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  1. #1
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    An overview: Friction Stir Processing / Friction Stir Forging

    The very first metal knives and cutting tools were made by casting, molten metal being poured into an earthenware or stone mould and allowed to solidify into the desired shape. An edge would then be ground onto the metal tool by rubbing them against abrasive stone. The ancient Egyptians manufactured copper chisels in this way and these were a huge improvement upon the stone tools in use up to that point – indeed structures such as the pyramids could not have been created without the development of metallic tools to expedite the quarrying process. Copper alloys quickly replaced pure copper and were of such importance technologically that they gave their name to that era: the Bronze Age.

    Cast metals, however, can be far from perfect, frequently containing slag, voids, porosity, shrinkage defects and numerous other faults. Hammering the cast metal can alleviate many of these faults, closing down porosity and forcing inclusions such as slag to the surface from where it can be removed. In addition, hammering – “forging” - works the metal, bringing about alterations to its microstructure. The individual grains of metal are hardened, broken up, reduced in size and aligned in certain directions. It was the development of these skills by successive generations of smiths that led to the processes we now recognise as producing good cutting tools, almost totally replacing casting as the basic production method. Forging further developed as a process with the move towards iron as the basic knife material. Iron could not initially be smelted into its liquid form as the fires available to the smiths were not hot enough: casting was no longer an option. Iron was first worked as bloom, a soft spongy mass of iron and many other contaminants which the smith would beat into a solid billet and purify by hammering and carburising. In time, processes were developed for the production of molten iron but knives were rarely made by the casting route – the bladesmith instead preferring to take the iron ingots and forge them to shape because of the many benefits that arise from thermomechanically processing the metal.

    Recent technological advances have offered the possibility of taking knife forging to a new level. As a general rule of thumb, the smaller the grain size of a metal the closer to its theoretical strength it is possible to get. Most metals have a theoretical maximum strength several orders of magnitude greater than is observed in real world tests but because of the inevitable accumulation of defects and flaws in bulk material this theoretical strength is only observed in individual grains or fine whiskers of metal. A forging process that can refine the grain structure of a metal, whilst simultaneously inducing phase transformations - for example austenite to martensite – normally brought about by a separate heat treatment, would offer considerable benefits to the knifemaker. Such a process, friction stir welding, was invented and patented in 1991 by TWI at their Cambridge laboratories and a variant of it, friction stir processing was developed thereafter. Both processes are being further developed, particularly in TWI's Advanced Manufacturing Park research facility on the outskirts of Sheffield.

    In FSW, a rotating tool is driven with significant force into a metal, heating the metal by frictional effects until it becomes plastic but not molten. The rotating tool is then translated along the metal substrate. As the tool travels through the substrate, the plasticised material is extruded past the rotating faces of the tool and in this process the metal grains are broken down and the plasticised material deposited behind the advancing tool. As the tool moves on, the displaced material cools and recrystallises with a new, generally much finer, wrought microstructure. Furthermore, as the bulk of the substrate is not heated, the recrystallising zone is effectively quenched into the cold parent metal and phase transformations can take place. For example, with steel substrates, it is possible to induce a martensitic transformation. By controlling the many parameters involved in the process – rotation speed, forging force, tool design, translation speed and so on it is possible to create a strong, tough zone in the stirred material which is ideal for making a high hardness cutting edge.

    The diagrams show a schematic of the process and section through a typical processed section annotated to show:

    A. Unaffected material
    B. Heat affected zone (HAZ)
    C. Thermo-mechanically affected zone (TMAZ)
    D. Weld nugget (Part of thermo-mechanically affected zone)
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    Last edited by meles meles; 20-08-07 at 06:17 PM.
    Darth Badger
    CEO - Chief Exkavatin' Officer
    Badger Korporashun

  2. #2
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    Nov 2005
    St Petersburg, FL
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    Re: An overview: Friction Stir Processing / Friction Stir Forging

    I have been reading a bit about this process in the mags. I was familiar with friction stir welding because Eclipse is using it to weld the aluminum skin on their new Eclipse 500 ultra-light jet. From the description in the article, I would think that you would have to use an air hardening steel to get the martensitic transformation during the friction forging process.
    Erik Markman ees mijn hero beecuz he ees zo beek unt stronk!!

  3. #3
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    Mar 2005
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    Re: An overview: Friction Stir Processing / Friction Stir Forging

    Sorry, I clicked this post by accident!



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