Induction heating of aluminum strip possible?

[Ryoko]

TL;DR

I need to anneal a strip of aluminum 1" wide and 0.032" thick. The strip is 7 feet long. Using a propane torch is a pain. Can an induction heater work?

I need to anneal a strip of aluminum 1" wide and 0.032" thick. The strip is 7 feet long. The way I'm currently doing it is to use the old trick of using a Sharpy to draw a black line down the part, hanging the strip vertically, then heating the strip with a propane torch from top to bottom a few inches at a time starting at the top and working my way down to the bottom until the black Sharpy mark disappears. This is a really slow process and those propane cylinders get expensive after a while.

I'd like to be able to heat the strip faster and not have an open flame which is a little hard to control to get even heating. I'm seeing the induction heating wands all over Flea Bay and elsewhere. I don't know if they have enough strength to heat aluminum or how quickly they would work. It's also unclear what the duty cycle is on these things since doing a 7 foot strip would take a while. Thoughts?

 

[Averagesupernova]

I'm pretty sure induction heating is only possible with ferrous metals. I'm not sure if you are in a position to do some plumbing but you can aquire some fittings to adapt your propane torch to go on the end of the appropriate hose connected to a gas grill bottle. You'll have a few dollars invested but at least still have the equipment. I have purchase such things at a hardware/big box store. Propane hose with threaded ends is available and should not be substituted with something else.

 

[Baluncore]

The problem with aluminium is the electrical and thermal conductivity. I expect you could induction heat the surface of aluminium, quickly, at MHz frequencies.

There would be a benefit in changing the heat loss from your non-existent stove or furnace. Obtain an insulated ceramic tube that can hang vertically, containing the strip of Al. Apply the neutral flame into the base, like a fire at the foot of a chimney, wait for the exhaust at the top to exceed the required temperature. Quickly remove the strip from the furnace to cool the Al.

Consider using the ceramic tube as an induction furnace. A solenoid coil would heat the enclosed aluminium with circular eddy currents, but the coil would need to be made from copper, which would also heat, so would need to be a water filled tube. That game does not end happily.

An alternative would be to use electrical resistance heating of the Al. Clamp the ends of the strip to heavy electrical terminals, then momentarily apply the low-voltage output of a transformer, a 12V vehicle battery, or a cheap stick welder. The Al strip will rapidly heat throughout its length. You could use less current if you thermally insulated the strip by folding it in rock wool, or glass wool.

Examine the technology used to power a spot welder, (resistance welding). One or two turns of a flat copper strap are used as the transformer secondary.

By monitoring the voltage across the strip, (with auxiliary voltage sensing electrodes to avoid contact resistance, the 4 terminal method), and the current flowing, you could estimate the resistance, and so the temperature of the metal strip. The resistance of elemental metals is proportional to absolute temperature, but that does not hold for some alloys.

To anneal aluminium, you require 300°C to 410°C, say 360°C.
Room temperature: 20°C = 293 K.
Al annealing temp: 360°C = 633 K.
Resistance ratio = 633 / 293 = 2.16
So you need the average resistance of the strip to become about 2.2 times the initial value.
With a heavy low-resistance voltage source, you could watch the current fall from 100% to 45% as the strip heats.

 

[berkeman]

An alternative would be to use electrical resistance heating of the Al.

That was my first thought as well. Seems like a natural way to do it.

 

[Baluncore]

That was my first thought as well. Seems like a natural way to do it.

Yes, it would be my first choice for an 0.032" thick, 1" strip, that was 7 ft long. But what is the resistance of that strip?

Room temperature resistivity of Al is 2.82×10⁻⁸ ohm metre.
Length is 7' = 2.1336 m.
Width is 1" = 0.0254 m.
Thickness 0.032" = 0.813 mm = 0.000813 m.
Area = 2.065e-5 m².
Resistance = 2.82e-8 * 2.1336 / 2.065e-5 = 0.002914 ohms = 2.9 mΩ

For 12V/2m9 = 4000 amp, 48 kW, that is too much power for an amateur process.

1V / 2m9 = 343 amp = 343 W of heating, which will not be sufficient, especially when the resistance rises and the heat loss is greatest.

For a single lead acid cell, or a one turn secondary on a power transformer, 2V / 2m9 = 686 amp = 1372 W heating. That seems about right for this job.

A cheap stick welder will have an aluminium wound coil, designed for something like 24Vac at 150 amp. That is quite unsuitable for heating the 2m9 strip. The secondary coil would need to be replaced with a single turn, of flat copper strap.

The length of the connecting conductors needs to be minimised. Maybe fold the strip back on itself, or heat two aluminium strips next to each other, electrically connected in series.

 

[Ranger Mike]

TL;DR: I need to anneal a strip of aluminum 1" wide and 0.032" thick. The strip is 7 feet long. Using a propane torch is a pain. Can an induction heater work?

I need to anneal a strip of aluminum 1" wide and 0.032" thick. The strip is 7 feet long. The way I'm currently doing it is to use the old trick of using a Sharpy to draw a black line down the part, hanging the strip vertically, then heating the strip with a propane torch from top to bottom a few inches at a time starting at the top and working my way down to the bottom until the black Sharpy mark disappears. This is a really slow process and those propane cylinders get expensive after a while.

I'd like to be able to heat the strip faster and not have an open flame which is a little hard to control to get even heating. I'm seeing the induction heating wands all over Flea Bay and elsewhere. I don't know if they have enough strength to heat aluminum or how quickly they would work. It's also unclear what the duty cycle is on these things since doing a 7 foot strip would take a while. Thoughts?

How hot do you wish to heat it? pls advise temperature.

 

[berkeman]

How hot do you wish to heat it? pls advise temperature.

I think you can just use the temperature numbers suggested by @ Baluncore above.

 

[Baluncore]

To anneal aluminium, you require 300°C to 410°C, say 360°C.