Selecting the Correct Flux –WHAT?
The first requirement of an Aluminium brazing is to be chemically effective.
Fluxes are categorized as active (corrosive) and inert (noncorrosive).

Fluoride base fluxes → NON‐CORROSIVE
The fluxes leave the white gritty residue on the part. These fluxes include the higher temperature potassium
aluminium fluoride and the lower activated cesium aluminium fluoride Fluoride flux residues is tightly adhered to aluminium surface, relatively insoluble, not necessary to be removed. Fluoride flux operates by melting, spreading and dissolving of aluminium oxide layer.

Active fluxes ‐ Chloride base fluxes → CORROSIVE
The appearance of the part after brazing is bright and shiny. The chloride post braze residues must be removed by water washing or chemical treatment, to prevent the occurrence of electrolytic corrosion. These fluxes require a significant exposure to hot water to remove the corrosive flux residues. Attention must be given to the outside of the assembly and to any residues that have migrated to the inside of the part.
Chloride flux is reported to work penetrating aluminium oxides at weak points and breaking up the oxide/aluminium bond.

Selecting the Correct Flux –WHY?
Effect of Mg content on mechanical properties and braze ability of Al‐alloys.

Microsoft PowerPoint - HPa Flame brazing.pptx

0,3‐0,6% Σ(Mg+Cu)% -> 2% cesium flux
0,6‐0,9% Σ(Mg+Cu)% -> 6‐10% cesium flux
0,9‐2% Σ(Mg+Cu)% -> 100% cesium flux
>2% Σ(Mg+Cu)% -> 726/0726 pastes

Correct filler metal / Correct flux
Fluxes must be thermally matched to the melting phase of the braze filler base metal.

Microsoft PowerPoint - HPa Flame brazing.pptx

CORROSIVE flux paste for the flame brazing of aluminium materials
Al‐FLUX 0726/2zG – Flux Paste
• As the active component of Corrosive‐Flux‐Paste, AL‐Flux 0726 contains a mixture of LiCl,
NaCl, KCl, inorganic‐ and complex‐ fluorides.
• Organic carrier systems are used to prepare a wide range of pastes for the flame brazing of
aluminium materials.
• Al‐Flux 0726 Flux Paste is typically applied by dispensing and brushing for flame brazing. No
component mixing is required. Products are supplied as ready to use products, requiring short
remixing

Download product information.

NON‐CORROSIVE Flux Paste for the flame brazing of aluminium materials
NOCOLOK® 028/55 Cs2 ‐ Flux Paste
NOCOLOK® 028/55 Cs3 ‐ Flux Paste
NOCOLOK® 028 CsD ‐ Flux Paste
From low‐temperature brazing (450°C) to the brazing of high‐strength aluminium alloys.

Download product information.

To be continued…

Aluminium joining – Brazing ‐ Flux function



What is brazing?
Brazing is the joining of metals using a molten filler metal. On melting, the filler metal spreads between the closely fitted surfaces, forms a fillet around the joint and on cooling forms a metallurgical bond. By technical definition ‘brazing’ is a joining process in which the filler metal melting temperature is above 450°C, but below the melting point of the metals to be joined.


Aluminium brazing ‐ How does it work?
An aluminium oxide layer is formed instantly on aluminium in the presence of oxygen. This oxide layer has to be removed before brazing and the formation of a new oxide layer has to be prevented. The oxide layer is chemically dissolved by a FLUX.

 

use_of_flux

Aluminium joining – Flame Brazing



Flame (torch) brazing of aluminium involves locally applied heat typically generated by a slightly reducing oxy‐acetylene or oxy‐natural gas flame. (Gas mixture is preferred as it is cheaper and more comfortable).
Care must be taken to ensure even heat distribution. As with other aluminium brazing processes, close temperature control is important – it`must be closely monitored as there is no colour change in the aluminium to indicate temperature.


