Brazing and Soldering by the Numbers
Chapter 6 of 10 · 10 min read
Brazing filler metals
Compare brazing fillers from 1910 spelter to silver, copper-phosphorus, copper and nickel, read a melting range, calculate the filler a joint needs, and avoid cadmium and zinc fume.
A brazing filler has to do several things at once. It must melt below the parts, wet them, flow into the gap, alloy with the surfaces without eating them away, and then give a joint strong enough, ductile enough and corrosion-resistant enough for its service. No single alloy does all of that for every pair of metals, so brazing has developed families of fillers, each suited to particular metals, temperatures and processes. Choosing among them is one of the main decisions in any brazing procedure.
This chapter describes the main filler families, beginning with the copper-zinc spelter of the early manuals, explains what a melting range means for flow, calculates how much filler a joint needs and what it costs, and deals with the fume hazards of zinc and cadmium.
Spelter: copper and zinc
The brazing filler of Hart’s 1910 manual was spelter, an alloy of copper and zinc. Hart quoted a table from James F. Hobart’s 1908 book Brazing and Soldering that graded brazing alloys from hardest to softest; the hardest was three parts copper to one of zinc, and the hard spelter of general use was copper and zinc in equal parts. He added that English books described spelter as one part of fine brass to one of zinc, which amounted to two parts zinc to one of copper. The harder spelters, richer in copper, gave the strongest joints but needed the hottest flame and were harder to handle; softer spelters ran more easily and were cheaper to use, and suited joints that did not need to stand strain.
Copper-zinc fillers survive as the brass and bronze brazing and braze-welding rods used on steel, cast iron and copper alloys. They work at high temperatures, close to the melting point of copper, which is why they suit steel and cast iron but need care on copper and brass parts. The Open University’s brazing table gives a brazing temperature of up to about 1,120 °C for copper-zinc fillers.
Zinc and its fume
Zinc boils at about 907 °C, well below the working temperature of many copper-zinc fillers, so zinc evaporates from the molten filler and burns in the air to form zinc oxide fume, the white smoke familiar to anyone who has brazed with brass rod. The loss changes the filler, leaving it richer in copper and more porous, and the fume causes metal fume fever in people who breathe it, an illness like influenza. Hart noted that overheating brass work burned out its zinc, and recommended painting brass parts with a graphite preparation, except at the joint, to stop the zinc volatilising. Modern practice keeps the temperature no higher than needed, uses fillers formulated to fume less, and extracts the fume at the joint.
Silver brazing fillers
Silver-based fillers, usually alloys of silver, copper and zinc, often with tin or other elements, work at much lower temperatures than copper-zinc fillers, typically in the range of 600 to 800 °C, flow freely into fine gaps and suit copper, brass, steel, stainless steel and many other metals and combinations. They are the fillers of most hand torch brazing of copper and brass, of refrigeration and of carbide tipping. The Open University table lists a silver-copper filler at a brazing temperature of about 820 °C.
Silver is expensive, so silver fillers are offered with a range of silver contents. Higher silver content generally means a lower working temperature, freer flow and a narrower melting range, at a higher price; lower silver fillers cost less but run hotter and flow less freely. The choice is a balance between filler cost, heating cost, the risk to the parts from heat, and the ease of making a sound joint.
Cadmium in older silver fillers
Some older silver brazing fillers contained cadmium, which lowers the melting range and improves flow. Cadmium boils at a low temperature and its fume is highly toxic: the NIOSH Pocket Guide lists cadmium as a potential occupational carcinogen, with target organs including the respiratory system and the kidneys. Cadmium-free silver fillers are now widely available for almost every use, and makers label cadmium-bearing fillers clearly. Any workshop that finds old filler stock of unknown composition should check it against the maker’s data and safety data sheet before using it, and cadmium-bearing fillers should not be used unless there is no alternative and the controls required for cadmium are in place.
Copper-phosphorus fillers
Copper-phosphorus fillers, sometimes with silver, are used mainly for joining copper to copper. Their phosphorus removes copper oxide at brazing temperature, making them self-fluxing on copper, as Chapter 5 explained. They are economical and widely used for copper tube in refrigeration and plumbing. They must not be used on steel, nickel or nickel alloys, where the phosphorus forms brittle phosphides at the interface, and on brass and bronze they need a flux. Their joints are less ductile than silver-brazed ones and are generally not chosen where the joint will be heavily vibrated or bent.
Copper and nickel fillers
Pure copper is used as a brazing filler for steel, almost always in a furnace with a protective, usually reducing, atmosphere. It melts at about 1,085 °C and flows extremely freely in clean steel joints, so it suits very close or even interference fits, as the filler makers’ clearance guidance shows. Nickel-based fillers, containing chromium, silicon, boron or phosphorus, work at high temperatures and give strong, heat- and corrosion-resistant joints in stainless steels and nickel alloys; the Open University table lists a nickel-chromium-phosphorus filler at about 960 °C and a nickel-chromium-silicon-boron-iron filler at about 1,040 °C. They are mostly used in vacuum or atmosphere furnaces, for parts such as heat exchangers, turbine components and tooling.
Melting ranges: solidus and liquidus
A pure metal melts at one temperature, but most alloys melt over a range. The solidus is the temperature at which the alloy begins to melt; the liquidus is the temperature at which it is fully liquid. Between them, the alloy is a mush of solid crystals and liquid. Some alloys, called eutectics, melt at a single temperature, like a pure metal, because their composition sits at a special point where all their phases melt together.
