Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

piston metalurgy

Discussion in 'Technical and Troubleshooting Torque' started by muso280, Apr 19, 2006.

  1. Greetings All,
    Was wondering if any of you might be able to settle an argument for me?
    A friend and I have been debating the subject of the CB1300's pistons, namely their being described as "semi-forged"- my friend seems to think no such beast exists, that pistons can only be forged or cast. As a welder i know a fair bit about various hardening treatments, but mainly in relation to steel, so felt rather disadvantaged in the discussion with my friend who is a qualified motorcycle mechanic (gained his trade in barely over 2 years then left the industry but thats another story!). My thinking is that, by "semi-forged" they mean the piston is essentially cast, then the crown heated and compressed, to perhaps get the benefits of forging at less cost? Meanwhile, old Ralphy is insisting that doing this to a piston would create too many stress fractures and would only serve to weaken the piston.

    Sorry for the rather obscure question, but can any mechanics/metalurgists/smart people shed any light on what this process might involve and/or benefits?
  2. Not quite - semi-forged actually means that the part was forged but at a temperature somwhere between the point at which the metal softens and where it melts completely (therefore useless for eutectic alloys :wink:). The term is actually short for "semi solid" forging and bascially fits in between normal casting and forging processes. The results are not quite as good as what can be achieved with true forging, but parts are cheaper to produce and are still much better than cast components. So it's sort of like the process you describe, only there's no casting involved and the entire part is heated and compressed. Since the metal is in a semi solid state no stress fractures will form especially with steel (edit: or most Al alloys) since the crystal structure of the metal will change on cooling essentially "resetting" the metal - changing the rate/way in which it is cooled will determine the structure of the final part.
  3. The thixoforming process posesses advantages of both forging and casting in the manufacture of alloy or composite parts. The manufactured part has a "near-net-shape", meaning that very little addition processing is required to bring the part to its final form. The feedstock for thixoforming (or semi-solid metalworking) is usually in billet form and is heated to a high temperature but not quite melted. It is then injected into a die. Parts produced by thixoforming are as strong as cold-forged parts but at a lower cost. The structure is fine-grained, homogeneous, and virtually free of porosity. The part can be heat-treated to give mechanical properties superior to those of liquid castings.

    wow we are getting all tecnicalogicimal hey :LOL:

    so in essence you are right about the 'semi forged pistons'
    ( in fact you'll find em in most of todays internal combustion engines )
  4. Written by people who manufacture semi-forged parts - and not entirely true (difficult to compare since alloys suitable for one process generally don't suit the other).
  5. well i only install em i dont make em, so i can only go on the techno babble
    im given, so ner

    :LOL: :p
  6. :LOL: That's okay, I spent 4 years at Uni studying this stuff so it's nice to be able to put it to use :).
  7. Thanks for the swift replies guys- much appreciated!

    what you've described is what I've always (perhaps incorrectly?) understood '(high)pressure/injection-casting' to be, slightly less heat in casting material to avoid problems with porosity/grain structure; the other solution being vaccuum casting, as seen on the latest generation of bike frames, yes?

    thanks for the info- suitably armed I shall enter the fray for round 2 tommorrow ; )
  8. High pressure/vacuum casting is different since your dealing with the metal in it's liquid state - which is forced/sucked into a mould rather than relying on being poured in, reducing the porosity of the final structure (most molten metals will contain a significant amount of dissolved gasses). Don't know if you've ever looked at a phase diagram for a metal alloy before but basically depending on the composition many alloys will go through a "liquid + solid" state before fully melting (and occasionally 1 or more solid phases too). Cooling from a liquid (ie casting) generally means that it is the centre of the part that will solidify last and generally have a much coarser grain size, creating a weak zone (much like the non-clear centre of ice cubes). Cooling from a semi-solid reduces this problem since much of the material remains in a solid form. The part that does solidify will generally be more evenly distributed through the component and the rate of cooling can be much more easily controlled (reducing the final grain size). So basically the final part will have a far more even structure and a finer grain size, both of which increase strength.
  9. I was going to do that when I got home from work yesterday, but Neighbours was on.