Welding Right Angles

[Kevin Sevcik]

We've taken to riveting our frames specifically because of this. We're just too easily annoyed by everything going completely wonky after welding. So what I'm about to say is from theory and not actual practice welding robot frames.

Theoretically, the best answer to things warping during welding is more heat, not less. Allow me to explain for the uninitiated. Things warp during welding because you're dumping a lot of heat into a small area. That small area heats up and expands. Or tries to. There's all this cold, unexpanded metal around it that resists it expanding, you see. So it expands as much as it can, pushing against the metal around it, and if the force and stress exerted from this expansion exceeds the yield stress, then it yields. It yields and takes a set in its currently not quite as expanded as temperature requires state. And then it cools and shrinks down to smaller than its original size, on account of it's starting to shrink from a smaller size than it really should have been. There's also the fact that the filler metal itself is going to shrink an awful lot just because it froze at the solidification temperature, and it's going to cool down all the way to room temperature.

You can't completely prevent all this from happening, but you can reduce the effects by preheating your part. The closer the surrounding material is to the temperature of the welded area, the less warping you'll see, thanks to there being less difference in expansion between the hot metal and the cold metal. 6061 anneals at 775F, so you don't want to get that hot or your whole piece will get soft. 300F-400F seems to be the recommended range for a preheat temperature.

[AdamHeard]

It's an interesting topic really.

The more experienced the welder I'm talking to, the more they advocate to me that welding without post welding machining is NOT the best way to do things if you really, really want precision.

Precision is really the key. If you can design such that the warp and such doesn't cause an issue, you're golden. If not, be prepared to take a lot more effort during & after welding to straighten, tweak, thwack, etc. your frame into tolerance.

[MrForbes]

We haven't seen any warping problems in our welded chassis this year. Of course, we cheated by using steel....it's cheap, easily welded by a freshman with little experience and no formal training, and it keeps the bottom of our wide base robot heavy enough that it shouldn't tip over too many times in competition. We also designed it so that it doesn't really have anything critical as far as being in the right place, so even if it did warp some, it doesn't matter.

[kmccorm2000]

If you're welding aluminum it helps to preheat and use strong jigs.

[ratdude747]

for TIG, a trick I was taught once was to put the (pointed) end of the tunsten rod (make sure it is the "green" kind, not orange or red) about .25" INSIDE the porcelain gas shield...

also, for TIG, make sure you are set to AC, not DC if you are welding AL.

For any metal or welding type, be sure to use a wire brush to get the weld areas as oxide-free as possible... having an angle die grinder/drill with a wire brush makes this a lot easier.

Last, for TIG, this is kinda a basic thing, but make sure that you use the correct filler rod. for butt and inner angle joints, the thickness should match the thinner of the two edges being welded. for lap and outer angle joints, no filler rod is (generally) needed; sometimes it helps to "cheat" and use a bit of filler rod to get the puddle started.

[JamesCH95]

Originally Posted by ratdude747

for TIG, a trick I was taught once was to put the (pointed) end of the tunsten rod (make sure it is the "green" kind, not orange or red) about .25" INSIDE the porcelain gas shield...

also, for TIG, make sure you are set to AC, not DC if you are welding AL.

For any metal or welding type, be sure to use a wire brush to get the weld areas as oxide-free as possible... having an angle die grinder/drill with a wire brush makes this a lot easier.

Last, for TIG, this is kinda a basic thing, but make sure that you use the correct filler rod. for butt and inner angle joints, the thickness should match the thinner of the two edges being welded. for lap and outer angle joints, no filler rod is (generally) needed; sometimes it helps to "cheat" and use a bit of filler rod to get the puddle started.

Any benefit to pulling the tungsten into the nozzle? Maybe just to keep a new operator from dunking it in the puddle?

FWIW I prefer thoriated tungsten (Red, also radioactive! avoid unless you have vented grinding equipment!), lanthanated tungsten (orange), or zirconated tungsten (silver), over pure tungsten (green) for welding aluminum. Pure tungsten is a bit old-school and definitely not the best way to do it IMO as someone who earned an AWS welding certification in Al TIG.