Flamebrazing1


Flamebrazing2

 

Aluminium joining – Basic Requirements


CORRECT FLUX / CORRECT FILLER METAL / CORRECT DIFFUSION OF FILLER METAL
The first requirement of an Aluminium brazing is to be chemically effective.
Fluxes are categorized as active (corrosive) and inert (noncorrosive)

CORRECT BRAZING TEMPERATURE
Between liquidus and solidus of filler metal
Decide how to manage the small thermal window between the melting temperature of the brazing filler metal and thermal damage to the base metals

BRAZING THROUGH CAPILARITY / RIGHT GAP OF THE BRAZING JOINT
Apart from the selection of flux and filler metal, important process parameters are the cleanliness and proper geometrical alignment of the individual components.


To be continued…

Part: 1: Flux Paints and Pastes – Part 1

Flux and Braze Paste Characteristics

The purpose of flux and/or braze pastes is to provide an initial or supplemental volume of flux and/or filler metal in the form of a viscous liquid at or near the interface of two components:

  • Folded tube seams
  • Tube to header joints
  • Connector tube to manifolds
  • End caps
  • Blocks and fittings
  • Flame braze joints

Viscosity

Flux and braze pastes exhibit pseudoplastic viscosity, meaning to have shear thinning behavior. As the shear stress is increased, the viscosity decreases, but the relationship is not linear. When the shear stress decreases, the viscosity increases, again non-linearly. Other factors affecting viscosity are solids content and temperature. Generally speaking, as the temperature increases, the viscosity decreases. Conversely, as the solids content increases, the viscosity also increases. These dependences illustrate the importance of quoting the parameters under which the viscosity is measured.

The graph below shows the effect of temperature on viscosity:

gf_Temperature_viscosity_1_RGB

Note that at each temperature, a double curve is shown representing two sets of measurements: one curve shows the decrease in viscosity as the shear rate is increased and the other curve shows the increase in viscosity as the shear rate is decreased. This mirrored behavior as the shear rate is increased or decreased perfectly illustrates the pseudoplastic behavior.

The following curves show both: – the effect of temperature, and – solids content on viscosity:

gf_Temperature_viscosity_2_RGB
These properties allow a bead of flux paste to be dispensed on the surface of a folded tube consistently and continuously at line speeds ranging from 30 m/min to 120 m/min.
braze-paste

Influence of Carrier on Volatilization Behavior

Since the carrier is a main ingredient, the carrier burn off temperature must be considered.

Below are 3 TGA curves showing burn-off temperature of 3 different paste formulations:

Glycol based carrier

Glycol based carrier – burn off at 180°C @ 10°C/min

 

Acrylate based carrier

Acrylate based carrier – burn off at 400°C @ 10°C/min

Polybutene based carrier

Polybutene based carrier – burn off at 425°C @ 10°C/min

 

Delivery Systems and Recommendations

When designing a flux delivery system, choosing the right delivery pump is just as important. Below is a list of pros and cons for the associated pumps.

Diaphragm Pump

  • Low cost, low maintenance
  • High flow
  • Generally used for low viscosity fluids
  • Pulsation, viscosity sensitive
  • Poor flow metering

Air Over Liquid (Pressure Vessel)

  • Low cost, low maintenance
  • Temperature sensitive
  • Viscosity sensitive
  • Poor flow metering

Rotary/Gear Pump

  • High viscous liquids
  • Metered dosing
  • Pulsation
  • Does not handle solids

Rotor-Stator Pumps

  • High viscous liquids
  • No pulsation
  • Precision metering
  • Not viscosity sensitive
  • Highest cost

Rotor-stator pumps are the pumps of choice because of their ability to deliver a constant volume of material without pulsation over a range of flow rates. The best example of this is in the localized dispensing of a continuous bead/stripe in folded tube mills where speeds can range from 20 m/min to 120 m/min.

