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Arc welding

Arc welding

Arc welding is the process of joining separate metal pieces together with high heat from an arc that is generated by an electric current.

Overview

Arc welding is the process of joining metal pieces together using high heat from an arc that is generated and sustained by an electric current. The power supply, either an alternating current (“AC”) or a direct current (“DC”), creates an electric arc between a consumable or non-consumable electrode and the base material. A fusion welding process, the electric arc generates temperatures around 6500˚ degrees F, melting the metal at the joint between the two work pieces. Arc welding can be guided either mechanically or manually along the line of the join. Manual arc welding is a difficult technique to master, with operators requiring significant knowledge and skill. Arc welding offers high efficiency and speed as well as one of the strongest bonds between metals. There are a number of different arc welding processes depending on the application.

There are five basic components in arc welding:

  1. An electrode and electrode cable
  2. A ground cable and clamp (sometimes called a work cable)
  3. A power supply
  4. Metal workpieces
  5. An Arc
Basic components of arc welding.

The electrode either carries the current or conducts the current, melting into the weld pool to supply filler metal to the join. At high temperatures, molten metals can react with oxygen and nitrogen in the air, causing issues with porous or weak welds, excessive spatter, and reduced productivity. To prevent these problems, it is common to utilize one of two forms of protection to the molten pool of metal:

  • Shielding gas flooded over the hot weld at the correct flow rate to prevent atmospheric air from reaching the molten metal.
  • Flux that creates its own shield gas and slag when exposed to high temperatures. The flux's inert gas and slag surround the weld keeping atmospheric gases away.
Pros and cons

The following are the pros of arc welding :

  • Good impact strength
  • High productivity (reduces the cost per weld)
  • Strong and seamless welds with no air gaps
  • Portable equipment
  • Versatile, producing good results on a range of metals
  • Low-cost
  • No license required

There are also the following cons of arc welding:

  • Toxic fumes and the need for ventilation or repirators
  • Greater level of waste produced
  • The practice operators need to perform arc welding
  • Burn through when working with thin materials
Process

Learning to arc weld takes significant practice. The process can be simplified into four main steps:

  1. Creating an electric arc (also known as striking the arc) between the piece of metal you are working on and the electrode.
  2. Creating a bead from the metal that is being melted by moving the arc. The electrode bonds and flows with the metal being melted from the extreme temperatures.
  3. Giving your weld bead some texture and shape. To do this, move your arc in either a figure of 8 or a zig-zag pattern. The motion of the arc helps spread the metal to the desired width.
  4. Cleaning up the edges by brushing the weld and chipping away any jagged edges after every pass. This removes any excess melted material.
AC vs DC

DC power supplies can cause issues with magnetic fields disrupting the arc, preventing it from traveling the shortest distance between the electrode and the metal. This causes spatter, porosity, and incomplete fusion, often happening at the inside corners or the end of welds. This is called arc blow. With the current oscillating back and forth this issue is not present when using AC power supplies. However, DC arc welders provide a smooth and easy-to-control arc that works better with thin materials. This can reduce spatter, plus DC beads generally look better.

AC welders are cheaper and are good for situations prone to arc blow. They penetrate well and are good for thick pieces, such as shipyards. AC welders also work well with metals that have oxide layers on their surface (e.g. aluminum). As the current oscillates it efficiently removes the oxide layer that can otherwise interfere with the fusion process.

Types

Types of arc welding are classified as either consumable or non-consumable, depending on whether the electrode melts and become part of the weld.

Consumer electrode methods

Consumable electrodes melt and become part of the weld.

  • Metal inert gas (MIG) welding—uses a shielding gas to protect the base metals from contamination
  • Shielded metal arc welding (SMAW)—also called stick welding, the process uses stick electrodes coated with flux that acts as the filler material
  • Flux cored arc welding (FCAW)—similar to MIG welding, except the core of this electrode is hollow and filled with flux and other additives that generate the shielding gas when heated
  • Submerged arc welding (SAW)—uses a continuously fed wire electrode that is consumed in the weld but the weld pool is immersed in a layer of powdered flux
  • Electro-slag welding (ESW)—only used in vertical joints at least 1" thick. The gap between pieces is filled with flux and an automatically fed wire electrode is embedded within the flux
  • Arc stud welding (SW)—a special process used to weld fasteners (studs) in place
Non-consumer electrode methods

Non-consumable electrodes maintain their structure and are used in combination with filler metal or use the base metal to fuse the join.

  • Tungsten inert gas (TIG) welding—the operator feeds a separate filler rod with their second hand. The tungsten electrode alloy used varies depending on the metals being welded.
  • Plasma arc welding (PAW)—electric arc produced between a non-consumable electrode and an anode. The arc ionizes the gas in the torch creating a plasma that is pushed through a fine borehole in the anode.

Timeline

Further Resources

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References

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