Types of Welding

Welding is the most common, economical, and efficient way to join metal. It is the process of melting and fusing two different metal parts together by applying heat to form a permanent bond. Once the two workpieces are melted, a filler material is added to form a pool of molten material (weld puddle) that cools to become a strong joint. Because of its strength, welding is used in shipbuilding, automobile manufacturing and repair, aerospace applications, and many other manufacturing activities. Welding also is used to join beams when constructing buildings, bridges, and other structures, and to join pipes in pipelines, power plants, and refineries.

Arc welding is the most common type of welding. Methods such as Mig, Tig, and Braze are the most popular types of arc welding procedures that are used. The basic process of arc welding includes two large metal clips that carry a strong electrical current. One of the clips is attached to a part of the metal piece that is to be welded and the other is connected to a welding rod. When the metal piece is brought into contact with the rod, a powerful electrical circuit is produced. The massive heat that it creates causes the metal piece and the steel ore of the rod to melt together, forming a solid bond with quick cooling.

 

MIG (Metal Inert Gas) Welding

MIG welding is the most common form of welding accounting for about 50% of all welds. This process uses a consumable wire electrode fed through the welding gun and an inert shielding gas also supplied through the welding gun. The circuit formed between the electrode and the parent metal forms an arc which simultaneously melts the parent metal and filler wire (electrode) forming the weld pool. As the wire is continuously fed from a spool the process is sometimes call semi-automatic welding.

MIG, originally developed for welding aluminum, can also be used to weld mild steel and stainless steel. In general, the filler wire must be the same metal as the parent metal.

Metal Deposition in MIG

Short Circuit or Dip MIG Welding - In regular MIG welding the electrode makes contact with the weld pool by dipping into the pool causing a short circuit. This action melts the wire on contact. Short circuit is the “coldest” form of MIG welding and can sometimes lead to poor fusion of metals in thicker pieces. Also the nature of the arc leads to greater weld spatter.

Spray Transfer MIG Welding - In Droplet, or Spray MIG welding the higher voltage is used to melt the tip of the wire before it makes contact with the weld pool. The molten wire forms a spray of droplets about the size of the wire diameter which are projected across the arc. Care should be taken to avoid the molten wire simply dripping into the weld pool as this gives an uneven weld.

Pulsed MIG Welding - A variant on Spray Transfer welding where the current is pulsed to provide just enough force to project the molten wire droplet from the wire to the work piece. One droplet per pulse. The fluctuating current gives greater control leading to a smaller neater weld.

Shield Gasses

The shielding gas forms the arc plasma, stabilizes the arc on the parent metal and ensures smooth transfer of welds droplets to the weld pool. Ferrous metals use mixes of CO2 and Argon while non ferrous metals typically use mixtures of Argon and Helium.

MIG Advantages

• High productivity, because you don’t have to stop to change rods or chip and brush the weld frequently.
• Easy to learn and makes reasonably neat welds.
• Can be used on thick plate (with several passes)
• Can weld on stainless, mild steel, and aluminum.
• Can weld in all positions including overhead
• No flux to remove as shielding comes from the gas
• High welding speed can be attained (up to 4 times faster than stick)

MIG Disadvantages

• Not very portable – due to the cumbersome bottle of shielding gas.
• Causes weld spatter and greater local heating which causes oxidation and metal distortion.
• Cannot be used in drafty situations as the shield gas is blown away which causes porosity.

 

TIG (Tungsten Inert Gas) Welding

Unlike MIG welding, TIG welding uses a non-consumable tungsten electrode. The small intense arc is formed between the electrode and the parent metal being welded. A shielding gas is used to stabilize the arc and is supplied via the welding head. TIG welding can be done with or without a filler metal. If a filler metal is used it is applied separately as a stick. TIG welding requires a higher degree of dexterity to produce a good weld.

TIG welding is a popular choice of welding processes when high quality, precision welding is required.  This is because the arc formed is small and intense leading to highly localized heating. Since the heated area is minimal, TIG is the preferred technique for welding thin sheet where distortion can be a problem.

Arc Initiation

In TIG welding an initial problem is initiating the arc. This can be achieved by shorting the tungsten electrode on the work piece. Only when the ‘short’ is broken will the arc flow. The risk here is that the tungsten electrode will stick to the surface and leave a tungsten inclusion in the weld. An alternative method to start the arc is to use an extremely high frequency which produces high voltage sparks for fractions of a second. The sparks ionize the atmosphere around the electrode which facilitates the formation of the arc.

The shielding gases used in TIG welding are typically Argon and Argon/Hydrogen where hydrogen promotes arc formation and cleaner welds due to less surface oxidation also the arc tends to be hotter allowing faster welding speed. The downside of argon/hydrogen is the chance of porosity and cracking in carbon steels. Argon/helium mixtures give rise to greater heat generation and therefore deeper weld s but are more costly and make arc initiation more difficult.

