Friction Stir Welding Quiz

Advanced Joining Processes - Solid State Welding

• Solid state welding is performed below the melting point of the materials being joined, unlike fusion welding.
• These processes rely on temperature, pressure, or a combination of both, to create a plastic state enabling the materials to intermix.
• Filler material is often not necessary.
• Heat affected zones are usually minor.
• Distortion is greatly minimized.

Contents

• Introduction to solid state welding
• Friction stir welding (FSW)
• Diffusion welding
• Forge welding
• Explosive welding
• Ultrasonic welding

Friction Stir Welding (FSW) - Introduction

• Uses a non-consumable tool to join two adjacent workpieces without fusing the base material.
• Friction between the rotating tool and the workpiece generates heat and softens the area near the tool.

Friction Stir Welding - Process Description

• Stages in the process include: plunge and dwell stage, traverse stage, and retracting stage.
• Different zones affected by the process:
  • Parent metal unaffected
  • Heat affected zone (HAZ)
  • Unrecrystallised area
  • Recrystallised nugget
  • Thermomechanically affected zone (TMAZ)

Friction Stir Welding - Advantages

• Largely defect-free joining.
• Reduction in shrinkage and distortion due to low temperatures.
• No filler material, flux, or shielding gas needed for aluminium alloys.
• Environmentally friendly, producing no fume, spatter, or UV radiation.
• Easy to automate.
• Works in any position.
• Good mechanical properties, often equal to or exceeding competing processes.
• Energy efficient, with lower temperatures.
• Can join many non-weldable aluminium alloys.
• No special edge preparation in most cases.
• Extremely low energy consumption and CO2 emissions.

Friction Stir Welding - Disadvantages

• Exit hole left after withdrawing tool, which can be accommodated in design.
• Significant clamping force is needed.
• Tight gap control is needed since no filler materials are used.

Friction Stir Welding - Equipment and Tooling

• Initially developed for use in conventional milling machines, but suffer from power and stiffness limitations.
• Industrial applications use gantry machines / six-axis robots suited for 3D joints.

Friction Stir Welding - Process Parameters

• Key variables: welding (travel) speed, tool rotation speed, axial force, tilt angle, and tool design.
• Determine peak weld temperature, linear force, torque, and power.

Friction Stir Welding - Process Parameters - Specific Variables

• Peak weld temperature rises due to increases in rotational speed and axial pressure.
• High pressure can overheat, thin the joint while low pressure can inadequately heat.
• Higher travel speed leads to excessive linear force and tool breakage and power requirements rise due to greater axial pressure.
• Torque changes only slightly with travel speed as the flow becomes more difficult at lower temperatures.

Friction Stir Welding - Process Parameters - Torque

• Dependent on axial force, tool design, tilt angle, friction coefficients, slippage at the tool-workpiece interface, and shear stress at the interface.

Friction Stir Welding - Process Parameters - Peak Temperature

• Not greatly affected by travel speed.
• High speeds tend to reduce heat input.
• Torque may increase very little after increases in travel speed.

Friction Stir Welding - Process Parameters - Plunge force, Tilt angle, Heel plunge depth, Spindle speed, Shoulder diameter, Pin diameter

• Influence weld quality, temperature and material flow.

Friction Stir Welding - Process Parameters - Tool design

• Tapered and three equally spaced helical flutes, compared to cylindrical probes, result in faster welding speeds, reduced material displacement, greater weld quality.

Friction Stir Welding - Defects

• Tunnel defect (insufficient heat input and material flow).
• Flash defect (generated heat softens material and expels it).
• Void defect (due to insufficient forging pressure and high welding speeds).
• Cavity defect (insufficient forging pressure and high welding speeds).
• Kissing bond (insufficient stirring, or low heat input).
• Root defect (insufficient heat input and surface oxide layers).

Friction Stir Welding - Dissimilar Materials - Introduction

• Joining dissimilar materials is challenging due to different melting temperatures and intermetallic compound formation.
• Intermetallic compounds can create hardness and brittleness in the joint, which impacts the mechanical properties.

Friction Stir Welding - Dissimilar Materials - Intermetallic Phases

• A type of metallic compound resulting from an ordered solid-state mixture of two or more metallic elements.
• Often hard and brittle with good high-temperature mechanical properties.

Friction Stir Welding - Dissimilar Materials - Joining Methods

• Bimetallic, solid solution, alloy, intermetallic.

Friction Stir Welding - Dissimilar Materials - Aluminium and Magnesium

• Similar properties in melting point, thermal expansion, thermal conductivity.
• Different properties in crystal structure and formability.
• Applications in transportation industries.

Friction Stir Welding - Dissimilar Materials - Aluminium and Copper

• Fusion welding is not ideal as it leads to solidification and liquefaction cracking and intermetallic compounds.
• FSW is challenging but possible despite the energy reduction.