Wire brushes, particularly non-stainless steel brushes, have a very nasty habit of making aluminum and other non-ferrous metals very dirty if used in weld prep. One must also ensure that stainless steel brushes are designated for one material only, i.e. aluminum, steel, or titanium, and that they are not used on a different material. I have always preferred abrasive sanding discs for aluminum weld prep because they don't contaminate the base metal, even if they're used on a different material. Though AC welding current (for AL) will break through the oxide layer as it is, and a break-arc technique will clean out even the toughest oxide deposits like anodized surfaces. Just be sure a good degreaser is used BEFORE and AFTER any brushing or abrasive prep.

In thinner sheet metal you could probably get away with just making a fuse joint for lap and outside corner welds. However, you run the risk of under-cutting the base material and causing a weak joint and some nasty stress risers. I would be very careful about specifying the right weld bead for each joint knowing that fuse joints are prone to under-cutting.

[ratdude747]

Originally Posted by JamesCH95

Any benefit to pulling the tungsten into the nozzle? Maybe just to keep a new operator from dunking it in the puddle?

It always worked better for me. my guess is that it may have a "preheating" effect.

Originally Posted by JamesCH95

FWIW I prefer thoriated tungsten (Red, also radioactive! avoid unless you have vented grinding equipment!), lanthanated tungsten (orange), or zirconated tungsten (silver), over pure tungsten (green) for welding aluminum. Pure tungsten is a bit old-school and definitely not the best way to do it IMO as someone who earned an AWS welding certification in Al TIG.

I was always taught that the red and orange were for steel and green was for aluminum... whatever works for you... A lot of this is welder's preference

side note- thorium is not very radioactive... the high school shop where I learned to TIG didn't have vented grinders and we used red rods all the time... hopefully that won't cause problems down the road

Originally Posted by JamesCH95

Wire brushes, particularly non-stainless steel brushes, have a very nasty habit of making aluminum and other non-ferrous metals very dirty if used in weld prep. One must also ensure that stainless steel brushes are designated for one material only, i.e. aluminum, steel, or titanium, and that they are not used on a different material. I have always preferred abrasive sanding discs for aluminum weld prep because they don't contaminate the base metal, even if they're used on a different material. Though AC welding current (for AL) will break through the oxide layer as it is, and a break-arc technique will clean out even the toughest oxide deposits like anodized surfaces. Just be sure a good degreaser is used BEFORE and AFTER any brushing or abrasive prep.

I never did that... we used the same brushes used for everything... and I doubt they were stainless... I know, not good, but on a cheap public school budget...

[JamesCH95]

Originally Posted by ratdude747

It always worked better for me. my guess is that it may have a "preheating" effect.

It would force the arc to be longer, increasing the weld arc's work function energy, making the pool hotter. However, the longer arc is less stable and more heat can be added by turning up weld current, which is better in terms of arc control.

Originally Posted by ratdude747

I was always taught that the red and orange were for steel and green was for aluminum... whatever works for you... A lot of this is welder's preference

It's not just welder preference. The balled end of a pure tungsten electrode is molten and more likely to contaminate the weld with tungsten, particularly if the molten tungsten drips into the weld pool. This a REALLY bad problem if you want to do any post-weld machining, tungsten doesn't machine so well

The ground point on an red/orange/silver electrode control the arc better than the balled end on a pure tungsten electrode, and they can handle more heat/current.

[ratdude747]

Originally Posted by JamesCH95

It would force the arc to be longer, increasing the weld arc's work function energy, making the pool hotter. However, the longer arc is less stable and more heat can be added by turning up weld current, which is better in terms of arc control.



It's not just welder preference. The balled end of a pure tungsten electrode is molten and more likely to contaminate the weld with tungsten, particularly if the molten tungsten drips into the weld pool. This a REALLY bad problem if you want to do any post-weld machining, tungsten doesn't machine so well

The ground point on an red/orange/silver electrode control the arc better than the balled end on a pure tungsten electrode, and they can handle more heat/current.

well, since the shop had a large stock of really old stock electrodes, we used what we had first... all there was was red and green electrodes...

from what I have read green electrodes work well for AC AL TIG... FWIW, green rods are lower cost as well...