Process Window

When an external source of heat (e.g., a flame) is applied to an assembly, its surface will heat more rapidly than its interior. The rate of heating of the interior of the joint depends on a variety of factors, the most important of which are the mass of the components, the intensity of the heat source being used, and the thermal conductivity of the materials that compose the joint.

In other words, the rate of temperature increase in an assembly is directly related to the rate of heat transfer to the parts from that heating source.

(Further information on this fundamental aspect of brazing will be found in Chapter 4.)

A consideration of these basic facts leads to another of the fundamental rules of brazing:

In brazing, an assembly can be heated only as fast as the parent materials used in its construction can conduct the heat away from its point of application. If more heat is applied than can be conducted away, the parent metal will melt at the point of application of that heat.

Localized melting of the parent material is a situation that is specifically required to occur if the parts are to be joined by welding. When brazing, however, the occurrence of heat damage to the parent material, and perhaps even accidental melting of them, must always be avoided.

It is this consideration that points to the necessity of having a balanced heat input to the joint. This ensures that neither underheating nor overheat-ing will result — both features that will have a harmful effect on the pro-duction of a satisfactory joint. Once the filler metal has melted and the joint has been made, heating is discontinued and the component begins to cool.

TABLE 1.1

Possible Change in Composition Melting Range of the 88% Aluminum-12% Silicon Alloy Used to Braze an Aluminum-Rich Substrate at a Constant Temperature of 630ºC

Position in

Joint Start 25% Through 50% Through 50%+ Through Alloy

Composition

88 Al-12 Si 91 Al-9 Si 93 Al-7 Si No flow

Melting Range (˚C)

5 14 48 48+

Melting Range (˚C)

577–582 577–591 577–625 577–630?

A typical brazing cycle of this type is illustrated schematically in Figure 1.11.

Note particularly how relatively easy it is to overheat an assembly by heating it too rapidly and, more importantly, what the consequences of that action might be. This study has to be taken together with that which relates to the change in filler metal composition, and hence its flow properties, as a result of its dissolving some of the parent material during wetting and flow on the surface of the material over which it is spreading. The overall effect of the combination of these factors on the process only serves to underline why it is so essential to exercise close control of the temperature attained by the assembly during the brazing operation.

In reality, Figure 1.11 represents the general case of heating for brazing. In some applications (e.g., the brazing of copper and mild steel to themselves and to each other, using low-temperature silver brazing alloys), the magni-tude of the process window might be 250ºC or more. In these circumstances, precise temperature control of the parts during processing, although being clearly desirable, will not be a fundamental necessity. However, in the braz-ing of aluminum, the process window is never more than about 65ºC, and in many cases might be as short as 30ºC. If accidental melting of the assembly is to be avoided, very precise control of the process temperature becomes a primary necessity. This is one of the features of the process that makes the brazing of aluminum so different from all other engineering materials in everyday use. It also partially explains why the change of parent material from copper to aluminum for the construction of automotive heat exchangers and condensers in the mid-1990s was not as trouble-free an experience as had been originally contemplated by that industry.

FIGURE 1.11

Representation of the heating of a joint made by flame brazing.

Temperature of Exterior surface Temperature of the joint interior

Temperature

Poor temperature control Heat damage occurs

Process Window

Brazing alloy flows Flux becomes active

Pattern that must be developed

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This consideration inevitably leads to the conclusion that a brazing process that will provide precise control of the temperature gradient experienced by the components is always to be preferred when aluminum or its alloys are to be brazed. This tends to explain the following:

1. The wide use of mechanized brazing systems of varying complexity for the joining of aluminum and its alloys

2. The fundamental necessity of designing joints that are capable of being brazed automatically when components fabricated from alu-minum are being considered

3. The undisputed accuracy of a further fundamental rule of brazing:

It is almost never sensible to use the most intensive heat source available when selecting a heating method for brazing.

As discussed in Chapter 9, it is very important to follow this rule when flame brazing aluminum.

1.4.5 Dezincification

Depending on the environment in which the finished joint is to operate, in situations where either the brazing filler material or the parent metal contains zinc there is an ever present risk that the zinc component of the parent material or of the filler material will be preferentially dissolved.

This phenomenon is known as dezincification. In normal brazing practice, it is found that joints exposed to seawater are particularly prone to this form of accelerated failure. Research has shown that where silver-contain-ing brazsilver-contain-ing alloys are employed as the filler material when ternary Ag-Cu-Zn alloys are used, a minimum silver content of 43% is necessary to ensure that it will not corrode by this mechanism. For the Ag-Cu-Zn-Cd materials, the minimum silver content required is 50%. An addition of nickel to the filler material enhances the material’s resistance to dezincification failure in such situations.

The potential for dezincification to occur in assemblies exposed to seawater explains the wide use of copper–nickel alloys and specialized admiralty brasses as the parent materials of first choice in naval applications. Failure to select filler materials that satisfy the above criteria can lead to the type of situation illustrated in Figure 1.12. This illustration, which shows an excellent example of dezincification of a filler material in a brazed joint, is a photo-micrograph of a longitudinal section through the wall of a tube-to-tube joint brazed with a 30% silver-copper-cadmium-zinc filler material. In service, the tube carried seawater. Three problems exist here:

1. The joint design is a butt, yet good brazing practice demands the use of a lap joint. (See Chapter 2.)

2. To compensate for the unsatisfactory joint design, the operator is required to use the relatively long melting range of the filler material to build up a very substantial external fillet of alloy.

3. The filler metal that has been selected is prone to dezincification failure if exposed to seawater.