I don't think that is considered politically correct these days. :-)
Colin Bignell
I don't think that is considered politically correct these days. :-)
Colin Bignell
Aluminium forms a very tenacious nanometres thick aluminium oxide (Al2O3) layer on the surface. You need to remove and prevent the formation of this Al2O3 layer in order to succeed in welding it.
Titanium can be difficult to weld and so is stainless steels.
Lead
Thermocouple junctions are in fact created using electrodischarge welding. You charge up a capacitor bank to a couple of hundred volts, attach one of the thermocouple wires to one end of teh capacitor, the other wire is attached to the other end of of the capacitor. The other two ends of the two wires are then brought together and allowed to make contact. This results in a sudden discharge of the capacitor, with a nice big fat spark and the two wires then spot weld forming the thermoelectric junction.
Which does not stop it being a commonly used technique. There was lots of aluminium being welded when I walked around the QE2 while she was building and the very successful American M113 APC, which dates back more than half a century, uses welded rolled aluminium armour.
Colin Bignell
Well I've done them by spark discharge of a capacitor in the past, but tend now to use oxy-acetylene nowadays as you can form a much neater end.
Andrew
How easy is ni-chrome wire to spot-weld? I often find myself repairing toasters from around the family and friends, and I currently crimp it with small high temperature crimps, but I keep thinking about getting a spot welder (not just for this;-). I presume you would have to do a few spot-welds in parallel, so that one didn't become a hot-spot when the element is running?
Colin,
I didn't know that you had been involved in the QE2 ! We (Ferranti) had an Argus 400 process control computer (*) on board monitoring the turbines following that initial problem that they had with them. It was always a sort after call out when she docked at Southampton, as if a job was unfinished before she left you HAD to sail with her - great hardship :)
Andrew
(* lots of thermocouples there Stephen along with isothermal planes for the interfaces :) )
The problem with Ni-Chrome wire is that it gets very brittle in use. Example - the old radiant 1kW heater elements could easily be unwound and re-deployed before use. Get one that's been in service for a while and the wire will break as you unwind it. Not sure why this is as it is being annealed every time it's switched on. Perhaps out Material Technologist friend can expound on this curiosity? I expect its an oxidisation effect.
So coming to the point of your question, clean soft ni-chrome can easily be spot welded. A used toaster element may be more of a challenge as the surface needs cleaning, and the spot welding pressure will reduce the cross section and form a 'hot spot' in use.
Andrew
I wasn't actually involved with building the ship, but Glasgow University had close links with the shipyards and, as a member of the GU Engineering Society, I was one of the only group allowed aboard her during building. We also got to go around when she was finished and open to the general public, but she looked a lot smaller inside once all the furnishings were in.
:-)
Colin
An old mate of mine gets out on trips on the new cruise ships with podded propulsion systems, to monitor their tribology problems.
Thanks. Toaster elements are almost always a ribbon rather than circular profile wire. I'm always careful bending it, but it seems to bend across the ribbon just fine. I scrape the surface shiny where it will make contact in the crimp.
BTW, I'm assuming it's ni-chrome, but I could be wrong.
I've MIG welded ally at home successfully. It wasn't a critical weld, but works and looks ok. Use a stainless steel wire brush to clean it just before welding.
The use of inert gas helps to shield the now newly exposed aluminium from oxygen thus preventing the formation of the Al203 layer.
Incidentally, pure aluminium is actually extremely reactive. It is this oxide layer that makes aluminium appear to be sd passive chemically speaking.
it could be Kanthal or cupronickel instead.
I must get new glasses. I read trilobyte.
Colin Bignell
and it's hard and a good insulator.
I was breadboarding a circuit on the bench which used a pair of power MOSFETs to switch a mains heating load. I temporarily used a bulldog clip to hold the MOSFET onto a piece of aluminium for a heatsink, making a mental note not to touch the live heatsink. Circuit was working OK so far, and I added the second MOSFET, also clipped onto same piece of aluminium. All worked OK.
As I was disassembling it to build the final unit, which had proper insulating kits between the MOSFETs and the heatsink, it suddenly occured to me that the two MOSFETs actually had 240VAC between their heatsink tabs, and it was only the aluminium oxide layer which had stopped them going bang during the breadboarding.
This can also be an issue when you are trying to make an earth connection to an aluminium case.
yes that Al2O3 is also known as alumina, which is a ceramic.... and ceramics are hard and electrically insulating materials as well as chemically inert......
...
Why do I have this mental image of a grandmother with a basket of eggs?
Colin Bignell
And despite this aluminium is still very commonly welded - normally TIG I believe.
Ah!, Well thats what the hoi polli will assume when "welding" is mentioned in most circles;-!...
Were they the engines which subsequently got replaced by 9 MAN diesels?
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