Everything you ever wanted to know about titanium

Originally published in Bicycle Times Issue #31

OF BIRDS AND BIKES

Titanium burst onto the scene in the 1950s in military aerospace applications. Favorable ratios of strength, durability and toughness to weight—along with its corrosion resistance and high temperature capabilities—made titanium the material of choice in airframes and aerospace hydraulic systems. The first bird to feature titanium was the ‘50s era Douglas X-3 Stiletto experimental aircraft.

During the 10-speed boom of the 1970s, enterprising bike builders used available “commercially pure” titanium tubing designed for aerospace hydraulic systems to build the first Ti bicycle frames. Component compatibility forced builders to use Ti tubing sized similarly to steel bikes of the era. The inherent flexibility of titanium, combined with the small tubing diameter, produced bikes with more frame flex than most riders deemed acceptable.
The 1970s-era Teledyne Titan was one of the first Ti bikes commercially produced in the USA.
Aerospace design requirements eventually exceeded the capabilities of commercially pure titanium, which led to the development of Ti-3Al-2.5V and Ti-6Al-4V alloys. Adding aluminum (AI) and vanadium (V) to titanium improved the tensile strength yield strength, and toughness (among other mechanical properties).
In the later half of the 1980s, bikes fabricated with these mechanically superior titanium alloys resulted in bikes with improved ride qualities, and put early Ti brands on the map. Still, the available tube diameters (typically 1.25” max) were not optimized for bicycle frames. While there was a lot to like about the early titanium alloy bikes, they had yet to shed their “flexy” reputation.
Titanium alloy bicycle fabrication reached a tipping point during the 1990s and early 2000s when the tubing mills started producing thin-walled titanium alloy tubing with 1.5”, then 1.75” and eventually 2” diameter. Finally, the frame designers/builders had an array of tubing that allowed them to optimize frame strength and stiffness—from the rear stays, to the main tubes, to the bottom bracket, to the head tube.

GEOMETRY MATTERS

Tube geometry is crucial to the strength and stiff ness of a bicycle frame, no matter what material it’s made of. A rough rule of thumb is that a tube’s rigidity depends on the product of the wall thickness and the cube of the diameter; and strength depends on the product of wall thickness and square of diameter. By optimizing each tube’s size and wall thickness, the designer is able to tune the way the bike feels, while ensuring strength and durability.

TI-ALLOY COMPARISON

Both Ti-3Al-2.5V and Ti-6Al-4V have mechanical and physical properties suitable for making bicycle frames. While Ti-6Al-4V has superior strength, Ti-3Al- 2.5V is more malleable and is easier to fabricate into seamless, thin-walled tubing using conventional metal forming equipment and techniques. Due to ease of manufacture, Ti-3Al-2.5V became the alloy of choice for aerospace hydraulic tubing. This led to the wide array of tubing sizes available to bicycle frame builders.

By comparison, the selection of seamless Ti-6Al-4V alloy tubing is rather limited. Therefore, the material sees limited application in bicycle frames (typically in areas where its superior strength is an overriding benefit). Conversely, Ti-6Al-4V plate is readily available and is commonly used for making dropouts.

WELDING

Welding titanium requires white-glove cleanliness. Trace contamination, even oil or grease deposited from a careless fingerprint, could ruin a weld. Since titanium can absorb and become embrittled by oxygen, hydrogen or nitrogen at welding temperatures, extreme care must be taken to insure the material is completely encapsulated in an inert argon atmosphere. A brittle weld may fail (fracture) immediately upon cooling, or later under riding stresses.

Best welding practices typically involve purging the inside of the tube with argon, in addition to argon gas shielding of the TIG or MIG weld tool. In the 1970s, prior to the evolution of existing bicycle tube welding techniques, titanium frames were sometimes brazed. Another early fabrication technique involved welding inside a sealed chamber that was completely purged with argon (a.k.a. “glove box” welding).

WORK IT

Mechanical processing is challenging due to titanium’s high strength and its rapid rate of work hardening— which can be thought of as a continual decrease in malleability as deformation increases. While a proper amount of work hardening (a.k.a. cold working) will strengthen a metal, overdoing it could produce a brittle and weakened structure.

Bike companies typically purchase titanium alloy tubing in a “cold worked stress relieved” (CWSR) state which gives the material higher strength compared to the fully annealed condition, and leaves some room for additional cold working of the tubing during bike building. Malleability is particularly important when fabricating complex shapes such as S-bend chainstays.

ECONOMICS

Titanium alloy tubing costs more than comparable steel or aluminum alloy tubing. While raw titanium costs only $5-6 per pound, special processing and metalworking requirements boost the price of the finished CWSR alloy tubing to $50-60 per pound. For instance, titanium is highly reactive with oxygen, nitrogen and hydrogen at elevated temperatures; therefore melting, annealing and other high-temperature operations must be carried out in a vacuum or under an argon atmosphere. Titanium must be cold worked a small step at a time, with stress-relief annealing performed between each incremental step. This adds complexity and costs to the tube forming process.

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Check out our feature story of Lynskey Bikes, which makes titanium bicycles in Tennessee.