T-welds and undercut question

Date 07-02-2003 21:43

OK, so I know that one of the challenges in doing any T-weld or V-weld (ie. any right angle weld, be it a fillet weld or any other name) of, say, two pieces of flat stock or plate is preventing the molten metal in the vertical piece from "running" down into the pool on the horizontal piece. In other words, preventing "undercut". I guess the suggestion that I've heard most often (and what seems to work best for me doing primarily TIG) is to direct most of the heat to the horizontal piece and less to the vertical piece.

If you used some sort of fixture that put both pieces at a 45 degree angle from horizontal (rather than having one piece at 0 degrees and the other piece at 90 degrees), wouldn't this help? Or do you now have a tendency to undercut both pieces? I realize this would only be useful for smaller parts, but would this not do the trick? (Or at least make it easier?) I haven't actually tried this, but it sure seems like you'd end up with a nice looking and high quality weld on weldments where this orientation is actually possible. Thoughts?

Thx!
Dave

By bmaas1

Date 07-02-2003 21:49

It has always been my feeling that it always easier and better to produce welds in the flat position or -V- position if possible, it also helps eliminate undercut. But this is not always possible.

Brian Maas

By TimGary (****)

Date 07-03-2003 11:26

I have always found that the best way to combat this weld sagging and undercut is to direct the weld mostly toward the vertical piece, not the horizontal. Also the gun or electrode angle in relation to the vertical piece needs to be at or just below 90 degrees. This helps to force the molten puddle to stay in the crater dug by the arc, rather than sagging and undercutting.

Tim

By DaveSisk (**)

Date 07-03-2003 12:27

You must be referring specifically to MIG. I haven't found this to work with TIG.

Thx,
Dave

By TimGary (****)

Date 07-03-2003 12:56

Yes, I guess so...
Most of my expierience is with mig and stick...
Tim :)

By CHGuilford (****)

Date 07-03-2003 14:31

Dave,
To qualify my comments here, I am not a TIG welder (my career path just never went that way yet). So I don't know for sure if a 45 deg angle is best for TIG. But if you're dealing with SMAW, FCAW, or SAW, you'll find it works very well.

Placing your materials at a 45 deg angle gives you a true 1F fillet weld, and grooves are similar. If your heat (amps/volts) is correct for the amount of filler metal being added, you should have very little undercut (due to surface tension of the molten metal, you will nearly always have some undercut but it can be almost too small to realistically measure).

In fact, that is a technique used to increase productivity, whenever we find it practical. The position allows us to turn up the power and lay in a lot of metal while gravity helps shape the weld face. You do have to be sure to stay on the leading edge of the puddle to avoid slag and metal from rolling ahead; that would hinder penetration and promote slag entrapment.
I imagine TIG would be similar, but this is where someone else should jump in and explain the finer points.

Chet Guilford

By JINX

Date 07-03-2003 20:15

Dave,
To produce a undercut free fillet weld in the horizontal (2F) position it is important to have the proper amperage for the thickness of the base material. The GTAW torch angle, travel speed, and technique are essential as well. Please try the following:
a) Torch angle 45 degrees - this will apply an equal amount of heat into the horizontal and vertical member of the T-joint.
b) Grind tungsten electrode to a point with the grinding marks in line with the electrode - a contaminated tungsten will direct the arc force in most cases to the vertical member resulting in uncontrollable undercut. If the electrode becomes contaminated, stop welding grind a new point. As you grind Dave you will be able to see by the sparks when the electrode is clean and ready to continue the weldment.
c) Welding Technique - walking the cup or free handing - which ever you choose, keep the weave motion to a minimum. If you weave to wide, undercut will occur.
d) Adding filler metal to the puddle - dip the filler into the puddle or place the filler rod into the joint and melt into the puddle.
e) Choose the proper diameter ceramic cup - to large for the application will cause difficulty viewing the puddle.
f) Tungsten electrode stickout - the general rule of thumb is to have the electrode out past the face of the ceramic cup no longer than the inside diameter of the cup.
I hope these comments are helpful, and have a great 4th of July weekend! Jinx 67'-68'

By MikeR

Date 07-21-2003 08:29

If you're getting undercut with TIG, you're either too hot or using too small of filler. And from personal experience, you never need to weave using TIG. If you need a bigger fillet for strength, use a different process.