Brazing is a joining process in which two or more items are bound via a filler metal flowing into the joint. The filler material has a lower melting point than the target.
What is brazing?
Brazing doesn’t require melting the target material, unlike in many welding processes. The filler metal flows by capillary action and is brought above its melting point, always protected by a suitable atmospheric flux. It flows over the base metal and is then cooled. Brazing facilitates joining of the same or different metals with high strength.
Advantages and disadvantages of Brazing
There are many advantages as it doesn’t melt the base metal, allowing tighter tolerances and producing a clean joint without the need for a post finish. Brazing also produces less thermal distortion due to the uniform heating process. Complex structures can be brazed very cost-effectively and its perfect for mass production and automation.
One negative, is a lower joint strength due to the utilisation of softer filler metals. The strength of the joint is likely to be less than that of the base metal but greater than the filler metal. Brazed joints can be damaged under high temperatures and they require absolute cleanliness. It will require the use of adequate fluxing agents for cleanliness.
How does brazing work?
Quality joints require the base metal to be clean and free of oxides. Contamination will lead to a reduced flow, poor wetting. The two main methods are chemical cleaning and abrasive cleaning. In the case of abrasive cleaning, continuous maintenance of the proper surface roughness is paramount, as wetting on a rough surface occurs more readily than on a smooth surface of the same geometry.
Another consideration is temperature. As the temperature of the braze alloy rises, the alloying and wetting action of the filler metal increases. The temperature selected must be above the melting point of the filler metal. However, several factors influence the joint designer’s temperature selection. Criteria can vary for example, choosing the lowest possible braze temperature. Additional considerations could be to minimise filler metal/base metal interaction or reduce the life of any fixtures or jigs used.
Brazing vs TIG welding
There are a variety of factors in choosing which method to use for a specific process. A large assembly size would usually point to TIG welding, as brazing works by applying heat to a broader area. The metal thickness would lend towards a technique for thinner materials. Thicker base metals could actually work with both processes. A specific consideration for joint shape e.g. linear joints – brazing is preferred as it involves less manual work than welding. For spot welds, however, welding is more cost effective. Aesthetically, this produces a neat finish.
Filler materials used in brazing
A variety of alloys are used as filler metals. Braze alloys can be made up of 3 or more metals. The filler metal usually lends to its ability to wet the base metal, favour service conditions and melt at a lower temperature than the base metals.
Braze alloy is generally available as rod, ribbon, powder, paste, wire and preforms. Depending on the application, the filler material can be pre-placed at the location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used. In the case of furnace techniques, alloy is usually placed beforehand since the process is usually highly automated. Filler metals include: Aluminium-silicon, copper, copper-silver, copper-zinc (brass), copper-tin (bronze) and nickel alloy.
As it requires high temperatures, oxidation of the metal surface occurs in an oxygen-containing atmosphere. Various atmospheres include:
- Air. Many materials susceptible to oxidation. Flux counteracts the oxidation, but weakens the joint.
- Combusted fuel gas (87% N2, 11–12% CO2, 5-1% CO, 5-1% H2). For silver, copper-phosphorus and copper-zinc filler metals. For brazing copper and brass.
- Combusted fuel gas (73–75% N2, 10–11% CO, 15–16% H2). For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, monel, medium and high carbon steels.
- Combusted fuel gas (41–45% N2, 17–19% CO, 38–40% H2). For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.
- Ammonia. Dissociated ammonia (75% hydrogen, 25% nitrogen). Inexpensive. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals.
- Nitrogen+hydrogen+carbon monoxide, cryogenic or purified (70–99% N2, 2–20% H2, 1–10% CO). For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.
- Nitrogen, cryogenic or purified. At high temperatures can react with some metals, e.g. certain steels, forming nitrides. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. Also, for copper, brass, low-nickel alloys, Monel, medium and high carbon steels.
- Inorganic vapours. Used for silver-brazing of brasses.
- Noble gases. Non-oxidising, more expensive than nitrogen. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, monel, medium and high carbon steels chromium alloys, titanium, zirconium, hafnium.
- Vacuum. Requires evacuating the work chamber. Expensive. Used for highest-quality joints, for e.g. aerospace applications.
Brazing applications within industry
Fuel cells present challenges for this joining technology. Specialised heat exchangers looking at brazing metals to ceramics are being researched. Engineers at Dana Corp. have developed an ultra-clean nickel process that offers advanced thermal management.
