Casting Aluminum Pistons for Internal Combustion Engines

[timo_gross]

I did not yet read all of the posts. (they are long)
If I google metal injection molding I get something very different from what I wanted. Plastic (wax) with metal powder is injection molded. Then the plastic (wax) is burned away and the metalpowder sinters together.

I think those pen clips were done like this.

What I ment with metal injection molding is this. Metal into "heated syringe", then the liquid metal is shot into the mold with high speed at high pressure.


Greetings Timo

 

[timo_gross]

As shown in some of the videos, in some less sophisticated countries, sand casting pistons seems to be done with good success, although sand casting is a much slower and less efficient process.

In casting that boils a lot down to what you want to cast and how many parts. The advantage of sand casting is that you can produce as many molds as you like.
So in a mass production the sand molds can be made in sufficient number to keep the casting station busy. Also you can mix different types of parts in the production line. Automation grade in mass produced sand castings can be astonishingly high.

 

[GreenTwin]

I would guess that they pressure inject some metals in a molten state.
I don't know much about die casting.
For low temperature metals like Zamak, I would think it would be relatively easy to pressure inject.
For higher temperare metals, I am not sure how you would keep the metal fluid.

One of the videos I have seen is sand casting the finned cylinders for the radial aircraft engines during WWII.
Very tedious process, but used in mass production.

The methods and materials available these days for foundry work are vastly superior to the old greensand methods, but most folks still use some form of greensand on a hobby setting, due to the economy of it, and availability of materials.

There is very much an art to making, mulling, and molding greensand, which is why I don't use greensand.
I completely removed the "art" aspect from my mold making process.
The science part of it alone is challenging enough.

.

 

[L98fiero]

I did not yet read all of the posts. (they are long)
If I google metal injection molding I get something very different from what I wanted. Plastic (wax) with metal powder is injection molded. Then the plastic (wax) is burned away and the metalpowder sinters together.
View attachment 164846 I think those pen clips were done like this.

What I ment with metal injection molding is this. Metal into "heated syringe", then the liquid metal is shot into the mold with high speed at high pressure.


Greetings Timo

'metal injection molding' is called diecasting, at one time I worked for a company that made molds for transmissions among other things. What the guy doing the pistons was doing is called permanent mold casting

 

[GreenTwin]

Info per my aluminum "expert":

356 aluminum alloy is 7 to 9% silicon.
Pistons run from 9 to as high as 15% silicon.
The shrinkage rate isn't a lot different between piston alloy and 356 alloy aluminum, but piston alloy shrinks more the higher the pour temperature.
Piston alloy also tends to need more degassing.

.

 

[GreenTwin]

The complications of casting pistons in high silicon aluminum makes think perhaps ductile iron would be a good material for pistons.
Perhaps not necessary, but would seem to be more durable, sort of like going from a cast aluminum piston to a forged one.
I have found some ferromagnesium; I just need to find the time to make a reaction chamber.

Ductile iron would not be a more simple method than high silicon aluminum, but I need it for things like crankshafts, etc., so may as well consider all the parts where it may work well. I have seen some use it for flywheels.
.

 

[GreenTwin]

This information from Wikipedia.
This explains why modern aluminum pistons have a high silicon content.
I would guess that a cast 356 alloy piston would work well in a model engine as long as the correct expansion properties of 356 were taken into account, and sufficient clearance used.
If I used a piston cast in 356 alloy, I would temper it to an approximate T6 level to get better hardness and strength.

The article mentions that with some lower silicon cast aluminum pistons, you may experience piston slap until the piston heats up, and the clearance is reduced.


A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon content greater than the eutectic point of 12 weight percent silicon.[1] Most aluminum-silicon casting alloys are hypoeutectic, meaning the silicon content is lower than the eutectic point, and contain relatively fine elemental silicon crystals formed through the eutectic reaction during solidification. In addition to fine silicon crystals, hypereutectic alloys also contain large primary silicon crystals that form before the eutectic reaction. As a result it contains a much higher phase fraction of silicon. Consequently, hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances. The silicon content of these alloys is typically 16-19 weight percent, and above this content the mechanical properties and castability degrade substantially. Special molds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material.
By adding silicon to the pistons alloy, the piston expansion was dramatically reduced. This allowed engineers to specify reduced clearance between the piston and the cylinder liner. Silicon itself expands less than aluminum. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft aluminum piston is cold revved in a relatively dry cylinder on start-up or during abnormally high operating temperatures.

