American Welding Society Forum
Have been welding a lot of aluminum lately, and have been keeping my metal as clean as possible.
I still get a film, almost a group of tiny black flakes that will follow the weld pool. I am not getting any porosity at all not even a bit.
What doesnt my AC High frequency clean that? and what exactly is the high frequency cleaning? Im looking for more a science explanation, not "it cleans off the stuff on top"
Hello Jordan, the hi-frequency does "NOT" provide cleaning in the welding area, it is simply a carrier for the current upon start-up and, in the case of AC, to provide a bridge for the transition from DC- to DC+ when the current hit's it's zero point in the waveform.
The cleaning action in the arc comes from the DC+ side of the current and is what breaks up and dispells the oxides that are present. With the GTAW process some techologies have a current balance control to allow for more or less cleaning(balance to the cleaning side:DC+, with the balance to the DC- side: more penetration), some of the advanced process machines also have a hertz control that can somewhat increase or decrease cleaning to some degree due to being able to go from the traditional 60 hz current switching to say a 20 hz switching speed which essentially allows the arc to spend more time on DC+ for each switching phase. The hertz adjustment also "focuses" the arc and provides for a wider, shallower arc column(at lower hertz), or a narrower and more penetrating arc for a smaller focus and better penetration(at higher hertz settings) with AC current.
GMAW welding of aluminum also utilizes the DC+ cleaning action and is what allows it to work as well as it does. With the GMAW process a push progression is generally recommended to capitalize on the characteristic of DC+ polarity. I am sure that others will chime in with more and likely more concise information. Best regards, Allan
What am I seeing in the weld that is "cleaning" in the DC+ side of this?
Yes, as Al added in a fine technical manner, it is the reason that the oxides are able to be cleaned from the welding surface in the area of the weld pool as welding is progressing. Best regards, Allan
The AC "spalls" off flake of aluminum oxide during 1/2 the AC cycle. As Allen says, the high frequency makes starting and maintaining the arc easier.
Aluminum oxide forms immediate upon exposure to the oxygen in air. 90% of the maximum oxide layer thickness forms within the first 24 hours of exposure. So, even with cleaning the metal just before welding, it is oxidizing. As the aluminum is heated from the welding operation, the rate of oxidation increases as does the thickness the oxide layer. The oxide has a melting temperature that is approximately 3 times higher than the pure aluminum. This adds to the complications of welding aluminum because the oxide doesn't melt at the same temperature as the aluminum, but instead stay solid. It has the same approximate density as the aluminum, so it does not float to the surface as some oxides do.
If you weld aluminum with DCEN, you will see the "halo" from the cathodic cleaning disappear. Weld quality can be compromised if cleaning isn't performed immediately before welding to remove the heavy oxides present.
Best regards - Al
Is the "AC" waveform on your power supply adjustable? It might help to get more time on the DCEP portion of the wave.
I have seen tiny black oxide "dots" in aluminum welds before. My suspicion is that they are not that uncommon.
When we were welding aluminum aerospace parts we used to keep the aluminum parts stored in argon gas filled dry boxes. Even if aluminum is highly polished with the oxide removed, the oxide may return within 24 hours or less of cleaning to bright and shiny if it is stored in the open atmosphere.
I think that even if the DECP "breaks up" the oxide particles they are still prone to floating on the furface of the weld puddle - hence the black dots.
Black dots and grey smudgy stuff "floating" on top of an aluminum weld pool is a sign of wrongness
Two typical causes
1. AC balence control favoring DC- too much... Set your machine for about 55% DCEP if you have an inverter or about 7 on your balence control if you have a transformer...
2. contaminated gas
You say your prep is good so the contamination is either oxide that isn't being removed or oxide that is being introduced via poor shielding or bad gas.
Thank you all for your input on the subject. Tomorrow Ill be making some changes to see what comes up of it. One of the gas lens's had some crud in the screen and that was killing it for one of the machines...
Now if I am getting a goopy blackness when i finish off corners, is that because the gas flow around a corner is pulling in oxygen? So in order it clear it up i would have to wrap my torch perpendicular to the weld line to avoid it? Because sometimes I think it just starts into that mess.
Maybe Al can help with terminology here haha "goopy blackness" when the state of the aluminum is at a point when when I dip the rod and it pollutes, I can flick the rod while still under the hood and it will clear the garbage off the end.
Im just trying to find a consistant way to prep for large aluminum projects. My aluminum welds are great, dimes all day. But we have over 140 feet of welds this next week and here and there those oxides or halogens pop up and it makes me want to punch something. Frustrating.
Anyways Thanks ALL YOU
Jordan, Lawrence also included a number of important points, I would add that filler wire cleanliness and mastery is an important component of the "big" picture to successful aluminum tig welding. If the filler wires aren't oxide free/clean you can experience contamination. Additionally, the method of adding filler can be a contributor or detractor to successful bead work. If you are removing the end of the filler from the protective gas-shield during the welding progression(band leader method) you will introduce comtaminants into the weld zone due to the exposure of the molten/hot end of the filler wire to the atmosphere and the resulting oxidation caused by that exposure. Best regards, Allan
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