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Considerations
Fixtures are used to hold the braze assembly in place during brazing. Surfaces with molten filler metal are very “greasy” and the fixtures need to hold the shape and tolerances during heat-up. Fixtures may also be used to support attachments such as inlet or outlet tubes.
When considering the type or configuration of fixtures to use, there are a number of considerations to take into account. For example, differential expansion between fixture and braze assembly increases part compression significantly during heat-up. One must be acutely aware of the differences in the coefficients of thermal expansion between stainless steel and aluminum. Aluminum expands much faster than stainless steel and this must be taken into consideration when designing a fixture. This is important to prevent distortion of the heat exchanger at final brazing temperature.
It is also important to note that molten filler metal dissolves steel and stainless steel. It is important to minimize contact with filler metal. It is also possible for aluminum to braze to fixtures. It is therefore important to either use a brazing stop-off for surfaces in contact with aluminum or to oxidize the fixtures when new or after cleaning. This can be done simply by running the fixtures through the brazing furnace.

Permanent Fixtures
The most common type of fixtures for heat exchanger manufacturing are permanent fixtures, ones that are used over and over again. These are usually made and should be made from stainless steel to prevent rust contamination in the slurry tank. The preferred material for fixtures is AISI 309 or 316, but most stainless steel alloys are perfectly acceptable.
Springs may be used in the fixture to apply a certain “holding” pressure to the heat exchanger during brazing. However, the technique of using springs seems to be less common than in the past. More often now, fixtures are designed with fixed dimensions. The heat exchanger is compressed slightly and loaded into the fixture. When the source of compression is removed, the natural spring-back holds the heat exchanger in place against the fixture.

Cleaning
As flux builds up on permanent fixtures and may contaminate the flux slurry, it is necessary to routinely clean the fixtures to remove flux and other contaminants that may have accumulated. There is no convenient chemical cleaning method to remove flux residues. The most appropriate methods are by mechanical means such as wire brushing or grit-blasting.
Once the residues have been removed by one of the above methods, the fixtures should be oxidized by running them through the brazing furnace. Non-oxidized fixtures are likely to stick or even braze to the work piece.

Steel banding
An alternative to permanent fixtures is the use of disposable steel banding. Since mild steel can be used, material costs are kept to a minimum. Wax coated mild steel bands are often used to prevent the banding material from rusting that can contaminate the flux slurry and discolor the heat exchanger. The steel bands are used only once and are disposed of after brazing.
Steel banding requires experimentation to determine the appropriate tension and positioning. Thereafter, an automatic banding machine should be used to ensure consistency.

The European Association for Brazing and soldering — EABS for short — together with experts from Solvay Fluor, holds technical training seminars in which the theory and practice of flame and furnace aluminium brazing are communicated in detail.

40 interested participants from all over the world gather for the two day seminar in Hannover, Germany: technical staff, design and production engineers as well as production engineering managers.

EABS Seminar

More information about the seminar.

How to obtain?

A lot of information can be gained from heat exchanger brazing cycle temperature profile. It is probably one of the most important pieces of information that the brazing engineer can use to fully understand his process. A temperature profile will provide information such as heating rate, maximum peak brazing temperature, time at temperature, temperature uniformity across the heat exchanger and cooling rate. No other tool can provide so much information.

The simplest method for obtaining a temperature profile is to attach thermocouple wires to various parts of the heat exchanger and graphing the resulting profile on a chart recorder. The disadvantage of this method is that the thermocouple wire must be long enough to traverse the length of the furnace. One must also ensure that the wire does not become entangled in the mesh belt.

The second and more common (also more expensive) method of obtaining temperature profiles is with the use of a thermally insulated data pack. The data pack is a stand-alone unit capable of withstanding brazing temperatures. The thermocouples wired into the data pack are attached to various parts of the heat exchanger. The data pack then travels on the belt with the heat exchanger through the brazing furnace. At the end of the run, the data stored in the data pack is downloaded into a computer where graphs can be generated. The sophisticated software allows the user to determine quickly a number of parameters such as maximum temperature reached by each thermocouple.

Recent advances in thermal profiling allows getting information in real time. The thermally insulated data pack transmits data in real time from inside the brazing furnace to a computer situated outside the furnace using the latest radio telemetry technology. Changes to the furnace settings can now be seen instantly1.

