). However, the process doesn’t use electrical resistance as a heat source, but a gas shielded arc, therefore the molten flux pool above the weld is not necessary. Used for thick sections of carbon steel.

Economic considerations

  • Full automation and integration with component assembly relatively easy.
  • High production rates possible due to short weld times, e.g. RSW =20 spots/min, RSEW =30 m/min, FW =3 s/10mm² area.
  • Automation readily achievable using all processes.
  • No filler metals or fluxes required (except ESW).
  • Little or no post-welding heat treatment required.
  • Minimal joint preparation needed.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to moderate.
  • Equipment costs low to moderate.
  • Direct labor costs low. Skilled operators are not required.
  • Finishing costs very low. Cleaning of welds is not necessary typically, except with Flash Welding (FW), which requires machining or grinding to remove excess material.
  • High deposition rates for ESW, but can still be slow.

Typical applications

  • RSW: car bodies, aircraft structures, light structural fabrications and domestic appliances
  • RSEW: fuel tanks, cans and radiators
  • RPW: reinforcing rings, captive nuts, pins and studs to sheet-metal, wire mesh
  • FW: for joining parts of uniform cross section, such as bar, rods and tubes, and occasionally sheetmetal
  • ESW: joining structural sections of buildings and bridges such as columns, machine frames and on-site fabrication

Design aspects

  • Typical joint designs: lap (RSW and RSEW), edge (RSEW), butt (FW and ESW), attachments (PW).
  • Access to joint area important.
  • Can be used for joints inaccessible by other methods or where welded components are closely situated.
  • Spot weld should have a diameter between four and eight times the material thickness.
  • Can process some coated sheet-metals (except ESW).
  • Same end cross sections are required for FW.
  • For RSW, RSEW and PW:
    • Minimum sheet thickness =0.3mm
    • Maximum sheet thickness, commonly =6mm
    • Mild steel sheet up to 20mm thick has been spot- and seam-welded, but requires high currents and expensive equipment.
  • For FW, sizes ranging 0.2mm thick sheet to sections up to 0.1m² in area.
  • Unequal thicknesses possible with RSW and RSEW (up to 3:1 thickness ratio).
  • ESW applied to sheet thicknesses of same order from 25 up to 500mm using several guide tubes and electrodes in one pass, but down to 75mm for a single set. Vertical welds can restrict design freedom in ESW.

Quality issues

  • Clean, high quality welds with very low distortion can be produced. Although a heat affected zone always created, can be small.
  • Coarse grain structures may be created in ESW due to high heat input and slow cooling.
  • Surface preparation important to remove any contaminates from the weld area such as oxide layers, paint and thick films of grease and oil. Resistance welding of aluminum requires special surface preparation.
  • Welding variables for spot, seam and projection welding should be pre-set and controlled during production, these include: current, timing and pressure (where necessary).
  • Electrodes or platens must efficiently transfer pressure to the weld, conduct and concentrate the current and remove heat away from the weld area, therefore, maintenance should be performed at regular intervals.
  • Spot, seam and projection welds can act as corrosion traps.
  • RSW, RSEW and PW welds can be difficult to inspect. Destructive testing should be intermittently performed to monitor weld quality.
  • Depression left behind in RSW and RSEW serves to prevent cavities or cracks due to contraction of the cooling metal.
  • Possibility of galvanic corrosion when resistance welding some dissimilar metals.
  • High strength welds are produced by FW. Always leaves a ridge at the joint area which must be removed.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of the welds fair to good for RSW, RSEW, FW and PW. Excellent for ESW.
  • No weld spatter and no arc flash (except ESW initially).
  • Alignment of parts to give good contact at the joint area important for consistent weld quality.
  • Repeatability typically ±0.5– ±1mm for robot RSW.
  • Axes alignment total tolerance for FW between 0.1 and 0.25 mm.

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