Friction Stir Welding - Dissimilar Materials - Steel and Aluminium

• Highly susceptible to intermetallic compound formation.
• Form a layer full of discontinuities, which is reduced with heat treatment.
• Offset pin is employed to explore large plastic flow of aluminium.

Friction Stir Welding - Dissimilar Materials - Titanium and Aluminium

• Similar characteristics to steel & titanium.
• Requires specialized pin to accommodate titanium & aluminium material.

Friction Stir Welding - Thermoplastics

• Soften when heated and recover stiffness when cooled.
• Process parameters and tool design differ from metals due to their complex molecular characteristics.

Diffusion Welding

• Solid-state bonding technique for similar or dissimilar metals.
• Atoms in two solids intermix under high pressure and temperature.
• Suitable for joining high strength, refractory metals, difficult to weld by other methods.
• Temperature operation 50 to 75% of fusion temp.

Diffusion Welding - Advantages

• Simple process, less operation cost, clean joints, free of discontinuities and porosity.
• Good dimensional tolerance, suitable for complex, high-precision components.
• Limited plastic deformation.

Diffusion Welding - Disadvantages

• High initial setup cost.
• Time-consuming compared to other techniques.
• Critical surface preparation.
• Equipment limits weld size.
• Extremely dependent on welding parameters.

Diffusion Welding - Applications

• Micro-heat exchanger design.

Forge Welding

• Two pieces of metal joined by heating them to a high temperature and hammering them together.
• Simple and versatile, suitable for dissimilar metals.

Forge Welding - Advantages

• Simple process, inexpensive equipment for small-scale work.
• Works with similar and dissimilar materials.
• Welded area maintains properties similar to base material.
• No additional materials required.

Forge Welding - Disadvantages

• Suitable only for small components.
• Large joints require expensive equipment.
• Skilled worker required to avoid damage.
• High likelihood for weld defects.
• More suited for iron and steel.
• Relatively slow process.

Forge Welding - Applications

• Shafts
• Fasteners

Explosive Welding

• Joining overlapping metal sheets using detonation of explosives creating high compression force.
• Joining is continuous due to the local plastic deformation of the contact area.

Explosive Welding - Advantages

• Joins dissimilar metals.
• Reduced manufacturing costs by applying thin layers of expensive materials to mass produced components of less expensive materials.
• Uses simple jigs and fixtures.
• Works with various thickness values.
• No change in mechanical properties.

Explosive Welding - Disadvantages

• Base metals must resist impact.
• Noisy and dangerous process (requires special chambers, sand/water protection).
• Suitable only for simple geometries such as plates or cylinders.
• Requires thorough cleaning and preparation of the surfaces.
• Use of explosives is limited in strict adherence to regulations.

Explosive Welding - Applications

• Pressure vessels
• Large heat exchangers

Magnetic Pulse Welding

• Fixes one component, and an outer component surrounds it.
• High-amperage alternating current flows through conducting coils creating a powerful magnetic field that accelerates the outer component into the interior.
• Weld is made by the kinetic energy and heat from the collision.

Magnetic Pulse Welding - Advantages

• Stronger than weakest base material.
• No protection atmosphere, filler, or additional materials
• No heat affected zone
• Workpieces can be unclamped immediately after welding
• Very fast production rates
• No release of heat, radiation, gas, or welding fume

Magnetic Pulse Welding - Disadvantages

• Outer material should be electrically conductive.
• Overlapping configuration is required.
• Part geometry might require alteration.
• Multi-part coils might be needed if parts don't entirely fit.
• Brittle materials may shatter from the large mechanical shock.
• High setup cost relative to the lower cost of the manufactured part.

Magnetic Pulse Welding - Applications

• Electrical conductors
• Gears

Ultrasonic Welding

• Uses mechanical vibrations above the audible frequency to soften/melt thermoplastic materials at the joint line.

Ultrasonic Welding - Principle

• Pieces assembled in a fixture.
• Horn placed to contact the piece and transmits mechanical vibrations.
• Pressure applied using a driven press.
• High frequency ( ~ 20 kHz) vibrating horn to generate heat.
• Pressure released, horn withdrawn to remove welded part from fixture.

Ultrasonic Welding - Advantages

• Exceptionally fast process.
• Safe.
• Highly reliable equipment with minimal human input.
• Clean and precise joint with no plastic flash or deformation.
• Works with thermoplastics and metals.
• Low-cost with low material usage.

Ultrasonic Welding - Disadvantages

• Not all thermoplastics usable.
• Low moisture content is needed.
• Limited length per joint for high power requirements.
• Only usable for lap joints.
• Custom tooling needs for very slow setup time.
• More expensive than traditional welding equipment.

Ultrasonic Welding - Applications

• Shoes
• Printed circuits
• Medical equipment
• Electrical connectors