In lower demanding applications where precision dispensing and flow control is not so critical, for example in tube to header fluxing, the lower cost options (air over liquid, diaphragm pumps) are more than adequate.

Concept and Definitions

In addition to the standard methods of applying flux to heat exchanger components (wet fluxing and dry fluxing), there is an increasing trend to using sophisticated flux formulations for selective pre-fluxing of components and/or localized fluxing of complicated geometries. The driving force behind this trend is multi-faceted: heat exchanger manufacturers are seeking to out-source flux application, to partially or completely eliminate certain process elements (fluxer, degreaser) and the movement away from seam-welded and extruded tubes to folded tube technology.

Before describing flux pastes and paints in more detail, a few definitions are noteworthy:

  • Flux Paint: Mixture of various powders mixed with a binder which is applied to as substrate in a thin layer. The coating is then converted to a solid film during a subsequent drying (curing) operation thereby adhering to the substrate.
  • Paste: Any mixture of various powders mixed with a carrier. Generally used for application where flux and/or alloy is required for a target location on a heat exchanger assembly or component. The viscosity is adapted to fit the application.
  • Binder: Complex organic compounds that upon curing, reacts to provide adhesion of flux particles to the coated surface.
  • Additives: Organic or inorganic substances used to modify the rheological properties of a fluid.
  • Curing: Drying of the flux painted parts usually with hot air (150°C). Liquid carrier, i.e., water and/or organic solvent(s) evaporate and the binder reacts to provide adhesion.
  • Adhesion: Qualitative or quantitative measure by which bonding strength of the flux particles to the coated surface is determined.
  • De-binding: Process of binder removal from the painted surface either in air or in the furnace atmosphere by the treatment with high temperature.
  • Viscosity: a measure of resistance to gradual deformation by shear stress, corresponding to the informal concept of thickness.
  • Thermogravimetric Analysis (TGA): A technique in which the mass of a substance is monitored as a function of temperature or time as the sample specimen is subjected to a controlled temperature program in a controlled atmosphere.

Paint Flux Characteristics – Viscosity

Viscosity is an essential parameter for the paint flux and is used to determine a suitable application process. A change in viscosity usually requires a change in the design of the application technique or equipment. All paste and paints used in fluxing are non-Newtonian fluids, meaning that the correlation between the applied shear stress and the shear rate is not linear. Many liquids, including paints are typical shear thinning fluids whose viscosities decreases non-linearly with shear stress. When providing a viscosity value, the methodology, specific shear rate and temperature must be provided.

gf_dependence_viscosity

Settling Behavior

Flux powder has very low solubility in water and organic solvent based paint mixtures. During storage, the solid flux particles will eventually settle out, causing a separation of solids and liquid/carrier. The rate of settling and the ease for remixing is therefore an important practical characteristic. The photo below shows an example of different settling behaviors:

Note that the higher value of the settled volume at left means a slower process of settling.

Note that the higher value of the settled volume at left means a slower process of settling.

The settling rate can be affected by several parameters such as binder concentration, flux solids content, storage time and temperature. To ensure complete homogeneity prior to use, a thorough remixing is necessary. Just shaking the container is usually not sufficient. It is recommended to use of a gyroscopic mixer which rotates the container around two perpendicular axes resulting in intensive material flow. These shear forces ensure optimal mixing without affecting the structure of the material.

Adhesion

The degree of paint flux adhesion varies depending on the heat exchanger manufacturer’s requirements. If paint fluxing is performed off-site and the material needs to be transported over long distances, a higher degree of adhesion is required than if the material is coated in house and simply needs to be transported from one station to the next. The degree of adhesion is typically controlled by the binder concentration in the formulation.

While there are standard methods for measuring adhesion (ASTM D 3359 Standard Methods for Measuring Adhesion by Tape Test), some paint flux users have developed their own in-house methods. The advantage of employing standard methods for measuring adhesion allows for a higher degree of inter-laboratory precision and comparison.

