Friction stir processing
Friction stir processing (FSP) is a method of changing the properties of a metal through intense, localized plastic deformation.:7:1117 This deformation is produced by forcibly inserting a non-consumable tool into the workpiece, and revolving the tool in a stirring motion as it is pushed laterally through the workpiece.:5 The precursor of this technique, friction stir welding, is used to join multiple pieces of metal without creating the heat affected zone typical of fusion welding.:5, 7
When ideally implemented, this process mixes the material without changing the phase (by melting or otherwise) and creates a microstructure with fine, equiaxed grains.:1117 This homogeneous grain structure, separated by high-angle boundaries, allows some aluminium alloys to take on superplastic properties.:7 Friction stir processing also enhances the tensile strength and fatigue strength of the metal.:1117 In tests with actively cooled magnesium-alloy workpieces, the microhardness was almost tripled in the area of the friction stir processed seam (to 120–130 Vickers hardness).:565
In friction stir processing (FSP), a rotating tool is used with a pin and a shoulder to a single piece of material to make specific property enhancement, such as improving the material’s toughness or flexibility, in a specific area in the micro-structure of the material via fine grain of a second material with properties that improve the first.(MA) Friction between the tool and workpieces results in localized heating that softens and plasticizes the workpiece. A volume of processed material is produced by movement of materials from the front of the pin to the back of the pin. During this process, the material undergoes intense plastic deformation and this results in significant grain refinement. (Misha) FSP changes physical properties without changing physical state which helps engineers create things such as “high-strain-rate superplasticity”. The grain refinement occurs on the base material improving properties of the first material, while mixing with the second material. This causes for the base material’s properties. This allows for a variety of materials to be altered to be changed for things that may require other difficult to acquire conditions. The processes branches off of friction stir welding (FSW) which uses the same process to weld two pieces of different materials together without heating, melting, or having to change the materials’ physical state.
The tool has a crucial part to creation of the final product. The tool consists of two main functions:
- Localized heating
- Material flow
The tool at its most simplest form consist of a shoulder, a small cylinder with a diameter of 50 mm, and a pin, a small threaded cylinder similar to a drill. The tool itself has been modified to reduce displaced volume of the metals as they merged together. Recently two new pin geometries have arisen:
- Flared-Triflute – introducing flutes(large carving vertically on the pin)
- A-skew – the pin axis being inclined to the axis of the spindle.
The FSP is used when metals properties want to be improved using other metals for support and improvement of the first. This is promising process for the automotive and aerospace industries where new material will need to be developed to improve resistance to wear, creep, and fatigue. (Misha) Examples of materials successfully processed using the friction stir technique include AA 2519, AA 5083 and AA 7075 aluminum alloys,:7–8 AZ61 magnesium alloy,:562 nickel-aluminium bronze:7 and 304L stainless steel.:33
Metallic parts produced by casting are comparatively inexpensive, but are often subject to metallurgical flaws like porosity and microstructural defects. Friction stir processing can be used to introduce a wrought microstructure into a cast component and eliminate many of the defects. By vigorously stirring a cast metal part to homogenize it and reduce the grain size, the ductility and strength are increased.
Friction stir processing can also be used to improve the microstructural properties of powder metal objects. In particular, when dealing with aluminium powder metal alloys, the aluminium oxide film on the surface of each granule is detrimental to the ductility, fatigue properties and fracture toughness of the workpiece. While conventional techniques for removing this film include forging and extrusion, friction stir processing is suited for situations where localized treatment is desired.
Fabrication of metal matrix composites(MMC)
Friction stir processing(FSP) can also be used to fabricate MMC at the nugget zone where we need the change of properties. Al 5052/SiC and some other composites were successfully fabricated. Even nano composites can also be fabricated by FSP.
Mg based nano-composites
FSP was used to modify a Mg alloy and insert nano-sized SiO2. The test was conducted a total four times with the average grain size varying from 0.5–2μm. This nearly doubled the hardness of the Mg and also increased the super-plasticity. At room temperature, the yield stress of the FSP composites was improved in the 1D and in the 2D specimens signifying a larger resistance of the product metal under high stress conditions without deforming. The tensile strength was shown to increase along with the yield stress.
FSP has benefits for when two materials’ would be needed to be mixed. “FSP is a short route, solid state processing technique with one-step processing that achieves microstructural refinement densification and homogeneity”(MA) FSW helps modify materials so that metaling down or changing the material drastically does not have to take place. FSP, for example, can easily change the form of a piece of material as sheets of metal, where before it may have had to be melted down before and put into a mold for it to cool and form. (Smith,Mishra) “The microstructure and mechanical properties of the processed zone can be accurately controlled by optimizing the tool design, FSP parameters an active cooling/heating.”(MA) The same sheet of metal can be modified to fit various situations with the proper modification of the tool. FSP has shown to make metallic alloys bendable as for example an alloy modified with FSP would be able to bend to 30 degrees as before it could only bend to seven.
