Friction Stir Welding

What is Friction Stir Welding and How Does It Work

Friction stir welding is a solid-state joining process that combines the principles of frictional heating and plastic deformation to create high-quality welds. Unlike conventional welding methods that involve melting the base materials, FSW utilizes a non-consumable rotating tool to generate frictional heat and plastically deform the material, resulting in a solid-state joint. FSW involves no melting of the base materials, significantly reducing the risk of defects such as porosity and distortion. This innovative method produces high-quality, defect-free welds, making it a preferred choice in various industries. 

What is Friction Stir Welding?

Friction stir welding refers to a solid-state joining process that utilizes a non-consumable rotating tool with a specially designed probe and shoulder to join two-facing workpieces. The tool plunges into the abutting edges of the workpieces to be joined and traverses along the joint line, generating frictional heat and a softened region near the FSW tool. This process effectively intermixes the materials from the two workpieces and forges the hot and softened metal through mechanical pressure exerted by the tool, akin to the way clay or dough is joined. In this way, FSW creates a seamless and robust weld without melting the base materials. 

The primary application of FSW is on wrought or extruded aluminum, particularly for structures that require high weld strength. It is also effective in joining various materials, including aluminum alloys, copper alloys, titanium alloys, mild steel, stainless steel, and magnesium alloys. In addition, it has been successfully employed in the welding of polymers and joining dissimilar metals, such as aluminum to magnesium alloys. The technique is widely utilized in industries such as shipbuilding, train manufacturing, and aerospace applications.

Friction stir welding

How does Friction Stir Welding Work?

Friction Stir Welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two-facing workpieces without melting the workpiece material. The following is how the whole friction stir welding process works:

Friction stir welding uses a rotating cylindrical tool with a profiled pin, known as a probe, which has a smaller diameter than the tool’s shoulder. In the welding process, the tool is plunged into a butt joint between two secured workpieces. This continues until the probe penetrates the workpiece and the shoulder makes contact with the workpiece surface. The length of the probe is slightly less than the necessary weld depth, allowing the tool’s shoulder to rest on the work surface. Following a brief pause, the tool is advanced along the joint line at the predetermined welding speed.

Frictional heat will be produced between the wear-resistant tool and the workpieces. This heat is combined with the heat produced by the mechanical mixing process and the adiabatic heat within the material. All this heat softens the stirred materials without causing them to melt. As the tool progresses, a specialized profile on the probe pushes plasticized material from the front to the back, where the high forces aid in a forged consolidation of the weld. The process involves the tool moving along the weld line within a plasticized tubular shaft of metal, leading to significant solid-state deformation that includes the dynamic recrystallization of the base material.

What Materials Can Be Welded by Friction Stir Welding?

The friction stir welding process has been widely used for various types of materials, particularly for those that are difficult to weld using conventional fusion welding methods. Here are some materials that can be welded using the FSW process:

Aluminum Alloys

FSW was initially developed for welding aluminum alloys, which are challenging to weld with traditional methods due to issues like hot cracking and porosity. It is particularly effective for aerospace applications where aluminum alloys are prevalent. In addition, the aluminum parts with friction stir welding demonstrate superior mechanical properties compared to other types of welding aluminum parts.

Steel

While FSW is primarily used for welding non-ferrous materials, it has also been applied to certain types of steel, including stainless steel, mild steel, and some high-strength steel. It is beneficial for joining heat-sensitive alloys by minimizing the heat generation during the welding process. Generally, FSW steel parts have fewer defects which are economical for production.

Magnesium Alloys

FSW is an effective method for welding magnesium alloys, which are challenging to weld using fusion welding due to their high reactivity and tendency to form oxides. FSW helped to overcome this difficulty. Therefore, magnesium parts can easily be welded with FSW welding.

Titanium Alloys

While other welding techniques can be utilized for titanium parts, issues arise when there is subpar weld quality or distortion of the workpiece. Friction stir welding, on the other hand, does not encounter such problems and stands out as the optimal choice for welding titanium parts.

Copper Alloys

Due to their high electrical and thermal conductivity, welding copper parts used to pose significant challenges. However, FSW can make it easier to weld copper and its alloys and provide good mechanical properties in the welded region.

Dissimilar Materials

One of the advantages of FSW is its ability to join dissimilar materials, such as aluminum to magnesium alloys or different types of aluminum alloys.

Polymer Materials

In some cases, FSW can also be used for joining thermoplastic materials, although this is less common than for metallic materials.

Hafnium and Zirconium

In industries like power generation, oil refining, and petrochemical sectors, both hafnium and zirconium find applications. Within these industries, FSW proves to be a valuable method for easily and efficiently joining hafnium or zirconium parts.

