Alternative for A356 6

[izax1]

I am a structural engineer working with a (vibration) fatigue loaded structure sand cast A356. When I consider the mechanical properties of this material, it is really not suited for fatigue applications. For the next version of this product, I have argued not to use A356, but the answer I get from the materials people are that this material is widely used in the automotive industry for loaded structures. I can't think of many automotive loaded parts that are not fatigue loaded.

Is this a commonly used automotive Al alloy??? If so, why?????

 

[TVP]

Yes, A356 is a commonly used aluminum alloy for automotive castings. For components that operate under extreme fatigue conditions, like suspension control arms or road wheels, this alloy (or the plain 356 alloy on which it is based) is usually cast into metal permanent molds, which gives the final product a finer, more uniform microstructure, and this results in better mechanical properties. Squeeze casting is used to produce the highest quality castings, essentially eliminating microporosity (due to solidification shrinkage) and greatly reducing macroporosity due to entrapped gas.

There has been quite a bit of study on the fatigue strength of these alloys in recent years. I recommend you do a search on google for freely-available information, and at the SAE website (

http://www.sae.org)

for technical papers on the subject. Significant improvements can be made, based upon a few characteristics:

1. Modification of eutectic silicon. Strontium is the most common method for doing this, which changes the morphology of the silicon from angular blade shapes, into a more rounded shape.

2. Elimination of porosity.

3. Grain refinement. Small grain size, along with a well-modified eutectic, can significantly improve the fatigue strength.

4. Proper heat treatment. The solution phase of the process is quite important, so that all of the second phases are dissolved, and then properly re-distributed during the aging step. Minimizing the quench delay is absolutely imperative as well.

Having said all of this, one still only gets a fatigue strength on the order of 100 MPa at 10[sup]7[/sup] cycles. And since this alloy does not feature substantial ductility, predicting fatigue life under variable amplitude loading featuring excursions into the plastic strain region can be difficult.

As the last part of my reply, I will actually answer the question that you posed:

a. Because it is well-known. Alloy 356 was introduced in 1930.
b. Because it is cheap -- less than $1.76 / kg.
c. Because it has good foundry characteristics -- good fluidity, solidification range, etc.
d. Because it is heat-treatable to moderate strength levels.

I'm sure there are others, but this should give you an indication of why. The low density of aluminum, coupled with the ability to drastically reduce bending stresses by modestly increasing material thickness/section size, allows for lightweight castings to be economically produced. Which is not to say that there aren't other ways to achieve the same objectives...

 

[izax1]

TVP, thank you very much. Since this is not a high volume product, sand casting is the only commercial alternative.
We have actually conducted some fatigue tests for A356 ourselves, and we got approximately what we expected. Bur we have difficulty in getting acceptance for an elongation of more than 3-4% from the foundry. And that is worrying me. If I am really interested in an alternative, what would be your recommendation??

 

[TVP]

Unfortunately, you have stumbled upon a fundamental problem for aluminum alloys-- poor fatigue strength, especially for the casting alloys. As I mentioned in my previous email, it is imperative that you eliminate porosity, keep the grain size as small as possible, and have a properly modified eutectic. I forgot to mention inclusions-- non-metallic inclusions, dross, and intermetallic compounds are all detrimental to fatigue performance, so you must eliminate them in order to improve fatigue performance.

Two alloys from the aerospace industry that have improved mechanical properties are A201 and A357/D357. Alloy A201 can have twice the nominal tensile properties of A356, and yet the fatigue strength is not really that much higher than that of A356. The mechanism for reduced fatigue strength is the formation of persistant slip bands (PSB's) at the interdendritic triple points. This is why, in the absence of gross defects like microshrinkage, macroporosity, or inclusions, fatigue strength is fairly similar.

My recommendations would be the following:

1. Investigate Hot Isostatic Pressing (HIP) as a post-casting process step. This process seals internal porosity, and can greatly improve fatigue strength.

2. Investigate the use of A357 or A201. Small improvements over A356 are possible, as long as casting quality is maintained.

3. Investigate the use of shot-peening or roller burnishing. Castings typically do not respond that well to these treatments, due to the numerous defects that I have already mentioned. However, defect-free castings can benefit from this treatment, although not to the same degree as wrought aluminum does.

 

[etch]

you could always try EN2726, this is an aerospace dirivative of A357, is got less impurities and hence better chance of reaching valuses recorded, in sand castings with a test piece taken from the castings in the fully heat treated state we obtain properties of 330 UTS 250 ish 0.2% proof stress and elongation from 7- 10 %

 

[bruv]

Bernt

For another discussion on A356 mechanical properties, go to thread330-20699 which may have some answers you need.

 

[majordud]

May I complement TVP on his knowledge of the alloy. This person knows about metals.
Question - has bernt reviewed his concerns with the foundry? Absolutely no question that the fatique performance is improved/managed/controlled by porosity, grain size & modification. Call in other possible vendors. Not all foundries have the same capabilities.

 

[izax1]

Majordud

Yes, we have discussed our concerns with several foundries. The best option we have is to define critical zones in the structure where we know (or suspect) the stresses to be high. What they can offer is to monitor the cooling process (I think) in order to improve elongation locally in this area. Of course that is a function of costs. Also the addition of strontium has been discussed. Some foundries are, for some reason, not keen on that at all. So we have some options.

It seems like we have to try and get what we can from what we have using A356.

Thanks for all the useful tips!

 

[goodrich]

Anyone here can share the fatigue data / curve of A356 with me? I am conducting some research on it. But I can not find any usefull data on it?

Thanks a lot.

 

[TVP]

As I mentioned in an earlier post, searching the SAE website (

http://www.sae.org)

is a good way to find recently published articles about fatigue of aluminum castings. Another good place to search is Scirus (

http://www.scirus.net),

which is linked to Elsevier, one of the leading publishers of scientific journals (International Journal of Fatigue, Engineering Fracture Mechanics, etc.). Teksid (a casting company) publishes a journal called Metallurgical Science and Technology that has had papers on Al fatigue. Use the following link for more information:

http://www.teksid.com/science.htm