What does semi-forged really mean?

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What does semi-forged really mean?

Postby fivebob » Thu Jan 06, 2005 5:02 am

I thought it was about time we had this perennial argument again, but please, before you rush to post, read the whole thing through, or else you may regret posting in haste...

Ok before we get into arguments about technical processes let’s define what this debate about the term “Semi-Forged” really means. Let’s start with a few definitions.
The term is made up of two parts namely a prefix:
Semi; - Synonyms - half, partially, partly. Antonym – fully.

And the word, Forged; in this case meaning -To form (metal) by a mechanical, or hydraulic, press.

Ok, some people contend that an object e.g. a piston, can’t be Semi-forged, it can only be forged. If you use one connotation of the prefix semi, they are indeed correct, in that once you form it by applying pressure it is forged. It’s not half forged, the object has had pressure applied to it from a press, and, therefore, it’s forged. Nothing semi- about it.

That’s fine, but what if forging wasn’t the only process used to create the item? Such that it’s partially, or partly, cast and partly forged, so in the English language it can rightly be called Semi-Forged, or for that matter Semi-Cast if you so choose. Such a process is squeeze casting, which usually involves pouring molten metal into the bottom half of a mould, and then pressing the top half in and squeezing the metal into shape. Depending on how long you wait to press the two (or more) parts of the mould together, would determine if this process could be described as “Semi-Solid” forging.

Which brings me to a quirk of the, ever evolving, English language, namely the tendency to drop parts of words or phrases. In the name of brevity, or perchance marketing, such deletions can completely change the meaning of the word/phrase. Hence “Semi-Solid Forged”, becomes “Semi-Forged”. Which is a misnomer for a process like thixo-forming, because the item has never been subject to any other forming process other than forging, but would be correct if the product was squeeze cast.

Now to the question of the correct description for Toyota pistons. Well for a start they are “Forged”, as can be seen from the grain pattern it’s easy to tell that the piston was formed in a press, and, given that they are hypereutectic, which is not easy to form in a solid state, it’s a fairly safe assumption that they are forged in a semi-solid condition. IMO they are unlikely to be squeeze cast because hypereutectic alloys are supersaturated with silicon, which tends to crystallise on cooling creating pockets of pure silicon, if you don’t use special, expensive, fluxes.

So the correct term form the pistons in question is “Semi-Solid Forged”, “Semi-Forged” is marketing speak, stemming from the aforementioned need for brevity in getting your message across, and easily remembered, by your average petrolhead. Judging by the literacy standards displayed on most automotive forums, that’s not an easy task. In fact it was probably a petrolhead who first coined the term, when he/she forgot the word when he was telling his mate about the you beaut new wheels that they had purchased “Yeah mate,they um…Semi..umm…umm, Semi Forged, yeah that’s it, Semi-Forged. Buggered if I know what it means, but it sound cool”, and before you know it a new term has entered the English language.

Dropping the word “Solid” might just be all that’s needed for a potential customer to remember that your product is better than a cast one, and, depending on the spin you put on it, may even be as good as a forged one. Given the right marketing spiel it could even be touted as being better (and IMO in some cases it is better).

So now we have a solution to the age old hotly debated topic of whether or not semi forged is a true and correct description. Namely, there is no clear answer unless we know the exact process used. In the case of pistons, it’s probably not right, but wait around a few more years for the marketing BS to insinuate it’s way into the vernacular and it soon will be. For those vehemently opposed to this foul deed being perpetrated on the Queen’s English, just make sure you know how to use the big words before they’re all eradicated from vocabulary that we all abuse each and every day. :twisted:
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Postby Dr-X » Thu Jan 06, 2005 8:44 am

Good post. The reason I resent the use of the term semi-forged, is because people then believe to think there is such a thing as semi forged, which of course there isnt. Thixoforming has more in common with casting, than with forging (hence common use of the term squeeze casting, or semi solid casting).
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Postby Virtual Genocide » Thu Jan 06, 2005 10:00 am

Does this process (thixoforming) change the ammount of Pearlite grains in the steel? If not, how does it make it harder?
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Postby Chickenman » Thu Jan 06, 2005 12:34 pm

Five Bob... I was starting to get pissy with all the "What would I know" stuff in your posts but after reading that... well, lets just say if everything was explained that clearly there would b a lot less $&#$% running around. (maybe not, but it's ok to dream).

Thanks for that dude.


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Postby fivebob » Thu Jan 06, 2005 12:48 pm

Dr-X wrote:Good post. The reason I resent the use of the term semi-forged, is because people then believe to think there is such a thing as semi forged, which of course there isnt. Thixoforming has more in common with casting, than with forging (hence common use of the term squeeze casting, or semi solid casting).


