Pressure welding

Types and mechanisms of pressure welding

Pressure welding uses friction or explosion to heat the joining section of metal workpieces and join them under pressure. The process is also called solid-state welding. Pressure welding is a generic term for welding methods that weld workpieces by applying mechanical pressure on the joining section (weld joint).
The use of mechanical pressure allows numerical control of the process. Pressure welding has been used widely in FA (factory automation).
Major methods include gas pressure welding, friction welding, resistance welding, diffusion welding, ultrasonic welding, and explosion welding. Friction stir welding (FSW), a variant of friction welding, has become increasingly popular. This process can improve joint efficiency by using a rotating tool to stir the base materials with rotational friction while applying strong pressure on the joining section.

Pressure welding type
Pressure welding type Welding method
Gas pressure welding - -
Friction welding Friction stir welding (FSW) -
Resistance welding Lap resistance welding Resistance spot welding
Projection welding
Seam welding
Butt resistance welding Upset welding
High-frequency induction welding
Butt projection welding
Flash welding
Butt seam welding
Diffusion welding - -
Ultrasonic welding - -
Explosion welding - -

Gas pressure welding

This method is often used to join steel frames for buildings. The joining surfaces of the base materials are pressed against each other and the area is heated with acetylene gas and oxygen. When the base materials heat up and start melting, they are further pressed against each other. Impurities in the joining surfaces are pushed out and the surfaces are joined.

Gas pressure welding

Friction welding

This method generates high-speed friction between the base materials (metal or resin) to soften them with the heat generated by the friction and then applies pressure to join them. It is said to be an eco-friendly joining method because it does not need a heat source other than friction heat, removes the need for welding rods or flux, and does not produce spatter or gas as compared to arc welding or gas welding.
Friction welding can be also performed based on three factors of friction thrust (pushing force), rotation speed and time. Since all of these can be controlled numerically, friction welding can be controlled automatically without human intervention and is widely used in FA (factory automation).

Friction stir welding (FSW), a variant of friction welding, has attracted a lot of attention. The process rotates a cylindrical tool with a probe (protrusion) at high speed and moves the tool so that the probe digs along the joining section with high pressure. The rotating tool softens the base materials and stirs the area around the weld to cause plastic deformation and atomic bonding between the materials.

Friction stir welding (FSW)
Friction stir welding (FSW)
  1. Probe
  2. Rotation
  3. Weld zone
  4. Pressurization by the tool
  5. Butt surfaces
  6. Plate movement
  7. Shoulder

Resistance spot welding

Weld materials are held together from above and below with copper electrodes for energization connected to the welding power supply. When a current passes through the section to be welded, the heat generated by electrical resistance (Joule heat) melts and joins the materials. In FA (factory automation), automatic resistant spot welding machines have been used widely in joining processes on manufacturing lines.
Seam welding, which uses a series of overlapping weld spots, and projection welding, which causes concentrated resistance heat on projections created on the joining surface of one material, are variations of resistant spot welding.

Resistance spot welding
  1. Pressure force
  2. Flow of electric current
  3. Electrodes
  4. Weld materials

Projection welding

This method is used for welding nuts/bolts to steel plates. Electrodes for resistant spot welding are applied to the projection(s) provided on one of the base materials. Heat is concentrated on the projection(s) to soften the material and the welding starts. As the welding proceeds, the spot(s) becomes larger. Although this decreases the current density, the electrical resistance is increased by the raised temperature, which maintains high heat generation to allow welding. Consequently, the weld has high quality as compared to welding without using projections.

Projection welding is broadly divided into two types: Embossed projection welding and solid projection welding.
Embossed projection welding uses projections worked on the base material to concentrate current flow on the projections. Creating multiple projections allow simultaneous welding of multiple weld spots. Practical applications include welding gas tank reinforcements, shock absorber brackets, and oil filters. Unlike embossed projection welding, solid projection welding does not use projections created on a flat plate. It uses existing projections such as the corners of plates or crossed round bars. Practical applications include welding nuts and bolts on plates, or brake drums.

Embossed projection welding

Before welding

Embossed projection welding - Before welding

After welding

Embossed projection welding - After welding
Solid projection welding

Before welding

Solid projection welding - Before welding

After welding

Solid projection welding - After welding

Seam welding

Weld materials are held from above and below with circular electrodes. A current is passed while the electrodes are rotated, and the heat generated by electrical resistance joins the weld materials continuously. The method is also called lap seam welding.
Making a line of overlapping spot welds ensures leak tightness. This is cost effective because the welding speed is fast and no gas is used. Since sparks are not produced during welding, there are no safety problems and no need for protective equipment.

Practical applications include welding parts or sections requiring leak tightness or waterproofing such as fuel tanks. In addition to lap seam welding, seam welding has variations such as butt seam welding that continues heating butted surfaces by applying pressure and passing a welding current to weld the seam, and mush seam welding that uses roller electrodes to mush up slightly overlapped edges of base materials by passing an electric current and applying pressure to weld the seam continuously.

Seam welding
  1. Roller electrode
  2. Weld
  3. Welding power supply

During the production of small crystal oscillators and gyroscope sensors, it is necessary to seam-weld their lids in vacuum to improve product performance and suppress performance degradation over time. This is done with a vacuum seam welder that can provide leak-tight sealing using roller electrodes in a vacuum.

Upset welding

This method holds base materials with their joint ends together, applies pressure and then passes a current through the abutting surfaces. The materials are joined with the heat generated by electrical resistance (Joule heat).
Unlike resistant spot welding that holds two overlaid base materials with electrodes, upset welding welds the joint ends of the materials. Upset is formed around the weld.
This method is used for welding metal wires or rods with small cross-sectional areas.

Flash welding

This method holds base materials with their joint ends lightly held together and then passes a current through the abutting surfaces. The materials are joined with the heat generated by electrical resistance (Joule heat). Unlike upset welding, pressure is not applied to the base materials when the current is passed. After the current is passed and the generated Joule heat heats the material to the joining temperature, pressure is applied to weld the materials.
Similar to upset welding, this method is suited for welding metal wires or rods.

Mechanism of upset welding/flash welding
Mechanism of upset welding/flash welding
  1. Pressure
  2. Electrode
  3. Joined section

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