Technical Supplement: Welding Techniques
Proper welding technique is one of the most important steps in the construction of a titanium frame, since titanium has a tendency to alloy with anything it touches. Fortunately, this is only a problem when it is in its molten state during the welding process (over 1700 degrees C).
Oxygen, nitrogen, and hydrocarbons are the three most damaging elements to a molten titanium weld bead. If any of these substances enters and mixes with a titanium weld bead, the fatigue life and structural integrity of the joint is greatly compromised.
Therefore, Seven Cycles uses a proprietary TIG welding process to build our titanium frames. Specially modified TIG welding torches, lenses, and cups ensure that the welded joint has the highest possible level of strength and purity. We have also specially modified our welding machines to optimize weld strength in all of our frames.
Since even fingerprint oils (hydrocarbons) will dramatically degrade a weld joint's fatigue life, we prepare and clean each tube prior to welding. Our machinists wear cotton gloves during the welding preparation stages, even before any welding is performed, and welders also wear cotton gloves while welding and handling any frame to keep the tubes from becoming contaminated.
To keep airborne contaminants out of the weld area during welding, we bathe both the inside and outside of the frame's joints in a completely inert gas, thereby protecting the molten metal from the high levels of nitrogen and oxygen that naturally occur in the environment. A positive pressure purge system creates a purely inert atmosphere inside the frame. Outside, the modified TIG torch produces a bath of inert gas to shield the bead from possible contamination. The inert gas maintains the titanium's purity, and does not react with the metal.
Individuals who weld titanium must have very steady hands to avoid both flaws and contamination in the weld zone. Even something as simple as inappropriate weld wire movement can actually create vortices that stir up nitrogen and oxygen and allow them into the molten TIG bead, thereby compromising the joint's strength.
To minimize heat distortion and optimize bead penetration, Seven uses various weld wire sizes and bead sizes, depending on the joint and tube selection. Heat distortion must be minimized because it can create problems with frame alignment that affect the bike's handling and tracking during riding. Nonetheless, we still perform five frame alignment checks throughout the welding process to maintain precise alignment. If a slight alignment adjustment is necessary, the welding steps can be modified to bring the frame back into alignment.
To work with Origin™ tube sets, Seven's welders must successfully pass an in-house certification process that typically takes more than four months to complete. Steel frames are also protected from contamination by using the same positive pressure purge system used on our titanium frames. Dispersing atmosphere while welding steel frames is not necessary; however, removing potential contaminates allows for the strongest welds possible.
While there are several methods of joining steel frame tubing, all of Seven's Origin™ tube sets are assembled using a TIG welding technique that is tailored specifically to Seven's requirements. Seven prefers a TIG welded joint over the more common lugged design for two primary reasons:
Fine-tuning of the frame's ride characteristics can be tailored to a greater degree without the tube diameter size limits imposed by lugged construction.
A frame can be customized without geometry constraints. Lugs are available in limited sizes, or must be modified to accommodate even slight variations from typical geometry.
TIG welding also creates the lightest joints possible, and, with the use of special weld wire, produces joints that are very ductile and extremely strong. The result is excellent service life for our steel frames.
For enhanced strength, Seven silver-brazes all of the small parts—cable stops, shifter guides, water bottle bosses—on our steel frames. Silver brazing is the strongest type of brazing possible. Although silver is much more expensive than brass, which is typically used, it requires less heat. Therefore, the strength of the base metal is maintained. Silver also flows better than brass, so the likelihood of voids in the brazed area is reduced, thereby maximizing joint strength.