What Is Fusion Welding?

Welding has evolved from a simple, arc welding process into a complex procedure used to join even the most advanced metals. Today, there are dozens of welding methods available, differing in their working principles, heat sources, gas usage, electrodes, and many other aspects.

Fusion welding is a term that encompasses all welding methods that melt the base metal, often with the addition of filler metal. As everything cools down, it creates a uniform, strong, and everlasting bond.

In this article, we will further address fusion welding, its working principle, characteristics, and examples. Understanding the differences between fusion and solid-state welding is crucial in making an informed choice for your welding projects.

Fusion MIG Welding Process
Photo by @brett.jozsa (TikTok)

Fusion Welding Explained

Fusion welding is a term used to describe all welding methods that melt base pieces to create a bond. Depending on the gap, processes may use additional filler metal, which also melts and fuses with the base pieces.

Due to the high heat, everything melts and blends together. Molten pieces and filler metal commonly have a similar composition, allowing them to form a consistent bond. As the heat dissipates, everything solidifies, creating a strong connection between the two pieces.

Unlike non-fusion methods, fusion welding doesn't need the external application of pressure to form a bond. The heat is intense enough to melt and join the pieces, without applying additional pressure.

Fusion Welding Explained

How Does Fusion Welding Work?

Fusion welding is a relatively straightforward procedure. Here is how it works in a few simple steps:

1. Melting: The heat melts base metal and filler metal (if used) inside the joint. Fusion welding methods utilize various heat sources, including an electric arc, gas, a laser, an electron beam, or a plasma arc, to melt the pieces. The heat should be sufficient to reach the melting point of the pieces and penetrate the base metal.

2. Molten pool: As the pieces and filler metal melt, they form a small pool of molten metal, often referred to as a weld pool or weld puddle. Welders must constantly control the size, width, and fluidity of this puddle to produce sound welds. As the heat moves, the puddle moves across the joint.

3. Solidification: As you move the heat source away from the joint, the weld puddle solidifies. The solidified pool forms a connection known as a weld bead, or simply, a weld. The produced fusion weld is consistent, containing traces of molten base metal and filler metal.

Fusion Welding vs Non-fusion Welding

Keep in mind that not all welding methods melt the base metal to form a bond. In contrast to fusion welding, which does so, there are non-fusion welding methods, known as solid-state welding, that use pressure, heat, or both to achieve metallic continuity at the joint.

Fusion Welding vs Solid-State Welding
Source: https://www.youtube.com/watch?v=9i_VZGmDQP0

Solid-state welding can utilize heat (as in friction stir welding), pressure (as in cold pressure welding), or a combination of both. However, the heat does not reach the melting point of the base metal. The heat helps in the formation of the bond, but it is often pressure that enables the connection to form.

Making a joint without melting pieces is ideal for thin and heat-sensitive metals such as stainless steel, aluminum, copper, and magnesium. Working at lower heat reduces the risk of distortion, burn-through, or solidification cracking. Since pieces don't melt, the base metal preserves original properties, but it also makes dissimilar metals easier to weld.

Nonetheless, fusion welding is much more suited for thicker metals, bridging wide gaps, and heavy-duty applications. The equipment is straightforward, so most fusion welding today is also part of home, hobby, and DIY applications.

TIG Welding Thick Metal
Photo by @nijinojmcjk (TikTok)

Common Examples of Fusion Welding

As noted, fusion welding encompasses dozens of welding methods that utilize high heat to melt and join pieces. These methods can be further divided based on the heat source or the use of filler metal.

Here are some common examples of fusion welding, categorized by the source of the heat.

Arc Welding Methods

Arc welding methods are undoubtedly the most popular and widely used fusion welding techniques. The four most commonly used fusion welding processes that use the heat of an arc are:

• MIG (GMAW) welding: Metal Inert Gas, or MIG welding, is one of the most straightforward processes. The arc forms between the automatically fed wire and the base metal. You press the trigger, and the wire mechanism automatically feeds the wire inside the joint, making it highly convenient for beginners and new welders.

• Flux Core Welding (FCAW): A variant of MIG welding that also uses automatically fed wire. However, the wire contains flux, which migrates to the surface and creates a protective layer of slag. The slag protects the molten weld puddle from contamination, allowing you to weld without the need for external shielding gas.

• TIG (GTAW) welding: Tungsten Inert Gas welding forms an arc between a non-consumable tungsten electrode and the base metal. Welders manually feed the TIG rod with one hand, and control the puddle with the other. The process is renowned for its precision, accuracy, and temperature control. It yields the highest quality welds, but it is more challenging to master and utilize.

• Stick (SMAW) welding: One of the oldest, but widely used methods in industry, pipeline, and outdoor repairs. The arc forms between the self-shielded electrode and the metal. Stick is renowned for its low cost and ability to work outdoors, but it can be a bit messy.