No, you've got it wrong, thixo-forming has more in common with forging than casting, since the metal is never completely in it's liquid state and really the only forming process used is the forging press. Squeeze casting is a totally different process involving metal in it's liquid state, hence the "casting" designation (though it's sometimes still referred to as semi solid forging).

And to clarify what I said above... Squeeze casting can rightfully be called semi forged, as in partially, or partly cast and partly forged. So IMHO the term is valid in this case, just not really true when it comes to the process that is probably used to form pistons i.e. thixo-forming.
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Postby fivebob » Thu Jan 06, 2005 1:03 pm

Virtual Genocide wrote:Does this process (thixoforming) change the ammount of Pearlite grains in the steel? If not, how does it make it harder?


Sorry don't know much about steel and I've not come across much documentaion about the the use of thixo-forming with anything other than Aluminium alloys.

Pardon my ignorance, but perlite?? Do you perhaps mean austentite & martensite formation. I thought perlite was a completely different substance. i.e. siliceous rock (see www.perlite.net )
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Postby fivebob » Thu Jan 06, 2005 1:48 pm

CozmoNz wrote:you calling me the dunce or x?

i was just saying it because people seem to be proud that they have toyota forged pistons.... but that seems to mean they arnt as good as aftermarket (i dont quite understand how), but thats all im saying..


jeeze dr x, isnt it time to get that tampon outta there?


Sheesh, is the standard of english comprension that bad today, or am I expecting too much :roll: My comment was directed at Dr-X, though if the cap fits... (j/k)

I'm not sure whether or not they're as good as aftermarket, they're different and designed for a different purpose, i.e. 250,000km+ street engine vs <10,000km race engine and in the street environment they are, IMO, better. It just that they have poor tolerance for repeated detonation which gives them a bad rep in modified engines.

For my part I'd use them in a High HP application if they had more squish band for better detonation control, however the trend these days is to use tumble & swirl to control detonation and low squish clearance seems to have take a back seat. All well and good if the engine has been designed for operation at the levels of boost, and therefore flow, that you are running, but not so good when you start operating outside those levels and the flow patterns change somewhat. Incidently that's one of the things that TVIS was supposed help in, increased swirl from having only one port open, but somehow that fell out of favour too.

Oh and one more interesting fact, did you know that Offenhauser engines (60's Indy car turbos) ran better and produced more HP on cast pistons than on forged. So even the humble cast piston can be better than forged in some applications. BTW Dr-X in case you're wondering my source for this little tidbit of info comes from a good friend of mine, ex rocket scientist, and part time top fuel racer from the 60's so I'm afraid it's not on google anywhere :oops:
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Postby vvega » Thu Jan 06, 2005 2:00 pm

fivebob you knew this discussion was flamebait to begin with
but i must for a nice change im loving it :D

but in regards to the topic

would it not be easyier to explain what alloys are used for each process
MS75
A-390
F-132
RR58 (2618A)
4032

http://www.azom.com/details.asp?ArticleID=1392
that may help with the understanding of the diffrent casting prcedures

just my thoughts and from what i learnt ...the hard way :D

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Postby vvega » Thu Jan 06, 2005 2:14 pm

althought its not entirely in context
it interesting to note the blacktop 20v head has open combustin chamber(well almost) and higher compression whilest the silvertop head has a good squishband and lower compression

dont think its even revevant other than squish comments


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Postby Mr Revhead » Thu Jan 06, 2005 2:26 pm

ok lets see if iv got this...
concerning toyota pistons, namely 4agze...
they are hyperturetic* (sp :oops: ) semi solid forged pistons?
ie made from a hyperturetic (high in silicone) alloy thats forged (pressed under pressure) while in a semi molten state? or is that semi solidified? :?

*i think in a previous thread itwas decided they were hyperturetic
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Postby fivebob » Thu Jan 06, 2005 2:33 pm

vvega wrote:althought its not entirely in context
it interesting to note the blacktop 20v head has open combustin chamber(well almost) and higher compression whilest the silvertop head has a good squishband and lower compression

Not exactly on topic but it's my thread so what the heck :D

Hmm interesting, don't know much about 4A series engines but it sounds like the designer of the silvertop was more old skool :D

Got a pic showing the differences?

