Colours of (very) HOT Solids!

[Macavity]

Recently I was pondering "early life experiences" of bending
Aluminium sheet - I used to make chassis(es) for radios etc.
As like-minds may remember, bending 16 s.w.g. Aluminium
tended to *crack* the surface of the metal on the outside of
the bend! Not sufficient to lose real strength, but notable!  

Then I happened across this Youtube video where the guy is
bending 1/8" (3mm!) Aluminium aided by heating it... Great
NERDY appeal anyway?! But the thing I noted was that HOT
Aluminium (as he states) does not *glow* "red hot" like e.g.
a bar of ferrous material at the similar(?) temperatures. He
uses carbonised WAX as a temperature indicator instead!

BUT THIS also interested me from a "science" point of view!
Hey, I also watch TV programs where people (maybe you!!)
do a bit of Blacksmithing or Welding in your spare time... 

Set me pondering things like Black Body Temperature and 
Electronic Structure? I have no REAL ideas re. beginning a
(any) discussion, but I was genuinely intrigued by this... 

Any thought / comments? 
 

 

[Davey-T]

Used to weld aluminium in my youth using oxy acetylene very tricky and took a bit of practice otherwise you ended up with an aluminium puddle 

Got all this new fangled  eletrimicity stuff now I believe.

Dave

[Macavity]

Heheh. I note that Aluminium MELTS at about 660 degrees C... 
Maybe that's only a DEEP RED? Compared to a Oxy-Acetylene!
I only had a "Skool Bunsen" as reference. Claimed 1000 deg? 

[xtreemchaos]

mig and tiged most of my life, done mosts other types welding too, I think the hardest thing to weld is cast iron I find it a nightmare. charl

[Zakalwe]

Rubbing a bar of soap on ally before heating it also works well. As soon as the soap blackens then the ally is hot enough to bend.

 

26 minutes ago, xtreemchaos said:

mig and tiged most of my life, done mosts other types welding too, I think the hardest thing to weld is cast iron I find it a nightmare. charl

Lots of pre-heat and lots of asbestos blankets to wrap it in afterwards (or a kiln if you have one big enough). The damn thing will still crack all too often though

[xtreemchaos]

agreed Zakalwe, used to do quite a few tractor blocks, ended up giving up or brasing or at a last resort liqid metal. charl.

[Davey-T]

Arc welded a Ford Consul block once that had been frozen solid, no one more amazed than me that it worked afterwards 

Dave

[Macavity]

Part key to colour temperature is the:
https://en.wikipedia.org/wiki/Emissivity
But a bit early for Theoretical Physicists?

[Davey-T]

Aly don't glow it just melts without warning, you have to keep prodding it with the rod until it's soft 

Dave

[SilverAstro]

31 minutes ago, Macavity said:

But a bit early for Theoretical Physicists?

Hahaa you beat me to it ! was going to say -> be careful, the last bloke who worried about Black Bodies ended up inventing Quantum Mechanics in order to avoid a dreadful escalating energy paradox

PS not sure which I prefer "like-minds" or "mature"

[xtreemchaos]

heres a tip if ya do do the last resort and use some liqid metal , mix some wire wool in with it, it lasts just about forever on cast iron blocks water jackets. charl.

[andrew s]

3 hours ago, Macavity said:

But a bit early for Theoretical Physicists?

Quite right we need time to wake up sorry I mean consider before responding. I dug out my copy of Theory of Reflectance and Emittance Spectroscopy but alas nothing on bending Aluminium Alloy.

Had it been a Plank of aluminium it might have been of more use.

It is interesting that metals have a high reflectance but low emissivity must be to do with the electron band structure of metal alloys or some such hand waving.

Regards Andrew

PS I was an experimental Physicist as I never got past playing with expensive toys bit. Come to think of it I still am!

[robin_astro]

Google Draper point.  I would be surprised if glowing aluminium looks significantly different from any other glowing metal at the same temperature (but then again nothing really surprises me these days!) aluminium would melt before you would see any glow in a lit room.

Robin

[robin_astro]

The observation in the video that aluminium looks different from steel where the flame hits the surface is interesting though. The local surface temperature here could be much higher. I checked the thermal conductivities and steel has ~5x lower conductivity than aluminium so perhaps the heat is conducted away faster so you dont see a glowing hot spot like you do with steel. Also you never see the raw metal surface with aluminium as it immediately oxidises so perhaps the emissivity of the oxide layer has a part to play here.

Robin

[Zakalwe]

18 minutes ago, robin_astro said:

The observation in the video that aluminium looks different from steel where the flame hits the surface is interesting though. The local surface temperature here could be much higher. I checked the thermal conductivities and steel has ~5x lower conductivity than aluminium so perhaps the heat is conducted away faster so you dont see a glowing hot spot like you do with steel. Also you never see the raw metal surface with aluminium as it immediately oxidises so perhaps the emissivity of the oxide layer has a part to play here.

Robin

It's more to do with the low melting point of aluminium. It melts at  about 660C whereas it takes nearly 700C to get steel to glow even a dull red. You can make aluminium glow if it is heated high enough, but normally by the time it hits that temperature it is a molten puddle on the floor.

In comparison, it takes about 1400 C to melt steel.

 

[Stub Mandrel]

Using soft soap and heating it until it blackens is the standard way to anneal most aluminium alloys. They have the benefit of age hardening, anneal, bend, leave alone and they will get their hardness back, unlike steels that may need three heatings (anneal, harden, temper) or copper and brass that barely age hardens and stays soft unless work hardened.

I have some aluminium welding rod, it melts into, partly dissolving, the aluminium at well under red heat. To remelt it needs to be much hotter (but still not enough to melt the aluminium).

 

[Alien 13]

+1 for soap as an indicator but I never managed to bend the thick stuff successfully, preferred drill and tap to join or machine out of a single block (for audio projects).

Alan 

[Macavity]

We got there by collective effort? I now suspect that metals
tend to appear the same colour at the same temperature...
The rest is down to conduction... emissivity (or whatever)!

Aluminium (below its melting point) is probably a tad dull:

As a bonus, I include a favourite image of the HEX colours of STARS
without the effect of the earth's atmosphere! As we know, the Sun
is pretty much "White Hot" in reality!

[Stub Mandrel]

41 minutes ago, Macavity said:

We got there by collective effort? I now suspect that metals
tend to appear the same colour at the same temperature...

They do indeed, look up black body radiation.

There's a separate effect found (only) in steels caused by the variable thickness of the oxide film that can give temperatures within ~10 degrees which are invaluable for tempering and heat treatment.