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No sparks


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As any fule kno, you get lots of sparks when cutting or grinding steel.    Red hot and glowing, I suspect that some of them are actually burning in the oxygen of air, else how could tiny specks  of metal (they must have a very large surface area in proprotion to their mass),stay so hot, long enough to cause significant grief when they end up in your ear or up your sleeve.

Yesterday, I was cutting some aluminium, and no sparks!  None at all.   Just powder on my overalls.     Now, contrary to appearances, aluminium is a very reactive metal, so reacative that it's a component of thermite mixtures!   Is it so reactive that it burns to the oxide at once -the white powder - or,  aluminium having a high thermal conductivity, does it cool so quickly, that it can't burn?

Another one, more academic this time, for the Common Room of Sideways U!

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Interesting question. I doubt it's the thermal conductivity, because when your speck of aluminium is airborne it has nowhere to conduct heat except the air. 

Do you reckon it's to do with how soft ali is? Think about effort if you're hacksawing through it - much less heat generated in the first place, so it never gets red hot. 

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Ummmmm.   Less heat?   I find that a small piece of aluminium, say 2" square, which is what I was cutting to make brackets, gets quite hot when hand sawn , and VERY HOT when cut with a grinder!   Aluminium has a high thermal conductivity, so the heat spreads out more quickly than in an equivalent steel piece.  They are too hot to handle after cutting off with the grinder!     

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A few thoughts. Al is much softer, so tends to break away in bigger chunks. And they have a much smaller ratio of surface area to size, so less area for a reaction. Next is the fact al oxide is extremely unreactive, yes, bare al does react fast, but stops immediately as it gets a surface coating of oxide. This seems likely. 

However, we know al WILL burn, remember the Falklands when we had some aluminium ships that burnt? Extreme circumstances yes, but it was catastrophic. Nearly lost a friend, but he got lucky.  

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6 minutes ago, JohnD said:

Ummmmm.   Less heat?

Yes John! Why the ummmm??? When cutting metal, why does it get hot? Friction, I think we'd agree. Aluminium, being significantly softer than steel, incurs (?) less friction per distance cut, and less heat is generated. If it doesn't get red-hot, it doesn't glow and it isn't a spark. 

That's my idly assumed explanation. Not convinced there is an oxidation reaction occurring with your steel sparks, I think they're just very very hot. Although I take your point about their small mass. 

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Hi John,

consider the mechanics !!! there is a massive heat sink for the Ali swarf speck to dissipate its heat to. The hack saw blade.

Indeed it gets hot but not that hot. And in turn the hacksaw puts its heat back into the work piece. 

The Ali work piece gets hotter and hotter because the hacksaw blade can only do so much.

If you grind the Ali into a fine powder and expose it to Oxygen then you should get the fire your looking for.

As for sparks - you need some iron in the mix. It is the Iron that spark. The harder the iron alloy the brighter the spark. Wrought iron is dull red.

 

Roger

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1 hour ago, zetecspit said:

remember the Falklands when we had some aluminium ships that burnt?

Not just any aluminium, but high magnesium alloys. Light and strong, but a really odd choice for a warship. As well as being corrosion prone around salt water, magnesium burns like crazy........ and adding water just makes thing worse. Someone hadn’t thought that through!

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1 hour ago, zetecspit said:

Pete, burning is oxidation

Ah is it?! OK I won't pretend I know what I'm talking about. But you can make steel hot enough to glow without it burning, right? That was what I was musing on - if you heat anything sufficiently it will give off visible light. Red-hot, white-hot etc. So when you get sparks off steel, the steel debris is glowing because it's hot not necessarily because it's burning. Whereas ali doesn't get hot enough because it's softer... That was my theory. 

Edit: I just held a sliver of aluminium foil in a candle flame (WFH...) and it did glow. But didn't burn. I don't think that tells us anything useful or new! 

Edited by PeteStupps
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43 minutes ago, PeteStupps said:

Ah is it?! OK I won't pretend I know what I'm talking about. But you can make steel hot enough to glow without it burning, right? That was what I was musing on - if you heat anything sufficiently it will give off visible light. Red-hot, white-hot etc. So when you get sparks off steel, the steel debris is glowing because it's hot not necessarily because it's burning. Whereas ali doesn't get hot enough because it's softer... That was my theory. 

Edit: I just held a sliver of aluminium foil in a candle flame (WFH...) and it did glow. But didn't burn. I don't think that tells us anything useful or new! 

You need to find some wire wool, entertaining!

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I recall a friend demonstrating how, with a gas welding set, he could burn steel!      Hot flame, get the metal white hot, then turn off the acetylene!   Showers of sparks, and he could continue to cut the metal using its own heat of combustion!     Saw him cut through a six inch bar that way!    Sure, that's the power of pure oxygen, and I'm seeing sparks in air, so probably not relevant.

But why does iron spark, and not (?) other metals?    Is it the carbon?  But even 'high-carbon' steels have less than 2% carbon, which doesn't seem much.   But I'm  wrong!   The Wiki entry for "Spark" says,  "In the case of iron, the presence of carbon is required"  !  It also says, "Metals with low thermal conductivity are especially good at producing sparks"    So that must be true, then!

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