AC vs DC for GTAW. Please explain.

Date 11-28-2009 18:13

awspartb
I am not knowledgeable enough to give you a good answer but here, http://www.aws.org/wj/9-98article/klingman.html
Dennis Klingman is Director of Technical Training, Welding and Cutting Technology Center, The Lincoln Electric Company gives a lot of pertinent info.
Good Luck
Marshall

By Tommyjoking (****)

Date 11-28-2009 19:55

There are not a whole lot of AC ONLY machines out there.......if we are talking CC (constant current) machines for GTAW and SMAW. An AC only machine would limit you to Aluminum tig (with a high frequency unit) and very poor selection of stick welding that you could do. So the benefit of an AC DC machine is that you can do AL and some special stick welding as well as the normal DC tig and stick. If you are looking to portable machines with the capability to do both....in general it really raises the price and some more maintenance issues to deal with.

By Lawrence (*****)

Date 11-28-2009 19:54

Hey partb

The advantage (in GTAW) is gained by having an AC/DC power supply.. This way you have flexability to run a variety of currents.

Further advantage is gained if the AC/DC GTAW power supply has High Frequency and AC balence control

Even further advantages can be gained by inverter power supplies that consume less power and can produce a variety of True Square Wave forms that can accentuate AC balence control or even control amperage on each side of the AC half cycle (asymmetric AC power)

Don't think there are any AC only power supplies on the market strictly made for GTAW.

There are some mini-inverter DC only GTAW power supplies that are very handy for construction use... Smaller than a lunchbox... No AC for aluminum though.

It would take more time than I have to explain in detail GTAW power supply selection... But here are some key words you can search for in your textbook or even here in the forum.

Transformer Rectifier

Inverter

Asymmetric power

Balence control

Sine wave

Traditional Square wave

True Square wave

AC Cleaning Action (Cathodic Bombardment)

PS:
Follow Marshall's advice and read anything Klingman puts out.... I like Dennis. We have worked together for years.. Spent some time with him at the Expo two weeks ago.. He is doing very well.

By awspartb (***)

Date 11-29-2009 03:03 Edited 11-29-2009 03:17

Thanks. I was asked this on a job interview in front of a hiring board. I said AC would be the better choice for aluminum welding. I wasn't sure what else to say as I've had little experience using AC machines. Here's the question they asked me:

"What are the differences, benefits and drawbacks of using an AC vs DC machine using GTAW?" Huh? I'm just a welder and a damn good one at that. I also passed the CWI test on the first try. I'm no dummy but this question seemed pretty technical for the job I was being interviewed for. The job was pretty basic GTAW welding.

I think it was a weed out question as they already had the person they wanted to hire picked out and had to go through the motions. This is one brutal job market we are in guys.

By Lawrence (*****)

Date 11-29-2009 06:09

I don't think it's a high level question at all...

On the other hand, I think you answered it perfectly in the context of a job interview.

By Metarinka (****)

Date 11-30-2009 16:14

in all my interviews as a welding engineer I don't think I was ever asked a specific technical question like that. maybe they assumed I knew the difference already.

In AC 50% of the energy give or take goes into the tungsten and it tends to ball up much faster. you need to run higher amperage for the same thickness material and high frequency is a must in order to keep the arc established.

in DC about 80% of the heat goes into the work piece and the tungsten will stay sharper. The bead profile is deeper and more narrow, compared to a shallow wider bead for AC. This can be good or bad depending on the goals.
AC also has much more cleaning action as the polarity reverse causes contaminants to be pulled away from the surface. This is the reasoning for using it on aluminum in the first place.

for GTAW it's useful for aluminum and also as a "trick" when welding low melt point metals such as copper to limit the molten puddle temperature for a given current. We would often run copper AC or DC reverse polarity to help lower the temp.

By SWP

Date 12-03-2009 15:24

"for GTAW it's useful for aluminum and also as a "trick" when welding low melt point metals such as copper to limit the molten puddle temperature for a given current. We would often run copper AC or DC reverse polarity to help lower the temp. "

This makes no sense to me. First, copper has such a high thermal conductivity, you need higher current than steels to form a puddle, so unless the cleaning action of AC works on copper and is beneficial, I see no logic in using AC or DCEP on copper. Second, the temperature of the molten puddle is largely limited by the melting point of the material, and using AC or DCEP is not going to change that. If you want less heat input to the puddle, instead of using a "trick" or AC or DCEP, just reduce the DCEN current.

By Metarinka (****)

Date 12-03-2009 17:31

We used it to great affect on very thin copper material in the range of 16 ga and thinner. This was years ago so I'm getting a little rusty. Your right about the thermal conductivity but the copper was preheated (for expansion and fittup reasons) and it's a code requirement so forming the puddle wasn't so bad.

Also remember that running DCEP will ball the electrode up immediately. My guess is that this lowered the areal current density of the welding arc, which gives a much broader low penetration bead. WE found it very helpful. That copper sucks up the heat soo much, by the time you get a puddle form you blow thru and keyhole.