Schlagwortarchiv für: CAB

Filler Metal Management 2

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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.

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Filler Metal Management 1

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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.

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Video about EABS Technical Training Seminar

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.

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HF Generation – Mechanisms and Sources

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

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Furnace Temperature Profile

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)

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Furnace Brazing Conditions

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.

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Flux Transformations

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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.

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Flux Application: Wet Fluxing

In its simplest form, a slurry is held in a reservoir tank and continuously agitated to prevent settling. The slurry is pumped, usually with air-diaphragm pumps to the flux slurry cabinet where the heat exchangers moving on a conveyor are sprayed with the slurry. After spraying, the excess flux slurry is blown off in a separate chamber with high volume air. The over spray and blown off slurry is recycled back to the reservoir tanks, again using air-diaphragm pumps.

Depending on the sophistication desired, a second flux spray chamber may be installed after the first chamber to deliver a higher concentration slurry to problem areas such as tube to header joints in condensers and radiators. This second spray chamber would have a separate flux delivery system and a separate reservoir tank to contain the higher concentration flux slurry.

The components of the flux delivery system including reservoir and agitators should all be constructed of stainless steel or chemically resistant plastics (nozzles for instance). There should be no mild steel or copper containing components – includes brass or bronze – in contact with the flux slurry. The schematic below shows the components of a generic fluxing station:

Wet Fluxing

Note that splashing will occur inside the fluxing cabinet and cause an accumulation of dried flux on the walls. Therefore the cabinet is washed with water periodically to remove this accumulated flux. The frequency of this maintenance operation is up to the manufacturer, but could be anywhere from once per shift to once per month.

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Flux loading

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?

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Controlled Gas Plasma Depostion (CGPD)

A new coating technology for the functionalization of semi-finished aluminium products for heat exchanger (HEX) applications has been developed by Erbslöh Aluminium GmbH with assistance from Solvay Fluor. In contrast to binder-based flux coatings, it is now possible to apply various kinds of NOCOLOK® fluxes free of any adhesives for the Controlled Atmosphere Brazing (CAB) process to aluminium surfaces, gaining economic and technological advantages. Today’s application examples include coatings on extruded condenser and evaporator tubes, and internal brazing of B-type tubes by integration in tube mills. Other applications, e.g. for selective local coatings, are conceivable.
Several aluminium substrates were coated with different NOCOLOK® fluxes/flux materials and examined in pre and post-brazed condition. As the results show, the new technology has a considerable potential in substituting or even replacing common pre-fluxing processes for CAB in HEX manufacturing.
Controlled Gas Plasma Deposition (CGPD) represents an innovative method for pre-fluxing of semi-finished products. It allows the application of pure flux by means of a plasma source with comparable adhesion properties as achieved with binder-based coatings. Due to the absence of any binder or other redundant chemicals, CGPD is not limited to any drying or hardening time, opening the way for high speed flux application. This comes hand in hand with the higher environmental friendliness of a solvent and binder-free process.

Illustration of the CGPD process

Benefits of brazed aluminium HEXs in Micro Multiport (MMP) design are:
■ Cost reduction
■ Improved performance with downsizing potential
■ Weight reduction
■ Lower refrigerant charge
■ Better corrosion performance
■ Recycling advantages
Improvements in CAB:
■ Use of pre-coated components
■ Process, quality, cost
■ Binder-free CGPD coating for easy use and high speed applications

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