Strength of Cast aluminum

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Weyland

Strength of Cast aluminum

Postby Weyland » Tue Nov 11, 2003 11:05 pm

Greetings all.

I'm thinking of learning about casting aluminum (primarily),
and have a question on the strength of it, as cast.

(actually, I've bought a few books already)

I'd like to make duplicates of some forged steel pieces, cast of aluminum, but am
concerned about the structural integrity of them as they are for a motorcycle frame.

In general, I would expect tht the type of material I start with will have much
to do with my results, but am looking for more info than that if anyone can help.

Can anyone give me their experiences with anything similar, or direct
me to someplace or a publication that might answer my questions?

Thank you.

Best,

Weyland

jpfalt
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Re: Strength of Cast aluminum

Postby jpfalt » Tue Nov 11, 2003 11:43 pm

Start with Machineries Handbook. The values you are interested in to start are yield strength, ultimate strength and modulus of elasticity. The modulus directly reflects the stiffness of the material. You will find that aluminum is much less stiff than steel and you will need to design aluminum parts much heavier than an equivalent steel part to maintain stiffness.

One other concern is that aluminum will eventually fail in fatigue where a steel part may not.

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MikeC
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Agreed.

Postby MikeC » Wed Nov 12, 2003 8:06 pm

You will probably come out better with steel on a M/C frame unless it is not a stressed member.

There is no real ansew to your question,as it was asked, though. That's like saying "how strong is a piece of wood?" Tensile strength, compressive, shear...? There are many varieties of cast aluminum, and any of the recognized commercial grades are probably far stronger than my special blend of old VW pistons, water pump housings, etc... Mine works for what I need, but I wouldn't ride on it. :-)
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

Weyland

Re: Agreed.

Postby Weyland » Thu Nov 13, 2003 8:01 pm

Hi guys,

Allow me to back up for a minute and add some background information.

I've been in Machinery's Handbook, already, and have read the data.
I have a few actually - I'm a Toolmaker by trade.
I just don't know anything about home-style casting yet, but am intrested in learning about it.

Now, as far as motorcycle frames go, people have
been riding ( and racing ) aluminum ones for a very long time.

I've even built one myself.
The problem isn't Aluminum, for me, as I've machined out lots of frame
parts with it, but rather whether something sand cast would be strong enough.

As far as what type of Aluminum, I'm completely open to buying good stuff if someone
can make recommendations on various types and their casting characteristics.

For example -
I make some rear axle plates (and steering necks) for a company out of 2" thick bar 6" x 6" (6061 -T6).
Would casting them be as strong as machining them?
Doubtful, but you get where I'm heading with this.
(as in would they be strong enough?) (without going TOO far into stress diagnostics)
(as in just HOW much weaker would the castings be...?)

Now, just for the record, this isn't for any commercial purpose.
This is simply an idea I've had kicking around for a while that I got
to thinking about again and decided to try and find some information about.

My idea is to take some older style steel forgings, make molds of them, then cast them in
Aluminum and use Aluminum pipe instead of steel pipe to construct a frame for one of my bikes.
The idea is to have the frame look OEM, but not be...

Thanks for the replies, and any insights you guys may have~!

Best,

Weyland

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mrb37211
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Re: Agreed.

Postby mrb37211 » Thu Nov 13, 2003 9:04 pm

Well, when you put it that way... the proof is in the pudding. Why not cast a few parts in the different shapes you will need and see what it takes to break them? Varying sprue sizes and locations and venting and runners could give you a feel for shrinkage effects on strength. Gingery's book on the charcoal foundry (which you probably already have) makes the point that the do-it-yourself approach means you haven't lost much if it doesn't pan out. Charles

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MikeC
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Re: Agreed.

Postby MikeC » Fri Nov 14, 2003 11:19 pm

Yup, folks make airplanes out of aluminum, too... but not cast. In the home foundry, we generally don't reach the level of technical expertise required for critical castings like steering heads. If it breaks, you're screwed.

