Friction Stir Welding Techniques

Advanced Joining Processes - Solid State Welding

• Solid state welding processes always rely on temperature, pressure, or a combination of both.
• No fusion occurs, but materials become sufficiently plastic for intermixing,creating a joint.
• Filler material is usually unnecessary.
• Heat affected zone (HAZ) exists, typically minor.
• Distortion is greatly minimized.
• Solid-state welding is carried out below the melting point of the materials being joined. This differs from fusion welding.

Contents (Solid State Welding Processes)

• Introduction to solid state welding
• Friction stir welding (FSW)
• Diffusion welding (DW)
• Forge welding (FW)
• Explosive welding (EW)
• Ultrasonic welding (UW)

Friction Stir Welding (FSW) - Introduction

• FSW uses a non-consumable tool to join adjacent workpieces without fusing the base material.
• Friction between the rotating tool and the workpiece generates heat, softening the region near the tool.

FSW - Process Description

• Stages of FSW include:
  • Plunge and Dwell
  • Traverse
  • Retract
• Various joint geometries are possible with FSW (butt, two-sided butt, circumferential butt, lap, combination lap-butt, lateral offset, T-joints, corner joints).
• Different zones affected include:
  • Parent metal unaffected by weld
  • Heat affected zone (HAZ)
  • Unrecrystallised area (in aluminum alloys)
  • Recrystallised nugget (in aluminum alloys)
  • Thermomechanically affected zone (TMAZ)

FSW - Advantages

• Largely defect-free joining method (no hot cracking, porosity, or solidification cracks).
• Reduction in shrinkage and distortion due to low temperatures.
• No filler material, flux, or shielding gas required for aluminum alloys.
• Environmentally friendly (no fume, spatter, or UV radiation).
• Easy automation, highly repeatable, and reduced need for skilled welders.
• Works in any position.
• Good mechanical properties, often equal to or exceeding competing processes.
• Energy efficient with lower temperatures.
• Can join many "non-weldable" aluminum alloys (2xxx and 7xxx series).
• No special edge preparation in many applications.
• Extremely low energy consumption and CO2 emissions.

FSW - Disadvantages

• Exit hole after tool withdrawal, usually accounted for in design or run-on/off tabs.
• Significant clamping force and traversing forces needed.
• Gaps between joined parts need tight control.
• Initial setups for FSW machines can be costly.

FSW - Equipment and Tooling

• Initially developed for use in conventional milling machines, but these machines often have limited power and stiffness.
• Industrial applications often use dedicated XYZ gantry machinery or six-axis robots (for 3D joint lines).

FSW - Process Parameters

• Main variables are welding speed, tool rotation speed, axial force on the tool, tilt angle of the tool, and tool design.
• These variables influence peak weld temperature, linear force, torque, and power.

FSW - Process Parameters (continued)

• Peak weld temperature increases significantly with rising rotation speed and axial pressure.
• High pressure leads to overheating and thinning of the joint, while low pressure can result in insufficient heating and void formation.
• Rising travel speeds lead to higher linear force, potentially causing tool erosion and breakage.
• Axial pressure increase leads to increased power requirements.
• Torque depends upon axial force, tool design, tilt angle, friction coefficients, slippage between the tool and workpiece, and local shear stress at the tool-workpiece interface.

FSW - Process Parameters (continued)

• Peak temperature is not significantly affected by travel speed.
• High travel speeds reduce heat input to the workpieces.
• Torque increases slightly with increasing travel speed, since material flow becomes more difficult at lower temperatures.

FSW - Process Parameters (continued)

• The tool is often tilted, ensuring the trailing edge of the shoulder penetrates the workpiece applying additional forging pressure.
• Scroll shoulders (with machined scroll features) pull in material from the outer edge to the probe root; this facilitates faster welding of some aluminum alloys.

FSW - Defects

• Tunnel defects
• Flash defects
• Void defects
• Cavity defects
• Kissing bond
• Root defects

FSW - Joining of Dissimilar Materials

• Joining dissimilar materials is challenging due to differing melting temperatures and intermetallic compound formation.
• Intermetallic compounds formed at the interface are often hard and brittle, decreasing the mechanical properties of the joint.
• Specific applications like Al-Mg, Al-Cu, and steel-aluminum are noted as examples.
• Formation of intermetallic compounds might necessitate a heat-treatment process.
• FSW is still applicable to dissimilar-metal joining, but special considerations such as tool geometry, offset pins, and potential heat-treatments might be needed for optimal results.

FSW - Joining of Thermoplastics

• Thermoplastics soften and flow upon heating, then regain stiffness when cooled.
• They are composed of long chain molecules, leading to flow characteristics different from metals.
• Process parameters and tooling will differ greatly from those used for metals.