EWI and TWI Ltd (Cambridge, UK) have been researching assembly of fuel cell components with adhesives, brazing and laser welding. Research includes work on ceramic-reinforced braze systems for ceramic-to-ceramic and ceramic-to-metal joints, and glass-ceramic systems for improved thermal expansion matching.
Space exploration may also provide innovation in this joining technology. NASA has been studying how electron beam brazing can be used to cost-effectively to assemble large truss structures in space. The structures would be used to support antennas, satellites, solar panels and telescopes.
It is used in specialised operations and splits into three categories, including manual, machine, and automatic torch brazing.
This is a procedure where the heat is applied using a gas flame. The torch can either be hand held or in a fixed position. Manual brazing is used on small production volumes. The disadvantage is the labour cost associated with the method as well as the high skills required. The use of flux is required to prevent oxidation.
Machine torch brazing
Used in a situation where a repetitive braze operation is being carried out. This method is a mix of both automated and manual operations with an operator often placing brazes material, flux and jigging parts while the machine mechanism carries out the braze. The use of flux is also required as there is no protective atmosphere, and it is suited to small or medium production.
Automatic torch brazing
This method eliminates the need for manual labour. The main advantages of this method are; a high production rate, uniform braze quality and reduced operating costs. The main difference is that the machinery replaces the operator in the part preparation.
Furnace brazing is a semi-automated process used mass production. One advantage is the ease with which it can produce large numbers of small components. Other advantages include a low unit cost and the ability to braze multiple joints simultaneously. The disadvantages of this method include a high equipment cost and more complex design. There are four main types of furnaces used in brazing operations as detailed below.
A batch type furnace
This has a low initial equipment cost and can heat each part load separately. These furnaces are suited to medium to large volume production and offer a degree of flexibility in the types of parts that can be brazed. Either controlled atmospheres or flux can be used.
Continuous type furnaces
These are best suited to a steady flow of similar-sized parts. Components are conveyor fed, moved through the hot zone at a controlled speed.
These furnaces make use of a sealed lining called a retort. The retort is sealed with either a gasket or welded shut and filled completely with the desired atmosphere and then heated externally. The retort is usually made of heat resistant alloys that resist oxidation. Retort furnaces are used for batch production.
This is a relatively economical method of oxide prevention and is most often used to braze materials with very stable oxides (aluminium, titanium and zirconium). Vacuum brazing is used with exotic alloy combinations unsuited to atmosphere furnaces. The component cleanliness is critical.
Silver brazing uses a silver alloy based filler. These consist of many different percentages of silver and other metals, such as copper, zinc and cadmium. One type of silver brazing is called pinbrazing. It has been developed especially for connecting cables to railway track or for cathodic protection installations. The method uses a silver- and flux-containing brazing pin, which is melted in the eye of a cable lug.
Braze welding is the use of a bronze or brass filler rod coated with flux to join steel workpieces. Since braze welding usually requires more heat than brazing, acetylene or methylacetylene-propadiene (MAP) gas fuel is used. The name comes from the fact that no capillary action is used.
Braze welding has many advantages over fusion welding. It allows the joining of dissimilar metals and reduced heat distortion. Additionally, since the metals joined are not melted in the process, the components retain their original shape. Another effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures.
Cast iron welding
The welding of cast iron is usually a brazing operation, with a filler rod made of nickel. Ductile cast iron pipe may be also cadwelded, a process that connects joints by means of a small copper wire fused into the iron when previously ground down to the bare metal, parallel to the iron joints being formed. The purpose behind this operation is to use electricity along the copper for keeping underground pipes warm in cold climates.
Vacuum brazing is a material joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle. Products that are most commonly vacuum-brazed include aluminum cold plates, plate-fin heat exchangers, and flat tube heat exchangers.
Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.
Dip brazing is especially suited for brazing aluminium because air is excluded. The parts to be joined are fixtured and the brazing compound applied to the mating surfaces. The assemblies are dipped into a bath of molten salt (typically NaCl, KCl and other compounds), which functions as both heat transfer medium and flux.
Differences between soldering and brazing
Soldering involves joining of materials with a filler metal that melts below ~450 °C. It generally requires a relatively fine and uniform surface finish between the faying surfaces. The solder joints tend to be weaker due to the lower strength of the solder materials.
Brazing utilises filler materials with a melting temperature above ~450 °C. Surface finish tends to be less critical and the braze joints tend to be stronger.
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