Brazing Handbook - Corrosion Resistance and Special brazing processes

Cleaning after brazing

Normally, no cleaning is needed after the brazing operation.

lnternal corrosion

The corrosion resistance of CuproBraze radiators is generally better than that of soldered radiators an d very competitive with that of aluminum.

When different metallic materials are used in the same cooling system, questions sometimes are raised about possible micro- galvanic corrosion risk, considering that the noble metal (copper) deposits on the unnoble metal (aluminum) surfaces. Inhibitor systems in the coolant are designed to prevent all kinds of corrosion in the cooling systems, including microgalvanic corrosion and, for this reason, the maintenance of the coolant is important. In general, however, copper alloys are less sensitive to a bad coolant than aluminum.

Corrosion test results in cool ants for CuproBraze materials (including tube brass SM 2385 and header brass SM 2464) are similar to test results for copper-based materials used in soft-soldered radiators. In a study on a mixed-metal cooling system, there was no indication of micro-galvanic corrosion on aluminum caused by copper. Therefore, the coolants th at fulfill the standard requirements with copper materials are considered compatible with materials used in CuproBraze radiators, and vice versa. CuproBraze heat exchangers are compatible with mixed-metal cooling systems.

External corrosion

The risks of external corrosion caused by galvanic attacks is minimized by the fact that the materials in CuproBraze radiators, including the copper-fin alloy, brass- tube alloy and the brazing alloy, have about equal mutual nobility. The brass-tube alloy contains 85 percent copper, which means that the alloy is less sensitive to stress corrosion cracking and dezincification than conventional brass alloys.

The brazing alloy OKC600 (CuNiSnP -type) also provides an extra protective and mechanically strengthening coating for tubes. Corrosion test results have been published. The references were soldered copper-brass radiators and brazed aluminum radiators. According to the results from four different kinds of accelerated corrosion tests, CuproBraze radiators were generally more corrosion resistant than soldered copper-brass radiators and very competitive with aluminum radiators. Generally, aluminum radiators were more prone to localized corrosion forms, whereas the corrosion form on CuproBraze was usually even and thus predictable.

Coatings

CuproBraze radiators are mechanically strong and facilitate thickness reductions High-performance coatings further improve corrosion resistance and make thickness reductions possible without risks from corrosion.

There are different ways to increase external corrosion resistance when reducing thickness. The easiest way is to leave the commonly used cosmetic coating off totally. An uncoated radiator has a lifetime that is about 30 percent longer than the lifetime of a cosmetically spray-coated radiator.

Electrophoretic coating is the best technical solution to increase corrosion resistance. It increase s the lifetime by 2.5 to 3 times compared to an uncoated radiator . A new option is powder coating with a multi-nozzle spray gun, which gives good results with respect to corrosion resistance and thermal performance and has a lower cost compared to an electrophoretic coating.

High-performance coatings cover the entire external radiator surface (not just 10 percent of it, as is the case for most conventional cosmetic-spray coatings). They clearly prolong the lifetime of the radiators. These technologies are commercially available.

One shot brazing

One shot brazing is when all or most of the joints in the heat exchangers are brazed simultaneously in one brazing cycle and there are no metallurgical differences between one shot and other brazed joints. If the brazing parameters for all joints are fulfilled, the joints will be satisfactorily brazed. There are some points to be noted. If the one-shot brazed heat exchanger includes inner fins or other kind of joints inside the component, it must be ensured that the atmosphere around all joints fulfill the recommendations for the atmosphere stated in chapter 6.1. In most cases, purging inside the heat exchanger with nitrogen is necessary. To ensure good brazing results in all joints, the joint geometry as well as fixturing and paste application should be designed for furnace brazing. During the brazing process some movements between parts in the joints can take place due to stress relieving in stamped parts and also due to differences in thermal expansion between the parts . When designing for one shot brazing th is should be taken into account. If possible, self-fixturing of the parts should be used.

The normal design for soldered tank-header joints, in most cases, has gaps that are too large for satisfactory brazed joints. The design for brazed joints should be in the recommendation earlier in this handbook, an optimal joint gap of 50 µm to a maximum 100 µm.

Figure 28 shows a norm a l geometry for soldering of header-tank. In figure 29 some improved header-tank designs are shown.

Figure 28. Typical joint for soldered header-tank. Gaps are too large

Figure 29. Alternative joint geometries for header-tank.

Many of the joints in “one shot” brazing are placed upside down or horizontally, it is therefore important to use a brazing paste suitable for this kind of joints.

The brazing paste should (if possible) be applied just outside the joints. During brazing the molten filler metal will b e drawn into th e joint by capillary force.

