Definition, Processes, and Types of Welds

Welding is the process that combines materials such as metals, thermoplastics, and/or wood by using heat or pressure to melt the parts together. Allowing them to cool enables fusion.

Different materials require unique processes and techniques. There are some materials that are considered unweldable. While not a term in the dictionary, it is well known by engineers as a term that means a certain material is not able to weld.

Parent materials is a term used to describe the separate parts that are joined together. Examples of parent materials are pipes and plates. This is different from filler or consumable material which is added to help join the parent material. Examples of filler materials are wire, metal, or consumable electrodes.

Typically, consumable materials are selected to be comparable in composition to the parent material. This enables them to create a homogeneous weld. However, there are circumstances like welding brittle cast irons that require a different composition. In this scenario, these welds would be referred to as heterogeneous.

Lastly, the finalized welded joint is known as a weldment. A weldment is a unit created by welding together a bunch of smaller pieces.

How Does Welding Work?

Joining Metals

Welding, brazing, and soldering all result in the permanent joining of the parent materials. The brazing and soldering processes do not melt the base metal. Compared to brazing and soldering, welding is a high heat process that melts the base material with the addition of a welding filler material.

The high heat creates a weld pool of molten material. This molten material is cooled to form the joint. The resulting joints that are created are extremely strong, sometimes even stronger than the parent materials. Pressure can be used in conjunction with heat to weld two materials together.

When welding a metal if it is exposed directly to air, the Oxygen in the air will react with the metal and form oxide. This contaminates the metal and will result in poor welding. The best way to prevent this from happening is to use a shielding gas.

Joining Plastics

Plastic welding is a process that creates a molecular bond between two thermoplastics that are compatible. This process typically uses heat, except for solvent-welding, to join the materials. Depending on the process used, joining methods for plastics can be divided into external or internal heating methods.

Joining plastics happens in 3 steps:

  1. Pressure is often used throughout the heating and cooling stages in order to hold the parts in the correct placement and improve melt flow across the edges.
  2. Heating enables the intermolecular diffusion from one part to the other.
  3. Cooling creates fusion which solidifies the newly created bond.
Joining Wood

Wood welding joins materials by using pressure and heat created from friction. The wood materials that are to be joined together are placed under pressure and then rubbed back and forth at high speeds which creates heat that bonds the materials together.

This is a rapid process that creates a high-strength bond that happens within seconds and does not use any form of adhesive.

Common Joint Configurations

Butt Joint

This is the joining of two pieces at the ends or edges that makes a right angle to one another of 135 to 180 degrees. This is the simplest joint to make because it only involves cutting the material at a certain length and then butting the two pieces together.

T Joint

The T Joint is commonly referred to as a welding point of two materials. It is a connection between the end or edge of one part and the face of another. The parts end up making a 90 degrees angle at the region of the joint. This joint forms the letter T, just as its name implies.

Corner Joint

The corner joint is a joining between the ends of two segments. This connection makes an angle to one another of more than 30 degrees, but less than 135 degrees in the area of the joint. This joint looks like the letter L when it is formed.

Edge Joint

The edge joint is formed by uniting the edges of two segments to form an angle to one another that of 0 to 30 degrees in the area of the joint.

Cruciform Joint

The cruciform joint is formed by welding two flat bars or flat plates together at right-angled and on the same axis. This joint resembles a cross when it is formed. 

Lap Joint

The lap joint is the joining of two overlapping segments to make an angle to one another of 0 to 5 degrees in the area of the joint.

Types of Welding Joints

Welds Based on Configuration
  • Slot weld. This is the joining of two overlying parts that is created through placing a completely or partially fillet weld around the edge of a hole in one component. This joins a piece to the exterior of the other piece which is exposed through the hole.
  • Plug weld. This weld is made by filling a hole in one piece with a filler metal in order to combine it to the exterior of an overlapping piece that is exposed through the hole. The hole can be an oval or circular shape.
Welds Based on Penetration
  • Full penetration weld. Also known as complete joint penetration weld, is an extremely thick weld. This welded joint consists of two pieces of metal being joined together with no gaps. The welded metal is completely penetrating the joint with full root fusion. This results in higher strength than a partial penetration weld.
  • Partial penetration weld. Also known as partial joint penetration weld, is a grooved weld resulting in an incomplete joint penetration. This weld only fills a part of the gap and is only used when it is not necessary to develop the full strength of the connected parts in order to transfer the load.
Welds Based on Accessibility
  • Single side weld. This is a joint that is only welded from one side. It is not to be used for structural or panel replacement. It is extremely effective in cosmetic patching.
  • Double side weld. This is a joint that is welded from both sides.

