Fillet Brazed Titanium ?

[ferrus]

Any reason why this couldn't be done?

Presumably it would be a lot easier and cheaper than welding, as any steel frame builder who fillets' could do it.
Aside from any difficulties with mitering or bending Ti, couldn't this be useful for repairs etc? (I quite fancy a custom brazed bar-stem combo)

 

[Sundayjumper]

It needs to be scrupulously clean (medical grade cleanliness), and in a vacuum. Not very practical for small volume / low budget production.

A couple of £k for a bike frame is very low budget compared to the aerospace applications

 

[Elysarian]

From http://www.azom.com/article.aspx?Articl ... 9#_Brazing :

Brazing

Brazing operations are limited by the chemical and metallurgical properties of titanium since filler materials tend to alloy with and attack the base metal forming brittle intermetallic compounds. Alloys of aluminium or silver have been used on occasions but the technique is not widespread with titanium. For torch brazing, it is essential to use a reactive flux containing, for example, a barium chloride-sodium chloride-lithium fluoride-zinc chloride mixture. Furnace brazing in either an inert gas or in vacuum can produce better joints by operating at higher temperature for longer periods and using a thin film of copper or cupro-nickel which melts and diffuses into the titanium to give a relatively ductile and strong titanium rich joint. With induction brazing, on the other hand, time at high temperature can be kept very short, resulting in less contamination, less alloying and, consequently, improved ductility.

 

[Chopper the ex Copper]

Even if it were feasible, why would you go to the efforts of building a shoe lightweight titanium frame only to then add unnecessary weight by bronze welding it?

 

[ferrus]

Thank's for the replies, that's a bit disappointing news.
Peculiar stuff titanium in some ways, it's certainly sensitive to contamination when it comes to joining - almost ironic when you consider the stuck seatposts, BB's and headsets where it can't wait to fuse!

Incidentally does any one have any idea how the Ti dropouts on DynaTech MT4/Ogre's are joined to the steel seat stays?? it's bugged me for years!

 

[ferrus]

Even if it were feasible, why would you go to the efforts of building a shoe lightweight titanium frame only to then add unnecessary weight by bronze welding it?

well if it was do-able, any old steel builder could have a bash, so it could've been a convenient alternative.
As for the weight issue, I doubt small fillets weigh much at all, and Ti frames are so light there's plenty of margin. Personally I'm not that fussed about weight, it's more about its ride-quality, durability (thick gauge tubing) and appearance.

 

[hamster]

I think the issue is the stable oxide layer that forms on the surface, like aluminium. You have to remove it to get the braze to stick. Steel is sufficiently inert that you can clean up the end with wire wool and flux. However, Ti and Alu react intensely with air so it's not possible without an inert atmosphere. It can be plated with something that will wet out for brazing in air.

Even for welding an inert atmosphere is necessary - in Russia and China it's with blokes in chambers with aqualungs, elsewhere a safer approach is used with inert chambers.

 

[ferrus]

That's interesting hamster. The hope was that the lower temperature of brazing wouldn't contaminate the interface and require purging. It's surprising to learn that Ti oxidises so readily (without heat?), I was under the impression that it was distinctly inert, hence it's usage in marine and chemical environments as well as joint replacements and body implants..?
I've heard mention of surface oxidation before, but again it seemed to contradict the notion of Ti being non-reactive, is this the blueish tarnish you see on polished frames that have been a bit neglected?

 

[hamster]

Ti is inert but only because of its tightly bonded oxide layer. Ti with dry chlorine is extremely dangerous for example, similarly with liquid oxygen - fine until the Ti gets scratched, then all hell breaks loose!

 

[ferrus]

is it important to retain the oxide layer then, should I resist the Xizang's seductive pleas for a good buffing!