slowster wrote: ↑20 May 2021, 7:15pm
It would be interesting to see what an expert metallurgist or similar would make of those cracks, i.e. what exactly were the causes.
I'm not a metallurgist, but I deal with failures in metal on a regular basis, and it looks like a fatigue failure. The problem with that is that the 'root cause' of such a failure can be a design issue, a quality issue, use outside of design, or abuse. Determining more than that would require having the frame in hand, and the possibility to do some destructive testing. Then, I could determine if the quality of the tubing was suitable, if the welds were correct, etc. The number of failures and from relatively early in life would suggest either quality issues with welding or materials, or abuse.
That said, I have some experience with titanium... (disclaimer: I don't know anything about Enigma or their processes & I am not suggesting that the welds are necessarily a problem)... titanium requires considerably more care to weld than steel because it is what's known as a reactive metal. At room temperature, it reacts with oxygen. The titanium dioxide that forms must be removed prior to welding because it melts at a much higher temperature, which substantially reduces weld integrity. In addition, when heated, titanium reacts more with oxygen, nitrogen, hydrogen and carbon to form oxides that embrittle the weldment (or heat affected zone). Everything within the heat-affected zone (the part that gets hot from welding) must be shielded from the atmosphere until the temperature drops below the temperature at which it is highly reactive.
That means, if a shielding gas such as argon isn't used, or impurities are introduced in the process, the titanium will react with oxygen from the atmosphere. This is the first place I look in examining premature failure issues with welded titanium. It is more difficult to control than welding steel, and in my experience somewhat more likely to produce quality issues.
Repairing titanium is even more likely to produce this sort of failure, and while a factory repair under the same stringent process controls should be of similar quality to the original, a repair in a bike shop is unlikely to be.
edit: NB, that's not to say that titanium isn't a good frame material; it is, after all heavily used in aerospace, but it does require better process controls than steel
edited again for clarity & disclaimer
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