A Brief History

Titanium was discovered in 1790 by William Gregor, a clergyman and amateur geologist in Cornwall, England. However, it was not purified until 1910, and was not refined and produced in commercial quantities until the early 1950s. Since then, titanium production has grown by about 8% per year, and since the early 1960s its use has shifted significantly from military applications to commercial ventures.

Although pure titanium was valued for its blend of high strength, low weight and excellent durability, even stronger materials were needed for aerospace use. In the 1950s, a high-strength alloy called 6-4 (6% aluminum, 4% vanadium, 90% titanium) was developed, and found immediate use in engine and airframe parts. But 6-4's low ductility made it difficult to draw into tubing, so a leaner alloy called 3-2.5 (3% aluminum, 2.5% vanadium, 94.5% titanium) was created, which could be processed by special tube-making equipment.

Today, virtually all the titanium tubing in aircraft and aerospace consists of 3-2.5 alloy. Its use spread in the 1970s to sports products such as golf shafts, and in the 1980s to wheelchairs, ski poles, pool cues and tennis rackets.

In the 1970s, commercially pure, or CP, titanium was used for the first time in bicycle frames. The frames were light and resilient, but they were not nearly strong enough to withstand the rigors of racing. In 1986, the first frames made from 3-2.5 titanium were manufactured by Merlin Metalworks. 1990 saw the first double-butted 3-2.5 seamless tube set, also created by Merlin.

Cost of Titanium

Titanium is expensive, but not because it is rare. In fact, it is the fourth most abundant structural metallic element in the earth's crust, after aluminum, iron, and magnesium. It is extremely common in the form of titanium dioxide, and is widely used as a whitener in pigments, paper, and food colorings.

Titanium's high cost arises from three main factors:

1. REFINERY COSTS - titanium is never found in its pure form. It must be extracted from other compounds, which requires a significant amount of electrical energy and human labor.
   
   
2. TOOLING COSTS - whether pure or alloyed with other metals, titanium is a tough material that requires specially made forming equipment, and an oxygen-free atmosphere for heat-treating and annealing (heating and cooling at a controlled rate to eliminate work-hardening and restore ductility).
   
   
3. PROCESSING COSTS - titanium work hardens easily, and so must be annealed a number of times during the tube forming process.

Unfortunately, there are no market forces at work to cut prices significantly in the foreseeable future. The slowdown in the aerospace and defense industries has created a slight surplus in capacity, which in the short term should cause more competition and lower prices. However, if these industries keep shrinking, as all signs indicate, the market for titanium will also shrink. In addition, there are design forces at work, including fly-by-wire systems, that will further reduce the total consumption of titanium alloys in the aerospace industry. It is unlikely that the titanium sports industry can make up the difference. One Boeing 747 uses about 95,000 pounds of titanium, which is an eight-year supply for bicycle frames at current usage rates.

The Grades and Sources of Titanium

Titanium alloys vary widely in their properties and appropriate applications. The alloy most suitable for bicycles is 3-2.5, due to its strength, resiliency, and durability. In addition, 3-2.5 can be drawn readily into small-diameter tubing. Merlin bicycles also employ 6-4 titanium plate in the dropouts, and 6-4 or CP titanium for some non-load-bearing fittings.

CP

Commercially Pure, or CP, is titanium in its purest form, unalloyed with any other elements. It is available from many sources in the United States, Europe, Russia, and the Far East. It is relatively easy to form into tubing, and it is currently used in a few bonded bicycle frames in Europe and Taiwan.

Although CP has many industrial applications (primarily arising from its excellent corrosion resistance), its strength-to-weight ratio is substantially below that of 3-2.5, and actually worse than many modest steels. There are four grades of CP in the U.S., which are distinguished primarily by oxygen content. CP's yield strength ranges roughly from 25 to 65 ksi (thousand pounds per square inch). Grade 4 has the highest yield strength; Grade 1 is the weakest.Only Grade 4 is useful for bicycle frames, and only in areas that see minimal stress.

3-2.5

Titanium 3-2.5 is an alloy of 3% aluminum, 2.5% vanadium, and 94.5% pure titanium. The strongest grade, called AMS 105, has a minimum yield strength of 105 ksi, and a minimum ultimate tensile strength of 125 ksi. It has an annealed elongation of 15-30%, and a cold-worked minimum elongation (ductility) of 10%. It does not respond well to heat-treatment. Instead, increases in strength come solely from cold working.

Its fatigue strength-to-weight ratio is roughly twice that of the 4130 chrome-moly steel used in bicycles.

It has excellent resiliency, which can be controlled by changes to the tube diameter and wall thickness, allowing the bicycle designer to accurately tune the ride. This latitude is a direct result of titanium's superb margin of fatigue strength, and is unique to the metal; neither steel nor aluminum enjoys the same "tunability."

As with most titanium alloys, 3-2.5 is corrosion resistant, and it does not need to be painted.

6-4

6-4 alloy (6% aluminum, 4% vanadium, 90% titanium) was the original miracle metal of the aerospace industry, due to its outstanding strength-to-weight ratio. Its primacy is such that it currently represents 50% of all titanium alloy usage in the U.S.

However, 6-4 has several severe drawbacks as a bicycle frame material. Compared to 3-2.5, 6-4's ductility is roughly 30% lower, which makes it extremely difficult to draw into seamless tubing. In fact, there is no such thing as seamless 6-4 tubing in the sizes needed for bicycles. All small-diameter 6-4 is made from annealed sheetmetal, which is rolled into a tube shape and welded.

Fatigue strength in tubing made from sheet is also compromised. The weld area suffers from a random crystallographic texture (grain structure), with reduced fatigue endurance (see Butting considerations in titanium on page 9). And the texture in the sheet cannot be controlled, as it is in seamless tubing.

In addition, 6-4's shear modulus (stiffness in torsion) is considerably lower than 3-2.5's, which is problematic in a bicycle frame that is repeatedly stressed in torsion.

Finally, it should be noted that cost is a limiting factor, too; 6-4 is more expensive to machine and process.

Russian Titanium

Russia has recently been identified as a possible source of low-cost, high-strength titanium alloys. The appeal seems to be twofold:

1. First, in theory, Russia's costs of labor and electricity are lower than the West's. However, costs are also lower because those manufacturers offering tubing for sports applications have not invested in up-to-date equipment and processes for optimum quality.
   
   
2. Second, Russian producers reportedly have a more extensive array of high-strength alloys. This, however, is a misunderstanding that arises from Russia's labeling system for its 200 alloys. In fact, many Russian alloys are similar to U.S. alloys, but carry different names or slightly different formulations. For example, Russia's equivalent to 6-4 is called VT-6. The properties of these alloys are nearly identical. And Russia's VT-5 alloy has similar performance specifications to 3-2.5.