I have no problem with lighter machined billet replacements for cast aluminum parts. I also have no problem with replacing non-stressed members and non-critical items with home-brew castings. Replacing forged steel structural members with backyard cast aluminum replicas, and welding up an aluminum frame with them would be quite another story. The entire frame assembly really needs to be re-engineered and correctly built to take full advantage of the properties of aluminum to avoid it being weaker and/or heavier than it already is. But then, wouldn't you come out cheaper and possibly lighter just making it out of 4130? If money is no object, go titanium.
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

Weyland

Re: Agreed.

Postby Weyland » Sat Nov 15, 2003 12:21 am

Yup, folks make airplanes out of aluminum, too... but not cast.


Good point.

In the home foundry, we generally don't reach the level of technical expertise required for critical castings like steering heads. If it breaks, you're screwed.


True again.
Which begs the question -
What would it take to make a casting reach that level of strength?

I have no problem with lighter machined billet replacements for cast aluminum parts. I also have no problem with replacing non-stressed members and non-critical items with home-brew castings.


Agreed again.

Replacing forged steel structural members with backyard cast aluminum replicas, and welding up an aluminum frame with them would be quite another story.


Yes... very good point.
You've worded it perfectly for me better grasp what I'm looking at here... thanks.

The entire frame assembly really needs to be re-engineered and correctly built to take full advantage of the properties of aluminum to avoid it being weaker and/or heavier than it already is. But then, wouldn't you come out cheaper and possibly lighter just making it out of 4130? If money is no object, go titanium.


It's not about the money, nor does that mean it isn't an issue. (:>)
For me, the project is about that fact that I think it would be tre' kewl
to have a frame that looked OEM, yet be made out of Aluminum.
If it was *JUST* about Aluminum, I can make a frame right now that I know will work.
But I happen to love the look of the OEM forgings...
The idea being, to leave the frame unfinished, so that at first it would appear
to be simply an OEM frame that hadn't been painted, until closer scrutiny...

Thanks for talking this out with me, Mike.
I'm learning a little and enjoying a lot.

More thoughts?
Suggestions?

Best,

Weyland

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MikeC
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Re: Agreed.

Postby MikeC » Sat Nov 15, 2003 8:38 pm

"What would it take to make a casting reach that level of strength?"

Two things: 1. A high strength alloy, produced by careful and highly technical alloying, melting and pouring methods. 2. The casting would undoubtedly have to be far thicker and more massive than any forged steel part it was replacing. It would probably end up actually weighing mare than the steel part, if it is to be of the same strength. That's the bad part about cast parts. You have to severely overbuild them to allow for porosity, voids, inclusions, etc...

Now...you like the forged look. The late Ken White (R.I.P.), who built the Hughes R-1 air racer replica that crashed on the way home from Oshkosh last year, came up with a great solution. You machine the part out of billet and then sandblast it to slightly roughen the surface. It was VERY convincing on the landing gear "forgings" on that plane. They were actually CNCed from billet. You would never know it to look at them.

You might want to check on the possibility of titanium, though. It would be MUCH stouter, lighter and (IMHO) far cooler than aluminum OR steel. If you can find drops and surplus, it's about the same price as high grade aluminum or 4130. If you have a TIG welder, you can weld it. I have not personally had a go at titanium welding, but everyone I have talked to about it says it is no harder than TIG-ing 4130, which is a breeze compared to aluminum. You just need a second bottle and regulator, and a glove box (can be made of about anything) to ensure an inert atmoshpere. For plate you can get a long "trailer cup", but that won't get it on tubing clusters.
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

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Richard_W
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Re: Strength of Cast aluminum

Postby Richard_W » Sat Nov 29, 2003 5:29 pm

At work we use aluminum castings for hydraulic tools. We ave the castings tempered to give them the strength of the tempered bar stock. We also have one casting we heat to put liners in, but not over 300 Degrees F. or we lose the temper.
So I would think if you are using 6061 T6 tempered material for casting, may be you just need to temper the casting.

