Horale Caballero!
So you're really getting into it!!!
The "thing "you're having a problem with could be two "things".
Before I explain the two things, I have to ask you a question:
Are you spacing the members of the joint with pieces of the same diameter wire you're using to weld? Or are you "butting" the two members together? The reason why I ask is because, if you're butting them together, you have to spend more time on the tack weld in order to get enough penetration.
If you're spacing the joint the way I just mentioned, then you'll get a decent amount of penetration, and actually be able to close the amount of separation at the opposite side of your first tack by depositing your next tack weld 90 degrees from your first tack, then 180 degrees from your second tack, and finally 180 degrees from your first tack!!! ( if this is a critical joint then I recommend a gas purge for the inside (root)of the weld so that it does'nt get contaminated.
so getting back to those two "Things":
One is that you are welding too slow of a tack which would cause excess separation at the opposite side of the tack weld.
Two is that you're welding the correct size tack, and you're still getting separation at the opposite side of the tack weld, where the joint is'nt tacked yet.
Regardless of the two choices, it's normal to have separation at the opposite side of the tack weld because, both members surfaces that make up the joint are being pulled closer together at the location of the tack weld as it cools, and at the opposite side of the joint where it's not tacked, the two members separate from each other. The slower you weld the tack, the more heat input to the joint. This will separate the opposite side of your first tack even more. Pipe clamps help minimize and sometimes even eliminate this!!!
The key to any tube/pipe joint is fit-up!!! If you're off in your fit-up in any way, then it wo'nt matter about how much or how less the heat input you're putting into the joint because, your end result will be distortion, and a noticeable "Kink or Pinch" at the joint!!!
One way to tell if you're spending too much time on your first tack is by looking at the amount of discoloration (Heat Affected Zone) around the tack weld.
Generally speaking, if you see that the combined (from both members) heat affected zone is at least 5 times the area of the tack weld then it could be that you're spending too much time welding the tack. (this is just generally speaking!!!)
If you have access to a pipe clamp then, it will certainly help minimize your problem so much so that you could place your tacks into the joint with the least amount of separation
If your first tack is welded at the six o'clock position in order to close the opposite side (twelve o'clock), then you should deposit a tack at either the nine or three o'clock position of the joint, then finally at the twelve o'clock position of the joint, take off the clamp, and then weld the pipe up in the same sequence as you did the tacks. You should first grind or "Feather" both ends of each tack beforehand in order to achieve good fusion where the rest of the welds meet at the tacks!!!
There are some other details missing that could help in giving you some more suggestions so, I'll ask you some questions:
What is the diameter wire you use?
What size tubing/pipe? Wall thickness? Outside diameter?
How long are the members of the joint?
Is the joint beveled? On each bevel, are they chamfered?
Describe your machine to me (brand name/model)...
What is the voltage set to on your machine?
What's the wire feed speed set to?
In other words, Hometown!!! You need to feed me some more dope on this dillemma you're having holmes.... Ahhhyighttt????
Respectfully,
SSBN727 Run Silent... Run Deep!!!