"keyhole" shape?

Michael,

I think of it as the welding version of the vortex the water makes as it goes down the drain in a bath tub. That's my "clean and wholesome" visualization of it. But it also reminds me of a certain part of female anatomy....

As you move along it moves along as the trailing weld bead winks shut.

It is the harbinger of "complete penetration" .

Vernon

The term keyhole applies to the circular hole that precedes your puddle. I have only seen it with stick and TIG, unsure about other processes. It readily appears with V joint preparation or similar (corner joint). It has nothing to do with the puddle, except that when it is visible, you know you have 100% penetration. If you turn the stock over, you can see a small bead on the backside.

Where does the "hole" appear? Spread between both pieces that one is trying to weld? It seems to me I get this doing a lap joint on aluminum, but the keyhole appears only on the top piece... I figured it was a sign that I was getting too much heat on the top piece.

Peter

You are correct, the hole is evenly spread across both pieces. The heat from the arc is melting both roots, surface tension is drawing the liquid metal (from electrode and root) back into the puddle. I don't believe keyhole welding applies to anything but butt and corner welds.

IMO, the best way to experience keyhole welding is to take two 3/8" (fairly thick) plates and tack them into a corner joint. A natural V will be formed on the outside. Use a light-coating electrode like

6010/6011 and normal heat (95 amps or so). Stick the electrode down into the groove, almost touching the sides of the groove. Use a reasonbly slow travel. The electrode will almost immediately burn through the roots and form the keyhole. The puddle will form right behind it. Don't try to go too fast, the thick plate will absorb any tendency for the keyhole to get away from you. After you get going, you can stick the end of the electrode down, flush with the the keyhole.

You will have less success with heavy-coating electrode like 6013/7014. The slag tends to prevent the keyhole from forming.

Keyhole welding is a way to guarantee a full penetration weld by punching either the arc or flame right through the material.

It is a common way to weld sheet metal or pipe with a gas torch. It is also used in pipe welding for the root pass of an open-root pipe weld, and can be done with TIG or Stick.

The one thing they all have in common is that you can SEE all the way through the weld joint as you progress. The "puddle" is the molten metal that surrounds the hole. As the arc, or flame, progresses down the seam the metal flows around the hole and fuses together behind.

It is a very aggressive way to weld and requires excellent technique in any process.

On sheet metal it works very well because the weld is made evenly on both sides of the metal so there is less warpage.

So how does one cause the puddle to flow together after the arc passes, as opposed to blowing through the metal? Any guides to the technique on the web somewhere?

Peter

Hi everyone

In keyholing you place yourself in the hands of the control of surface tension. Surface tension is the dangling bonds of the atoms at the surface kind-of "linking arms" for company and pulling a bit harder than the bonds in volume. That pull tries to draw a liquid to a smallest surface area. That's why a free-falling liquid droplet tends to be sperical. And why a thin flow of water from a tap is so close to cylindrical. In the case of keyholing it draws metal back to fill the weld root gap.

Placing trust in surface tension is a good trust. You run at a rate which keeps the keyhole well-formed, with nature and physics being the higher controlling authority. Surface tension draws the metal to the back of the weld, making a full-penetration (ie. full-height) joint. There is filler metal coming off the welding rod to make up the volume of the root gap and the extra small amount of penetration bead. So if you were out-of-sync: too slow and the keyhole would choke up with liquid metal (?); too fast and the keyhole would become big and malformed, thirsting for liquid metal. Keyholing is a lovely to technique to get the hang of and is a big sense of satisfaction in progression of skills having mastered smooth fillet runs. Well I think so :-)

Richard Smith

Just as I sometimes remind my wife how, back in 1988, I washed the dishes without being asked, I fondly remember how, about three months ago, while gas welding up a "cube" as a welding class project, I produced not just one or two, but TWELVE perfect welds. For each of these, I found that magic "keyhole" groove.

That's now 13 times I've welded "perfect" beads. Twelve with gas. One with a Maxstar 140 inverter machine cranked to full power burning 1/8"

7018.

But generally, my welds look like recycled refried beans.

V

I think "keyhole" refers to the shape of an old door set keyhole. To me, it refers to the proper shape of the opening when making the first pass on an open V root.

Steve

Steve's description is nice, I'd certainly agree. The shape of a mortice lock key - with a cylindrical shaft and a flat "plate" coming off to one side. With a key this is the "presence of metal" shape. When keyholing the root, the shape is in the opposite sense of being the "absence of metal" shape. The "plate of the key" is off to the side you are going in as you weld. The trailing side is closed off by the cyclindrical "top" surface of the "shaft of the key" hole through the root.

When it is said, "keeping your keyhole open" what is mean tis keeping the cyclindrical hole through the root well-formed. If you are keyholing you are *guaranteed* penetration and that the penetration bead will be a nice low rise off the plate on the backface.

There is a sound of the arc blowing through the keyhole which confirms all is well.

Anyone care to confirm or refute this point - the arc blowing through the back of the weld into clear space carrying vapourised metal - as it cools the metal vapour will condense as an oxide smoke which is why cellulosic root-running makes a lot of "smoke"?

Richard Smith

You also see key hole when gas welding butt joint.