Toymods Car Club Inc: Tech & Conversions => 4AGZE broken piston analysis part 1

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oldcorollas
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Kita-Ku, Sapporo, Japan
Registered:
January 2003

Re: 4AGZE broken piston analysis part 1

Tue, 21 October 2003 02:18

Heya Mr Witzl,

forged you can see in the grain structure, the grain 'flow'. this is from the forging process, where a slug of metal is basically squashed into shape. the grains will curve around the pistons curves and basically follow the directiont hat the metal squished. the grains are also very elongated in areas of high flow (like the skirt).

for cast, there will be a layer of small grains near the edges, then roughly equal sized, regular shaped (equiaxed) grains in the centre of larger cross-sections.

hyper-eutectic pistons are also cast are they not??

hyper and hypo eutectic, means that the composition is either over (hyper) or under (hypo) the eustectic composition. in the case of casting alloys, i think it is referring to the silicon content of the aluminium, but i'll check up on that.. i should be able to see which is which by the microstructure.

the difference is that either side of the eutectic point, there will be a different second phase in the microstructure.

i will have to look up some articles on piston alloys before i blab on more Wink

LOL, np man, i will try and get some interesting articles with pics and stuff and make up a sort of information pack, with some background on it all...

the main difference between forged and cast (apart form composition, as cast needs more silicon to be able to flow into all the nooks and crannys of the mold), is that the forged piston will have the previously mentioned grain flow... this is effectively 'work-hardening' the aluminium as it forms the piston shape. the elngated grains and the grain flow lines give the piston more strength, because the material has been deformed and there are 'residual stresses' and it is also harder for a crack to propogate thru this work hardened structure.

with a cast piston, it is literally 'as cast' with no work hardening, and no deformation of the grains. this also means it is easier for a crack to grow around the nearly round grains....

anyhoo, thats a bit for now Wink

i'll get some more info from the pistons first.
Cya, Stewart

ps, these pics were taken after grinding to about a 2000 grit finish!! i polished them to 1 micron finish, but the grains were nto as obvious without etching.... and i need to organise access to the hydroflouric acid, so it might be a week or two before i can do that part.

THE WITZL wrote on Tue, 21 October 2003 00:17

Stewart,
im interested to know, how would you be able to compare and identify a forged piston vs a Hypertuetic piston vs a cast piston?
How would the grains vary?
I don't mean to sound rude etc, im just curious is all Smile

P.S. you rock dude! Cool

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