The biggest drawback of adding silicon to pistons is that the piston becomes more brittle as the ratio of silicon to aluminum is increased. This makes the piston more susceptible to cracking if the engine experiences pre-ignition or detonation.

.

 

[GreenTwin]

Also from Wikipedia.
Steve Chastain mentions how high-silicon piston alloy may solidify differently that lower-silicon alloy (if I am interpreting what he said correctly).

A eutectic system or eutectic mixture is a type of a homogeneous mixture that has a melting point lower than those of the constituents.[2]
The lowest possible melting point over all of the mixing ratios of the constituents is called the eutectic temperature.
On a phase diagram, the eutectic temperature is seen as the eutectic point.

Non-eutectic mixture ratios have different melting temperatures for their different constituents, since one component's lattice will melt at a lower temperature than the other's.
Conversely, as a non-eutectic mixture cools down, each of its components solidifies into a lattice at a different temperature, until the entire mass is solid.

A non-eutectic mixture thus does not have a single melting/freezing point temperature at which it changes phase, but rather a temperature at which it changes between liquid and slush (known as the liquidus) and a lower temperature at which it changes between slush and solid (the solidus).

.

 

[59dfuls]

Diecasting a piston.
This does not have all the risers and such that olfoundryman uses, so it makes me wonder if those dual risers are really necessary.

Sand molds need the risers. The metal mold for a die casting cools and solidifies the molten aluminum rapidly. A sand mold has a very slow cooling rate. In a sand mold, the casting thin sections solidify first and under go a reduction in volume which draws molten metal out of thick sections, such as wrist pin bosses, leaving a shrink or void. These heavy sections must be fed with a riser to feed molten metal while this section solidifies. My employer had software that would model solidification rates across the casting and made recommendations on number and size of risers. In the backyard foundry I make one test mold with a single simple gate to feed the casting. After pouring the mold, look at where the casting had shrinkage and add a riser to that area larger than the heavy section. It’s about directional solidification, all sections of the casting must solidify first, and then the feed system solidifies last so it can feed the reduction in volume as it cools.

 

[Jasonb]

Grey iron will be fine for casting model pistons you don't need ductile.

Likewise for Aluminium pistons for open crank hit and miss type engines your regular casting grade aluminium will be fine. Many a model engine uses 6081/2 not high silicon alloys for pistons cut from solid and nothing special if cast.

If you were building higher revving engines where the weight of the piston may come into it then aluminium would be a better choice over iron be it grey or ductile

 

[TrollForge]

There is another video, in which olfoundryman mentions a book by Steve Chastain called "Making Pistons for Experimental and Restoration Engines".

I found this book online in pdf format.
I don't know how to post the link here, since when I click on the link, it does not go to a website, but rather downloads a pdf of the book.

Steve Chastain has a BS in Mechanical Engineering and Material Science from the University of Central Florida, and has written several books about backyard casting.



I may not know enough about Machining or anything about running a mill, but to copy a Link that opens a PDF, right click and choose "Copy Link" then paste where you want it. T.
.
View attachment 164832

 

[Tom-AMS]

This information from Wikipedia.
This explains why modern aluminum pistons have a high silicon content.
I would guess that a cast 356 alloy piston would work well in a model engine as long as the correct expansion properties of 356 were taken into account, and sufficient clearance used.
If I used a piston cast in 356 alloy, I would temper it to an approximate T6 level to get better hardness and strength.

The article mentions that with some lower silicon cast aluminum pistons, you may experience piston slap until the piston heats up, and the clearance is reduced.


A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon
content ...

My company makes hi-performance big-bore piston & cylinder kits for later model Harley-Davidson motorcycle. We use hypereutectic pistons (~16% Si) so we can run tighter piston to cyl wall clearances - These air-cool cylinders typically have a 4" bore, with short skirted pistons, so to eliminate piston slap noise and significantly reduce any blow-by, we run 0.0008" clearance. You are correct about brittleness, however, 2the FM244 alloy is tough, and can withstand quite a bit of detonation, unlike early 356 hyper pistons from the 80's. We also apply a ceramic-metallic thermal barrier coating on the crown and a moly-based dry film lubricant on the skirts.
A friend , JV Brotherton and his sons own a company in Mena, Ark that cast aluminum heads and engine blocks for race engine. their foundry uses a permanent mold process, except, as soon as the casting s are poured, the conveyed into a Hot Isostatic Pressure chamber, (about 25k psi), which removes gas porosity and significantly strengthens the casting.