Diffusion Welding (DW) - Introduction

• Diffusion welding is a solid-state welding technique capable of joining similar and dissimilar metals.
• Solid-state diffusion occurs when atoms of two metals intermix.
• This occurs over time, under high pressure and temperature.

Diffusion Welding (DW) - Advantages

• Simple process with low operation costs
• Joints maintain the base material's characteristics
• Clean joints, no discontinuities or porosity
• Capability to join similar or dissimilar metals
• Good dimensional tolerance
• Limited plastic deformation

Diffusion Welding (DW) - Disadvantages

• High initial setup cost
• Time-consuming process
• Critical surface preparation
• Weld size limited by available equipment
• Extreme dependence on parameters (temperature, pressure, surface finish, materials)

Diffusion Welding (DW) - Applications

• Micro-heat exchanger design

Forge Welding (FW) - Introduction

• Forge welding joins two metal pieces by heating them to high temperatures and hammering them together.
• It's among the simplest of joining metals.

Forge Welding (FW) - Advantages

• Simple and inexpensive process
• Can join similar or dissimilar metals
• Welded area properties resemble base material
• No additional material required

Forge Welding (FW) - Disadvantages

• Suitable only for small components
• Large-joint welding requires large (expensive) presses and furnaces
• Skilled operator required to avoid material damage
• High likelihood of defects
• More suitable for iron or steel
• Relatively slow process

Forge Welding (FW) - Applications

• Shafts and fasteners

Explosive Welding (EW) - Principle

• Explosive welding utilizes an explosive force to join overlapping metal plates through local plastic deformation.

Explosive Welding (EW) - Advantages

• Can join similar or dissimilar, even unweldable, metals
• Reduces manufacturing costs (thin expensive coatings onto larger, cheaper components)
• Simple jigs and fixtures
• Use for large joining areas
• Applicable to a wide range of metal thickness.
• No change in material properties.

Explosive Welding (EW) - Disadvantages

• Based metals need ductility to withstand impact.
• Noisy and potentially dangerous (special chambers or protection needed).
• Suitable mostly for simple geometries (like plates and cylinders).
• Surface cleaning and prep is critical.
• Explosives' use has limitations and strict regulations.

Explosive Welding (EW) - Applications

• Pressure vessels (often with clad layers of different metals).
• Large heat exchangers

Magnetic Pulse Welding - Introduction

• A component is fixed, with a surrounding outer component
• A large alternating current (high amperage) is applied to a coil, creating a strong magnetic field.
• This magnetic field accelerates the outer component towards the fixed component with considerable speed.
• Welding occurs through the impact fusion of the pieces, rather than through melting.

Magnetic Pulse Welding - Advantages

• Stronger weld than the weakest joined material
• No protections required (no atmosphere, fillers, or other materials)
• No heat affected zone
• Workpieces can be processed right after welding
• Very rapid production rates
• Emission free welding.

Magnetic Pulse Welding - Disadvantages

• Outer parts need excellent electrical conductivity
• Overlap of joined surfaces is essential
• Geometry may need adjustment for the magnetic field and coil.
• If components don't fit, a complex multi-part coil might be required.
• Parts needing welding may need to be more closely integrated.
• Large setup costs.
• Some brittle materials might be damaged by the impact.

Magnetic Pulse Welding - Applications

• Electrical conduction
• Gears

Ultrasonic Welding - Principle

• Ultrasonic welding utilizes ultrasonic vibrations to join thermoplastic materials that are placed in a welding fixture.
• Vibration softens or melts the materials of parts at the joint line.
• The weld is solidified through applied pressure.

Ultrasonic Welding - Advantages

• Very fast process due to rapid welding and cooling times
• Very secure process due to highly targeted ultrasonic energy
• High level of reliability with minimal operator intervention
• Clean weld, no plastic flash, or deformation
• Affordable, with low material usage
• Works well with a wide range of metals and polymers (esp. thermoplastics)

Ultrasonic Welding - Disadvantages

• Lacks applicability to many thermoplastic materials (some plastics cannot tolerate the required moisture content).
• Not suitable for materials needing larger joints; high power may be needed.
• Mostly applicable to lap joints.
• Non-trivial setup time, necessitating custom tooling for optimal results.
• Significantly higher initial cost than traditional welding processes.

Ultrasonic Welding - Applications

• Shoes
• Printed circuits
• Medical equipment
• Electrical connectors

Test your knowledge on the principles and mechanics of friction stir welding with this quiz. Explore the advantages, environmental benefits, and key characteristics that define this innovative welding technique. Perfect for students and professionals looking to deepen their understanding of welding processes.