pdlpsher1 wrote: ↑Fri May 10, 2019 10:02 pm
mattr wrote: ↑Fri May 10, 2019 8:27 pm
Bit like 753 used to be BITD. Too hot, too cold, too dirty, too long, too short. Then you get a crack.
Yeah I remember the 753. I have a friend who had a 753 bike and that bike was oh so light with tubs. One has to be certified by Reynolds to braze 753 tubes. I don't think the 753 ever caught on. Great tubing but it wasn't intended for mass production.
Yes Ti requires a lot of care during welding. Generally a thicker tubing will be more tolerent of any welding defects. When you build an ultralight Ti frame then you start to see problems. There are also two types of Ti welds- single and double pass. Double pass requires less heat but it's more time-consuming. Single pass uses more current so the heat-affected zone is greater. The first pass of a double pass weld is called the fusion weld, where no filler material is used. Then a second pass called a 'dress pass' is done using a filler rod. If you are using a thicker tubing then single pass is OK. But most importantly the welding area has to be extremely clean and contaminant free whether it's single or double pass weld. When it's properly designed and manufactured Ti is very durable. But just like any material it can fail when it's not properly designed and manufactured.
All the above is true. But as many probably already know, the problem isn't just the skill of the welder, nor impurites (natural oils, lubricants, dirt, etc) on the surface of the titanium.
It's the fact that titanium itself reacts to atmospheric elements, forming a natural layer of protective oxidization when exposed to oxygen, the reason for its corrosion resistance. However, this layer needs to be removed before welding begins, because it melts at a higher temperature and can contaminate the weld. Once welding begins, and the heat-affected zone spreads rapidly, titanium will feed on any oxygen and hydrogen present, further weakening the integrity of the weld. A skilled welder can mitigate this using sheilding gas coverage (usually a 75/25 mix of argon and helium), but not be sure of eliminating it entirely. In other words, the quality of the weld is not determined solely on single/double pass and avoidance of over-heating, but ultimately, what chemical elements are present in the weld.
The only way to completely mitigate atmospheric contamination is to weld titanium in an inert gas (argon) environment within a hyperbaric chamber. Ensuing no oxygen is present during the heating and cooling process means atmospheric contamination is mitigated, allowing the gasses already present in the metal to be released and purged.
I know both Bertoletti and Nevi weld titanium this way. I would never personally purchase a titanium frame, especially a custom one, not welded under hyperbaric conditions.
Best,
853guy
There was a spate of cracked lynskeys a few years ago (2014/15 i think) then another batch about 8-9 years ago as well.