TIG Advantages

• Can be used on all metals but is especially good for aluminum.
• The sharp and rigid tungsten tip allows for high precision, high quality welding.
• The small arc produced by TIG welding is ideal for welding thin materials.
• No weld spatter.
• No material is added to the weld unless it is required.
• Because the weld bead is smaller less grinding is required post operation.

TIG Disadvantages

• A greater degree of operator skill is required.
• A separate filler material must be used if additional material is required.
• The HF may cause electromagnetic interference to other nearby electrical equipment.
• As with tig requires a gas bottle – so not very portable.
• Lower metal deposition rates compared to MIG.
• TIG units tend to be more expensive than MIG units.

 

Stick Welding

Unlike MIG and TIG welding, Stick welding does not use a bottled gas supply so is inherently more portable. The equipment is simple and inexpensive. For this reason stick welding is often used on sites where it would be difficult to bring MIG and TIG equipment to the job.

In stick welding the electrode, which is consumed during the welding operation, is sheathed in a solid flux. As the flux melts it produces the shielding gas and slag which protects the molten weld pool. Because the gas is produced at the weld the technique is less susceptible to drafts. The flux can also be used to introduce scavengers, deoxidizers and alloying elements to the weld. The slag generated during the welding process has to be removed after the operation.

Stick Advantages

• Low cost equipment
• Highly portable
• Not susceptible to drafts
• Can be used on all metals

Stick Disadvantages

• More difficult than MIG and TIG to produce a clean weld
• Welds are usually less even requiring more finishing work
• Slag needs to be removed after the weld has been completed
• Method susceptible to weld splatter

 

Robotic Welding

Robotic welding is the use of a mechanical device (a robot) that completely automates the welding process by completing the weld and handling the part. Robots work well for repetitive tasks and have the distinct advantages of precision and productivity. The most popular robotic welding process, used in almost 75 percent of applications, is the solid wire gas metal arc welding. This process is best for most high production situations because no post-weld cleanup is necessary. Robot welding is commonly used for arc welding in high production applications, for example the automotive industry.

There are 2 common types of industrial welding robots: rectilinear and articulating robots. Rectilinear robots move in line in any of three different axes. There is also a wrist attached to the robot that allows rotational movement in addition to the linear movement of the robot along the axes. Articulating robots use arms and rotating joints. They resemble a human arm with a rotating wrist at the end.

 

Filler Metals

A filler metal is the material that is used in welding to join the two metal pieces together.  Its main purpose is to fill the gap between the two pieces or to add extra reinforcement metal. Filler metal melts, flows into the weld puddle, and freezes just as the base metal does to become a part of the completed weld. There are three different types of filler metals: Covered electrodes (used in stick welding); Bare electrode wire or rod (used in MIG & TIG welding); Welding fluxes (used in brazing).

Covered Electrodes are the most popular type of filler metal and are primarily used in shielded metal arc welding. They have a coating of a special composition material over them, which determines the electrode’s properties, the welded mold, and the usability of the electrode. Additives are added to the electrode to improve overall performance as well as weld performance. Common additives include: cellulose, metal carbonate, titanium dioxide, calcium fluoride, iron power, and sodium silicate (most popular).

Bare/Solid Electrode Wires are used in gas metal arc welding (MIG) as opposed to bare/solid electrode rods that are used in gas tungsten arc welding (TIG). Generally, the wires have a thin copper coating which enhances the current pickup.

Welding Fluxes are used in submerged arc welding.

 

Welding Gases

Certain external types of gases are used or produced during the welding process for the purpose of shielding the weld. The following are the gases that are widely used for this task.

Argon is colorless, odorless, tasteless and non-toxic. Used to create an inert gas shield during welding. 

Carbon Dioxide is a tasteless, colorless, odorless, nonflammable gas. Commonly utilized with argon as a shielding gas during welding.

Helium is colorless, odorless, tasteless, nontoxic and chemically inert. Used to create an inert gas shield and prevent oxidation during welding of metals.

Hydrogen is colorless, odorless, tasteless and nontoxic. Used to enhance plasma welding and cutting operations.

Nitrogen is colorless, odorless, tasteless, and nontoxic. Small additions to argon based shielding gases can be used for welding stainless steel by the Gas Metal Arc Welding (GMAW or MIG) process.

There are also mixtures of gasses that are used in welding. Common mixes include: Argon/Carbon Dioxide (used for stainless steel); Carbon Dioxide (used for mild steel); Argon/Helium/Nitrogen (used for aluminum and stainless steel).