Binder Removal

For successful brazing of paint fluxed aluminum components, the binder must be removed before reaching brazing temperature. In the production process, the paint flux carrier is removed immediately after coating in the dry-off / curing operation. When paint fluxed components are put into the brazing line, the increasing temperature is then responsible for decomposing and removing the binder by evaporation. The temperature range at which the binder is removed is determined by Thermogravimetric Analysis (TGA). With this technique, a simulated braze cycle is used to determine at which temperature the binder is removed. The TGA curves below shows the de-binding temperature for a typical paint flux formulation.

gf TGA air nitrogen
Note that whether in air or nitrogen, the binder removal temperature is in the range of 250°C to 450°C. This means that in this case, at least part of the binder will be removed in the brazing furnace. In continuous tunnel furnaces, this is not an issue since the binder evaporation products will be swept away by the counter flow of nitrogen. In semi-continuous or batch type furnaces, the potential influence of binder removal on equipment must be individually considered depending on each brazing line design. In most semi-continuous or batch type furnaces, binder removal takes place during the drying or preheating step in the presence of air – at temperatures below 400°C (to avoid formation of high-temperature oxides).

Special consideration must be given to paint fluxed components which are not boldly exposed to the furnace atmosphere. These areas are usually enclosed spaces such as inside manifolds, sandwiched evaporator plates and turbulators for charge air coolers. In these cases, de-binding products may remain trapped in the enclosed spaces and result in discoloration and black carbon residue deposits. In these cases, it may better to sacrifice some adhesion (lower binder concentration) in order to ensure adequate binder removal.

Paint Flux Machines

Paint Flux MachinesAs the trend towards paint fluxing has increased, so has the sophistication of the paint flux machines. The industry has seen the evolution of paint fluxing from simple hand held paint sprayers, to semi-automatic machines incorporating a degreasing chamber, a paint flux spray chamber and drying/curing chamber. Today, the most sophisticated flux paint spray machines can be fully automated and fully integrated from the stamping operation through to core assembly. Machines with conveyor widths of 1500 mm, conveyor speeds of greater than 3.5 meters/minute which can spray top and bottom and be fully integrated with stamping and assembly are not uncommon. An example of such a machine is shown below:

 

To be continued…

Flux Paints and Pastes – Part 2

Flux-Green-Filler-Stop (GFS) “stops” molten brazing filler metal from flowing into areas where it is unwanted, thus the surfaces remain clean and free from the presence of any filler metal.

Brazing filler metals do not like to bond with, or flow over, any dirt, grease, or oxides so the presence of any of such contaminants can prevent the filler metal from flowing over the surfaces of those parts to be brazed where these contaminants are located.

Green-Filler

Therefore, GFS compounds are very effective at preventing molten filler metal flowing into protected areas. The GFS compounds are mixed with a liquid carrier solution and can be applied onto metal surfaces by using a small brush or by spraying or dipping.

For more information, please download the new brochure.

Brazing aluminum products such as radiators, condensers, evaporators, etc. for the auto industry is a mass production process. The brazing operation is generally carried out in a mesh belt furnace under a nitrogen atmosphere and is commonly known as ‘CAB’ – Controlled Atmosphere Brazing.

entering furnace 1

Accurate temperature measurement of the product throughout the furnace can be critical. Using a ‘through furnace’ temperature profiling system to measure product temperature is common practice within the industry, and the benefits are well established. There are also some known disadvantages to using these types of systems and here we look at recent developments to overcome these problems.

profile 2

The ‘through furnace’ profiling system measures temperature by connecting thermocouples at specific points on the product which feed temperature information back to the data logger. The data logger is protected from the heat of the furnace by a ‘hot box’ or thermal barrier, allowing the system to travel through the furnace together with the product, storing valuable temperature data which is analysed at the end of the process using specialized software.

