Why bother with brazing?

What is brazing? Fun, for starters. Melting and joining metal is the exciting part of old-school frame building and the oldest method, brazing, remains the joining technology favoured by most custom frame builders working with steel. That’s partly because the equipment needed is relatively inexpensive. More to the point, perhaps, is its relationship with lugs and the scope they offer for the expression of skill and artistry.

Frame lugs and brazing go together like fish and chips, simply because the brazing process relies on a tiny gap between the two surfaces to be joined. Brazing is the process by which two parts, each composed of a ‘base’ metal, are joined by melting the filler metal, which is drawn by capillary action into a gap between the base metal parts. The base metal necessarily has a higher melting temperature than the filler and, unlike in welding, is not melted during the process.

It’s a process widely used in industry to join a wide variety of base metals, using a wide variety of fillers. Some engineering metals, such as titanium, have so far proven near-impossible to braze successfully – meaning resulting in a strong, reliable joint – but complex aluminium structures such as motor vehicle radiators are brazed, as are small parts such as brake cable stops to aluminium cycle frames, the filler metal being an aluminium alloy with a lower melting point than that of the alloy used in the base metal parts to be joined. The difference may be a few degrees Celsius, making accurate temperature control critical if the whole lot is not to melt.

By definition, brazing takes place at temperatures above about 450degC/840degF; the similar process using filler metals that melt at lower temperatures is known as soldering. This should not be confused with silver-soldering, which is a form of brazing, often referred to as silver-brazing, using a silver-based filler rod that melts at upwards of 630degC. Fillet-brazing is, strictly speaking, not brazing at all since it does not rely on capillary action (although this is used to create a small fillet on the inside of the join) and is referred to in industries such as custom motorcycle frame manufacture as braze- or bronze-welding.

Crucially, brazing relies on capillary action to pull the filler metal into the joint gap. This, in turn, relies on a close fit of as little as 0.03mm between the parts to be joined. Too large a gap, and the filler won’t be drawn as far into the joint; too small, and there’s no room for the metal to flow. Bike frame lugs are made to a suitable fit with the various standard tube sizes and brazing with them is straightforward provided the correct procedure is followed.

The surfaces to be joined must be cleaned by degreasing with a solvent and benefit from ‘mechanical’ roughening with abrasive cloth before being coated with a suitable flux. The primary task of flux is to allow the filler to ‘wet’ or flow across, the base metal by reacting chemically with oxides forming on the metal surfaces from the heat of brazing through chemical reaction, and it can become ‘exhausted’ – run out of reactive compounds - if heating goes on too long. The flux must be chosen to match the heat required to melt the filler; it becomes ‘active’ and reacts with oxides at some specific temperature and won’t work below that, so a flux suitable for brazing with brass filler rod at around 900degC won’t do for silver brazing at 650degC. Silver solder flux has a maximum working temperature beyond which it ‘burns’ and not only fails to work but ruins the joint. Keeping the temperature of the joint to within the range of the flux is one of the trickier aspects of silver brazing.

Once the work is brought up to the required temperature, the filler rod can be applied. In the case of steel brazed with brass rod, the base metal will need to glow a dull cherry red; at this point, the rod will melt quickly on contact with the hot base metal and will flow nicely. Applied to the gap between tube and lug, it will be drawn by capillary action into the space between the surfaces and towards hotter areas, making it possible to draw more filler into the joint as needed by manoeuvring the heat source - usually an oxy-acetylene flame.

The object is to get the filler to fill the space between tubes and lug without leaving any voids or excess, which is best done by feeding it in from one edge and 'drawing' it, using heat as noted, through the lug to the far edge. This also ensures the flux escapes by flowing ahead of the filler; if it can’t escape, it will stay put and prevent complete filling of the joint.

Careful manipulation of the torch allows the operator to move filler around as needed to create a neat ‘meniscus’ or fillet along the edge of a lug; clumsy or careless work can leave voids or blobs of excess filler, which mar the appearance of the finished frame even if the joint is sound. Leaving a neat fillet not only enhances the look of the frame but saves spending time on cleaning up by filing away the excess, which never looks as good.

A good brazed joint does not rely on some sort of mechanical or chemical adhesion for its strength, which may surpass that of the parent metal, but on a metallurgical bond. An alloy between at least one of the constituent metals of base and filler metals is formed at the interface between them, leaving a brazed joint one comprised of solid, continuous metal.  Just how strong the bond is can be judged by trying to remove brass filler from steel…

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