 

[SmithDoor]

This information from Wikipedia.
This explains why modern aluminum pistons have a high silicon content.
I would guess that a cast 356 alloy piston would work well in a model engine as long as the correct expansion properties of 356 were taken into account, and sufficient clearance used.
If I used a piston cast in 356 alloy, I would temper it to an approximate T6 level to get better hardness and strength.

The article mentions that with some lower silicon cast aluminum pistons, you may experience piston slap until the piston heats up, and the clearance is reduced.


A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon content greater than the eutectic point of 12 weight percent silicon.[1] Most aluminum-silicon casting alloys are hypoeutectic, meaning the silicon content is lower than the eutectic point, and contain relatively fine elemental silicon crystals formed through the eutectic reaction during solidification. In addition to fine silicon crystals, hypereutectic alloys also contain large primary silicon crystals that form before the eutectic reaction. As a result it contains a much higher phase fraction of silicon. Consequently, hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances. The silicon content of these alloys is typically 16-19 weight percent, and above this content the mechanical properties and castability degrade substantially. Special molds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material.
By adding silicon to the pistons alloy, the piston expansion was dramatically reduced. This allowed engineers to specify reduced clearance between the piston and the cylinder liner. Silicon itself expands less than aluminum. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft aluminum piston is cold revved in a relatively dry cylinder on start-up or during abnormally high operating temperatures.

The biggest drawback of adding silicon to pistons is that the piston becomes more brittle as the ratio of silicon to aluminum is increased. This makes the piston more susceptible to cracking if the engine experiences pre-ignition or detonation.

.

I do not know what alloy piston are.
But when using piston for a sub for 356 it was not same alloy.
Tessalloy is closer I like that alloy but my supplier did have Tessalloy. The stock alloys for aircraft industry only. So I used 356 or A356.

I may have miss spell Tessalloy.
The good part about Tessalloy it age harden in 30 days and is harder most other aluminum alloys after heat treatment.
I was told the aircraft industry did not like the age harding alloys.
This data is over 40 years old.

I switch the grade 45 cast iron cost more but last foe long time on wheels for doors. Eastemated life for the wheels is over 100 years on daily uses under full wind load .

If making piston casting I go to a tractor rebuild and buy his old pistons. They like alloy what ever it is . Low cost too they happy to get good price and you happy because it is a low cost. Most tractor rebuilders will sell you scraps I found some do not

Dave

 

[Tom-AMS]

it's interesting to see the use of 3D printing for core and pattern making. Many commercial foundries use 3D printed cores. About 6 -7 years ago I reclaimed/manufactured "stage plates" for 3D laser sintering printers for a company call concept lasers. I had the opportunity to print a small piston for an old 049 model aero engine, from Inconel (only because an inconel stage and inconel powder were already loaded in the printer).
The printer had a "well", into which the stage fit very precisely (think of a piston and cylinder, only instead of a round bore, it was a 200mm square with 15mm radiused corners, Ta servo /ball screw lower/raised the stage in the well. Initially the top of the stage was flush with the printer bed. A VERY fine powdered metal was spread in a 0.6mm layer. then the laser sintered the shape of the part, essentially welding it to the stage,, The servo lowered the stage 0.6mm, another layer of PM was spread, then sintered - rinse & repeat. Once I bandsaws the piston loose from the stage. And finished machining its features. I refaced the stages in my lathe. I made stages from Stainless, Inconel, Tool steel, aluminum and Titanium. 1st pic shows a Titanium stage before recliaming; 2nd pic is after. New stage is 25mm thick, and were reused until about 8 to 10mm thick. Typically i only needed to face off ~0.005" - usualy a 0.003 ruff pass, folled by a 0.002" finish pass. 3rd pic is a stainless steel hoppers I modified to contain the powered metal. The powder was so fine, it was necessary to wear a respirator while reclaiming the stages. last pics show an aluminum 3D printed water neck vs a sand cast version. The down side to 3D metal print? The 049 piston took about 5 hours to print.