As previously stated the benefits of using temperature profiling systems are well known, however there are some disadvantages, these are:

  1. The thermal barrier normally has a very limited life span as parts of the insulation package are subject to acid attack from chemicals within the flux.
  2. Oxygen can leak from within the thermal barrier while it is in the furnace, potentially contaminating the nitrogen atmosphere.

A. Acid attack

During the braze cycle, moisture in the air inside the ‘hot box’ or thermal barrier will combine with chemicals in the brazing flux to form hydrofluoric acid which attacks the high temperature cloth covering the microporous insulation. Once this cloth begins to break down, the unprotected insulation at the entrance to the ‘hot box’ wears away increasing the aperture where the thermocouples enter. This allows heat in, potentially damaging the data logger, and lets oxygen escape into the furnace atmosphere, which may affect braze quality.

damage

The life of this type of thermal barrier is severely reduced leading to high maintenance costs. The solution uses a robust ‘drawer’ design rather than the traditional ‘clam shell’ type.

combination5

This eliminates exposure of the high temperature cloth to the aggressive flux atmosphere, and significantly increases the life of the barrier. This new type of thermal barrier has been used in daily production since April 2011 at many leading automotive parts suppliers, with one major North American auto manufacturer reporting over two thousand uses without any wear problems.

B. Oxygen leakage

Whether the thermal barrier is a ‘clam shell’ or ‘drawer’ type it will contain air. As the system travels through the furnace the air begins to warm up and expands. As it expands it begins to leak out into the furnace atmosphere, which may be an issue to some users.

air in barrier4

There are two areas within the thermal barrier where air will accumulate – within the microporous insulation, and in the spaces around the data logger and heat sink. A ‘two stage’ approach has been developed to remove this air.

Firstly eliminating the air deep within the microporous insulation is achieved by heating the whole thermal barrier or ‘hot box’ in a high vacuum, then back filling with nitrogen. This operation is carried out as the last stage in the manufacturing process.

Secondly, as an option for users with sensitive processes, all remaining air in the spaces around the data logger can be purged with low pressure nitrogen just prior to placing the system in the brazing furnace.

purge

The nozzle for the nitrogen purge has been designed to allow free flow of the gas through the barrier, but by use of strategically placed internal ‘baffles’, heat penetration is minimized during the brazing process.

Conclusion

Although using a profiling system to monitor the product temperature in a CAB furnace has generally been considered high maintenance, it was judged that the value of the data obtained was worth the extra cost. However through careful system design a solution has been engineered that successfully overcomes these problems, saving maintenance costs and allowing the ‘hot box’ temperature profiling system to be used on a more regular basis.

Dave Plester, Director
Phoenix Temperature Measurement
www.phoenixtm.com
sales@phoenixtm.com

Approach to non-corrosive fluxes for further reduced residue solubility and improved magnesium tolerance
Technical Information by Ulrich Seseke-Koyro, Hans-Walter Swidersky, Leszek Orman, Andreas Becker, Alfred Ottmann
We split the article in four parts:

  1. Abstract and Basic Experimental Laboratory Procedures
  2. Reduced Flux Residue Solubility
  3. Improved Magnesium Tolerance
  4. Summary and Outlook

Summary

Our research activities so far have been focusing on flux blends with additives to validate lower water solubility of post braze flux residue. Another objective of this work was to allow for brazing of Al alloys with increased Mg levels using non corrosive fluxes.

First steps have been made with selected flux blends.  This paper reflects the current project status, but more work still needs to be done for further improvement.

Low flux residue solubility

It has been shown that the flux residue water solubility is reduced by combining KAlF4 with AEFs (“KAlF4 compound concept”); among them BaF2 being the most promising candidate.

Fluxes for higher Mg tolerance

Flux blends containing KAlF4 plus CsAlF4 and Li3AlF6 seem to be a promising approach to improve brazing of higher Mg containing aluminium alloys.

Aluminium coupons samples (AMAG 6951 with 0.68% Mg) for the base coupon and the angle (1.36% Mg in the joint interface) require flux loads as high as 15g/m2 for successful brazing.

