Casting Aluminum Issues 2

[maxwolf]

I’m working on a streetlight with some cast aluminum parts. The base connecting the pole to the foundation is cast and I’ve spec’d 771.0-T71 up to this point as it has the required fu=48 and fy=45 psi required by the calc’s. But 771.0-T71 is only listed for sand casting so far as I’ve discovered, and if this thing goes into production, it will probably require a die casting process because of high production volume (>300,000) and tight tolerances in areas.

As an alternate to 771.0-T71, I’ve found casting aluminum 201.0-T6 listed on a couple of die casting foundry websites. 201 has the strength I need: fu=68 and fy=60 and an elong. at failure around 7% (important if it gets hit by a car).

Any advice on high strength, relatively ductile casting aluminum with acceptable machinability? And reasonable cost.

Thanks,

 

[CoryPad]

In addition to the alloys you have listed, you can consider 204, 206, and 390. High pressure die castings usually have very low fracture strain due to entrapped gas porosity. You should probably stick to gravity die casting. If your castings have porosity and low fracture strain, you could consider Hot Isostatic Pressing added after the casting process.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

For such large volumes,sand casting process is ruled out. Gravity die casting or may be low pressure die casting will be suitable. With so many foundries today operating on a low key,you can certainly get a great deal.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[swall]

201 is not die castable, as it is prone to hot tears. Also, with die castings, you can't do a T6 heat treat, unless done in conjection with HIP. 201 is one of the most expensive aluminum casting alloys and does not have the corrosion resistance that some of the others have. I would scratch that one off your list. 206 is less costly than 201 but still has all of the other issues.

 

[swall]

To add to my post, I would suggest you see if you can find a foundry that permanent mold casts A357. It is basically a jazzed up version of A356. Mech propterties are dependant on what supplier and purchaser can work out, but I think you can get to where you need to be.

 

[TVP]

I agree with swall that A356 or A357 will be easier to obtain and have better corrosion resistance than the 2xx or 7xx series alloys. Vacuum die casting can reduce or eliminate porosity problems. Gibbs Die Casting has a number of alloys it offers. The Alcoa Vacural process is another variant to investigate, although I'm not sure if it is still owned by Alcoa.

 

[CoryPad]

maxwolf,

I don't think you will get > 300 000 castings per year of A357 with yield strength [≥] 45 ksi.

TVP,

Alcoa sold their automotive casting division to a private equity firm. Here is a news story:

http://www.foundrymag.com/news/news/75545/alcoa_agrees_to_sell_automotive_castings_division

This included the Pressure-Counter Pressure process, but I don't think Vacural was included in this.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CoryPad]

I meant VRC/PRC in my previous post, not Pressure-Counter Pressure.

 

[maxwolf]

Thanks for all the feedback.

Permanent mold looks to have a limit of around 3,000 before ‘redressing’ is required, but a lifetime of about 20,000 castings. Also, I should have been more clear about the estimated 300,000 volume anticipated. That is the initial target range, but it may occur over say, 3 years.

357.0-T6 seems to have fu=52 ksi and fy=43 attainable with perm. mold casting, which is very close to the 771.0-T71 I was using initially. I have a least a shot at making that work with little change to the section modulus.

I only find 357 under perm mold casting, not die casting. Can 357 be die cast, or does the die cast process decrease fu and fy because of increased porosity from trapped gas? Also, all the alloys I find under die casting have fu and fy’s that are way too low to work.

Question: if I spec 357 for now, is it reasonable to also spec perm mold casting when I know they’re going to have to redress the mold or molds probably every two or four weeks? And that they will go through roughly 300,000/20,000 = 15 perm molds before they reach the 300,000 target? Is that reasonable?

Thanks again.

 

[swall]

Two probems with trying to diecast 357 alloy. 1--It just does not have the fluidity to diecast well (even though there has been some limited production of 356 die castings over the years). 2--as you will need a T6 heat treatment to reach your strength objectives, you won't be able to use a die casting process any way. If you expose a die casting to the elevated temperatures required for solution treating, it will blister badly. Although it would be possible to heat treat a die casting in a HIP chamber and heal the blisters, this would increase the cost tremendously.

 

[TVP]

maxwolf,

Yes, it is reasonable to expect that the molds will require some amount of maintenance and/or complete replacement up to something like 15 molds for a production quantity of 300,000 pieces. These types of molds are not nearly as complicated as high pressure die casting tools, so they are less costly to manufacture and to maintain. The lowest cost solution is likely to be a permanent mold (gravity die cast or low pressure die cast) casting out of alloy A356 or A356 that is subsequently heat treated to the T6 temper. Vacuum die casting would only be a viable option if you could reduce the yield strength requirement, but then the section modulus (thickness) would likely increase beyond the point where hpdc is optimal (< 3-5 mm).

 

[maxwolf]

Thanks swall and TVP for the helpful comments.

It looks like permanent mold is the only way to go. My only other question - now that I checked in the Aluminum Design Manual (that AASHTO references for streetlight design) is that there's no fy listed for 357.0-T6. Only an fu=45 ksi.

But there is 354.0-T62 listed with fu=50 and fy=42 for 'designated areas'- locations which I can specify that must meet those values. Any objections to 354? Corrosion resistance is good w/r to stress corrosion cracking and 'ok' for NaCl spray exposure(actually a 'C' rating on a scale of A-F; unprotected A and B ratings are considered ok for seacoast exposures). But the base will be painted.

Best,

 

[swall]

At this point, you need to talk to a foundry that casts 357 and see what properties they can provide. What is obtainable with 357 depends on many things in the process and the heat treat; that is why you don't see published data. 354 is a specialized alloy with good elevetated temperature fatigue resistance for items such as cylinder heads. No reason to go there for your application and if you did, you would find fewer sources.