Brazing of parts with internal turbulators (CAC)

If fin tip application is used, fum e from the binder is form e d during the first part of the brazing cycle. Experience has shown that it is difficult to evaporate all fum e s from the inside of the tube s be fore the brazing takes place, consequently having a bad influence on the brazing result. Therefore to avoid the risk of scraping off the paste on the tips during assembling, brazing foil is recommended to be used as filler metal for internal turbulators, especially for charge air coolers (CAC). The foil can be inserted together with the turbulator s from the end of the tube, see figure 30.

Figure 30. Brazing foil used for internal fin brazing.

When turbulators similar to norm a l fins for heat exchangers are used, they should be designed to take up some elastic movements during the brazing cycle. If very stiff turbulators are used they can form a split in the sample after brazing.

Splitter-fin together w ith CuproBraze

Splitter-fin is a method to solder thin copper fins to a centre strip, which together forms a strong fin-module, which is easy to use and has improved heat performance (figure 31). The splitter-fin-module is so far not possible to produce with CuproBraze brazing-paste but soldered splitter-fins are sometimes used in CuproBraze heat exchangers. In this case the fin material is SM 0502 (the normal CuproBraze fin ma terial) and the soldered joints (lead -free) are used as “pre-joining). W hen t he soldered joints between the fins and the centre strip are heated up to brazing temperature, the joints will be transformed to a copper-tin phase with similar composition appearing at the brazed fins in the CuproBraze process.

Figure 31. Splitter-fin modules to the left and the module inserted in a core to the right.

Brazing of steel parts

Brazing any steel parts is not recommended, due to formations of brittle Fe-P-film between the filler and the base metal.

Brazing Handbook - Selecting a furnace

Selecting a furnace

Factors to consider when select ing a suitable furnace are production volume, part size, available floor space, capital expense, and operating cost. Based on these specifications, the CuproBraze heat exchangers can be processed in batch, semi-continuous or continuous furnaces.

Selecting a suitable furnace requires knowledge of the temperature, time and atmospheric conditions of the process. All of the furnaces have heating and cooling sections. Batch furnaces and semi-continuous furnaces are suitable for any part size but limited with respect to production volume. Continuous furnaces are suitable for high volume production.

Batch furnace

A batch furnace uses the same door to load and unload the part. These furnaces can only produce one batch at a time. A load is purged with nitrogen then moved into the brazing chamber; after brazing, the load is moved back into the purge chamber where it is cooled.

Semi-continuous furnace

In a semi-continuous furnace parts are indexed from the loading area to the purge chamber, where the part is purged with nitrogen and then moved into the next chamber. The furnace simultaneously moves the purged part into the brazing chamber and a new part into the purge chamber. This type of furnace is suitable for large parts or intermediate volume production.

Continuous furnace

A continuous furnace uses a conveyor-belt to continuously move parts through the furnace where they are continuously purged with nitrogen, brazed and then cooled. This type of furnace is for high-volume production. A continuous furnace is not recommended for parts longer than 1000 mm because when the front of the part enters the heating zone, it conducts heat to the rear of the part. As a result, the trailing section of the part is held at temperature for a much longer time than the leading edge of the part.

Heating source

It is possible to heat all three types of furnaces with electricity, natural gas, or propane/butane. In many countries, natural gas and propane are a cheaper source of energy than electricity, but they require more maintenance and have a higher initial cost. Gas burners also require a gas-tight barrier between the combustion products and the brazing atmosphere. Such a barrier can be radiant tubes or a muffle.

Process emissions

Process emissions are generated when the binder is volatilized during the first part of the heating cycle. These emissions must be properly managed to prevent contamination of the atmosphere in the furnace. The constant flow of nitrogen normally expels the vapour from the brazing atmosphere. As there is no oxygen available to burn the gas products in the furnace, the gases mu st either be burned outside the furnace or diluted with ambient at mosphere, according to the local regulations, requiring, in most cases , some kind of afterburner. The laws vary from country to country and state to state, so one must check with the local authorities before designing a furnace. The generated emission is influenced by the binder system and could be totally different between paste ma nufacturers. Further information regarding the emissions, and further handling of them, are obtainable from paste manufacturers.

Important: Most of the binders form emissions. Contact the paste manufacturer to check if any kind of afterburner must be used.

Brazing Handbook - Brazing operation

Brazing operation

Because the amount of flux in the CuproBraze process should be zero or absolutely minimal, and the parts should be oxide free after brazing, an inert atmosphere is needed to prevent oxidation of the parent and filler materials. As the brazing temperature is much lower than the melting points for the copper and brasses, the temperature differences in the parts during the brazing process are not critical. As the CuproBraze process covers parts from around 100g up to more than 100kg it is not possible to advise exact settings of the furnaces.

Atmosphere

The primary function of the brazing atmosphere is to prevent oxidation. Furnaces for the CuproBraze process use high-purity nitrogen to displace oxygen from inside the furnace. The atmosphere of the furnace must have an oxygen content of less than 20ppm. The brazing powder is very sensitive when the binder starts to evaporate, If moisture and oxygen levels ar e higher than these levels, the powder and the base material have a risk of oxidation at temperatures exceeding about 200ºC the and the result is very poor joints. Thus the starting point of the brazing cycle is as sensitive for oxygen-content as the rest of the brazing cycle.

