Leonard's Blog: Day 2

Short summary-

Today the first half of the morning was spent on a demonstration of a Gruelon weld, followed by a long lecture on frame geometry. The rest of the day was spent prepping and practice welding T joints. Later in the day I went for a run around the town of Ashland, and spent the evening trying to figure out final bicycle design that was to be drawn tomorrow.

To create a T joint, pre-mitered tubes were first deburred using various files and other tools. A hole in the non-mitered tube was then drilled to allow the flow of argon between tubes. Tubes were then prepped ("sanded" with scotch-brite, than cleaned with acetone to give that shiny Ti finish and eliminate contaminates). Tubes were then set in a small jig, and were "tacked" on both sides which held the tubes in place without the jig. Jig was removed and tubes were welded in small 1 inch increments.

My first T joint:

My second T joint-

Third T joint- (done Wednesday)

Fourth T joint- (done Thursday)

Its no Moots, but its getting closer to looking like those Ti weld bike porn shots...

You'll have to read further in the Long Technical Section to understand what's at work here...

Tomorrow I'll cover bicycle geometry

Long Technical Section- Welding Ti

To build my cyclocross frame, I will be using Gas Tungsten Arc Welding (GTAW), also commonly referred to as Tungsten inert gas (TIG) welding. TIG welding produces an electric arc maintained between a nonconsumable tunsten electrode and the part to be welded. The head affected zone, the molten metal and the tungsten electrode are all shielded from atmospheric contamination by a blacket of inert gas fed through the TIG welder's torch. For titanium and steel you weld with Direct Current (DC), which produces energy that is conducted across the arc through a column of highly ionized gas and metal vapors known as plasma. For aluminum frame construction they use Alternating Current. TIG torches for our frame construction will reach up to 3,000 degrees Fahrenheit, and the Titanium will reharden into a solid state at around 800 degrees Fahrenheit.

Lugged frame construction (used in steel bikes) involves brazing together two steel tubes that are joined by either brass or silver filler metal. The lug greatly increases the strength of the joint by distributing the molten filler material over a larger surface area via capillary action.

TIG welding differs from lugged frame construction in that the tubes are heated to the point that they melt and bond together. Sometimes filler wire of the same material is used to increase the size of the molten pool of liquid to form the joint. It is more complicated than lugged frame construction, as theres nowhere to hide mistakes, and the welding action itself is on display for all to see. It doesnt take much to burn a hole straight through the Titanium and takes a real artist to create a beautiful looking weld.

Beautiful welds are not any stronger than the ugly ones we have been making. However, 2 things do strongly matter when considering the strength of a welded joint. The biggest one is contamination. Anything other than molten Ti in your weld is creating weakness, so every effort is made to eliminate everything around the weld. Tubes are carefully scotch- brited and cleaned with acetone before welding to remove dirt and oils on the tube service. During the weld and for a few seconds after, argon gas is focused around the weld in the cone shaped torch to create a virtual no oxygen/CO2/nitrogen environment that could contaminate the weld. Coloration on a weld shows the infiltration of oxygen and other atmospheric gases as the weld cools down below temperatures of 800.

(Photo: my first T joint, with a nice chunk melted away from the mitered tube, contamination of all different levels just about everywhere, and an inconsistent weld that sometimes runs hot, sometimes too cold all around...yikes)

Not only are you conscious of atmospheric gases outside the weld, but consideration for inside the tubing must also be taken into consideration. An argon gas line (called purge) are inserted into the inside of the bicycle frame to purge oxygen from the inside of the frame, which could also cause contamination.

(Quick digression- Tungsten tips are used for welding because they have the highest melting point temperature among the common metals. Argon gas is used because it is a noble gas, so it doesn't like to react with anything, and is relatively safe).

The other consideration with welding is welding temperature. Welding too hot can easily burn a hole through the tubing and can weaken the surrounding joint. Welding too cold is even more dangerous, as it may not fully melt the Ti tubes and creates a weak joint that is characterized by filler material that seems to bead.

The welding is set to a pulsing mode that concentrates large amounts of heat and energy for a short period of time that is immediately followed by a low current that allows the material to cool while still maintaining the electric arc. Moving your welding torch evenly across a weld creates a nice even look, while moving your hand more incrementally to the beat of the welder creates the appearance of a stack of coins. The differences are mostly cosmetic and the appearance of the weld can be modified by playing with how much time per second is spent at high energy, and how much cooling time is given.

So your playing with 3,000 degrees Fahrenheit torches, needing to hold the torch millimeters away from the material in order to minimize contamination, and a half second is the difference between burning through the tubeset and a cold weld. Now imagine doing all of this in the near complete darkness of a welder's helmet. Because welding is done in such bright light, special visors must be worn that bring everything except the weld into complete darkness. Its difficult to set filler material along two tubes when you can't even see them.

(Picture: Andy, a student in my class welding a Gruelon joint)