One thing that always impressed be about Ferrari F40 pistons was their generous squish and smooth combustion chamber with no sharp edges on the piston the valve recesses. Ferrari engines seem to be able to tolerate a lot of boost without detonation probably because of the clean chamber, even the 11:1 355 engine seems to tolerate 12psi+ without complaint. Try that on a Toyota and see how long it lasts :( (me thinks a couple of CT20b's on my 308 should prove interesting though)

Toyota pistons, at least the 3S-GTE ones I've seen have minminal squish and sharp detonation producing edges, and incidently from the damaged pistons I've seen all the cracks propogate from the intake valve pocket area so I suspect it's a major contributing factor to the detonation.
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Postby vvega » Thu Jan 06, 2005 2:37 pm

three types of alloy
hypereutectic
hypoeutectic
eutectic

high silicon resists scuffing has high melting point BUT is very difficult to die cast due to the "pocketing" in the materail during cool down
this is what the gtz pistons are made from
the alloy is called A-390

google will help you
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Postby fivebob » Thu Jan 06, 2005 2:40 pm

Mr Revhead wrote:ok lets see if iv got this...
concerning toyota pistons, namely 4agze...
they are hyperturetic* (sp :oops: ) semi solid forged pistons?
ie made from a hyperturetic (high in silicone) alloy thats forged (pressed under pressure) while in a semi molten state? or is that semi solidified? :?

*i think in a previous thread itwas decided they were hyperturetic


Almost right but the spelling is hypereutectic (pronounced Hi-per-u-tek-tik).

I can't be sure of the process used, but I'm of the opinion that it's from a semi-molten state (i.e. it was never liquid, AKA thixo-forming) not from a semi solidified state (i.e. it was once liquid, AKA squeeze casting).
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Postby vvega » Thu Jan 06, 2005 3:00 pm

blacktop
Image
Image

silvertop
Image
Image

both pistons
Image

and a gtz piston with some interesting det marks on it on the exhaust side of the pistion
looks like its been bead blasted :D
and has a couple of cracks
Image

v
Last edited by vvega on Thu Jan 06, 2005 3:13 pm, edited 3 times in total.
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Postby fivebob » Thu Jan 06, 2005 3:27 pm

vvega wrote:both pistons
Image

and a gtz piston with some interesting det marks on it on the exhaust side of the pistion
looks like its been bead blasted :D
and has a couple of cracks


Interesting... what's the extra machining on the right hand (silvertop) piston for, looks like a valve pocket but ????
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Postby vvega » Thu Jan 06, 2005 3:39 pm

silvetop has the 5 valve pockets and facing you on the ex side is a little ramp
im merly guessing at the fact that it is a clearance for the the squish platform between the 2 ex valves on the head
i know its not massive squish but givin the 5v design they certainly tried
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Postby fivebob » Thu Jan 06, 2005 4:00 pm

Mr Revhead wrote:haha nice
iv just figured out what cars in your avatar as well :wink:


Yes, that was my car in happier times, right after the previous owner had installed the titanium exhaust system, just before he crashed it, but not before it was filled it up with 90 litres of 100 Octane, bonus :). Titanium exhaust sounds wonderful (http://users.actrix.co.nz/richardm/308.wav), but I may fall foul of the exhaust noise rules, at least I got the stock exhaust system with it (another bonus) and the turbos will quieten it down somewhat.


i know where u can get a good deal on new "ct20bs" as well......

Steel turbine ones as well I presume?? Putting a ceramic turbine though a Ferrari is an expen$ive proposition and scares me somewhat.
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Postby fivebob » Thu Jan 06, 2005 4:03 pm

vvega wrote:silvetop has the 5 valve pockets and facing you on the ex side is a little ramp
im merly guessing at the fact that it is a clearance for the the squish platform between the 2 ex valves on the head
i know its not massive squish but givin the 5v design they certainly tried


Ahh I see now, they tried a lot harder than they did with the 3S, which is a pity, though I guess that the 4A is a more developed engine in it NA form than the 3S is in the turbo variant
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Postby spencer » Thu Jan 06, 2005 5:00 pm

[quote="fivebob]
Ahh I see now, they tried a lot harder than they did with the 3S, which is a pity, though I guess that the 4A is a more developed engine in it NA form than the 3S is in the turbo variant[/quote]

yea but in the blacktop which is later model they went away from the squish to a more open chamber as vvegas pics point out. Which do you guys think is better for n/a use? as I have both sitting here for a project and dont know which to use?
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Postby fivebob » Thu Jan 06, 2005 5:09 pm

ke70wagon20v wrote:yea but in the blacktop which is later model they went away from the squish to a more open chamber as vvegas pics point out.

Yes like I said before, current thinking is to use tumble and swirl to control detonation and use minimal squish. This should give more power as there is less trapped, and unburnt, charge and less turbulence in the combustion chamber for a more controlled burn, or so the theory goes. Works fine within the design parameters of the engine but once you change the flow characteristics all bets are off.

Which do you guys think is better for n/a use? as I have both sitting here for a project and dont know which to use?


I'll leave that question to the 4A experts, for forced induction I'd be inclinded to use the silvertop, but NA... don't know.
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