Types of welding for bicycles

There are many welding methods, but I’m going to look at the ones used on bicycles. Welding for bicycles centres on two techniques: TIG and fillet brazing. They’re also known as the modern technique and the old-school technique.

Within each one I’ll include an explanation of what the technique is like and how it joins the metal, the procedure described simply without going into great detail (it’s accessible even if you’re not an expert), and the pros and cons of each.

Let’s get started.

TIG welding

What does TIG stand for?

Tungsten Inert Gas (TIG) or Gas Tungsten Arc Welding (GTAW).

What is TIG welding?

TIG welding is a method widely used today in many industrial processes. The method consists of generating an electric arc within an inert-gas atmosphere; argon is used most often, though there’s also the option of using helium or mixes of these with other, cheaper gases. That said, pure gas is always recommended.

The arc is created by the electrode fitted at the tip of the welding torch; in this case the material used is tungsten. The electric arc spans from the metal workpiece to the electrode.

This type is defined as fusion welding: it avoids spatter and creates clean beads, resulting in very strong joints. It produces strong, more ductile welds that are less prone to corrosion. The latter is achieved thanks to the gas atmosphere, which prevents oxygen from contacting the molten metal and the filler material.

It’s ideal for welding aluminium and stainless steel.

How does TIG welding work?

The equipment consists of a torch with its power supply unit and the gas circuit.

The tungsten (wolfram) electrode has a single function: to produce an electric arc. Tungsten is used because it has a melting point of 3410 °C, which means it withstands the heat far better than the materials it has to melt.

The electric arc generated by the electrode heats it and turns the material molten. The electrode is fitted with a nozzle that keeps the area of molten material — where the filler will be added — inert. Keeping this area inert is critical, since molten material in contact with oxygen produces violent oxidation.

As the material becomes molten, the welder moves the torch forward and, when needed, adds filler material to join the two pieces.

Each material and each weld will require different working voltages and different filler material. But the recommended gas flow rate for all situations is between 6 and 12 litres per minute, depending on the nozzle and the electrode.

Maintaining a steady gas flow is important to keep the arc steady and avoid excessive wear on the electrode.

Practice and experience are among the most important factors when working with TIG. Two welders with the same equipment and the same pieces to join will use different parameters for voltage, gas flow and filler material.

Advantages of TIG welding

• Welds alloy steels, aluminium, magnesium, copper, nickel, titanium and zirconium.
• Produces no slag.
• The arc is stable and concentrated at a single point.
• The weld beads or lines are smooth and even.
• Thanks to the filler material, excessive weld build-up is avoided.
• The welder can see the molten pool at all times and control it perfectly.
• The arc holds at very low voltage, so it can be used across a wide variety of thicknesses.
• Controlling penetration is very straightforward: it helps to make a root pass and then complete the bead.
• Heat-affected zones are minimal.
• When no filler is used, the chemical composition of the base metal is not altered.
• No post-weld cleaning is required.

Disadvantages of TIG welding

• The deposition rate is much lower than with other welding processes.
• Tolerance for contaminants is minimal, so the base material must be well prepared.
• The skill required is far greater than with other techniques.
• In draughty conditions it’s hard to achieve adequate shielding at the weld area.
• It requires a continuous gas flow.

Fillet brazing

Fillet brazing is the oldest joining method used in bicycles. This type of brazing doesn’t require technologically advanced equipment.

What is fillet brazing?

Fillet brazing consists of joining two pieces by adding a filler metal. The filler metal must have a lower melting point than the base metal. As it liquefies, the filler penetrates the gap between the two pieces and distributes itself to form a joint through capillary action.

It’s a very versatile brazing method and provides great strength to the joint. With the right filler material, joints can be achieved with strength greater than that of the base material itself.

How is fillet brazing done?

To braze a bicycle using the fillet brazing method you need a torch, filler rod, flux, oxygen and acetylene.

The parts to be joined must be properly cleaned and sanded to help the filler penetrate. Next, apply flux to isolate the area we’ll heat from oxygen and so prevent oxidation.

Once everything is ready, we heat the area with the torch: it mixes oxygen and acetylene to produce an oxyacetylene flame. The flame must have specific characteristics for the braze to be effective.

The ideal working flame has two clearly defined zones. The first is the cone or dart, white in colour, where combustion takes place. The second is the plume: in this zone the primary combustion of the unburnt gases with the oxygen in the air takes place. This is the zone used for brazing and at the same time it shields the joint from direct contact with the oxygen in the air.

There are other types of flame:

• Pure acetylene: obtained by burning acetylene with air. It is a yellow flame that turns reddish at its tip. It produces soot particles and is not suitable for brazing.
• Carburising or reducing: the one containing excess acetylene. By regulating the oxygen supply, a pale green zone becomes visible after the dart, known as the acetylene plume. It disappears when the proportions are balanced.
• Oxidising: flame with excess oxygen; it makes the dart narrow and reduces the flame at the nozzle outlet.

Once we have a neutral flame, we proceed to heat the working area until it reaches the right temperature for laying down filler.

When the area has reached working temperature, colour changes appear in the base material, and that is when we must judge the moment to begin adding filler with the rod required for the base material we are using.

This brazing technique requires great skill, and equally calls for experience to recognise the various parameters and key moments in the process.

Advantages of fillet brazing

• As the base material is not melted, the process can be controlled much more precisely and good final tolerances achieved.
• The tensile strength of correctly made joints usually exceeds that of the metals being joined.
• The joints are ductile, so they can withstand shocks and vibration.
• Dissimilar metals can be joined.
• No further process is required to improve the surface finish.
• The joints distribute stress well: the fillet is ideal for resisting fatigue.
• It is ideal for joining small parts and thin materials.
• It is used for complex joints thanks to the capillary effect, joining the parts by atomic attraction and diffusion.
• It generates no thermal residual stresses.
• No zone is thermally affected, so the base material does not overheat, deform or melt. Its mechanical properties don’t even change; you can read more about this at this link.

Disadvantages of fillet brazing

• Final cleaning of the flux is always required.
• Preparation of the base materials takes more time.
• Tolerance to contaminants is minimal, so the base must be well prepared.
• The skill required is far greater than with other techniques.
• It needs a continuous supply of oxygen and acetylene.
• Ventilation is advisable.
• A gas mask is required as part of the PPE.

Conclusions

Either technique is effective in the bicycle building process. When TIG is mastered, it is possibly the faster technique, since it doesn’t require the polishing step applied to fillet brazing; but if you achieve a good fillet, the two techniques are on a par.

The finish of fillet brazing is more attractive: the tube joints look clean and the braze is not visible. At the same time, a good TIG bead can be considered a work of craft.

Scientific studies are being carried out by the International Bicycle Engineering Research Group (IBERG), and they are clarifying many aspects that were previously unknown.

One of the most important concerns brazing for bicycles and its effect on the mechanical properties of the base material. Testing is still under way and there are no definitive conclusions, but the fillet brazing method is believed to be more efficient than TIG as it doesn’t affect the mechanical properties of the steel, and the tests carried out over the past few years bear this out.

You can view the tests and publications on their website: IBERG.