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What are Semi Forged Pistons?

in detail

Today we’re talking about semi-forged piston and we will try to clear away some of the misconceptions surrounding semi-forged pistons, and even other semi-forged stuff like semi-forged wheels.

To understand what semi forged pistons are we first have to understand how they’re made. So how are they made? Well…they aren’t. There’s no such thing as semi-forged pistons. It is physically impossible to semi-forge something. It’s impossible to forge things half-way. A piston, just like pretty much anything else, is either forged or cast.

In many cases when people speak about semi-forged pistons they actually speak about hyper-eutectic cast pistons. The term semi forged is common in the Subaru community and many people also like to call the 2JZ-GTE OEM stock pistons semi forged. In reality they’re cast hyper-eutectic pistons.

Hyper-eutectic pistons are usually a step up from the more common gravity cast eutectic pistons because they do have a higher silicon content (more than 12.5%) which gives them better wear resistance, enables a tighter piston to bore fit and reduces emissions and blow-by). Although hyper-eutectic pistons need high pressure casting to be made properly, there is still zero forging going on during their manufacturing process. A molten aluminium alloy is fed under pressure into a mold. So the term semi-forged, although used often is definitely unjustified in the case of high pressure cast hyper-eutectic pistons. So what about semi-forged wheels? Well, in their case the semi forged term is sort of justified. A semi forged wheel starts as a cast wheel with a thinner than specified rim. After it’s cast it’s set into a special machine that rolls the rim section into the final width of the wheel. This gives the rim section forged-like properties, however the segment of the wheel where the spokes are remains fully cast with cast mechanical properties.

What about pistons where the semi forged term is sort of justified? Well, the OEM 4AGZE Toyota pistons are one such case. These pistons aren’t made using conventional low or high pressure casting methods. Instead, they are made using semi-solid casting. Semi solid casting relies on thixotropy, a property of certain fluids to be more viscous when agitated and less viscous when at rest.

Aluminium is also thixotropic at certain temperatures and the semi solid casting process relies on this property to create parts of very high quality, great surface finish and excellent mechanical properties. Because aluminium is forced into the mould when semi solid there is no slushing or slurring of the liquid which results in zero porosity for the final product. On top of this semi solid parts have an excellent grain flow and a very fine micro structure, similar to that of forged pistons. The process also creates parts with a very smooth finish that don’t shrink as they cool.

This is why the undersides of semi solid cast pistons look very similar to that of forged pistons, without any parting lines or cross hatches. This makes semi solid cast pistons excellent all-rounders giving them properties of both cast and forged pistons. They are usually made from a hyper-eutectic alloy with around 17% silicon in it, so they have low thermal expansion, but are strong thanks to their excellent micro-structure.

Of course. a fully forged piston made from the 2618 alloy will still take more abuse and be stronger and more ductile, but semi solid cast pistons are much better all rounders.

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