Mixing the brazing atmosphere with small amount of hydrogen (H ₂) is not generally recommended. Hydrogen can sometimes react with the organic binder forming products which can have an influence on the brazing result. Before using hydrogen or hydrogen mixed atmosphere, the paste manufacturer should be consulted.

Important : The oxygen content should be controlled from the time when the heating starts and no heat should be applied if the atmospheric conditions are not met.

Temperature and time

The difference in the melting points for the filler metal and the copper and brass materials is more than 300ºC, which means that there is no risk to destroy the parts by melting. The temperature above the melting point for the filler metal (600ºC) should be as short as possible but still gain a satisfactory brazing result. It means that the furnace must be able to heat up the load in the brazing zone with a steep ramp. A common value is more than 30ºC per minute. The furnace must be able to operate up to 700ºC. Figure 27 shows a principle temperature-time curve for the CuproBraze process.

Figure 27. Pr inciple temperature-ti me curve for the CuproBraze
process.

In the part A, the sample is slowly heated up and the binder evaporates and/or is decomposed. When the binder disappears, it leaves the particles in the brazing paste without any protection from oxidation if the oxygen content is too high. The oxygen content should therefore be controlled from the time when the heating starts and no heat should be applied if the atmospheric conditions are not me t. The brazing result will be poor if the brazing powder is oxidized before it starts to melt. Note that some big heat exchangers as well as “one shot” parts can include a lot of s mall half closed volumes, which could influence necessary time to reach satisfactory oxygen content. Good convection in the furnace is therefore recommended.
By the rather slow heating rate in this zone, the temperature differences in the samples are minimized, and the distortion of the sample due to heat expansion can also be minimized.

In part B th e whole co re will be p reheated to just under the melting point of the brazing metal. To minimize the temperature-difference in the core at brazing, the part B should be designed so the temperature in the whole sample is as equal as possible when the brazing part is entered. In some batch furnaces, there is no pre-heating and in that case furnace settings that minimize the temperature differences in the sample have to be used.

Part C is the brazing period. When the temperature exceeds 600ºC, the brazing filler metal (powder or foil) starts to melt. When it melts, metallurgical reactions (diffusion) starts and the extent of the filler-substrate interaction is the most important. The filler interaction on the fin material is when it starts to be alloyed with tin, forming a copper-tin alloy close to the joint. It does not influence the performance except for exceptional long brazing times, where some loss of thermal performance (up to 10 %) of the heat exchanger can occur. The governing factor for the brazing cycle in most cases is the brazing of the tube-header joints. In chapter 5 figure 11 it is shown that the brazing temperature has a big impact on the possibility to satisfactory fill gaps. In practice it has been found that to satisfactorily wet the surfaces and fill the joints, you should ensure that the temperature in the joints reaches 650ºC or for some cases even 670ºC.

As the alloying reaction starts when the molten filler metal wets the surfaces, the time above 600ºC should be as short as possible but long enough to reach complete brazing in tube-header joints., For small radiators, 3 to 4 minutes is typical. For bigger parts the time is guided by the tube-header brazing.

To reach short brazing times, usually the setting of the brazing part (A) of the furnaces is higher than 650ºC.
The effect of the brazing cycle on the tube-to-fin joints cannot be seen by the naked eye. During optimization of the brazing cycle, overshooting of the brazing temperature can sometimes happen, but it will not lead to any noticeable visual effect on the brazed heat exchanger as the melting point for copper and brass is far higher than the brazing temperature.

To minimize the risk for distortion of the joints during cooling (part D), it is recommended to have a low cooling rate down to around 550ºC, typical value 1ºC/s.

To prevent discoloration of the brazed parts, they should not leave the inert atmosphere until the part temperature is below 150ºC. In places where the ambient humidity is high the exit temperature should be even lower to prevent discoloration. Note: This discoloration is only a cosmetic effect and it will not deteriorate the brazed joint.

At least during optimization of the brazing cycle, it is highly recommended that some kind of measurement of the temperature with thermocouples mounted in the core should be used. To have full control on the brazing process, it is recommended to also have this equipment available to check the process every now and then during normal production. If it is not possible to use the thermocouples together with equipments outside the furnace, it is recommended to use them with a tracker following the sample through the furnace.

The brazed part should be cooled down as uniformly as possible at least in the first phase to prevent deformation. The temperature drop should be equal in the whole core. One way to achieve this is to slow down the cooling to around 550ºC. At that temperature, the filler metal in the joints is no longer molten.
To satisfactorily wet the surfaces and fill the joints, ensure that the temperature in them reaches 650ºC or for some new types of header pastes even 660ºC.