Keep in mind that there are more fusion arc welding methods, such as submerged arc welding (SAW) and plasma arc welding (PAW). However, you are likely to encounter the four techniques we mentioned above.

Plasma Welding with Powder Cladding
Source: https://www.youtube.com/shorts/ouE4NrCt0QQ

Gas Welding Methods

Gas welding methods utilize the heat generated by a mixture of gases, most commonly oxygen and acetylene, to melt and fuse the pieces. The most common gas fusion welding is Oxy-Fuel Welding, also known as Oxyacetylene Welding.

Welders use a handheld torch or blowpipe and a mixture of oxygen and acetylene. The mixture combusts, creating a flame that reaches high temperatures. The heat melts and fuses the pieces, or it can even completely cut through them.

Oxy-Acetylene Gas Welding
Source: https://www.youtube.com/watch?v=IwkSTfNy1Nw

Resistance Welding

Resistance welding is a fusion welding process that utilizes pressure and heat generated by the high current and electrical resistance of the metals inside the joint. The most common resistance fusion methods are:

• Resistance spot welding: Electrodes tightly clamp and apply pressure to two pieces. The electrical current passes through them and focuses the energy into a tiny spot, known as a weld nugget. Due to the electrical resistance of metals, the current heats them, and pressure helps form a fusion bond.

• Resistance seam welding: This is a variant of spot welding that produces a series of overlapping nuggets in a continuous joint, resulting in a seamless weld. Instead of conventional spot welding electrodes, this process utilizes wheels that rotate as the workpieces are fed between them.

Resistance Spot Welding vs Resistance Seam Welding
Source: https://wsiweld.com/supplies/seam-welding-wheels/

High Energy Welding Processes

Unlike traditional welding, these fusion welding techniques utilize extremely high energy sources to melt and join pieces. The most common high-energy fusion welding processes are:

• Laser beam welding (LBW): This welding method utilizes the laser's heat to melt and fuse the pieces. A series of lenses and mirrors focuses laser light into a tiny dot, yielding extremely high energy density. This high energy melts or completely evaporates the pieces to create strong, fusion welds.

How Does Laser Welding Work
Source: https://www.stylecnc.com/blog/laser-welding-vs-plasma-arc-welding.html

• Electron beam welding (EBW): Electron beam welding is a fusion welding method that uses a high-velocity beam of electrons. As they hit the metal surface, electrons heat up, melt, and fuse the pieces. However, this is a highly specialized technique that requires a vacuum environment.

Electron Beam Welding (EBW) Principle
Source: https://www.mdpi.com/1996-1944/10/9/1033

Fusion Welding Based on Filler Metal

As noted, fusion welding can use additional filler metal if needed. That's mainly the case on wider joints, where additional weld metal is required to fill the larger gap between the pieces.

Based on the filler metal use, there are three primary fusion welding types:

• Autogenous fusion welding: This is a fusion welding method that doesn't use additional filler metal. The process relies on fusing the edges of the base metals without any added metal. For this to work, the process requires a very tight fit-up and high precision. However, autogenous welding can deliver some of the highest visually appealing welds.

TIG Autogenous Fusion Stainless Steel
Photo by @vanthefabricator (TikTok)

• Homogenous welding: This fusion welding type uses a filler metal with the same composition as the base metal. Filler metal adds further weld metal inside the joint to make a strong bond on thicker pieces and wider joints.

• Heterogeneous welding: This type of fusion welding also uses an additional filler metal to fill the gap, but the rod has a different composition than the base metal. Welders use it when welding dissimilar metals or in special applications.

TIG Welding Dissimilar Metals
Source: https://www.youtube.com/watch?v=Ug_cToF_ryA

Advantages, Drawbacks & Applications of Fusion Welding

Fusion welding is undoubtedly one of the most widely used types of welding, particularly in everyday, home, and hobby applications. Its advantages also make it valuable in the industries across the globe, but it does come with some drawbacks.

Let's see what the most notable pros and cons of fusion welding methods are.

Fusion Welding Process
Photo by @...contain (TikTok)

Advantages of Fusion Welding

• Strong and permanent joints: Fusion welding produces high-strength welds that can match or even exceed the strength of the base materials. Fusion creates a metallurgical bond, not just a mechanical connection. Fusion welds are highly resistant to wear, tear, and bending.

Image of TIG High-Strength Welds
Photo by @_cody_lally_ (TikTok)

• Can weld various materials and thicknesses: Welders use fusion welding to join a wide range of metals and alloys (steel, aluminum, nickel, titanium, etc.) Depending on the method, it is suitable for both thin sheets and thick sections.

• No need for external pressure: Unlike solid-state welding, fusion welding does not require mechanical pressure to join materials. Heat provided by various sources is enough to form a metallurgical bond between pieces.