Features of Completed Welds

  • Parent metal. The metal components that are being joined together by a molten filler metal through the process of brazing, welding, or braze welding.
  • Filler metal. The metal that is added while surfacing, welding, brazing or braze welding.
  • Weld metal. This is all the metal that is melted and hardened during the formation of a weld and is preserved in the weld.
  • Heat affected zone (HAZ). This is the segment of the parent material, either a metal or a thermoplastic, that experiences changes in its material properties as a result of high temperatures from welding or thermal cutting heat. This segment is not melted.
  • Fusions line. This term is typically not used to describe weld junction but is the boundary between the welded metal and the HAZ in a fusion weld.
  • Weld zone. This is the area of a weld that includes both the weld metal and the HAZ.
  • Weld face. This is the exposed surface of a weld on the same side that the welding was completed on.
  • Weld root. This is the area where the back of a weld intersects with the surface of the parent metal. Just as a tree’s roots penetrate the earth deeply, this is the part that has the deepest penetration.
  • Weld toe. This is the junction of the weld face and the parent metal or between runs. This feature is crucial because the toes are areas of extraordinary stress concentration. The weld toes area also starting points for various kinds of cracks like cold cracks and fatigue cracks.
  • Excess weld metal. This is also known as reinforcement or overfill. It is the extra weld metal that is outside the area connecting the toes. Even though it is sometimes referred to as reinforcement, the term does not really describe what excess weld metal is because excess weld metal does not make the weld stronger. Design throat thickness is the actual term used to describe the thickness of a weld and it excludes excess weld metal.
  • Weld run (pass). This term is used to describe the metal that is melted during a single “run or pass” of a blowpipe, torch or electrode.
  • Weld layer. This is the result of a weld pass or run.

Energy Sources

Depending on the energy source used, various processes with a range of techniques are available. One of the earliest forms of welding was forge welding. Later, arc welding was created. Today, electric arc, lasers, gas flame, ultrasound, friction, and electron beam are used to weld. When working with each process a person needs to be very careful because they can cause burns, damage to vision, eclectic shock, inhalation of poisonous welding gases and fumes as well as exposure to radiation.

What are the Different Types of Welding Methods and What are They Used for?

The different types of processes with unique techniques include:

Arc Welding
  • Includes: manual, semi-automatic, and automatic processes.
  • Includes: metal inert gas (MIG) welding, metal active gas (MAG) welding, stick welding, tungsten inert gas (TIG) welding, gas metal arc welding (GMAW), flux-cored arc welding (FCAW), plasma arc welding, shielded metal arc welding (SMAW), and submerged arc welding (SAW).
  • Uses filler material.
  • Used mainly for joining metals like stainless steel, nickel, copper alloys, aluminum, titanium, and cobalt.
  • Process used across power, aerospace, automotive, and oil and gas industries, just to name a few.
Friction Welding
  • A technique used to join materials using mechanical friction.
  • Performed in a lot of different ways on various welding materials like aluminum, steel or wood.
  • Mechanical friction creates heat in order to soften the materials so they can be mixed to develop a bond while they cool.
  • Does not need filler metals, shielding gas, or flux.
  • The way in which joining happens is contingent on processes such as friction stir welding (FSW), linear friction welding (LFW), rotary friction welding (RFW), and friction stir spot welding (FSSW).
  • Ideal for combining non-weldable light-weights aluminum alloys and can be used to bond wood without adhesives or nails.
  • This process is typically used in the aerospace industry.
Electron Beam Welding
  • The process uses a mean of high-velocity electrons to combine materials.
  • Kinetic energy from electron changes into heat once the impact is made with the work piece. This results in the materials being melted together.
  • Performed in a vacuum to prevent the beam from scattering.
  • Used to join thick sections so it can be applied to different industries like aerospace, automotive, rail, and nuclear power.
Laser Beam Welding
  • Used to combine pieces of metal or thermoplastics.
  • The process uses a concentrated heat making it ideal for high welding rates we well as narrow and deep welds.
  • Easily automated.
  • Ideal for the automotive industry because the process is performed at high weld speeds making it perfect for high volume applications.
  • Performed in air.
Resistance Welding
  • Process joins metals by applying pressure and passing a current through it for a period of time.
  • Process is extremely cost-effective because no other materials are needed to develop the bond.
  • Commonly used in the automotive industry because of its fast process.
  • The process can be separated into two forms:
  1. Spot welding. This process uses heat that is delivered between two electrodes. This is then applied to a tiny area as the work pieces are clamped together.
  2. Seam welding. This form of welding is comparable to spot welding but replaces the electrodes with wheels that turn as the segments to be welded are pushed between them to create a leak-free weld. 

Where is it Used?

Welding processes are typically used across a wide variety of industries such as automotive, construction, energy, and aerospace. These processes are used to join wood, metals, or thermoplastics for a large range of applications. More recently, it is being used to by artists to form artwork.