Cast vs forged pistons, what is the difference? Both types use the same alloys but it’s the manufacturing process that gives difference results.
Forged vs cast piston manufacturing
The difference in manufacturing process is cast pistons are made from pouring molten alloy into a mould which is allowed to solidify in a controlled way. Forged pistons are manufactured by using force and pressure to mould a heated slug or ingot into a die.
So, how does a different manufacturing process alter mechanical properties of the same material? The microstructure of the cast piston is influenced by the solidification process used and can often be of high quality. So this allows the microstructure of the forged alloy to change and become finer and aligned during the process.
The outcome of simply allowing the microstructure to align is anywhere between 10-20% increase in tensile and yield strength of the alloy over casting. See below figure for caparison between microstructure of cast vs forged pistons.
Forged pistons are heavier because of the material used
A common myth is that forged pistons are heavier because of the materials used. This in false and the additional weight of forged pistons over cast is from the limitations on the forging process. The shape of the die is limited by a vertical pressing action and does not give opportunity for efficient pistons shapes. Cast pistons form to the shape of the mould and the shape of the mould can be more complex saving on weight.
Cast piston treatments
While forged pistons exhibit stronger mechanical properties over cast, there are some treatments used to help them out. Most cast pistons are T5 treated which is a process to ‘age’ the piston in a heated environment to relieve stresses then the piston is quenched. Another treatment is to cast in steel control struts near the piston pin to assist in regulating differential expansion if the cylinder block is a different material ie. iron.
Cast and forged piston materials
Now a little bit on materials and the interesting bit. The same piston alloys are used for both cast and forged pistons. The standard alloy is a eutectic alloy. What does that mean? It’s an aluminium alloy which has a little less than 12% silicon as well as 1% each of copper, nickel and magnesium. The other common alloy is a hypereutectic aluminium alloy. Hyper eutectic ‘piston’ alloys are a mix with greater than 12% silicon, normally 18%. Copper, nickel and magnesium are all still 1% each.
The difference between them are:
- Eutectic has a higher tensile capacity and yield strength than hypereutectic.
- Hypereutectic alloys are a tiny bit less dense, expands less and conducts a little less heat than eutectic alloys. So each have their own advantages.
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