• Can join complex shapes: Fusion weld methods can weld irregular, curved, or difficult-to-machine parts. Flexibility makes them useful for intricate structures in the automotive, aerospace, and shipbuilding industries.

TIG Overhead Welding Process
Photo by @blakegeorge77 (TikTok)

• Easy to automate: Processes such as MIG, TIG, and laser welding are easily automated. Automation results in enhanced speed, precision, and repeatability, and can help with the rising skilled welder shortage.

• Cost-effective in mass production: Once set up, fusion welding can be more economical than mechanical fastening (bolts, rivets) due to reduced material usage. Hard automation is highly cost-effective in repetitive tasks.

• Can work without additional materials: Autogenous fusion welds can be performed without filler material, thereby reducing costs and complexity. However, the use of filler metal depends on the metal thickness and joint fit-up.

Autogenous Fusion Welding
Source: https://www.youtube.com/watch?v=x98C922CLf0

Drawbacks of Fusion Welding

• Heat-related issues: High heat input that melts the pieces can cause distortion, warping, or residual stresses in the material. Heating and fast cooling can alter the structure of the Heat-Affected Zones (HAZ) and other metallurgical properties of molten, then solidified metals.

• Safety hazards: Melting pieces require intense heat that often creates sparks, fumes, and ultraviolet radiation. These are just some of the hazards fusion welders face daily. Minimizing risk involves the use of protective equipment and strict adherence to safety measures.

Use proper protective gear and follow strict safety practices during welding.
Photo by @soflasheetmetalworker (TikTok)

• Risk of weld defects: A lack of control and improper welding parameters can lead to defects. Fusion welds are prone to porosity, cracks, incomplete fusion, or spatter, especially in less experienced welders. Defective welds have reduced strength and often require expensive rework.

• Some materials can be very challenging to weld: Some metals (like certain high-carbon steels, copper alloys, or cast iron) are difficult to weld with fusion methods. Rapid heating and cooling create a brittle microstructure that is prone to cracking. In some cases, getting the heat right to produce proper fusion without burning through can be a nightmare.

Cast Iron TIG Welding Repair
Source: https://www.youtube.com/watch?v=jCmhYqOctXc

• It can be skill-dependent: The quality of fusion welds often depends on the expertise of experienced welders or precise automation. In manual welding methods, a lack of experience can lead to defects and costly rework.

• Equipment and setup costs: Processes such as electron beam or laser welding require a significant investment in setup. Machines, shielding gases, a vacuum environment, and regular maintenance can get quite pricy. Equipment complexity is often not economical for small jobs.

Laser Welding Process
Photo by @welding.asmr (TikTok)

Applications of Fusion Welding

• The automotive industry utilizes Fusion welding methods, including spot, MIG, and laser welding, to readily weld body panels, chassis, exhaust systems, and engine components.

Welding Car Body Panel
Photo by @renerose15 (TikTok)

• Aerospace and defense: Due to their precision, TIG and laser welding are perfect for aircraft frames, fuel tanks, turbine components, and spacecraft structures.

• Shipbuilding and offshore structures: The high strength and durability of the Stick/arc welding method are perfect for hulls, decks, pipelines, and pressure vessels in the shipbuilding industry.

• Construction and infrastructure: Fusion welding ensures reliability in load-bearing structures, such as steel frameworks, bridges, pipelines, and heavy machinery.

• The oil, gas, and energy sector: SAW and Stick are ideal for pressure vessels, boilers, heat exchangers, and pipelines in oil and gas transportation systems.

Stick Welding Pipelines
Photo by @918_rigwelder (TikTok)

• Manufacturing and fabrication: Fusion welding is ideal for both small-scale fabrication shops and mass production lines.

• Electronics and precision equipment: Laser welding offers high precision and exact heat control for the fine welding of small components, sensors, and medical devices.

• Repair and maintenance: Fusion welding restores functionality without replacing whole components, as well as cracks, broken machine parts, worn-out tools, and damaged vehicle parts.

• Home, Hobby, and DIY: Small welders, such as the YesWelder DP200 , are perfect for home and hobby welding, including DIY projects and garage work.

Firstess DP200 Multi-Process DualPulse™ MIG Welder

Final Thoughts

Fusion welding is a fantastic metal joining process that involves melting the pieces and filler metal together. Once everything solidifies, it creates a strong bond that is not only mechanical but also metallurgical in nature.

Some of the most common fusion welding methods include MIG, TIG, Stick, gas welding, resistance welding, laser welding, and electron beam welding. They can cover a wide variety of material types and thicknesses.

The advantages make fusion welding a vital part of industries worldwide. You can see its methods in applications such as car, ship, or airplane manufacturing, as well as pipeline, metal fabrication, and everyday repair and work.