Selective Pre-Fluxing with Adhesives – Fashion or Progress

Summary

Over the last 15 years, selective pre-fluxing – also called paint fluxing or binder-based fluxing – has evolved as an alternative method for applying flux powder in aluminium brazing industry. There are many activities to define process parameters of fluxing with adhesives.

The first part of this paper outlines key features of pre-fluxing. The methodology for measurements of physical characteristics of binders and paint flux mixtures are described. General rules for behaviour of flux paints in brazing process are discussed together with some examples of flux paint features.

In the second part a case study is shown to illustrate common challenges when brazing with flux paint. The last part of this paper provides a cost comparison as guidance for choosing the right fluxing method for two different cases, one being extremely negative and the second as a positive case.

1. Introduction

In aluminium brazing industry fluxing is one of the most important steps in the production process. Flux water slurry application is considered as a standard and as a matter of fact very robust methodi. We believe that about 70% of overall fluxing activities are done by flux water slurry spray. This method has however certain disadvantages like troublesome slurry preparation and requirements for large and sometimes expensive machinery.

In order to lower production costs, new fluxing technologies have been introduced. One of them is called paint fluxing or binder-based fluxing which allows for elimination of wet fluxing process from the brazing line. Actually, the prefluxing process can be even done by an external company/subcontractor. It is nevertheless still a process of coating, which is done to particular component surfaces of the whole assembly – usually before the components are assembled.

In the industrial practise – particularly when one is in contact with many different users of the prefluxing technology – it is quite important to define the basic features and properties of discussed technology.

Flux Painting Booth

Flux Paint:
A mixture of brazing flux with binder, demineralised water or organic solvent, and thickener (the latter not always obligatory)

Binder:
Organic complex compounds, which are activated by curing – to provide adhesion of flux particles to the painted surface.

Thickener:
Organic substance, which is used to adjust viscosity and to facilitate re/mixing of the flux paint.

Curing:
Drying of the painted parts – usually with a blow of hot air (about 150°C). During that process liquid carrier (water or organic solvent) is evaporated and binder becomes activated to provide adhesion.

Adhesion:
Qualitative or quantitative measure by which strength of the flux particles bonding to the painted surface is determined. There are many different methods to describe adhesion of the flux paint. At Solvay we are using a simple quantitative method: A coated and cured coupon is placed in a holder positioned on a scale, a steel wedge is moved along the coupon with a gradual pressure increase. The weight at which the first scratches appear is a numerical value for adhesion.

Debinding:
Removal process of the binder from the painted surface done by treatment with high temperature, either in air or in ambient atmosphere.

Binder must be removed before reaching brazing temperature; otherwise the carbon residue will interfere with the brazing process – leaving both black stains on the part surfaces and very often leading to lack of brazing. Removal of the binder is done by applying high temperature to the assembled parts. In most cases the process of binder removal is realized in the brazing line both in the dryer and brazer.

2. Basic Rules for Flux Painting

The process of coating can be done by spraying, roller coating, brushing, or dipping. Uniformity of coating is very important, any agglomerates and lumps must be avoided. The flux paint can be prepared without thickener. Practically it is required in the mixture when a longer storage is expected (i.e. more then few days). There is an optimal temperature for curing resulting in maximum adhesion. Curing at ambient temperature is possible, but it will yield lower adhesion. Curing at too high temperature can lead to significant loss of adhesion.

Apart from the curing condition, adhesion also depends on the ratio of binder in the mixture (the more binder in the mixture the stronger adhesion), and the type of the binder (as a rule of thumb: the higher flash point of the binder the stronger adhesion). The most common method for prefluxing with binder mixture is using an atomized spray method on a machine which performs degreasing, painting and curing.

Will be continued soon.

This technique, also known as dry fluxing is gaining popularity as an alternative fluxing practice and therefore will be described here in some detail. Dry fluxing is a technology whereby the flux is electrostatically charged and applied to a grounded work piece, in our case a heat exchanger or individual heat exchanger components. The electrostatic attraction causes a layer of flux to be deposited on the work piece. A typical flux application system consists of a powder feed system, the electrostatic spray gun, the gun control unit, the grounded work piece and finally the flux recovery system.

The advantages of such a system over conventional wet fluxing are evident. Since the flux is applied dry, there is no need to prepare flux slurries, hence no need to monitor flux slurry concentrations. There is also no wastewater generated therefore more environmentally friendly. The dehydration or dry-off section of the furnace may be eliminated since the heat exchangers enter the furnace already dry. However, one must keep in mind that this is a relatively new fluxing technique and there are some minor drawbacks. Flux adhesion is not as good compared to that of wet fluxing. The flux also tends to accumulate on the leading edges of the heat exchanger and because of the Faraday cage effect, may have some difficulties in coating into corners or more specifically, in tube to header joints.

