The "Cold Welding" Dilemma of Titanium Fasteners: When Threads Weld Themselves Together

In mechanical assembly, there is a problem that frustrates both engineers and assembly workers alike: a titanium alloy bolt goes in smoothly, but halfway through tightening it suddenly seizes – neither advancing nor backing out. It's not rust, it's not cross‑threading – it's that the thread surfaces have microscopically welded together.

This phenomenon is called cold welding, also known as galling. It is extremely common in titanium threaded connections, yet surprisingly little known outside specialist circles.

How Does Cold Welding Occur?

Titanium alloys have high chemical activity. Their natural oxide film is only a few nanometres thick. During tightening, the contact pressure between threads is sufficient to rupture this oxide layer, exposing fresh titanium surfaces directly. At contact points on the scale of 2‑3 microns, pressure and the frictional heat generated by relative motion cause titanium atoms to diffuse into the mating surface – bonding like a weld.

Research data tells the story clearly: under identical conditions, TC4 titanium alloy fasteners are significantly more prone to galling than steel fasteners; in galling failure processes, the contribution of thread surface friction coefficient accounts for 60.7‑73.4% of the total. In other words, the root cause of galling is uncontrolled friction.

The "Three Mountains" of Titanium Fasteners

Galling is only the surface symptom; the root causes lie in three inherent shortcomings of titanium alloys:

1. Poor thermal conductivity. Titanium's thermal conductivity is only one‑seventh that of steel. The frictional heat generated during tightening cannot dissipate and accumulates at the thread contact points. The higher the temperature, the more severe the chemical reactions.

2. Strong chemical affinity. Titanium reacts with almost any material at elevated temperatures – including itself. TC4 threaded directly against TC4 carries the highest galling risk and must be avoided.

3. Poor wear resistance. The surface hardness of titanium alloys is insufficient. Slightly rough thread surfaces produce wear debris that acts as an abrasive paste, further intensifying friction.

Surface Treatments: Putting a "Jacket" on Titanium Threads

The most effective way to prevent galling is to prevent titanium‑to‑titanium contact altogether.

Anodising is the most common treatment. It produces an oxide layer on the titanium surface through electrolysis, with controllable thickness and increased hardness. Among variants, pulse anodising (PA) offers the best combined friction‑reducing and wear‑resistant performance. However, anodised layers are only a few microns thick, and alone they cannot completely eliminate galling.

Solid lubricant coatings are the true "anti‑galling weapon." Molybdenum disulfide (MoS₂) dry‑film lubricant is now the standard for aerospace titanium fasteners. After MoS₂ coating, the friction coefficient of the thread pair is significantly reduced. Under a constant tightening torque, a lower friction coefficient results in greater preload. Studies also show that both thread rolling and MoS₂ coating significantly improve the fatigue life of titanium fasteners.

Aluminium coatings offer another route. Coating titanium threads with aluminium provides lubrication, isolates titanium‑titanium contact, and also mitigates galvanic corrosion when connected to aluminium alloy components. A Chinese‑developed aluminium coating technology meeting Hi‑shear 294 and Hi‑shear 397 standards has been successfully applied in large commercial aircraft.

Silver or nickel plating is suited for high‑temperature or repeated‑disassembly applications. Silver offers excellent lubricity but may tarnish in sulphide‑containing environments; nickel layers are hard and wear‑resistant, but thickness must be controlled to avoid compromising thread fit.

A typical process route for aviation primary‑load titanium bolts is: thread rolling + shot peening + anodising + dry‑film lubricant. Four layers of protection, each performing its own role.

Assembly Strategy: "Software" Protection Beyond Coatings

Surface treatments address the hardware side; assembly practices address the software side.

The torque‑angle method is more reliable than pure torque control. Studies show that the standard deviation of preload using the torque‑angle method is about one‑third that of the pure torque method, significantly reducing preload scatter.

Tightening speed also matters. An appropriate increase in rotational speed can stabilise the torque coefficient.

Preload decays. A certain amount of preload loss occurs shortly after tightening. For high‑reliability joints, step‑by‑step tightening or secondary tightening strategies are often employed.

Repeated tightening requires caution. As the number of tightening cycles increases, the lubricant coating on the thread surfaces gradually wears away, and the friction coefficient changes.

The galling problem of titanium fasteners is essentially a conflict between material strengths and processing weaknesses. Titanium's lightness, strength, and corrosion resistance are its strengths; its poor thermal conductivity, strong chemical affinity, and insufficient wear resistance are the processing challenges that must be addressed.

The solution is a combination of measures: anodising as a base, solid‑lubricant coatings for isolation, thread rolling for strengthening, and precise tightening strategies. Missing any one of these, and galling may return.

A titanium bolt takes only seconds to tighten. But to ensure those seconds go without incident, materials scientists and engineers have devoted over half a century of research.