Furnace brazing is almost always undertaken in some sort of protective atmosphere, the composition of which depends on the materials to be joined and the alloy used to join them. Although brazing can join materials other than metals, particularly ceramics, this article only considers metal-to-metal joining. A vacuum is also a kind of atmosphere and is widely used, especially for the more reactive metals and for large assemblies made in low volumes. Vacuum processing is a specialized subject and is not discussed here.
Fluxed BrazingFor most metal joining processes, the role of the furnace atmosphere is to create or maintain a clean surface both on the parent metal and the brazing alloy so that wetting and, hence, bonding can take place. However, this is not the case in one important area - the brazing of reactive metals like aluminum. Aluminum is so reactive that, in practice, it is not possible to create an atmosphere that will reduce its oxide and allow brazing to take place. In this case a flux has to be used to reduce the oxide first. A protective atmosphere is still needed to protect the flux from water and oxygen, which would overwhelm its reducing power before it could reduce the alumina.
The maximum allowable water and oxygen contents of the atmosphere are only one side of the successful brazing triangle shown in Figure 1.
The first leg of the triangle is the heating rate. If the heating rate is too fast, the brazement will not be at an even temperature right through when some sections reach brazing temperature. Some parts may not reach brazing temperature at all, resulting in dry joints. If the temperature is raised to compensate for this, then the hotter areas can melt. Conversely, if the heating rate is too slow, the flux will have a longer time to react with any oxygen present in the atmosphere and may become completely depleted before the brazing temperature is reached. It is generally recommended that the heating rate should be greater than 20°C (36°F)/minute, provided that a uniform temperature of ±3°C (5°F) is maintained across the brazement at the brazing temperature.
The second leg of the triangle is flux loading. It is generally accepted that the minimum amount of flux loading should be used. If the flux loading is increased to compensate for a poor atmosphere, not only will it raise the cost, but the extra flux will react with the atmosphere to give high levels of gaseous species such as KAlF4 and HF, leading to corrosion and deposition problems.
The third leg of the triangle is the furnace atmosphere. The content of water vapor and oxygen are critical to success. It is recommended that the dew point should be less than °'40°C (-40°F) and the oxygen concentration below 100 ppm. Both are important because both react with the flux and prevent it from reducing surface oxides during the brazing process.
Fluxless BrazingIn joining other metals, the furnace atmosphere acts as the flux so the assemblies leave the furnace clean and bright. Some typical examples of furnace-brazed assemblies are shown in Figure 2.
The fluxing agent is predominantly hydrogen, though some atmospheres generated by controlled combustion of hydrocarbon gases also contain some carbon monoxide. Atmospheres made by mixing high purity nitrogen and hydrogen have the advantages of being non-toxic, clean, consistent and flexible.
The reducing power of a given atmosphere is not controlled simply by the proportion of hydrogen present, but by the ratio of hydrogen to water. Figure 3 shows that each element has a critical ratio at each temperature, above which any of its oxide present will be reduced. In practice, it is impossible to reduce the water level in a mixture to very low levels because of the tiny residue of oxygen in industrially available gases and more importantly because air leaks into the furnace. Therefore, the more reactive the metal, the more hydrogen is needed to reduce its oxide.
For copper brazing of mild steel, iron has the highest requirements, but this is easily supplied with only a few percent of hydrogen. The components shown in Figure 5 were brazed in an atmosphere containing only 5 percent hydrogen.
Controlling WettingIn some assemblies, particularly mass produced heat exchangers, a large number of joints have to be made. The fit is often variable, resulting in some tight and some wide gaps to be filled by the braze alloy. In such assemblies, the braze alloy is normally placed at one position in the joint only and is expected to "wick" round the remainder of the joint when molten, but the variable gaps make wicking difficult to achieve.
The other extreme can also be a problem. If the surface of the assembly is too active, the braze alloy can wet the whole surface - this is known as flashing. Not only can this be unattractive, but it can it can starve the joint of braze alloy and result in poor bonding. The phenomenon is caused by the hydrogen/water ratio being too high and can be corrected by adding additional oxidant to the atmosphere.
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