Friction stir welding aluminum

The Advantages of Friction Stir Welding

Friction Stir Welding offers numerous advantages over traditional fusion welding techniques, contributing to its widespread adoption. Some of the key advantages include:

Solid-state Process: As a solid-state process, FSW avoids the formation of undesirable phases, porosity, and solidification-related defects associated with fusion welding.

Low Distortion and Residual Stresses: FSW generates lower heat input compared to fusion welding, resulting in minimal distortion and residual stresses in the welded components.

Improved Mechanical Properties: The fine-grained, recrystallized microstructure produced by FSW often leads to improved mechanical properties, such as increased joint strength, fatigue life, and corrosion resistance.

Energy Efficiency: FSW requires lower energy input compared to fusion welding processes, contributing to improved energy efficiency and reduced operating costs.

No Consumables: FSW does not require consumable materials like filler metals or shielding gases, reducing material costs and environmental impact.

Versatility: FSW can be used to weld a wide range of materials, including dissimilar metal combinations, which is challenging with traditional welding methods.

Improved Safety: The absence of an intense arc or molten metal pool reduces the risk of exposure to harmful fumes, spatter, and UV radiation, enhancing workplace safety.

Easy to Operate: FSW can be easily automated on simple milling machines, resulting in lower setup costs and less training required. It can be operated in all positions (horizontal, vertical, etc.), as there is no weld pool.

The Disadvantages of Friction Stir Welding

While Friction Stir Welding offers numerous advantages, it also has some inherent limitations and disadvantages:

Down Forces: Large down forces are required with heavy-duty clamping necessary to hold the plates together.

Exit Hole: At the end of the weld, a small exit hole is left behind, which may require post-weld machining or repair.

Keyhole Formation: In some cases, keyhole defects can form, particularly when welding thicker materials or when weld parameters are not optimized.

Tool Wear: The rotating tool experiences wear during the welding process, requiring periodic replacement or refurbishing.

Traverse Rate: Usually, this process has a slower traverse rate than some fusion welding techniques, although this may be offset if fewer welding passes are required.

Initial Investment: The equipment and tooling required for FSW can represent a significant initial investment, particularly for small-scale operations.

Size and Geometry Limitations: FSW may not be suitable for certain geometries or component sizes, particularly those with complex contours or limited accessibility.

The Important Welding Parameters in Friction Stir Welding

Friction stir welding has several key parameters that significantly influence the welded joint’s quality and properties. Here are some of the important welding parameters in FSW:

Tool Design

The tool design plays a crucial role, as an effective tool can enhance both the weld quality and the maximum achievable welding speed. Ideally, the tool material should possess adequate strength, toughness, and wear resistance at the welding temperature. Additionally, it should exhibit good oxidation resistance and low thermal conductivity to reduce heat loss and minimize thermal damage to the machinery along the drive train. 

Tool Rotation and Traverse Speeds

It’s very important to consider how fast the tool rotates and how quickly it traverses along the interface. These two parameters ensure a successful and efficient welding cycle. Generally speaking, raising the rotation speed or reducing the traverse speed will lead to a hotter weld. To achieve a successful weld, the material around the tool must be hot enough, enabling the extensive plastic flow needed while minimizing the forces acting on the tool. If the material is too cold, voids or other defects may appear in the stir zone; in extreme cases, the tool could even break.

Friction stir welding process 2

Axial Force (Downward Force or Top Forcing)

The force applied along the axis of the tool is crucial for material consolidation and the overall quality of the weld. Insufficient force may not lead to proper material mixing, while excessive force can cause deformation or damage to the workpiece.

Tool Geometry

The design of the welding tool, including the pin profile, shoulder diameter, and pin length, affects how material is displaced and mixed during the welding process. The tool geometry must be optimized for the specific materials being joined.

Plunge Depth

The plunge depth refers to the depth of the lowest point of the shoulder beneath the surface of the welded plate. Plunging the shoulder below the plate surface enhances the pressure beneath the tool, facilitating sufficient material forging at the tool’s rear.

Tilt Angle

The tilt angle of the tool relative to the workpiece surface can affect the material flow, weld appearance, and residual stress distribution.

Material Properties

The thermal and mechanical properties of the welded materials, such as their melting points, thermal conductivities, and flow characteristics, are critical in determining the appropriate welding parameters.

Joint Design

The type of joint, such as butt joint, lap joint, or T-joint, and the fit-up quality (gaps, alignment) also influence the welding process and the resulting joint quality.

Welding Path

The specific path that the tool follows during the welding process, including any oscillations or specific tool path strategies, can affect the uniformity and quality of the weld.

