A wise man once told me (about welding), if it looks good, it is good. He was mostly right. If you assume the things you can’t see, such as: the metal is cleaned and prepped properly, the correct filler rod is used, there is adequate penetration and the weld is properly purged; then yes, if it looks good, it is good…
T.I.G (Tungsten Inert Gas) welding is more than just melting tubes together with an electrical arc, it straddles the line between art, science and dexterity. It requires a steady hand, excellent hand-eye coordination and an intimate understanding of metal in it’s liquid state and how it responds to the world around it, so in other words: experience. When I say “liquid in it’s metal state” you have to take a step back and look the world a little differently. Titanium is solid (essentially a frozen liquid) at room temperature. It becomes liquid above a little over 2000 degrees and quickly re-freezes when the heat is taken away. In it’s liquid state titanium is susceptible to absorbing many of the elements in the air around us and it must be protected. If it is exposed to any of these elements the extremely strong titanium becomes an oxide and becomes brittle, like a combination of chalk and glass. This is not good.
To protect the the titanium in its liquid state we use an inert gas (argon) to surround the weld puddle. The argon gas does not bond with the titanium molecules effectively shielding it from the potentially dangerous atmosphere that surrounds it. The torch, pictured above, has a steady stream of argon slowly blowing out of it, surrounding the weld and the tungsten electrode (the sharpened piece of metal protruding out of the cup). This protects the weld on the outside. To protect it on the inside we fill the frame itself with argon. The argon flows through the frame via a system of purge holes that are drilled in it before assembly. We get the argon into the frame through our heatsinks (all bout our heatsinks here). The tungsten electrode must also be clean and sharp. Any contamination on the electrode can cause the electrical arc to wander and results in a loss of control over the weld bead.
The tubing that we use for our titanium bikes is an aerospace grade alloy called 3al/2.5v. That translates to 94% titanium, 3% aluminum and 2.5% vanadium. This is one of the strongest alloys of titanium available in a seamless tube form and is available in a wide array of diameters and wall thicknesses, making it an ideal alloy for building bicycle frames. The filler rod we use to make the weld bead is actually an even stronger alloy called 6al/4v (6% aluminum and 4% vanadium). We use a stronger alloy for the filler metal because when you heat up the titanium to its liquid state and it cools down to room temperature the grain structure changes and it loses some of its strength.
We built-in purging fixtures into all of the jigs we use to tack weld the cut tubes together. Every time a tube is welded it must be purged. Inside and out.
There are a few ways to weld the tubes together. We employ the “double pass” method. There are two stages to this type of welding. The first stage is laying a root pass. The root pass essentially melts the joint together, sometimes with a little bit of the filler rod and sometimes without any rod at all. Pictured above is a root pass.The root pass serves a couple of functions. First, it guarantees that the tubes are fully welded together with the proper penetration. Secondly, it allows more material to be put in the joints that are deeper so that the proper fillet can be achieved in the second pass, essentially laying the foundation for the final dress bead.
If you look closely at a weld bead you will notice that it is composed of little crescent shapes. Each one of those crescent shapes is where a little bit of the filler rod is dabbed into the liquid puddle. The goal is to dab the rod into the weld at even intervals creating a uniform distance between each bead. Another thing to look for is consistency in the width of the weld beads. the edges of the bead should be parallel to each other throughout the length of the weld.
The color should also be shiny, not just because shiny bikes rule, but because the color represents the metals level of exposure to contaminants. The shinier the better. A little bit of yellowing on the finished bead is also acceptable and is just on the surface.
This post is actually a precursor to the release and unveiling of our first titanium bike. The bike that we built is for a very special event, very far away from Boston and should garner a lot of attention. Per customer request, we are not posting any pictures of the complete frame until the unveiling. We should be able to show pictures of the completed frame sometime next week. We are working on a few more frames right now that we will be posting in our gallery soon, so keep an eye out there.
On a side note, I just want to thank everyone that follows our blog. I am an avid tracker of our analytics and notice all of the attention that it has garnered. So if you like posts like these and want more…
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