View Full Version : What's the hardest alloy of aluminum?

10-10-2005, 03:16 PM
Even though it seems strange, I found drawings for a model airplane engine that calls for a hard aluminum piston. (Roy Clough's "Little Dragon 060) They also mention that some commercial engines have used aluminum.

What's the hardest alloy of aluminum that can be machined by a model engineer?


10-10-2005, 03:24 PM
Likely to be 7075 T651 or so.

The E. M. Jorgensen catalog says this is going to be around 150 BHn, which is hardly hard at all when it comes to machining.


10-10-2005, 04:12 PM
If the intent is improving aluminum wear resistance, hard anodizing is a common technique. (Clearly the very thin surface treatment adds nothing to structural strength ..)

10-10-2005, 04:25 PM
Most new cars have engine pistons that are aluminum. JRouche

Michael E
10-10-2005, 04:42 PM
I would check out Fortal 7075. It has proerties exceeding 1040 steel and is much lighter. I haven't used it, but will be messing around with some shortly. You can get off cuts on E-bay at a very reasonable price if you're making small parts.

Here's a link to some propeties of this material:

10-10-2005, 04:56 PM
7075 is the hardest standard aluminum alloy. Any of the tempering grades (T-6, T-62, T-651)produce 150 Bnh hardness. This is the same as Fortal for a lot less money. If you need harder with some wear resistance, anodize or ENC the part after machining.

10-10-2005, 04:58 PM
You cold probably get away with 7075-T651 if you need to .
I am sure it would work just fine.


10-10-2005, 05:13 PM
Here is some info from Federal Mogul. The basic weakness of a piston is the land on the "side" of the piston above the top ring. In production gasoline engines everybody wants to shorten this land...the gap is a "hiding place" for unburned fuel that contributes directly to hydrocarbon emissions while doing nothing to aid combustion. However, there is a groove parted under there which weakens the land and can cause chunks to fracture off and lead to major problems.

Other serious issues are managing the expansion of the aluminum relative to the typically ferrous bore walls, and making sure the piston doesn't scuff on the major thrust side.

Also, I think the heat of combustion is going to quickly cook the age-hardened temper out of 7075-T651.


Boldly excerpted from:

3. Hypereutectic
Contains 12.5 percent or more silicon content. Special melting processes are necessary to ‘super-saturate’ the aluminum with additional silicon content. Special molds, casting and cooling techniques are required to obtain finely and uniformly dispersed silicon particles throughout the material. Our hyper-eutectic material is also being used in light and medium diesel engines to replace some of the eutectic pistons with iron groove inserts used for additional heat and wear resistance of the ring groove. Heat and wear in the ring groove contribute to groove ‘pound-out. In the Sealed Power hypereutectic pistons, the hard, finely dispersed silicon particles serve as ‘micro-inserts’ at the surfaces of the piston, especially in the surfaces of the precision-machined ring grooves. The increased strength, heat and wear resistance provided by the high-silicon content hypereutectic piston material allows for elimination of the groove inserts in several piston applications.

10-10-2005, 05:20 PM
Here... This page (http://www.tennalum.com/AATT.htm) might help you out too for some info on hardness, machining, heat treat etc.


10-10-2005, 05:22 PM
One note on hypereutectic pistons, while they are used on some production cars and even in mild race applications they are not considered strong enough for full on racing applications where forged aluminum pistons are used. I don't know of any modern day car that uses something other than an aluminum alloy for pistons, however for cost reasons most pistons are cast aluminum. Having said that and depending on the size piston you need to make I have some "scrap" pistons from my old race car I'd be willing to give you. These are forged and where damaged by debris from a broken connecting rod. It's possible there is enough metal to machine a small piston out of one of these.

Frederick Harvie
10-10-2005, 06:40 PM
7075 aluminum is likly the hardest commonly available aluminum but it is by no means the hardest . Acording to American society for metals handbook . 7001 T6 and 7178 T6 are both harder

Wild West
10-10-2005, 07:39 PM
As we speak I have some 7075-T7351 running on the Haas. The 3/4 2fl em does just fine.

www.pivotlok.com (http://www.pivotlok.com)

10-10-2005, 08:46 PM
Since I have a keen interest in model engines. Take note of my avatar. I can tell you from experience that 7075-T6 is NOT the way to go. In my experience the engine case should be made from 2024-T6 the head may be of the same material unless you are planning to chrome the cylinder do not use aluminum for the cylinder and piston anodizing will not hold up long enough to give you the proper wear for good compression. If you are planning to actually start and run this engine and build it with minimum cost then a good piece of fine grain cast iron would be in order for the piston and liner. I can tell you that the most critical item in a model engine is the coefficient of thermal expansion. The crankshaft could be made from a piece of 4130 Rc 30 The connecting rod made from the same 2024 ans the case. Sorry, that is all, I can relate at the moment, but I will post several websites that may be of interest within the next day or two got a Boy Scout meeting to go to with my oldest boy.

Note: if you must use aluminum for a piston here are three materials which are excellent for pistons they are high silicon base aluminum but are machinable by the HSM. I know I have made some pistons from one of these Mahle 244 (24% silicon)Peek 30% silicon RSP 30% silicon granted it is not easy but well worth the effort. Any other aluminum without at least 16% silicon would be a waste of time.


10-10-2005, 08:59 PM
DO NOT anodize your piston after machining unless you want your bore reamed out while running. The best coating is to use a Tin (Sn) electroplating. You can do that in your home or shop. Or if you really want to get exotic, have it Teflon coated with a ceramic top.

10-10-2005, 09:13 PM
Check out this site:


10-10-2005, 09:16 PM

Are you familiar with Roy Clough's "Little Dragon?" Just curious to see if others have had luck with that design.


10-10-2005, 11:48 PM
Hardest aluminum alloy is Aluminum Nitride ceramic. You can buy it at Ceradyne.

Peter S
10-11-2005, 08:42 AM
Most new cars have engine pistons that are aluminum. JR, I am guessing you drive a Model T and consider anything built after 1930 'new'? :D

10-11-2005, 12:16 PM
Here are some sites you may want to check out, I do not build engines for show, just go, mostly high end F2C Team Race Diesels. It takes alot of time and committment to build one especially with out CNC gear.

modelenginenews (http://modelenginenews.org/index.html)
engine collectors (http://groups.yahoo.com/group/engine_collectors/?yguid=122098567)


10-11-2005, 07:55 PM
JR, I am guessing you drive a Model T A "T" with a big block chivy stuffed with Manley forged aluminum slugs :D :D