A356 vs. AS91 Which is better? 1

[mikemoore]

I am not a metallurgist or metallurgical engineer. I am an ME working in the Aerospace industry. I have my suspicions but need some advice, we are developing a modification of a 6 cylinder horizontally opposed, RECIPROCATING air cooled light aircraft engine. Presently the crankcases are sand or investment cast A356 aluminum. We wish to develop a modification to the case (an entire reproduction) but are looking in to other alloys. OUR NEW design will be CNC machined from a billet of some type of alloy. I was considering AZ91 or AS91 magnesium. Weight is very important as well as strength. Harmonics are a concern, the engine will not exceed 2800rpm and experience VERY high gyroscopic loads and a wide range of vibration. However the factory crankcase holds up well. Any suggestions?
Mike Moore
Director, Aerospace Product Development
Northcoast Technologies
140 Parker Court
Chardon, OH 44024
440-646-9741 home
440-477-6075 cell
440-285-2365 office

 

[TVP]

1. AZ91 is the correct designation for the magnesium alloy. There is no AS91 alloy.

2. AZ91 has extremely poor high temperature properties (yield/tensile strength, creep strength) which is why it is not used for engine blocks/crankcases.

3. Other magnesium alloy have been developed for higher temperature use, such as AE42, WE43, WE54, etc. Also, there is a new generation of high temperature alloys being developed for the automotive industry by magnesium producers Noranda, AMC, Dead Sea Magnesium, etc. The following websites provide more information:

http://www.magnesium-elektron.com/productsandservices/casting.asp

http://www.noranda.com/

click on "Our Businesses" and then "Magnesium"

http://www.dsmag.co.il/

http://www.austmg.com/

http://www.intlmag.org/db-mechanical.html

 

[mikemoore]

TVP,

I found the AS91/AZ91 delineation on the web. "They", (can't remeber who they are) stated that AS91 was a more stable hybrid of AZ91. My concern is qestion is strength. Do you think that any specific magnesium alloy is an acceptable replacement for A356 cast Aluminum? Also the crankcase temperatures do not exceed 210-230 degrees F. This is the hottest the oil will ever see. Plus, I placed about 20 RTD's in various areas on the crankcase, attached with a temperature conductive epoxy, and I never saw over 200F on a 90 degree day, at the slowest of airspeeds. Radiant, or emissive heat was the biggest enemy (from the exhaust system) which I was able to stop with a simple heat sheild of thin aluminum. If the strength requirement of the magnesium is negated by the weight penalty, I will just use 2011 or 2017 aluminum, i.e., why have a 50lb magnesium case as opposed to a 45lb aluminum case. Your thoughts?
Mike Moore

 

[etch]

i would say that a356 would be supplied in heat treated condition thus giving further improvements on strength and elongation. I cannot see why you would want to machine the part, when you can easily cast it.


I would stick with Aluminium. but hey i work with aluminium so a bit biased.

 

[TVP]

Mike,

The answer to your question "Do you think that any specific magnesium alloy is an acceptable replacement for A356 cast Aluminum?" is a qualified yes. Go back to the Magnesium Elektron website I included above and check the product bulletin for alloy WE54. This is a high-temperature alloy used in some aerospace and racing applications for this type of application. It should be close to an A356 high-quality aerospace type casting.

One thing to note is the surface hardness and wear characteristics. I would not recommend a piston run directly on the magnesium surface. Also coefficient of thermal expansion will be higher for the magnesium -- approximately 24.6 vs. 21.5 for aluminium. All of these items would need to be considered before switching a current Al design to a Mg design. Good luck.

 

[mikemoore]

Etch & TVP,

Thanks alot for your input. Thanks TVP for your links to the other websites, they have been very useful. Just FYI the piston will not ever contact the magnesium and this is a very low temperature application. This is simply the crank case, although similar, not the same as an "engine block". This case is very similar to a Volkswagen Beetle case. It has two halves split longitudinally down the length of the crank and camshaft. It is dowled and bolted around the mating surfaces, and cross bolted over and under the main bearings. The cylinders are "jugs" just like a harley davidson, or volkswagen. They do not share a common head like a VW, but like the HD. These are 550 cubic inch (horizontally opposed) and go up to over 4500 cubic inches(the Pratt and Whitney Radial 28 cylinder) It is actually just like a Porche 6 cyl air cooled design. The current case does not have much room for a larger stroked crank shaft, so we must make a case with room in order to get larger displacement (stroke) Currently the bore is 5.312 inches and the stroke is 4.375 inches for about 580 cubic inches (I am guessing for timeliness). Since this engine will never see mass production, and its intended use is for air racing and competition aerobatics, the anticipated volume is very small, so making the molds for such low volume would be costly. Plus, once casted, they must all be finish machined anyway, which included deep hole drilling and align boring/honing of the crank shaft bores. If we make 10 a year over the next 10 years I would be amazed. I have a good friend in the mold making business, and for 100k he would make them for me. The machining operation on these is around 2k each, and who knows what for the materials. So I think CNC production is the way to go.
Mike