Heating Rate

A minimum average heating rate of 20°C/min up to the maximum brazing temperature is recommended. With very large heat exchangers such as charge air coolers, lower heating rates may be used, but with higher flux loadings. Once the flux starts to melt, it also begins to dry out. With slower heating rates, it is possible that the flux can be sufficiently dry as to loose its effectiveness when the filler metal starts to melt or before the maximum brazing temperature is reached.

Heating rates up to 45°C/min in the range of 400°C to 600°C are not uncommon. One could say that the faster the heating the better. However, temperature uniformity across the heat exchanger must be maintained especially when approaching the maximum brazing temperature and this becomes increasingly more difficult with fast heating rates.

Maximum Brazing Temperature

For most alloy packages, the recommended maximum peak brazing temperature is anywhere from 595°C to 605°C and in most cases around 600°C.

Temperature Uniformity

During heat up, there may be quite a variation in temperature across the heat exchanger. The variation will tighten as the maximum temperature is reached. At brazing temperature it is recommended that the variation should not exceed ± 5°C. This can be difficult to maintain when larger units are processed which have differing mass areas within the product.

Time at Temperature

The brazed product should not remain at the maximum brazing temperature for any longer than 3 to 5 minutes. The reason is that a phenomenon known as filler metal erosion (core alloy dissolution / Silicon penetration into the base material) begins to take place as soon as the filler metal becomes molten. And so the longer the filler metal remains molten, the more severe the erosion is.

The graph below shows an actual temperature profile for a heat exchanger brazed in a tunnel furnace. One characteristic feature of all temperature profiles is where the curve flattens out when approaching the maximum peak brazing temperature (area shown in blue circle). The plateau in the temperature profile is associated with the start of melting of the filler metal at 577°C, known as the latent heat of fusion. It is called latent heat because there is no temperature change when going from solid to liquid, only a phase change.

Temperature profile for a heat exchanger brazed in a tunnel furnace.

1 D. Plester, Datapak Ltd., International Congress Aluminium Brazing, Düsseldorf (2002)

Furnace atmosphere

The recommended furnace atmosphere conditions necessary for good brazing are as follows:

  • Dew point: ≤ -40°C
  • Oxygen: < 100 ppm
  • Inert gas: nitrogen

The most common source of nitrogen is that generated from liquid nitrogen storage tanks. A typical nitrogen gas specification from a liquid source indicates that the moisture content is <1.5 ppm (dew point = -73°C) and an oxygen level of <3 ppm. In brazing furnaces however, the normal atmospheric operating conditions almost always exceed incoming nitrogen contaminant levels. This is due to water and oxygen dragged into the furnace by the incoming products, by the stainless steel mesh belt and by the potential back-streaming of factory atmosphere through the entrance and exit of the furnace. The latter will occur when the exhaust and incoming nitrogen are not properly balanced.

Many furnaces are equipped with dew point and oxygen measurement devices. It is important that the measurements are taken in the critical brazing zone of the furnace because this is where these impurities will reach their lowest concentrations. Measuring dew point or oxygen levels anywhere else in the furnace may be of academic interest, but will not represent actual brazing conditions.

Dew Point Measurement

Measuring the moisture content in the critical brazing zone of the furnace has always been a key indicator of the quality of the brazing atmosphere. Moisture can substantially influence the quality and appearance of the brazed heat exchanger as well as the first time through braze quality (% rejects).

Chilled Mirror Technology

One of the more common principles of measuring dew point is using chilled mirror technology. The measurement of the water vapor content of a gas by the dew point technique involves chilling a surface, usually a metallic mirror, to the temperature at which water on the mirror surface is in equilibrium with the water vapor pressure in the gas sample above the surface. At this temperature, the mass of water on the surface is neither increasing (too cold a surface) nor decreasing (too warm a surface).

In the chilled-mirror technique, a mirror is constructed from a material with good thermal conductivity such as silver or copper, and properly plated with an inert metal such as iridium, rubidium, nickel, or gold to prevent tarnishing and oxidation. The mirror is chilled using a thermoelectric cooler until dew just begins to form. The temperature at which dew is formed on the mirror is displayed as the dew point.

The advantage of the chilled mirror dew point meter is that it is an absolute measurement with high precision. However, this measurement technique is sensitive to pollutants and corrosive contaminants which, in the brazing process, include KAlF4 condensation and trace amounts of HF gas. Consequently, the mirror requires frequent maintenance and replacement. “Dirty” mirrors can lead to false readings.