- Arora, H. S.; Mridha, S.; Grewal, H. S.; Singh, H.; Hofmann, D. C.; Mukherjee, S. (2014). "Controlling the length scale and distribution of the ductile phase in metallic glass composites through friction stir processing". Science and Technology of Advanced Materials. 15 (3): 035011. doi:10.1088/1468-6996/15/3/035011. PMC . PMID 27877687.
- Sterling, Colin J. (August 2004), "Effects of Friction Stir Processing on the Microstructure and Mechanical Properties of Fusion Welded 304L Stainless Steel" (PDF), Thesis (MSc), Provo, UT, U.S.: Brigham Young University, Department of Mechanical Engineering, retrieved May 16, 2010,
A variation of FSW, called friction stir processing (FSP), uses the same general setup and tools as FSW, but is used to selectively modify the microstructure of materials to enhance specific properties.
- Mahmoud, T. S. (2008). "Effect of friction stir processing on electrical conductivity and corrosion resistance of AA6063-T6 Al alloy". Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science. 222 (7): 1117–1123. doi:10.1243/09544062JMES847. OCLC 60648821.
During FSP, the metal is exposed to a combination of intense plastic deformation, mixing, and thermal exposure, resulting in a modified microstructure characterized by a very fine and equiaxed grain structure
- "Friction Stir Processing of Advanced Materials" (PDF), National Transportation Research Center website, Transportation for the 21st Century, Oak Ridge, TN, U.S.: Oak Ridge National Laboratory, U.S. Department of Energy, November 2002, archived from the original (PDF) on September 29, 2006, retrieved May 16, 2010,
The tool works heated materials into close contact, causing them to form a solid state joint. No melting of materials occurs; instead, the joint is formed by plastic deformation of the pieces.
- Du (杜), Xing-hao (兴蒿); Wu (武), Bao-lin (保林) (June 2008), "Using friction stir processing to produce ultrafine-grained microstructure in AZ61 magnesium alloy" (PDF), Transactions of Nonferrous Metals Society of China, 18 (3): 562–565, doi:10.1016/s1003-6326(08)60098-9, ISSN 1003-6326, OCLC 493811807, retrieved May 18, 2010,
Friction stir processing drastically increases the microhardness of AZ61 substrate. The mean value of Hv120−130 is almost three times higher than that of the AZ61 substrate.
- MA, Z.Y. (1 Feb 2008). "Friction Stir Processing Technology: Review". Springer. 39 (3): 1. doi:10.1007/s11661-007-9459-0.
- Misha, R.S. (2003). "Friction stir processing: a novel technique for fabrication of surface composite". Materials Science and Engineering: A. 341 (1–2): 1. doi:10.1016/S0921-5093(02)00199-5.
- Mishra, R.S.; Ma, Z.Y. (18 August 2005). "Materials Science and Engineering: R: Reports". Materials Science and Engineering: R: Reports. 50 (1–2): 1–78. doi:10.1016/j.mser.2005.07.001.
- Mishra, Rajiv, "Friction Stir Casting Modification", Friction Stir Processing website, Rolla, MO, U.S.: University of Missouri-Rolla, retrieved May 16, 2010,
FSP provides an unique opportunity to embed ‘wrought’ microstructure in ‘cast’ component by localized modification.
- Mishra, Rajiv, "Friction Stir Powder Processing", Friction Stir Processing website, Rolla, MO, U.S.: University of Missouri-Rolla, retrieved May 16, 2010,
Breakage of the aluminum oxide film on prior-particle boundaries by extrusion or forging is critical for ductility, fatigue and fracture toughness. Because of the material flow pattern, some microstructural inhomogeneity can not be eliminated in forging and extrusion. Friction stir processing provides opportunity to homogenize microstructure for subsequent forming operations or produce selectively reinforced regions.
- Lee, C.J.; Huang, J.C.; Hsieh, P.J. (2006). "Mg based nano-composites fabricated by friction stir processing". Scripta Materialia. 54 (7): 1415–1420. doi:10.1016/j.scriptamat.2005.11.056.
- Smith, Christopher; Misha, Rajiv (2014-03-21). Friction Stir Processing for Enhanced Low Temperature Formability: A volume in the Friction Stir Welding and Processing Book Series. Butterworth-Heinemann, 2014. p. 1. ISBN 9780124201835. Retrieved 30 October 2014.