Overview of Titanium Material
In recent years, the materials and manufacturing methods used for bicycle frames have become incredibly diverse, offering a wider range of choices. These choices are deeply tied to the performance of the frame—its strength, compliance, durability, and the overall experience of riding as a sport.
Seven Cycles believes that, among many materials, titanium is the ideal frame material. Titanium is lightweight, highly workable, possesses approximately twice the strength of iron, and offers excellent durability. These properties are not only attractive to builders, but also provide riders with a luxurious ride feel, improved drivetrain efficiency, and exceptional torsional rigidity.
However, titanium is also a delicate, high-performance material that requires advanced technology and expertise to handle. Furthermore, it is important to understand the characteristic that it is not rare at all, but rather expensive.
How Titanium Alloy is Made
While titanium is often thought of as a "rare metal," it is actually the fourth most abundant metal element on Earth. In terms of abundance in the Earth's crust, it is a plentiful resource, ranking behind only aluminum, iron, and magnesium.
However, titanium does not exist in its pure form in nature; it is mined as a component mixed within ores such as "rutile" and "ilmenite." Because this extraction and refining process is extremely difficult and costly, titanium is considered an expensive material.
Manufacturing Sponge Titanium
The first step in refining titanium is the production of "sponge titanium." Named for its sponge-like appearance, it is a mass of high-purity titanium with very few impurities.
- Titanium dioxide (TiO₂) is first mixed with coke and processed via chlorination to become a liquid called "titanium tetrachloride (TiCl₄)."
- After this liquid is distilled and purified, it is reacted with powdered magnesium and heated in an environment where oxygen and hydrogen have been removed.
- Through this reaction, magnesium chloride is separated, leaving behind pure titanium (sponge titanium).
From Compact to Ingot
Sponge titanium is then compressed by a hydraulic press into a mass called a "compact." These are welded together to form a massive "consumable electrode," which is re-melted in a vacuum arc furnace. The molten titanium solidifies inside the furnace to become an **"ingot."**
Incidentally, the inner wall of the furnace uses copper, which partially bonds with the solidified titanium, so it is also necessary to use a large lathe to shave off the copper.
Alloying: The Birth of 3AL-2.5V
At this stage, titanium is alloyed by adding aluminum (3%) and vanadium (2.5%) as needed. This is the **"3AL-2.5V" alloy, which is most widely used as a titanium material for bicycles**.
From Titanium to Tubing
The massive titanium ingot is **forged (hammered and elongated) in an oxygen-free environment**. This process homogenizes the internal structure of the metal, moderately adjusting its strength and flexibility.
The forged material becomes a rod approximately 8 inches (approx. 20 cm) in diameter, which is then pushed through an **extruder** to be processed into tubular titanium tubing.
After extrusion, the tubes are submerged in hydrochloric acid to remove surface oxides and impurities and then processed into thinner sizes.
Finishing and Inspection
A tube as extruded is not yet usable, so it undergoes precision finishing in a facility called a tubing mill.
- The tube's length, diameter, and thickness are measured, and its appearance and dimensions are inspected.
- Simultaneously, chemical composition analysis and checks for straightness are performed.
- Acid treatment is performed again to remove impurities and prevent oxidation.
Following this, the tube finally enters a heat treatment process called **"vacuum annealing."**
Why Vacuum Annealing is Important
- Removes stress accumulated during processing and makes the tube more compliant.
- Provides the proper softness before rolling, preventing damage during processing.
This process dramatically improves the machinability, bendability, and weldability of the titanium tube.
*1: Titanium tube strength is temporarily lowered to use rolling mills. When the titanium tube passes through the rolling mill, the tube's strength increases rapidly, making the titanium harder and stronger. Without the vacuum annealing process, the machinery would sustain significant damage.
*2: Increasing the tube's malleability allows it to pass through the rolling mill. This makes it possible to dramatically increase flexibility without losing strength. If the tube lacked flexibility, it would be highly unstable and unusable, and might even become difficult to machine or bend.
(Because titanium tubes may bend while increasing flexibility and strength during the annealing process, the tubes are straightened thoroughly before entering any further stages of work.)
Pilger Mill Processing
The next step is a process called "Pilger Mill," where the diameter and thickness of the tube are adjusted even more precisely.
- By passing through the mill repeatedly, the **grain structure** is refined into an ideal form.
- Invisible microscopic scratches and defects are removed through repeated ultrasonic inspection and pickling.
CSR (Contraction Strain Ratio) and Durability
One of the indicators used to evaluate the performance of titanium tubing is the CSR (Contraction Strain Ratio). This indicates the following properties:
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CSR value 1.7–1.9: The best balance of high fatigue strength and flexibility.
The grain structure is determined by the ratio of how much the diameter decreases relative to the thinness of the tube surface. It is possible to inspect the state of these grains and their texture by measuring the tube's CSR. As shown in the table below, a CSR value between 1.7 and 1.9 promotes the highest fatigue strength while enabling the maintenance of top-tier bending deformation characteristics. -
CSR value 2.0 or higher: Tends to be resistant to bending but weaker against fatigue.
A CSR value of 2.0 or higher maintains bending deformation characteristics, but fatigue durability decreases dramatically. The optimal CSR value is controlled during the Pilger Mill processing stage; however, it must be remembered that applying cold working (e.g., tapering, internal butting) to the titanium tube after this process can have a negative impact on the tube's durability.
If **cold working (such as butting or tapering)** is applied to a tube that has achieved an ideal CSR through a Pilger Mill, the crystal structure can collapse, causing the CSR value to drop; therefore, careful design is required.
If ordinary cold working is performed after a good grain structure has been finished through the cold-stress-relief process of Pilger milling, the carefully achieved grain structure may be degraded. For example, by stretching or tapering the tube, the grain structure causes molecules to shift out of radial alignment, and the CSR value drops. However, such processing, which results in a lower CSR value and reduced fatigue limit, is sometimes used to thin out main tubes or chainstays.
Final Finishing and Seven's Original Dedication
Finally, the tubes undergo the following finishing:
- Repeat acid treatment and ultrasonic inspection
- Final annealing: Slightly increases flexibility to provide the optimal balance for the end user.
- Alpha case (surface oxide layer) removal: The inner and outer surfaces of the tube are carefully cleaned with acid.
※ An alpha case is an oxygen-rich hardened layer that forms on the surface of titanium during high-temperature processing; it is brittle and has poor workability, so it must be removed.
This final step is sometimes omitted for general sports-grade titanium tubing, but Seven emphasizes this process to further enhance the perfection of our original tubing.
Final Inspection and Shipping
The following strict inspections are conducted before shipping:
- Eddy current test (internal defect detection via electromagnetic induction)
- Ultrasonic test (internal structure inspection not possible with the naked eye)
The Seven original titanium tubes completed in this way are delivered to Seven's workshop for frame production.