Bird's beak/fishmouth weld in a gas forge?

(with the note that I'm not great at forge welding) I don't see why you can't. Forge welding is about heat, scale control, and pressure. If you can control the scale as you get it to close up with a gas forge it should be possible.
 
I have always done it in a gas forge.

For those not familiar with what a "Fish Mouth Weld" is (AKA butterfly weld, bird's beak weld) -
It is when you make a V-shape cut in the end of a billet and then forge weld it shut to make the point of a sword or dagger. It is used when there is a pattern down the center of a damascus billet like feather or when there is a center stripe in a compound sword billet like a Viking sword. To be a perfect tip, the cuts should be two merging arcs., which is the reverse of the tip shape you want to create. I did a paper on it long ago. I'll see if I still have it around.
 
Here is the document. The drawings will not transfer to the forum format, so open the attachment file to see the weld drawings.



Butterfly Weld on a Sword or Dagger Tip​



If you simply forge the tip of a billet with a pattern down the center into a point the tip will not follow the pattern. This is avoided by doing what is called a butterfly weld. It is sometimes called a fish-mouth weld or a bird’s beak weld.



Billets that are in need of this technique are:

Feather pattern damascus and similar centerline patterns

Multi-bar complex/compound billets for Viking swords and daggers with a center segment of reversed twist and other bars and mono-steel edge bars.

Complex twist patterns and double twist “herringbone” billets.



How to do it:

Trace the end of the billet you have made on a piece of stiff paper. Draw the desired tip shape on the tracing. Cut the sketch out and then cut in half straight down the center. Reverse the two halves to make the butterfly/fish mouth. Glue on the billet tip and cut out the open center. File smooth and forge weld the mouth shut.



Tips:

Do the tip weld before forging any other shape to the billet. Leaving the billet a bit thicker than the final thickness is wise.

Make sure the tip is at full welding heat, but be careful of burning the thin points.

To assure a clean and tight weld, forge the mouth almost closed and clean up with a thin file before the final welds.

Work the weld form all sides to assure complete welding.

If it doesn’t come out right, re-cut the mouth and try again.

When the tip is done and checked by a quick grind and etch, forge the rest of the blade.



Billet with center pattern



Desired Tip Shape following pattern. The butterfly weld will look like this.



Butterfly cut to be forge welded​
 

Attachments

  • Butterfly Weld on a Sword or Dagger Tip.pdf
    394.5 KB · Views: 2
Thank you so much, Stacy! This is super helpful. My plan was to forge mostly closed and then soak in pH down overnight to clean up, but it sounds like a file might be simpler. Or maybe both.
 
Stacy, I have learned something new today thanks to you, and the OP for asking the question.
 
Both would be a good idea. The pickle will remove all scale on the weld area. The file makes a clean fresh surface just before the weld. The slight "tooth" left by the file also makes a better weld.

I heat to red, flux, heat the yellow, flux, heat again to bright-yellow/white, brush off quickly, and weld. Keeping it always above 1800°F is the trick to getting a clean weld and no trapped flux or scale.
Here is some science that most folks don't get taught about forging steel.
Steel welds at around 70-75% of the melting temperature. For our blade steels, that is a range of 1900-2100°F.
Scale forms on cooling from around 1900°F to 1600°F. Keeping the metal above those temps will allow continuous welding with only brushing the surfaces with a steel welding brush. Flux can be almost eliminated after initial fluxing by brushing the steel before and after every heat. A small sprinkle of new flux and brushing before a weld is OK as long as the steel is always kept bright red to yellow.

Most people weld too cold. Welding in high carbon steels is best at 2100°F, or white hot. As the carbon content goes down, the welding temperature goes down, too. 1084 and lower carbon steel welds around 2000°F. If it is any color of red, it is not ready to weld. The thicker the billet, the longer you need to heat it. While the surface may be 2000°, the center may only have reached 1800°. Allowing time for te billet to fully soak is the key to perfect one-heat welding. A couple additional heats to assure a solid weld is always best.

Metallurgically, forge welding is called Fire Welding. It is a solid-state process of welding, unlike arc/mig/tig/gas welding which are a liquid-state process. However, forge welding is actually liquid-state welding at the microscopic (grain boundary) level. Steel starts to melt around 400 degrees higher than the forge welding point, about 2500-2600°F. When you strike the 2100°F steel with a hammer it adds energy to the steel. This energy is transformed to the extra 400-500° heat need to melt the surfaces touching, raising the temperature at the joint to the melting point and allowing the two surfaces to flow together. The harder you hit the more energy is added and the more the steel melts ... so, too hard of a blow in the high heat range can squirt molten steel or cause mushing apart of the metal. Start with gentler blows and increase the force as the billet cools in the heat. Once fully fused, the billet can be worked in the 1800-2000°F range as hard as needed to shape and draw it out into a bar or blade.
Forging cu-mai has the same problem with the copper squirting out if you forge too hot. The copper melts at a much lower temperature than the steel. It is almost to its melting point at red heat. Add a heavy hammer blow and it becomes a liquid.

If you want to see this happen try these experiments:
First experiment - take a 1/4" thick bar of steel and heat it to red. Hit it hard with a forging hammer. The spot where the blow happens will become much brighter red.
Next experiment - make an alternating stack of three modern quarters (a copper alloy) and four steel washers about the same size. Use your mig/tig welder to tack them together two or three places on the sides.
WEAR A HEAVY LEATHER APRON, GLOVES, AND HAVING NO ONE CLOSE BY for the following hot part. Make sure nothing flammable is close, either.
Heat to red and smack hard with a hammer. You will see molten copper/nickel squirt out from between the washers. You may have to heat a little higher if it doesn't work the first time, as red heat looks different to different people.
Knowing what is happening in a weld is how to control the process.
 
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