Thread: Stoopid question time - Aluminium welding

You can TIG weld Al with DC (can't remember if it's DC- or DC+), but I think you need to put more heat into it and is prone to collapse if you don't move quickly. EDIT : I just did a bit more reading and I think I have that the wrong way around ... it is the tungsten that takes all the heat and unless it's "oversized" for the current being used, will disappear quickly.

This is my understanding:

For TIG: with straight DCEN about 2/3 of the heat is concentrated onto the +ve end of the arc (the job) and 1/3 on the tungsten. This is because the negative electrode benefits from the cooling action as electrons 'evaporate' off the tip of the electrode, whilst the job (being +ve) is heated comparatively more as the electrons 'condense' onto the surface. Obviously with AC it depends on the balance between the EN and EP cycles (typically something like 70/30%).

For MIG, I believe you run it so the wire is EP (for the cleaning action), but this means that 2/3 of the heat is concentrated at the wrong end of the arc (the torch end). This doesn't matter though because the wire is continually being renewed, and all the heat that is theoretically concentrated on the torch end gets transferred to the weld in the form of molten metal deposited on the job anyway.

Stick (MMA): whilst not really relevant to your question, the same thing as MIG happens with stick - you can often run in either polarity, because most of the heat ends up in the weld regardless. The polarity seems to have a bigger influence on how the flux behaves rather than heat input.

BTW, apparently it is not unusual to TIG weld aluminium with DCEN, but you have to run hot and having Helium or Helium/Argon shielding helps here. More common in the US because helium is so much cheaper there (which is another story).

Cheers

- Mick

Thanks guys. Great explanation. Makes much more sense than anything I came up with!

Just to add a little bit. You will often hear that the High Frequency used with AC tig is there for cleaning purposes. It is not, never was and never will be. The HF is purely to keep the arc going during the point where the voltage is zero between the +'ve and -'ve phases of the arc.

According to http://aluminium.matter.org.uk/conte...eid=2144416796 AC TIG with HF overlay isn't used now...
"These machines have now been replaced by pulse generators that do not constantly send out high frequency stress impulses, but instead supply a smooth sinusoidal voltage. This has the advantage that they are far less like to influence radio and TV receptions in the close environment and, as a consequence, do not have to be signed up at the federal post office."

The EWM welder documentation described high frequency pulse as "Metallurgic". "The kHz pulses (metallurgic pulses) use the plasma pressure produced at high currents (arc pressure) which is used to achieve a constricted arc with concentrated heat feeding."

czzer, I'm not sure at what level your curiosity is, but this has probably got way more than you ever wanted to know about arcs, cathodic cleaning, etc.
http://www.aws.org/wj/supplement/wj0110-1.pdf.
Karl is right about the HF, but that's for the old school machines. Karl's showing his long history with the industry. Square wave inverters don't need any HF assistance to maintain the arc during AC cycles (but it is bloody useful for initiating the arc at the start of the weld).