Coulometric Measurement Principle

The principle of operation for measuring is that an electrolyte is formed by absorption of water on a highly hygroscopic surface (e.g. P2O5) and the current level obtained to electrolyze the surface is proportional to the water content. The advantage of this principle of operation is that it is insensitive to aggressive media. The disadvantage is that the precision is not as high as chilled mirror technology. Some heat exchanger manufacturers have reported good success using this measurement principle in their CAB furnaces.

Relationship between dew point and moisture content

The relationship between dew point and moisture content is not linear. It is important to note that small changes in dew point will result in large changes in actual moisture content. This is evident from the graph shown below.

As manufactured, a non-corrosvie K-Al-F-type flux typically is a mixture of potassium tetra-fluoroaluminate (KAlF4), and also contains potassium penta-fluoroaluminate (K2AlF5). K2AlF5 exists in different modifications: potassium penta-fluoroaluminate hydrate (K2AlF5 · H2O), and hydrate-free (K2AlF5).
During the brazing process, the material undergoes essential physico-chemical alterations. While the chief component, KAlF4, is simply heated up, the compound K2AlF5 · H2O begins to lose its crystal water from 90°C (195°F) on. When the temperature is further increased within the ranges of 90° – 150°C (195°F – 302°F), and 290°C – 330°C (554°F – 626°F), two different crystallographic (structural) modifications of K2AlF5 are formatted.

When the furnace temperature is raised above 490°C (914°F), K2AlF5 begins to react chemically. According to the equation:

2 K2AlF5 → KAlF4 + K3AlF6 (Equation 1)

the exact amount of potassium hexa-fluoroaluminate (K3AlF6) necessary for a eutectic flux composition (i.e. mixture of two or more substances which has the lowest melting point; see phase diagram) is obtained from the original K2AlF5 content. At brazing temperature, the resulting flux composition has a clearly defined melting range of 565°C to 572°C (1049°F – 1062°F). The flux melts to a colorless liquid.
Due to a vapor pressure of 0.06 mbar at 600°C, some of the KAlF4 evaporates during the brazing cycle, particularly once melting temperature is reached. The total content of KAlF4 contained in the exhaust is depending on time and temperature. Based on results from TGA analysis (with a heating rate of 20°C/min), the quantity of volatile compounds in Flux between 250°C and 550°C (482°F and 1022°F) is approximately 0.2 to 0.5%. These flux fumes contain fluorides and have the potential to react with the furnace atmosphere, especially moisture, to form hydrogen fluoride according to the equation:

3 KAlF4 + 3 H2O → K3AlF6 + Al2O3 + 6 HF (Equation 2)

This is one of the reasons, why the brazing process should take place in a controlled atmosphere (nitrogen) with low dew point and low oxygen level (another reason is to minimize re-oxidization effects on the aluminum surfaces).

Directly after brazing has been completed, flux residues consist mainly of KAlF4 and K3AlF6. In the presence of moisture from the surrounding atmosphere, the K3AlF6 is converted back to K2AlF5 · H2O over time (several days) in a reaction reverse to the one described in equation 1 followed by a re-hydration step.

The schematic below illustrates the transformations that occur as the flux is heated to brazing temperature. Note that these phases are unstable outside the furnace atmosphere.

Very often, heat exchanger manufacturers increase the flux loading on components to be brazed to compensate for furnace atmosphere or other process related deficiencies. The flux is an excellent “band-aid” and can be used as such, but only while the true problems are located and rectified. Long term use of higher than recommended flux loads can lead to other problems.

Over fluxing causes more KAlF4 evaporation and condensation. This will load up the dry scrubber more quickly. White powder will accumulate more quickly on the curtains at the exit end of the furnace. If this is noticed, there is a very good chance that the dry scrubber is loading up more quickly.

There will be a more rapid build-up of the flux inside the furnace. This is a common issue with over fluxing whereby flux builds up on the muffle floor at the entrance to the cooling zone where it will solidify. This flux build up has been known to deflect the mesh belt.

There is more rapid build up of the flux on the fixtures which can significantly reduce maintenance intervals.

Over-fluxing can lead to visible flux residue on the brazed heat exchanger which may increase the incidence of flux residue fall-off. Excess flux residue dulls the appearance of a brazed heat exchanger and can also accumulate in the gasket areas causing problems with seals. Too much flux residue will also inhibit surface treatments such as painting or conversion treatments.

The theoretical amount of flux required to dissolve a 100 Å oxide film is about 0.02 g/m2
(1 Å = 10-10 m = 0,1 nm). For a 400 Å film, still only 0.08 g/m2 flux is required. These do not take into account losses to moisture, oxygen or poisoning of the flux by Mg alloy additions.

