Retemper Aluminum Street Light Pole 1

[greencircle]

I did try this on metallurgy forum, but believe this forum has a activity from variety of engineers.

This is in reference to pole production (aluminum street light pole welding to a cast aluminum base).
Generally post welding the pole weldment is subjected to precipitation hardening.

As you all may know this heat treatment is utilizes expensive setup with Large oven for enclosing complete pole, Time consuming etc.

I want to know your thoughts about induction heat treatment. This is a much localized heat treatment approach that doesn't need huge enclosures/equipment. I was thinking induction hardening was only applicable for magnetic materials like steel. Not sure how, turns out even aluminum can be heat treated.

My intention is to bring the temper back to T6 in the HAZ on the welded pole assemblies after this process.

Thoughts?

 

[JStephen]

My take-
You have an item that is initially heat-treated for high strength.
When you weld it, you may have areas in the weld or HAZ that are still high strength, or maybe too high strength. But you'll have areas farther away from the weld that didn't get that hot, that are essentially annealed by the welding, that are now too soft.
If you heat it locally, you could maybe temper it locally, but tempering wouldn't re-harden those areas that were weakened.
Or, if you heat and quench or heat and air-harden or whatever it takes, but just do it locally, you'll still have a transition area where you get a weak zone because it never got hot enough to harden.
Also, be sure to check if there are construction or fabrication codes or customer specifications that specify how this is to be done- in which case, it may be a moot point whether it could work.

 

[dhengr]

Greencircle:
I understand the cost, inconvenience and lead time need for large oven tempering. There would certainly be some advantage in doing these in quantities, an oven sized batch at a time. I assume you don’t make these one at a time, one every six months. Is there any cost advantage in buying the aluminum untempered, fabricating and welding the pole together and then heat treating/tempering the whole thing in one shot?

I would talk with a couple reputable heat treating shops regarding the induction hardening and the large oven treatment for that matter. They undoubtedly know more about the options, results and their own equipment and capabilities, than any of us do. With the advent of the internet, we have forgotten about actually talking with someone knowledgeable at our suppliers. Get to really know of few of their real smart guys, not just their salesman, they can really be helpful, and generally want to help, to earn our business. And, use the phone or a lunch meeting (they often, even buy), this can be much more informative than twenty back-n-forth e-mail chit-chats that don’t really say anything, and are often answered by the lowest paid typist. I’ve never been embarrassed to ask well thought-out, meaningful, questions of those type people, they know more about their business than we do, and are usually happy to help. Some of my most meaningful learning experiences have come in working with them, on my problem.

 

[dhengr]

Greencircle:
And, Jstephen has started your list of questions for you. His questions would have been some of mine too.

 

[drawoh]

greencircle,

Does the pole absolutely have to be aluminium? This is one of the things that makes cold rolled steel so cheap. What will cost to apply a decent anti-corrosion finish to it?

--
JHG

 

[berkshire]

Does the Pole have to be heat treated, The HAZ is only a small portion of the total area, do the loads approach the yield strength of the softened material?
B.E.

You are judged not by what you know, but by what you can do.

 

[dvd]

It would be a good exercise to determine how much the heat treating costs and then see what design changes could be substituted for the same cost or less. One idea which comes to mind is to thicken the baseplate and counterbore a socket into it to accept the pole, then weld from beneath the plate.

 

[drawoh]

berkshire,

Aluminium 6061 is very weak in the annealed condition. I can see why they want to heat treat. It sounds like the weak part is the part at maximum stress.

If they insist on aluminium, why not fabricate the pole out of 5086 sheet? This material is very corrosion resistant, and it is relatively strong in the annealed condition.

--
JHG

 

[berkshire]

Drawoh,
I am familiar with that. 7000psi annealed versus 37000 at T6. The unknown here is what is the cast aluminum, and what is its strength, is it 356 or A356. Which could come in at 20,000 psi or as low as 12000 depending on its temper or is it some other material?
Where I am going with this, is that tapered poles very often have a large base to spread the load, reducing the shear and tensile loads.
B.E.

You are judged not by what you know, but by what you can do.

 

[drawoh]

berkshire,

The trick with 5086 is to lose the casting. The annealed casting is weak too. The bottom of the pole is a flange with gussets. 5086 is what they make aluminium ships out of.

--
JHG

 

[berkshire]

Drawoh.
That would work, 5086 T0 is 38000psi It also bends around a tighter radius than 6061 T6.
If you could accept a slightly lower working strength 5052 at 28000psi would also do this job.
B.E.

You are judged not by what you know, but by what you can do.

 

[Ron]

I have designed many cantilevered columns in 6063-T6 where the weld at the bottom causes a significant reduction in allowable stress. I generally use gussets to compensate for the reduction. I have seen light poles (same design idea) with gussets as well.

I'm not a fan of welding extrusions to castings!

 

[greencircle]

As a company we haven't looked into using steel pole. Not sure why. But I will definitely consider looking into this. I can see some advantages.

Many large light pole manufacturers in US use 6063 T4 to weld on casting and heat treat the as-weld assembly to T6.

I haven't worked with 5052 or 5086. Don't they have HAZ after welding. I am thinking all aluminum alloy have this problem.

Ron- I don't like welded aluminum either. More work. How was the gusset attached to the column? Bolted connection?

 

[greencircle]

On point I forgot to mention. We often use aluminum arm extention to attach light fixtures. Now if we used steel then we will need to produce even arms in steel( to avoid dissimilar metal connection)

 

[Compositepro]

Why not eliminate welding altogether by casting a socket in the base flange? Use a press-fit and/or pin the pieces together, or tack-weld at the bottom of the socket where there is little stress.

 

[berkshire]

Green circle,
How are your light poles made ? Are they round tapered poles formed by the Dewey spinning process, Or are they segmented hexagonal or Octagonal poles produced by press braking? The answer to this may get you a better answer to your question here.
B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

Green circle,
In answer to your question about HAZ on welding 5086 and 5052, those numbers are for the annealed condition , These particular alloys are not heat treatable, and can only be hardened by strain hardening. So you are starting from a higher strength, than you would ,with a 6000 series alloy in the annealed condition.
B.E.

You are judged not by what you know, but by what you can do.

 

[drawoh]

I have designed many cantilevered columns in 6063-T6 where the weld at the bottom causes a significant reduction in allowable stress. I generally use gussets to compensate for the reduction. ...

I have done that on welded aluminium frames, although not on anything safety critical. A large gusset moves the weak weld away from the corner where it gets a nice lever advanage.

--
JHG

 

[greencircle]

I learnt that this is not possible by localised induction heat treatment. We will have to use a huge oven to solution heat treat the entire pole which is an expensive setup & handling. This even more expensive when we dont have an in-house heat treatment facility & have to be sent out.

Alternatively I am thinking of welding a cast base to a ~3ft taper pole. Now the the overall height of the weldment is not taller than 5ft. Which makes its easier to solution harden to T6. After heat treatment I want to lap-joint the tall pole to this short base. something like this :

I have seen some street light poles that have this kind Lap Joint connection & have a trough bolt & nut at the joint.

I have got this image by googling. But in aashto code, I don't see any formula for calculating minimum overlap.
Can any one see any drawback in this method?