Powder Feed Systems
Presently, there are two types of powder feed systems on the market. The first type begins with the flux being fluidized in a hopper. Dry compressed air is fed through a porous membrane in the bottom of the hopper. The air rising through the volume of flux makes it behave like a fluid since the powder is essentially diluted with air. A pick up tube attached to an air pump is extended in the fluidized flux. Powder flow is then regulated by controlling the air-flow to the pump which is then delivered through the feed system to the spray gun. This type of feed system works perfectly well for powder paints. However, the flux has very different physical characteristics than powder paints (particle size, morphology) and so is difficult to fluidize. This must be taken into consideration when the manufacturer designs a powder feed system that relies on fluidization.

Dry Fluxing – Mechanical Flux Transport

The second type of powder feed system works on the principle of mechanical delivery or positive displacement. This means that the powder feed rate to the air pump is controlled by a screw or auger. The flux is contained in a main feed hopper and delivered mechanically at a controlled feed rate to the air pump. Powder flow is thus regulated by controlling the auger feed rate. This powder feed system does not rely on the flux being fluidized. Nonetheless, modifications over conventional mechanical powder feed systems are still necessary to overcome the differences between the flux characteristics and conventional powder paints.

Dry Fluxing – Powder Fluidization

Japanese heat exchanger manufacturers have used the technique of dry fluxing for many years now. Within the last few years, North American and European manufacturers have also installed electrostatic fluxing stations. Experience with this technique is being accumulated at a rapid rate, given that the equipment manufacturers and flux suppliers are taking an active role in improving the technology.

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.

Sample Preparation – Orientation

The heat exchanger chosen for the purpose is a NOCOLOK Flux brazed radiator. To provide some orientation as to where the metallographic sections will be taken from, Figure 1 shows the water-side header area (top) and part of the finpack, sidesupport and header area.

Figure 1

In most metallographic investigations of brazed heat exchangers, the critical joints to examine are the tube-to-header joints and the fin-to-tube joints. For instance, a leak in a tube-to-header joint constitutes a failure. The fin-to-tube joint on the other hand, although not as critical as the tube-to-header joint, is the key area where heat transfer takes place. It is therefore necessary that the fin-to-tube joints are metallurgically bonded (i.e. brazed) for maximum heat transfer efficiency.

Sample Preparation – Sectioning

With a band saw, the radiator can be cut down through the center of the tubes (see Figure 2). This will keep the fins intact. If necessary, one can saw through the fins if the blade is kept as close to the outer tube wall as possible. That is the outer tube wall can be used to guide the saw blade. Otherwise, it is better to saw through the center of the tube where the inner walls will act as the guide.

Figure 2

Once the above sections have been obtained, the samples can be sawn longi-tudinally through the center of the tube and header as shown in Figure 2, right. The tube-to-header and tube-to-fin sections can then be cut. The size of the cut samples must fit inside a 30 mm or 40 mm mount.  Note that it is the cut face that will be grinded and polished.

Grinding and Polishing

The following section shows what the sample actually looks like after each grinding and polishing step. The intention is to help the metallographer track the progress of grinding and polishing with the help of visual aids.

Figure 3 shows what a section of „unbrazed“ brazing sheet looks like under the microscope after wet grinding with 220, 500 and 1000 grit SiC paper. In each case, the sample is ground until all the grinding lines appear in the same direction, across the entire grinding face. It also helps that the grinding lines go in the direction of, or perpendicular to the area of interest. In this case, the grinding lines all run perpendicular to the braze sheet after 220 grit paper. After 500 grit paper (rotating the sample 90°), the grinding lines all run parallel to the tube and after 1000 grit, once more perpendicular to the braze sheet.

Figure 4 shows what the braze sheet looks like after each successive polishing step. After the 6 µm diamond suspension, the microstructure of the braze sheet becomes visible (more on microstructure later). At this stage, there are still a number of scratches. After the 3 µm diamond suspension, the micro-structure is clearer and there are less scratches.  Only after the colloidal silica are all scratches removed. The 64x magnification in Figure 4 is too small to reveal fine micro-structural details after the colloidal silica polish, but what is evident is that the braze sheet is now highly polished and scratch free.

Figure 3


Figure 4

Article from the Newsletter of our sponsor Solvay Fluor:
New glass brazing furnace in the NOCOLOK Technical Center

Many visitors to seminars, trade shows or videos are already familiar with the test glass brazing furnace in the NOCOLOK Technical Center. The unique furnace now has a big brother. All components of the new test furnace, except for the radiant heater, were developed in own production at Solvay.

The fluorine research workshop in Hannover has done an excellent job, “The construction of such a furnace is only possible with the tremendous expertise of the colleagues in the test workshop,” says Andreas Becker, a Solvay Fluor research employee. “With the new glass furnace, it is possible to braze larger objects, such as aluminium wafers for refrigerant test series for automobile producers.”

Specially developed software can capture every stage of the brazing process as high-resolution images – so that not even the tiniest detail of the brazing process can escape the testers. The new furnace saves energy and time – test brazing series with larger objects no longer require the much larger Camlaw brazing furnace at the Technical Center.

The next stage of development is already being planned: in a unique process Solvay’s glass blowers have succeeded in forming a square glass body, which offers even more space for larger items.

An overview of all services from the NOCOLOK Technical Center is offered in the new brochure, which is available for download.

Case Study

A radiator core retrieved from service was examined for a suspected premature corrosion related failure.
Upon closer metallographic examination, no evidence of corrosion was found at the failed area.