In practice however, the recommended loading for fluxing is 5 g/m2, uniformly distributed on all active brazing surfaces. This is more than 250 times the theoretical amount required for oxide dissolution. To visualize what 5 g/m2 flux loading might look like, think of a very dusty car. As the heat exchange manufacturer gains experience with his products, he may find that a little more is required for consistent brazing or that he can get away with a little less flux.

Too little flux will result in poor filler metal flow, poor joint formation, higher reject rates, and inconsistent brazing. In other words, the process becomes very sensitive.

Too much flux will not affect the brazing results. However there will be pooling of flux which can drip on the muffle floor, the surface of the brazed product will be gray and there will be visible signs of flux residue. Furthermore, flux will accumulate on fixtures more rapidly which then requires more frequent maintenance. More importantly yet, using too much flux will increase the process costs.

In some cases, heat exchanger manufacturers use higher than recommended flux loadings to mask furnace atmosphere deficiencies. This should be viewed as a short-term solution and the furnace problems should be addressed.

See also: How to evaluate flux load?

Dust and dirt, condensates, lubricants and oils must be thoroughly removed. If the metal work pieces are poorly prepared, the flux will not spread evenly and the flow of filler alloy will be haphazard: it will either not spread properly or will discolour. The consequence would be an incomplete joint.

The first step is therefore: always clean the components of all oil and grease. The surfaces can be cleaned using either chemical, water-based or thermal cleaning techniques and substances.

Aqueous Cleaning

Aqueous or water based cleaning is a quite efficient and robust process, but still generates some waste water.

Aqueous cleaning starts off with a concentrated metal cleaning agent, which is subsequently diluted with water to 1% to 5% (v/v). The composition of a supplier’s cleaning solution is proprietary, but usually contains a mixture of surfactants, detergents and active ingredients such as sodium carbonate that serves to elevate the pH. Once diluted, the cleaning solution will typically have an elevated pH in the range of pH 9 to 12. There are acid based solutions, but appear to be less common.

The best water-based cleaners contain water, tensides, cleaning agent and active ingredients such as carbonates.

The cleaning solution works best at higher temperatures and is usually recommended to operate at 50°C to 80°C. Cleaning action is quicker at higher solution temperatures.

Thermal Degreasing

Thermal degreasing works by elevating the temperature of the work piece so that lubricants present on the surfaces will be evaporated. This procedure only works with special types of lubricants known as evaporative or vanishing oils. Vanishing oils are light duty lubricants used mostly for the fabrication of heat exchanger fins, although they are now finding uses in the stamping and forming of other heat exchanger components. Lubricants not designed for thermal degreasing must not be used. These could leave behind thermal decomposition products and carbonaceous residues which at higher level prevent brazing and have the potential to degrade product appearance and accelerate corrosion.

Brazing sheet comprises of a core alloy clad on 1 or 2 sides with a lower melting aluminum-silicon (Al-Si) alloy. This thin layer, usually makes up 5 % to 10 % of the total thickness of the brazing sheet.

It melts and flows during the brazing process, to provide upon cooling a metallic bond between the components. It is common that the braze clad alloy are from the AA 4xxx series or more particularly AA 4343 (Al-6.8~8.2 wt.% Si).

However, if larger fillets are desirable, or if in a situation where brazing is likely to occur at lower temperatures, AA‑4045 is the preferred choice.

Manufacturing

Cross-section morphology of a double-side clad tubestock.

Aluminium brazing sheet is manufactured by roll-bonding techniques to clad a core alloy ingot on one side or both sides with a low melting AlSi alloy. As an alternative, one side can be clad with a non-braze alloy, e.g. Zn-containing alloy.

Depending on the desired final properties, the core is either homogenized or not before the cladding operation.
The whole package is subjected to preheating, hot rolling and cold rolling down to the final thickness of the respective products. Depending on the requested final properties, the material is subjected to final annealing and / or intermediate annealing operation(s).

The core provides structural integrity. It is common to use a variety of aluminium alloys, examples being AA 3xxx (AlMn), more particularly AA 3003, AA 3005, AA 3105 and modified versions of AA 3003 or AA3005 (long life alloys).

