What is Friction Stir Processing?
Friction stir processing (FSP) is a technique for changing the properties of metals through localised plastic deformation. Developed from friction stir welding (FSW), which allows for the joining of materials without the creation of a heat affected zone, FSP involves the insertion of a non-consumable tool into the workpiece. This tool then revolves in a stirring motion as it is moved laterally along the workpiece.
The process mixes the material without changing the phase through melting and creates a microstructure with fine grains. Transforming a heterogeneous microstructure to a homogenous and refined grain structure, FSP offers superplastic properties to some aluminium alloys.
FSP also enhances the fatigue strength and tensile strength of the metals and there are several methods that can be applied to a variety of material shapes and sizes.
How Does it Work?
Friction stir processing has many similarities to friction stir welding in that both use a rotating tool with a pin and a shoulder. However, whereas friction stir welding used the set-up to join two materials without heating, melting or altering the physical state of the materials, friction stir processing enhances and improves the properties of a material. This includes properties such as toughness and flexibility in specific areas by altering the microstructure into a fine grain through the use of a second material.
Crucial to this process is the tool itself, which produces localised heating and material flow. As mentioned above, the tool consists of a small cylinder similar to a drill (with a diameter of around 50mm), a pin and a shoulder.
The tool creates localised heating through friction, softening and plasticising the workpiece. This causes processed material to move from the front to the back of the pin, during which the material undergoes significant plastic deformation, resulting in grain refinement in the base material.
Mixing this first material with a second material allows the base material to be altered without changing the physical state, allowing engineers to improve the material properties and deliver results including high strain rate superplasticity.
In many cases, the re-processed areas have superior strength and formability than the parent material, e.g. aluminium castings can be processed to consolidate voids, or extrusions can be improved in highly stressed areas. In combination with superplastic forming, FSP offers the potential to form complex-shaped parts at higher strain rates and in section thicknesses not possible using conventional superplastic processing.
FSP offers a technique for mixing materials in a solid state to achieve microstructural refinement densification and homogeneity. This means that the material doesn’t need to be melted and then remoulded to cool and form.
The microstructure and mechanical properties can be controlled in the processed zone by optimising the tool design. For example, FSP has been shown to modify metallic alloys so that they can be bent by as much as 30 degrees, whereas they would normally only be able to bend to 7 degrees.
Friction stir processing can be used with a range of different materials, including aluminium alloys, nickel-aluminium bronze, magnesium alloys, and stainless steel.
FSP can be used on both sheet and powdered metal objects.
Friction stir processing is used to improve the properties of one metal using another metal for support and improvement without exceeding the material melting points. It finds use in industries that require improved resistance to creep, fatigue and wear, including the aerospace and automotive industries.
Some examples of parts that can be improved by friction stir processing include:
- Aluminium Surface Composites: FSP can be used to enhance the mechanical properties and corrosion resistance of the surface layers of aluminium composites using friction stir processing parameters. The parameters, such as the diameter of the tool shoulder and the tool rotational speed, affect the material surface properties. In this instance, the lower the tool shoulder diameter and the faster the tool rotational speed, the higher the surface hardness.
- Casting: Casting is a relatively inexpensive way of producing metallic parts, but they can include flaws such as porosity or microstructural defects. A microstructure evolution can be delivered through FSP, creating a wrought microstructure for cast components while also eliminating defects. Reducing the grain size in cast metal parts through stirring will homogenise the material, increasing strength while lowering ductility.
- Metal Matrix Composite Fabrication: Metal matrix composites can be fabricated using FSP at the nugget zone to provide changed properties. This can be used for a range of composites including nano composites.
- Powder Metallurgy: FSP can improve the properties of powder metal objects, particularly those of aluminium powders that may have an aluminium oxide film on the surface of the granules. This oxide can harm the fatigue properties, fracture toughness and ductility of the workpiece. Friction stir processing can be used as an alternative to extrusion or forging where a localised treatment is required.