Sciencemadness Discussion Board

What's the cheapest metal powder to make?

John paul III - 2-2-2020 at 09:00

Im trying to weld two pieces of aluminium using an exothermic reaction. I want to use KNO3 as oxidizer, and a metal powder as fuel. What would be the easiest and cheapest metal powder to make on the scale of 100g? Would aluminium shredded on a cheese grater ignite and sustain when mixed with KNO3?

Amos - 2-2-2020 at 09:01

steel wool with potassium nitrate and a bit of sulfur burns extremely hot when briefly ground together and pressed down a bit

j_sum1 - 2-2-2020 at 12:38

If you are welding then you need compatible metals. So, in this case, aluminium.

wg48temp9 - 2-2-2020 at 20:26

I tried to repair the arm rest of a cast aluminium garden chair by welding the break with molten aluminium but failed. As aluminium melts at about 660C it is a relatively easy task to melt it compared to iron.

However there are two problems, the inevitable oxide coating and the high thermal conductivity of aluminium. The oxide coating melts at about 2,000C so the molten aluminium even at red heat will not fuse with the original casting as the oxide will remain in the way. I suspect a suitable flux could solve that.

The second problem is the high thermal conductivity of the aluminium tends to instantly freeze any added molten aluminium. Again the result is poor fusion. That could probably be solved by preheating the casting.

A thermite process will have the same problems.

PirateDocBrown - 2-2-2020 at 20:29

If you want to weld aluminum, you need to use inert gas welding.

clearly_not_atara - 2-2-2020 at 20:33

You may be able to use molten zinc as a “solder” for aluminium, as they are quite soluble in each other and zinc is very easy to melt. For a flux I’d guess you can try some fluoride salt.

wg48temp9 - 2-2-2020 at 21:00

Quote: Originally posted by clearly_not_atara  
You may be able to use molten zinc as a “solder” for aluminium, as they are quite soluble in each other and zinc is very easy to melt. For a flux I’d guess you can try some fluoride salt.


So with a suitable flux can I use zinc as a solder for aluminium in a similar way tin solder is used for copper ?

j_sum1 - 3-2-2020 at 02:49

Quote: Originally posted by wg48temp9  
Quote: Originally posted by clearly_not_atara  
You may be able to use molten zinc as a “solder” for aluminium, as they are quite soluble in each other and zinc is very easy to melt. For a flux I’d guess you can try some fluoride salt.


So with a suitable flux can I use zinc as a solder for aluminium in a similar way tin solder is used for copper ?


Probably. Soldering is not welding however. If you think it will do the trick then great. My experience with asnything zinc is that it breaks. And any impurities at high temperature will lead to embrittlement. At a joint of a chair there is going to be some stress loading and I would not be confident of success.

Aluminium welding is difficult for the exact reasons you state. My inclination under the circumstances would be to take the chair to someone with equipment and experience in welding aluminium.

wg48temp9 - 3-2-2020 at 04:24

Quote: Originally posted by j_sum1  

Probably. Soldering is not welding however. If you think it will do the trick then great. My experience with asnything zinc is that it breaks. And any impurities at high temperature will lead to embrittlement. At a joint of a chair there is going to be some stress loading and I would not be confident of success.

Aluminium welding is difficult for the exact reasons you state. My inclination under the circumstances would be to take the chair to someone with equipment and experience in welding aluminium.


Though the distinction between soldering and welding seems to have got fuzzy since I took Workshop Theory and Practice, I am old school so yes soldering is not welding and zinc is usually brittle.

I will not be soldering the arm rest back together that would be almost as useless as super glueing it.

DavidJR - 3-2-2020 at 06:38

I'm never quite sure where the boundary between soldering, brazing, and welding is...

TheMrbunGee - 3-2-2020 at 07:21

Quote: Originally posted by DavidJR  
I'm never quite sure where the boundary between soldering, brazing, and welding is...


Welding - joining two pieces by melting the pieces themselves.
Soldering - joining two pieces using relatively low melting metal/alloy, and not melting base pieces.
Brazing - higher temperature soldering.


Not an expert!!


[Edited on 3-2-2020 by TheMrbunGee]

DavidJR - 5-2-2020 at 12:15

Quote: Originally posted by TheMrbunGee  
Quote: Originally posted by DavidJR  
I'm never quite sure where the boundary between soldering, brazing, and welding is...


Welding - joining two pieces by melting the pieces themselves.
Soldering - joining two pieces using relatively low melting metal/alloy, and not melting base pieces.
Brazing - higher temperature soldering.


Not an expert!!
[Edited on 3-2-2020 by TheMrbunGee]


It's not quite as simple as that.

Welding sometimes is just melting the pieces themselves, but very often involves the use of a filler material (rod/wire) which may be of different composition to the base metal.

sbreheny - 7-2-2020 at 13:43

I have not tried soldering aluminum but I have cast things out of zinc with very good results. For that you do need flux and I used ammonium chloride. Use ventilation because the flux breaks down into NH3 and HCl during the process.

I have also cast aluminum and while it is impossible to prevent an oxide layer from forming in air, you can skim it off right before pouring and end up with relatively little inclusion of the slag in the cast part, because the newly-formed oxide layer is rather thin.

Aluminum is fairly easy to weld using MIG (metal-inert gas) and TIG (tungsten-inert gas) welding methods. The usual inert gas is an argon/CO2 mixture. The distinction between MIG and TIG is that the electrode in MIG is the actual filler metal wire but in TIG it is a tungsten rod which has been ground to a point and which does not melt during the welding. If needed, filler metal is supplied by holding a rod separate from the electrode. TIG welding is preferred for aluminum, especially if it is thin sheet, because you can reliably turn down the power and reduce the extent of the melt pool. I think that doesn't work with MIG because MIG needs to at least melt the wire electrode.

morganbw - 7-2-2020 at 16:30



[Edited on 2/8/2020 by morganbw]

elementcollector1 - 7-2-2020 at 18:49

The difference between welding and soldering (I think) is that for welding, at the working temperature, the components are fairly soluble in each other, dissolving to give a stronger bond. For soldering, the components are insoluble and are only physically joined.

You could, in theory at least, make an aluminum-based thermite with magnesium or something similar and stick both parts into that as it cools? Depends on your geometry needs... and also on if that can be reasonably done.

TheMrbunGee - 8-2-2020 at 08:28

Quote: Originally posted by DavidJR  
Quote: Originally posted by TheMrbunGee  
Quote: Originally posted by DavidJR  
I'm never quite sure where the boundary between soldering, brazing, and welding is...


Welding - joining two pieces by melting the pieces themselves.
Soldering - joining two pieces using relatively low melting metal/alloy, and not melting base pieces.
Brazing - higher temperature soldering.


Not an expert!!
[Edited on 3-2-2020 by TheMrbunGee]


It's not quite as simple as that.

Welding sometimes is just melting the pieces themselves, but very often involves the use of a filler material (rod/wire) which may be of different composition to the base metal.



But those are just different kinds of welding? Also while filling, one uses rod with the +/- same alloy, as bases? If that is true, then:

"Welding - joining two pieces by melting the pieces themselves using filler, sometimes. "

To make it as simple as that.

Tsjerk - 8-2-2020 at 08:33

Copper powder is easy and cheap to make.

stamasd - 14-2-2020 at 07:27

An usual solder for aluminum is zinc, and even better an alloy of 15% zinc and 85% tin, sometimes with 1% copper added. The alloy has better mechanical properties than zinc alone.