Phase diagram

Aluminum End of Al-Si Phase Diagram

The melting characteristics of the cladding alloys are governed by the Al-Si phase diagram. The eutectic composition, i.e. the amount of Si required to produce the lowest melting point is 12.6%. The melting point at this composition is 577°C. At lower Si levels the solidus or the point at which melting begins is also 577°C. However, melting occurs in a range and the temperature above which the filler is completely molten is called the liquidus. In between the solidus and liquidus, the filler is partially molten, existing both as liquid and solid. The difference between the solidus and the liquidus forms the basis for various filler metal alloys.

The table below shows the solidus and liquidus of common brazing alloys.
[table id=2 /]

The higher Si alloys (e.g. AA4047) have higher fluidity and a narrower melting point range while the lower Si alloys have less fluidity with a wider, higher melting point range. Erosion of the base metal occurs when the braze alloy dissolves part of the core alloy. The extent of erosion is increased by:

  • Higher Si levels in the braze alloy
  • Longer braze cycles
  • Excessive peak brazing temperatures
  • Excessive thickness fo the braze metal layer
  • A design which allows pooling of the braze metal to occur

For added strength and machineability, certain alloys contain Mg. Most notably are the 6XXX series alloys (up to 1% Mg) that are used for fittings and machined components and the so-called long life brazing sheet alloys (up to 0.3% Mg in the core). There is a limit to the amount of Mg tolerated in NOCOLOK® Flux brazing. Up to 0.5% Mg can be tolerated in furnace brazing while around 1% Mg is tolerable for flame brazing.
When an Al alloy containing Mg is heated, the Mg diffuses to the surface and reacts with the surface oxide to form MgO and spinels of MgO:Al2O3. The diffusion is time-temperature dependent and is rapid above 425°C. These spinel oxides have reduced solubility in the molten flux. Furthermore, Mg and/or MgO can react with the flux forming compounds such as MgF2, KMgF3 and K2MgF4. All of these serve to poison the flux and significantly reduce its effectiveness.
In flame brazing, higher Mg concentrations can be tolerated since the faster heating rates do not allow the diffusing Mg enough time to appreciably decrease the beneficial effects of the flux. Flame brazing components containing > 1% Mg may be possible under some circumstances with increased flux loadings and very fast heating rates (<20 second braze cycle).
It should be noted that when one speaks of the brazing tolerance to Mg, it is the total sum of the Mg concentrations in both components:

[Mg] component 1 + [Mg] component 2 = [Mg] total

The figure below shows the effect of Mg on fillet size and geometry:

0.1% Mg

0.4% Mg

If the user is experiencing difficulties brazing and suspects elevated Mg levels as the cause, there are a couple of ways to be sure. First, check with the supplier of the alloys or perform a chemical composition analysis on the suspect alloys. This is the most certain way. Secondly, look for a golden hue on the brazed product. This is an indication that Mg alloys are being used and the color is a result of the increased oxide thickness. Furthermore, there may be a very light, almost fluffy residue on the brazed component that can literally be blown off by mouth. These visual indicators can most certainly be traced back to poor brazing results due to the presence of Mg.

Improving brazeability
There are a few ways in which the brazeability of Mg containing alloys can be improved:

  1. Increasing the flux loading. A substantial improvement is gained when increasing the flux loading up to 10 g/m2 or more in furnace brazing. In cases where there is just one component containing Mg such as in a fitting, extra flux can be brushed around the area of the joint.
  2. Increasing the heating rate. Slow heating rates allow more Mg to diffuse to the surface thereby hindering brazeability. For furnace brazing Mg containing alloys, the fastest possible heating rates achievable without sacrificing temperature uniformity will increase the tolerance to Mg.
  3. Combining increased flux loadings and faster heating rates.
  4. Maintaining proper gap tolerances and joint designs.
  5. Increasing the nitrogen flow rate to minimize furnace atmosphere contaminants that also compete to reduce brazeability.

Tip: NOCOLOK® Cs Flux

Better results are reported when using cesium containing fluxes for aluminum alloys containing Mg up to 0.6 – 0.8% Mg. Fewer leaks are observed when compared with standard flux and less porosity is noted in the joint areas. Furthermore, standard flux loads and braze cycles can be used with Cs containing fluxes.

NOCOLOK® Cs Flux is a flux of the general formula KxCsyAlFz where Cs is chemically bound. It has a melting range of 558°C – 566°C. The maximum Cs content is limited to 2% to keep the cost of the flux down. Increasing the Cs content does not increase brazeability as shown below:

Cesium reacts as a chemical buffer for Mg by forming CsMgF3 and/or Cs4Mg3F10. The flux inhibiting factors of Mg are therefore reduced.