The Applications of Friction Stir Welding

Friction Stir Welding has found applications in various industries due to its unique capabilities and advantages. Some of the major applications include:

  • Joining of Aluminum 3D Printing Material: FSW has been demonstrated as a viable method for joining metal 3D printing materials. With the appropriate FSW tools and precise parameter settings, a strong, flawless weld can be achieved to join the metal 3D printing materials.
  • Aerospace Industry: FSW is extensively used in the aerospace sector for joining aluminum alloy components in aircraft structures, fuel tanks, and other critical assemblies, contributing to weight reduction and improved performance.
  • Automotive Industry: The automotive industry has embraced FSW for joining aluminum alloy components in vehicle bodies, chassis, and structural components, enabling lightweight construction and improved fuel efficiency.
  • Shipbuilding and Marine Industry: FSW is employed in the construction of high-performance marine vessels, including aluminum hulls, decks, and superstructures, offering improved corrosion resistance and fatigue life.
  • Railway Industry: Friction Stir Welding is used for joining aluminum alloy components in railway vehicles, such as passenger cars and freight wagons, enabling lightweight and durable constructions.
  • Energy and Power Generation: FSW finds applications in the fabrication of components for power generation equipment, including heat exchangers, pressure vessels, and other structures subjected to high operating temperatures and pressures.
  • Cryogenic Applications: The solid-state nature of FSW and the absence of solidification-related defects make it suitable for joining materials used in cryogenic applications, such as in the liquefied natural gas (LNG) industry.

FSW parts

Friction Welding (FRW) Vs. Friction Stir Welding (FSW): What are the Differences?

Friction Welding (FRW) and Friction Stir Welding (FSW) are both solid-state welding processes that join materials without melting them. However, they differ in their mechanisms, applications, and specific process parameters. Here’s a comparison between the two:

1. Mechanism:

FRW: Involves rubbing two surfaces together at a high speed to generate heat due to friction, which is then used to forge a joint between the two pieces. The process can be further divided into rotary friction welding, linear friction welding, and others. 

FSW: Uses a non-consumable tool that is plunged into the interface of two workpieces. The tool is rotated at high speed, and as it moves along the interface, the frictional heat generated softens the materials, allowing them to be mixed and joined. 

2. Applications:

 FRW: Typically used for joining shafts, pipes, and other rotary components. It’s well-suited for applications requiring high precision and is often used in the automotive and aerospace industries. 

FSW: Widely used for joining aluminum alloys and other materials that are difficult to weld with conventional methods. It’s particularly useful for structural components in aerospace, automotive, and high-tech industries.

3. Process Parameters:

FRW: Key parameters include the rotational speed of the workpiece, the pressure applied, and the dwell time (how long the components are held together after reaching the plastic state).

FSW: Parameters include the rotational speed of the tool, the travel speed (how fast the tool moves along the joint), the tool’s shoulder and pin profiles, and the plunge depth (how far the tool is inserted into the workpieces). 

4. Advantages:

FRW: Offers uniform and strong welds, is fast, and suitable for a wide range of materials. It also allows for a good price-to-joint ratio and is known for its precision.    

FSW: Produces fine-grained, recrystallized microstructure welds with no major change in chemical composition. It has lower power consumption, is suitable for thicker sections, and can be automated. 

5. Limitations:

FRW: Requires ductile metals at welding temperatures and is typically limited to butt welding. The process also necessitates square and machined smooth surfaces for effective joining.

FSW: Requires solid and rigid fixtures to hold the workpieces firm during welding. It may also face challenges when welding certain materials with high melting points or those prone to oxidation. 

6. Strength of Weld:

In both FRW and FSW, the strength of the weld is generally equal to or greater than the strength of the weaker of the two workpiece metals being joined, provided that the specified welding parameters are adhered to. 

Summary

Friction stir welding stands out as a distinctive and innovative technique for metal joining. This process utilizes frictional heat alongside precisely controlled forging pressure to create nearly flawless and exceptionally robust joints, featuring minimal heat-affected zone, minimal mechanical distortion, and superb surface finish. Its precision, quality, and adaptability make it a preferred choice in many industries. 

Reference

Friction stir welding – From Wikipedia

FAQs

Yes, friction stir welding is highly effective in joining dissimilar materials, offering a robust bond without the need for filler material.

Industries such as aerospace, automotive, marine, and railway extensively use Friction Stir Welding for manufacturing lightweight, high-strength components.

Friction stir welding differs from traditional fusion welding by joining materials in a solid-state form without melting them, resulting in superior mechanical properties and reduced defects.

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