33% tube core erosion in the failure area


Header: AA4343/ AA3005

Tube: AA4343/ AA3003
 
 
 
 
 
 
 
 
It was concluded that the cause of the failure was in fact a mechanical failure occuring in the thinned wall area.

The following sequence of events proposes a rational explanation for the eroded tube area:

In service radiators are subject to internal pressure fluctuations and expansion and contraction due to heating and cooling. Mechanical failure was imminent and occured in the weakest part of the tube, the thinned down tube wall area adjacent to the tube to header joint.

Conclusions

Erosion of the base metal is undesirable since it reduces the wall thickness of the brazed component.
In addition Si penetration in the grain boundaries is known to increase the susceptibility to intergranular corrosion. Therefore proper filler metal management practices should be observed to prevent undesirable effects. One such factor easily controlled by the brazer is maximum peak brazing temperature.

Experimental

The effect of temperature on filler metal erosion was studied using an automotive radiator core.

610 ° for 2 minutes

610 °C for 2 minutes - no thinning of the tube core

625 °C for 2 minutes

625 °C for 2 minutes - significant erosion of the tube core

In this case, joining progresses initially as expected. The cladding layer on the tube melts and flows by capillary action to the fin to tube joint and a normal fillet forms. However, as the peak brazing temperature is allowed to rise beyond the recommended maximum (605 °C) the following occurs:

  • The fluidity of the filler metal at the tube to header joint is increased and some of the liquid filler metal is released and flows to the nearest tube to fin joints.
  • Excess filler metal at the tube to fin joints accelerates dissolution of the tube core adjacent to the fin, eroding the tubewall thickness.
  • The excess filler metal pool is then drawn by capillary action in between the fins, particularly where the finspacing is narrow. The fins are drawn together by the strong capillary forces, displacing the fin from its original fin to tube position.
  • As the fins move together, drawing the filler metal pool from its original position, the denuded area is significantly reduced in cross sectional thickness.
Catastrophic Failures

In some instances the extent of filler metal erosion is so severe that the entire thickness of the tube is consumed resulting in catastrophic failures.

More about this topic in our next issue.

HF can potentially be formed during the flux brazing process. HF is very toxic, irritating to the eyes, skin and respiratory tract and cause severe burns of the skin and eyes. The threshold limit value (TLV) for HF is a ceiling concentration of 3 ppm (2.3 mg/m3), a concentration that should not be exceeded during any part of the working shift.

Drying ovens can be electrically heated or gas fired. In gas fired drying ovens, it is possible that any flux particles entrained in the moist air and passed through the high temperature flames may generate HF. The concern here is not so much with employee exposure, but that HF may be released into the atmosphere.

Similarly, flux particles coming in contact with the hot flames in a flame brazing station may also generate HF. Suitable local exhaust systems must be in place to capture vapors and fumes that may contain HF.

It is known that one of the components of the flux, KAlF4, has a measurable vapor pressure and the rate of evaporation increases rapidly once the flux is molten. With regard to CAB brazing (furnace brazing) where traces of moisture are always present even at below –40°C dew point, a number of compounds can be formed in the system K – Al – F – H – O. To our knowledge there has been no academic effort to create a thermodynamic model of this system. Thus, it is impossible to predict which compounds will and will not exist, and in what temperature or humidity regimes. This is why more than one mechanism has been proposed for the generation of HF, but no unique reaction mechanism has been identified:
3KAlF4 + 3H2O → Al2O3 + K3AlF6 + 6HF
2KAlF4 + 3H2O → 2KF + Al2O3 + 6HF

While the evidence above points to gas phase reactions between flux fumes and water vapor for the generation of HF, Thompson and Goad1) proposed that AlF3 dissolved in the flux melt is subject to hydrolysis according to:

2AlF3 + 3H2O → Al2O3 + 6HF

What is clear is that in all cases, HF is shown as a reaction product. As for the quantity, Field and Steward2) have indicated that the amount of HF formed is typically 20 ppm in the exhaust of a continuous tunnel furnace. Solvay’s own research work showed that even when flux on aluminum is heated in a bone-dry nitrogen atmosphere, a small quantity of HF is still generated3). A source of hydrogen must be made available for HF to be formed even under bone-dry conditions and this might include reduction of aluminum hydroxide, degassing of furnace walls, leakage or other less obvious sources. The work showed that even under ideal conditions, it is virtually impossible to avoid some HF formation. The graph below shows the relationship between dew point and HF formation:


The amount of HF generated depends on several factors such as:

  • Flux load going through the furnace – flux loading and component throughput
  • Temperature profile – heating rate and time at temperature
  • Furnace atmosphere conditions such as nitrogen flow and dew point

The HF is exhausted together with the nitrogen stream and absorbed by the dry scrubber.



1) Thompson, W.T., Goad, D.W.G., Can. J. Chem., 1976, Vol. 54, p3342-3349
2) Steward N.I., Field D.J., SAE 870186, 1987
3) Lauzon, D.C., Belt, H.J., Bentrup, U., Therm Alliance Seminar, Detroit, 1998

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.