I like titanium, so much I have two bikes made of it. But a quick search of the net shows there are issues with welding it and greater care is needed.
Here's an extract from one article:
the most widely used grade of titanium alloy, ASTM Grade 5 (Ti-6Al-4V), has a yield strength of 120,000 psi and a density of 282 lb/ft3. In comparison, ASTM A36 steel has a yield strength of 36,000 psi and a density of 487 lb/ft3, while 6061-T6 aluminum has a yield strength of 39,900 psi and density of 169 lb/ft3.
In short,
titanium is about 45 percent lighter than steel, 60 percent heavier than aluminum and more than three times stronger than either of them. While expensive initially, titanium lowers life cycle costs because of its long service life and reduced (or non-existent) maintenance and repair costs. For example, the Navy replaced copper-nickel with titanium for seawater piping systems on its LDP-17 San Antonio Class of ships because it expects titanium to last the entire 40 to 50 year life of the ship.
In addition to military applications, other common uses for this light, strong and corrosion-resistant metal include those for aerospace, marine, chemical plants, process plants, power generation, oil and gas extraction, medical and sports.
Shielding Gas Is Critical
Titanium falls into a family of metals called reactive metals, which means that they have a strong affinity for oxygen.
At room temperature, titanium reacts with oxygen to form titanium dioxide. This passive, impervious coating resists further interaction with the surrounding atmosphere, and it gives titanium its famous corrosion resistance. The oxide layer must be removed prior to welding because it melts at a much higher temperature than the base metal and because the oxide could enter the molten weld pool, create discontinuities and reduce weld integrity.
When heated, titanium becomes highly reactive and readily combines with oxygen, nitrogen, hydrogen and carbon to form oxides (titanium’s famous colors actually come from varying thickness of the oxide layer).
Interstitial absorption of these oxides embrittles the weldment and may render the part useless. For these reasons, all parts of the heat-affected zone (HAZ) must be shielded from the atmosphere until the temperature drops below 800°F (note: experts disagree on the exact temperature, with recommendations ranging from 500°F to 1000°F. Use 800°F as a reasonable median unless procedures, standards or codes indicate otherwise).
One of the most common mistakes when welding titanium is not verifying the many variables that contribute to good shielding gas coverage prior to striking the first arc. Make it a practice to always weld on a test piece before beginning each “real” welding session. To ensure that gas purity meets your requirements, AWS recommends using analytical equipment to measure shielding gas purity prior to welding. Gas purity varies by application.
Typical specifications require that the shielding gas (typically argon) be not less than 99.995 percent purity with not more than 5 to 20 ppm free oxygen and have a dew point better than –50 to –76°F.[/color]
Now I may be a simple soul, but for a bike frame shop to ensure 99.995% purity and atmospheric shielding below 800 degrees farenheit or risk embrittlement of the weld does sound like it's asking a fair bit! - you're not going to do that in the shed in your garden!
Which is why I'm happy to have a lifetime warranty on my frames from a reputable builder and bike shop - thank you very much indeed and goodnight.