The width of the range matters for brazing. A filler with a narrow range, or a eutectic, goes from solid to free-flowing liquid almost at once, and runs readily into fine gaps. A filler with a wide range stays mushy over a band of temperature, which helps it bridge wider gaps and build fillets, as in braze welding, but makes it reluctant to flow through tight ones.
Liquation
Wide-range fillers have a particular failure, called liquation. If such a filler is heated slowly through its melting range, as happens to a preplaced ring in a slow furnace or a large joint heated gradually by torch, the part of it that melts first, the low-melting constituents, can run off into the joint on its own, leaving behind a spongy skeleton of higher-melting metal that will not flow. The joint is then filled partly with a filler of the wrong composition and partly not at all. The cure is to heat quickly through the melting range, or to use a narrow-range filler wherever slow heating cannot be avoided, as in furnace brazing with preplaced filler.
How much filler a joint needs
A joint needs enough filler to fill the gap and form a small fillet, and little more. The quantity can be calculated from the joint’s geometry.
- d
- 15 mm
- L, socket depth
- 12 mm
- c, radial clearance
- 0.08 mm
- gap volume ≈ π × 15 × 12 × 0.08 ≈ 45 mm³
- fillet ≈ ½ × 1 × 1 × π × 15 ≈ 24 mm³
- total ≈ 69 mm³, about 0.65 g of filler
For the manufacturer’s 15 mm outlets, that is about 0.65 g of filler per joint, close to the 0.6 g that Chapter 1 used in its cost estimate. A 22 mm outlet with a 0.10 mm hot clearance needs about 1.1 g by the same calculation. Knowing the theoretical quantity lets a production engineer check how much filler is really being used: if the brazers are using twice the calculated amount, the excess is going into oversized fillets, drips and spatter, or into joints with gaps far wider than intended.
Filler cost
This is an illustration. At an illustrative A$1.80 per gram, excluding GST, for the manufacturer’s silver filler, a 15 mm outlet joint uses about 0.65 × 1.80 ≈ A$1.17 of filler. The manufacturer makes about 40,000 such joints a year, using about 26 kg of filler if every joint used the calculated amount, worth about A$46,800. When the production manager compared that figure with the filler actually bought, about 34 kg, the gap of 8 kg, worth about A$14,400 a year, was the cost of oversized fillets, feeding filler into joints that were not yet hot enough to take it, dropped and scrapped rod ends, and rebrazed joints.
Some of the excess was unavoidable, such as the short stub of each rod that cannot be used. Most was not. Better heating practice (Chapter 8), cut lengths of filler wire or preformed rings sized to each joint, and training on what a correct fillet looks like reduced purchases by about 5 kg the next year.
Forms of filler
Brazing fillers come in many forms. Rod and wire suit hand brazing, fed into the joint as it reaches temperature. Preformed rings and washers are placed on or in the joint before heating, giving exactly the right quantity every time, and suit production and furnace brazing. Foil and shim suit flat laps, such as carbide tips. Pastes combine powdered filler with flux and a binder, and can be dispensed automatically. Clad sheet, with a layer of filler rolled onto the surface of the parent metal, is used for aluminium heat exchangers. The form is chosen for the process, the joint and the quantity, and preformed fillers are one of the most effective ways of controlling filler use.
Choosing a filler
The choice of filler starts from the parts and works outwards:
- the metals being joined, which rule out some families, such as copper-phosphorus on steel;
- the service conditions, including temperature, corrosion and vibration, which set the strength, ductility and corrosion resistance needed;
- the heating method and atmosphere, which set whether flux is needed and how fast the joint heats;
- the parts’ tolerance of heat, which favours lower-temperature fillers for heat-treated or heat-sensitive parts;
- the clearance, which favours narrow-range fillers for tight joints and wider-range fillers for looser ones;
- cost and safety, including the price of silver and the avoidance of cadmium.
The filler maker’s data sheet, giving the composition, solidus, liquidus, recommended brazing range, clearances and suitable fluxes, is the authority for each product, and the safety data sheet sets out its hazards.
A failure: the ring that did not run
This is an illustration. When the manufacturer trialled furnace brazing of its steel headers with preplaced filler rings, many joints came out of the furnace with a rough, spongy lump of metal where the ring had been and only a little filler inside the joint. The filler chosen for the trial was one the brazers used by hand, a lower-silver alloy with a wide melting range.
In the furnace, the assemblies took many minutes to rise through the filler’s melting range, and the filler had liquated: the low-melting part had run into the joints first, and the remainder had stayed behind as a skeleton. Changing to a filler with a narrow melting range, suggested by the filler maker for furnace use, and raising the furnace’s heating rate through the melting range, gave joints filled from end to end.
Exercise: filler for a 22 mm joint
Using the method in this chapter, calculate the filler needed for a 22 mm tube in a 15 mm deep socket with a radial clearance of 0.10 mm at brazing temperature and a fillet with 1.2 mm legs, taking a filler density of 9.5 g/cm³. Then find its cost at A$1.80 per gram.
The gap volume is about π × 22 × 15 × 0.10 ≈ 104 mm³. The fillet is about ½ × 1.2 × 1.2 × π × 22 ≈ 50 mm³. The total is about 154 mm³, or 0.154 cm³, so the filler mass is about 0.154 × 9.5 ≈ 1.46 g, costing about 1.46 × 1.80 ≈ A$2.63, excluding GST.
The fillet is about a third of the filler. A fillet with 2 mm legs, the kind that looks generous to an inexperienced eye, would take about 138 mm³ on its own, almost three times as much, and add about 0.84 g, roughly A$1.50, of filler to every joint without making it any stronger or tighter.
