Welding Thin Magnesium on DCEP?

Date 12-24-2011 02:36 Edited 12-24-2011 02:42

I've been trying to get a decent looking weld on some .062 Magnesium with GTAW and EZ33 filler wire. I've been searching the forums for techniques and settings info for the procedure. I've had decent results on the thicker materials using AC High Freq. using pretty much the same settings as I would use on alluminum. The .062 thin material is giving me a bit of trouble!
I keep getting an etched looking edge on both sides of the weld bead no matter where I set the balance. The cathodic cleaning action of the arc wants to come off of the sides of the tungsten instead of the tip. and the balance settings don't make a difference at this low of an amperage. I am using 2% thoriated 1/16" with a truncated tip and linear grind and running about 60 amps. with straight argon. Everything is ground clean with a carbide rotary file and wiped with alcohol.
This morning I read that on thinner Magnesium , It could be welded with DC Straight polarity. I tried a couple of practice beads and got a clean looking bead .Much better appearance and no etched edges ! It is a very cloudy looking puddle and leaves a smoky looking finish that brushes off pretty easy. I did notice a transverse crack in one place where I stopped a little fast without sloping down slowly. I was wanting to get some opinions on what you guys with Mag experience think?
I read that an Argon Helium mix would make a difference, But I don't have a bottle of that yet. I was also wondering if Pure Tungsten would help or make any difference? I need to weld a test coupon on this material in the next week that will be X-Rayed. It is a groove joint and I am going to use a piece of Brass flat bar clamped on the backside for a heat sink. Would I be better off with a purge on the backside?
Thanks for any insight you can give me on the subject.

By Lawrence (*****)

Date 12-24-2011 05:42 Edited 12-24-2011 05:59

I'm confused

If you are doing a performance qualification test, how can you pick and choose polarity and shield gas? Won't your test include a WPS that you will need to follow? Your tag says your an aerospace welder so I'll assume AWS D17.1, where I know every welder performance qualification must be done to class "A" standards, no matter what the production work might be, and that means the welder doing the test must be provided a written WPS that itself has been qualified by even more extensive testing.

Technically speaking:

A 50/50 mix of He/Ar is good for DCEP welding of magnesium... But for a groove weld I don't think it would be a good choice. The benefit of DCEP for Mag is that it provides a wide and shallow weld pool, which is not always a great thing when you are trying to make a full penetration groove weld from one side. AC on the other hand, provides a more columner arc, which in turn tends to penetrate deeply and directionally, which is a benefit when welding groove welds in sheet.

Zirconium, Cerium or Lanthanium are better than pure for DCEP.. Thorium is a really bad choice for this polarity (especially under X-ray) because DCEP puts 70% of the arc energy at the tip of the electrode and only 30% on the work.. This means that you will need a larger sized electrode and the end will still ball up.. When thorium balls up you *MUST* assume that part of your electrode is transferring across the arc and will bust an X-ray... If the tip of the "thoriated" electrode is changing shape at all while you are welding, it's a bad thing. If I were taking the test and had a choice it would be Zirconium.

The copper/brass backup/heat sink is fine #ONLY# if you leave a recessed area directly below the groove in the coupon as there absolutly must be a space left for reeforcement on the back side of the weld... A heat sink placed directly in contact with the back of the groove will freeze the puddle and make full penetration next to impossible. Also keep in mind that typically the reenforcement on the back side of a magnesium groove weld will be even larger than that of Aluminum, which is expected to be 3X of steel.

Generally purge is not required,,,, But again... This should be covered in the Welding Procedure Specification (WPS)

In my experience with DCEP Mag welding (which is considerable) it's typically used for buildups on castings, where lugs have broken off or a bore needs to be repaired. The DCEP minimizes heat input and penetration often for the purpose of minimizing distortion on highly restrained weldments, or to minimize temperature differences on highly preheated castings with complex geometries. It can only be done in the flat position as the tungsten will be too large and soft at the end for even a horizontal weld that's more than an inch or two. I would expect that a 3/32" or even larger tungsten electrode would be best for DCEP even on .062 magnesium. Water cooled water cooled water cooled torch!

I'm not sure why you are worried about the Cathodic etch or "cleaning action" anyhow. If you don't have a procedure that calls out shield gas and polarity how can the inspection criteria be so strict as to disallow any etching at the toes of the weld?

Edit: Don't worry about the black "smut" on the DCEP welds.. That is also typical and simply may be removed post weld with a stainless steel wire brush.

By SEAPLANE (*)

Date 12-24-2011 14:54 Edited 12-24-2011 18:28

Thanks Lawrence,My Post title said DCEP, But I meant to say DCEN or Straight Polarity--Sorry for the typo! I was hoping you would reply ! The code is D17.1 as you stated. We have an SWPS for alluminum and will be getting one for the Mag if we can find one before we go into production . If not, we are going to have an independant CWI work with us on getting the weld procedure. At this point, I am just trying to get a heads up on the weld. When welding AC Hi Freq, The toes of the weld aren't just white, like alluminum looks, they are almost etched to the point of looking like a very fine porosity out to about 1/4" from the acual weld. It doesn't matter where the balance is set at.
When I changed over to DCEN--Straight polarity, I get the best results on the thinner stuff. I was playing with some thick housing parts with a 300 degree preheat. and I do not get the heavily etched edges that the lower amperage thin material receives on the AC settings.
Thanks for your time Lawrence, I've been reading your posts for years and you have helped me so many times without my asking and without your charging a penny for your time and knowledge!