Postby Brucey » 1 Feb 2015, 7:42pm
A few notes regarding Ti and welding of Ti:
Of the many different Ti grades and many different methods of welding Titanium, pretty much all bike frames are made via TIG welding using a mixture of CP (commercial purity) Ti and 3Al,2.5V Ti alloys. A few have 6Al,4V Ti alloy dropouts but 6,4 Ti tubing per se is rare and expensive.
Because of the cold war, and differences in Ti ore composition as well as available alloying elements, the Russians developed a whole range of Ti alloys that work in a different way to 'western' grades and some Russian-made frames are made in these unusual Ti grades to this very day. They are alloyed with different elements that similarly variously strengthen and stabilise the different phases, thus allowing both single phase and duplex grades. This has interesting implications in that if such a frame is weld-repaired in the west, chance are that no-one will know of or be able to source the correct filler wire. Under these circumstances (or with unknown Ti grades in use) using CP filler is probably safest because if you get the filler material wrong you can end up with a witches brew that might well be no good to man nor beast as it were.
Originally the most common 3Al,2.5V grade was manufactured by taking 'new' Ti sponge and mixing it with Ti scrap (which for a long while would have contained a lot of 6Al,4V material). Most Ti sold today will contain a proportion of scrap at least and some is likely to be mostly recycled scrap. Reports of 'bike frames made from recycled submarines' might be literally true; the Russian navy did build some subs using Ti for the pressure hull, which must have been a fantastically expensive thing to do.
Weld penetration in TIG welded Ti is very often less than you might expect. The reason for this is that heat is transported around the weldpool by convection. You might expect this convection to be the same in every case but it varies with the chemistry of the weldpool. Weldpools that convect 'normally' (i.e. rising in the centre) are in fact not normal per se in many materials; in most welded steels there is a phenomenon often known as 'Maragoni convection' (in which the surface tension varies strongly with temperature) which drives the convection in the reverse direction, increasing weld penetration in the weld centre. In addition there are other effects that can constrict the arc or flare it out.
All these things usually work against you in TIG welding of Ti. If you give a skilled TIG weld operator (who is used to steel) a piece of Ti and ask him to weld it, he'll produce consistently underpenetrating welds for a considerable length of time. In steel, if you take thin sheets and butt them together, you can make a fully penetrating weld that is only about twice as wide as it is deep. In many Ti grades, four times wider than the depth would be closer to it. If you want to make a narrow weld in Ti using TIG, you need to have a well-made V prep and then to weld it consistently well, or aim to make a good sized fillet. Even then it is very much the case that lack of fusion defects in the weld overlap regions are likely to be commonplace unless steps are taken to avoid them.
Keezx has asked 'why should they crack?'. IMHO a better question in welded Ti is 'why should they not crack?'.
However in welded steel, this would be a fairer question; you would (relatively speaking) need to do something cretinously stupid if you wanted to make a weld in steel that would crack very quickly in service without said weld being poor in appearance or there being an obvious deviation in welding parameters. By contrast almost any small deviation in procedure when welding Ti is liable to make a welded fabrication that will quickly fail by cracking in service, and the welds themselves may look 'normal'.
An example; if someone grinds some steel, anywhere in or near your Ti welding shop, there will be microscopic iron particles in the air. If any of these sit on the Ti parts to be welded, the welds will fail. If the iron particles are red-hot when they land on pieces of Ti, they will stick so hard that they cannot simply be removed by normal cleaning. The failure mechanism involves the formation of brittle Fe-Ti intermetallics on the Ti surface adjacent to the welds, in the region that gets hot but doesn't melt. Slightly grubby hands or gloves can easily transfer enough contaminant to cause the same kind of thing. Flakes of skin, hairs, spittle etc are all potential weld-wreckers, even in quantities that are so small you can't see them.
It takes a miniscule amount of contaminant to cause a problem; we are talking a few parts per million here. In this respect, welding steel is about x1000 times easier than welding Ti. Basically when TIG welding Ti there is one way to do it right and a million ways to do it wrong. It is sufficiently difficult to do well that I don't think there is a Ti welding shop that doesn't occasionally produce substandard product, even though most of the possible problems are well known; after all TIG welding of Ti has been practised industrially for over 50 years now.
When you see a Ti fabrication that has been welded properly, and/or see it actually being done, it can be magical. Good Ti welds can be things of real beauty. If you know what you are looking at and you see it being done wrong, or the results of it being done wrong, it can make your flesh crawl.
So yeah, best to get one with a good warranty, and to keep a good eye on it for at least the first year or so; that should weed out most of the rogue frames that might slip through the net.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Weight wise, my 62cm traditional diamond frame is 1625g naked, Bob Jackson's quote circa 1500g for a 56cm 853 so probably very similar. Newer compact ti frames can be quite light but for the sake of a few hundred grams the difference isn't worth worrying about one way or the other.