Weyland

Re: Strength of Cast aluminum

Postby Weyland » Sat Nov 29, 2003 10:10 pm

At work we use aluminum castings for hydraulic tools. We ave the castings tempered to give them the strength of the tempered bar stock.


Intresting.
Do you know what bar stock you start out with?
Also, do you know the specifics of the tempering?

We also have one casting we heat to put liners in, but not over 300 Degrees F. or we lose the temper.
So I would think if you are using 6061 T6 tempered material for casting, may be you just need to temper the casting.


Actually, I was thinking of 6061-T6 specifically.
I don't mind buying large bars of it to melt and cast, and was
indeed thinking if there was a way to heat treat and temper it afterwards.

Can you tell me any more...?

Thanks,

Weyland

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MikeC
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Re: Strength of Cast aluminum

Postby MikeC » Sun Nov 30, 2003 12:18 pm

6061 T6 is a heat treated wrought alloy. Much of the strength comes from the rolling process and subsequent treating/tempering. If you melt down pure 6061 T6 and pour it, you will not have a casting made of 6061 T6, even if you temper it. It will be a cast alloy similar in composition to 6061, but even that may change if some of your alloying ingredients react with the air at high temp and oxidize or evaporate (which happens).

Now, if you want to experiment with casting an alloy of ths type, here's how to get out cheap;-) Find somebody who does punch work and ask them to save the slugs for you. I have a 5 gal bucket of 3003 slugs that are 3/8x3/4" ovals, punched out at a buddy's shop. It is the perfect size for the crucible, doesn't require breaking up, melts really fast, and being a weldable alloy, it flows and pours like you wish everything would.

Once cast, it machines just like 3003; soft, gummy, and gooey. It makes very solid castings using the lost foam process in dry sand. Much better than scrap waterpump housings and old VW pistons. That may be the best alloy for you to use, actually. It's soft enough to avoid cracking and, as we all know is just rediculously easy to weld. It's not as strong as 6061, but you'll have a really tough time making a set of castings up that will mate well under load with 6061 T6 without the casting being as big as the engine block.

I still think the bike is going to end up weighing more than it would with a steel frame in the end... if it is to retain it's original stiffness and strength. Aluminum is light, but it's not as strong, especially cast aluminum, so you have to use more of it. If you want a nice, light and strong trick frame, build one up out of 4130 and have it plated or powder coated. That's what the old Rickman frames were. I'm still for the titanium frame myself;-)
18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

jpfalt
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Re: Strength of Cast aluminum

Postby jpfalt » Sun Nov 30, 2003 12:58 pm

I worked for a few years with two guys who built custom racing bike frames as a side business. Short of going to graphite composite, their preference was alloy steel tube that was silver sodlered at the joints. They used reinforcement at joints by using two tube sizes that nested and put on a tube seat of the larger tube and then inserted in the frame member to get a double wall at structural joints. They also used swaged tube that was thinner wall at midspan and heavier wall at the tube joints. The swaged tube was strictly special order only.

the reason for using silver solder was to avoid destroying the wrought grain structure at the joint. TIG welding would change the grain structure from wrought to cast structure where the metal melted, making a much weaker joint.

From experience with truck frames in the 70's, aluminum is no substitute for steel. Aluminum truck frames were made to reduce weight and increase payload in log and flatbed trucks. However, the aluminum frames inevitably cracked and had to have reinforcing plates installed, which made the frame heavier than the steel version and still prone to cracking. It is also much less stiff than steel by a factor of at least 3.

Last thing, 6061-T6 is cast, wrought, solution annealed and then precipitation hardened. The grain structure is not quite cast, but some of the wrought structure goes away during the solution anneal. Casting 6061-T6 with a precipitation hardening process won't be quite as strong as the T6 processing, but will be better than as-cast and will machine better. There is another type of processing, T8, which is cast, wrough, solution annealed, wrought again and then precipitation hardened which comes out stronger and harder than T6, but no stiffer.


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