Casting con rods

[kadora]

Good day guys
For my new engine i need to produce con rods //i would like to cast them//
but i do not know if casted con rods will be solid enough compare to con rods
produced from hard aluminium bar .
Is even normal to cast con rods from aluminium ?
Second question: When i melt "High Quality Hard aluminium" then i pour it to the mold will i have aluminium casting with the same properties after cooling?
Thank you for your opinion.

 

[jasonh]

Good day guys
For my new engine i need to produce con rods //i would like to cast them//
but i do not know if casted con rods will be solid enough compare to con rods
produced from hard aluminium bar .
Is even normal to cast con rods from aluminium ?
Second question: When i melt "High Quality Hard aluminium" then i pour it to the mold will i have aluminium casting with the same properties after cooling?
Thank you for your opinion.

>Is even normal to cast con rods from aluminium
Yes. I've done it. It works fine. I used scrap Al melted down from other Al castings. I think most "real" engines have forged Al conrods- and the forging process gives desirable mechanical properties to the product.

I'm not sure what "hard" Al bar is. You'd have to spec the alloy type. In any case it probably doesn't matter too much. Try it out. If you break a rod you'll know it's not good enough.

>When i melt "High Quality Hard aluminium" then i pour it to the mold will i >have aluminium casting with the same properties after cooling?

Strictly speaking- no. There will likely be contaminants causing compositional changes along with structural changes due to the way the metal has cooled/crystallized. You could probably do some heat treatment to alter the latter. If you want to run the engine at 1000's of rpm it may matter- I'm sure there are PhD's that have made whole careers out of the analysis of conrod manufacturing- but who has time for that.

One thing with Al- the big/small end will often be Al on steel. The Al bearing will tend to wear out. Some people use bronze as a conrod material because the bearing properties are better- but the bronze conrod will be heavier. You could also put bronze bearing sleeves into an Al conrod if you wanted to.

 

[Chiptosser]

If you are doing small rods for models, I would recommend using 7075-T4 or T6.

There are a lot of factors in smelting aluminum to get the properties that you would desire, for the application. I believe that starting out, you will be happier using a known grade of material. The smaller the piece, the better the quality of the material needed.

 

[kadora]

Thank you friends for your effort to help me.
Probably i will cast sample rods and then try how difficult is to bend them .
Have a nice day

 

[Till]

I think most "real" engines have forged Al conrods- and the forging process gives desirable mechanical properties to the product.

The conrods in real engines are made of (forged) steel, because the weak E-Modul (Youngs Modulum) of aluminium won't allow for stiff supporting structures around the bearing bushes.
The bushing will deform and thus wear out quickly. This is unimportant in model engines.


For illustration, imagine a small cube made of rubber (e.g. the size of your fist). Prick it slighly with an engineering scriber: it will show elastic deformation around the impactor.
Image a a large cube, e.g. trice the size, of the same material. Pricking it will show the same elastic deformation around the point, no matter how large that xxl cube is.


Most model engines aren't ment to deliver reasonable power, so using brass as a material for conrods will spare the affords of making and installing the bearing bushes. It works fine for smaller engines up to 7000...8000 rpm.

 

[jasonh]

The conrods in real engines are made of (forged) steel

Yes - I stand corrected. It looks like most commercial engines used forged steel for the conrods. But- some engines do use cast/forged/billet conrods made from Al. It looks like it's mostly in performance applications (race cars, dragsters, etc.) where the benefits of having a lower weight conrod outweighs the penalty of lower conrod life.

http://www.dragzine.com/tech-stories/engine/debunking-aluminum-rod-myths-with-grp/

 

[abby]

Because aluminium melts at such a low temperature , it can even be done in a wood fire , it is the usual starting point for foundry work and castings.
However sound aluminium castings are not so easy , most casting alloys require heat treatment of the castings , eg age hardening , grain refining , before they can be machined to a nice finish without pick-up
Many "engineering" grades of aluminium alloy are capable of heat treatment , with some , especially those used in the aircraft industry , it is possible to attain tensile strengths similar to steel.
For a simple shape such as a connecting rod , which even from a casting will be finish machined , I would machine them direct from a bar of suitable alloy.

 

[gus]

Here's my two cents worth. For small Ingersoll-Rand air compressors up to 7.5hp we use cast Aluminium Alloy Con Rods. I would recycle aluminium pistons. Our DIY Model Engines are not run at most for 2----10 mins at most and so heat is not a big problem. Important to degass and refine melt before poring. If the model IC engine is of the very small capacity, it is best to cut from bar stock.