Good joint formation can be achieved at 5g/m2 load on samples with 0.68% Mg content. Thus brazing of higher Mg level Al-alloys with appropriate flux mixtures at process-typical loads seems to be feasible.

Outlook

For the continuation of this project, we need to define the Mg range for real industrial aluminium heat exchanger needs. We think that this can best be done in a joint effort of HX manufacturer, Al material supplier and flux producer.


  1. P Garcia et al, Solubility Characteristics of Potassium Fluoroaluminate Flux and Residues, 2nd Int. Alum. Congress HVAC&R, Dusseldorf (2011)
  2. P Garcia et al., Solubility and Hydrolysis of Fluoroaluminates in Post-Braze Flux Residue, 13th AFC Holcroft Invitational Aluminum Brazing Seminar, Novi (2008)
  3. J Garcia et al, Brazeability of Aluminium Alloys Containing Magnesium by CAB Process Using Cs Flux, VTMS5, 2001-01-1763 (2001)
  4. H Johannson et al, Controlled Atmosphere Brazing of Heat Treatable Alloys With Cesium Flux, VTMS6 C599/03/2003 (2003)
  5. Handbook of Chemistry and Physics; Ref. BaSO4: 0.0025 g/l
  6. U Seseke, Structure and Effect – Mechanism of Flux Containing Cesium, 2nd Int. Alum. Brazing Con., Düsseldorf (2002)

Approach to non-corrosive fluxes for further reduced residue solubility and improved magnesium tolerance
Technical Information by Ulrich Seseke-Koyro, Hans-Walter Swidersky, Leszek Orman, Andreas Becker, Alfred Ottmann
We split the article in four parts:

  1. Abstract and Basic Experimental Laboratory Procedures
  2. Reduced Flux Residue Solubility
  3. Improved Magnesium Tolerance
  4. Summary and Outlook

Improved Magnesium tolerance

Mg additions to Al alloys contribute to higher strength properties. The ongoing trend in saving weight by down-gauging of Al sheet thickness requires sufficient mechanical stability. One option for the production of higher strength Al alloys is to increase the Mg content.

A disadvantage of Mg is the interaction with potas-sium fluoroamuminate fluxes during brazing, which results in poor joint formation [3] [4]. This effect, known as “flux poisoning”, is caused by the formation of high melting compounds. The addition of caesium and other metals to the flux helps to compensate to a certain degree the poisoning [6].

For the first set of laboratory brazing experiments we chose commercially available AMAG 6951 brazing sheet (0.68% Mg, 4343 clad) and clad-less AMAG angle material (0.68% Mg) to investigate the brazing performance and joint formation. In this situation the metal-to-metal interface adds up to 1.36% Mg (2 x 0.68%) in total.

Table 1 shows a list of representative flux combina-tions with NOCOLOK® types, KAlF4, Li3AlF6. CsAlF4, and AEFs.

We repeated all brazing tests with each sample three times.

Flux Type Load Fillet visual validation Comment
NOCOLOK® Cs Flux 10 g/m2 100% very small joint inconsistent seam
MD001212 LiCs24 10 g/m2 100% small joint weak seam
MD001223 LiCs43 10 g/m2 86% small joint inconsistent seam
AB039215 KAlF4/BaF2 10 g/m2 52% small joint inconsistent seam
NOCOLOK® Cs Flux 15 g/m2 100% weak seam
MD001212 LiCs24 15 g/m2 100% thicker than with NOC Cs Flux
MD001223 LiCs43 15 g/m2 100% thicker than with NOC Cs Flux
AB039215 KAlF4/BaF2 15 g/m2 98% weak seam slighly better than NOC Cs Flux

Table 1: Brazing trials: AMAG clad – AMAG clad-free angle different flux blends based on KAlF4 plus BaF2/Li3AlF6/CsAlF4

The angles from most of the AMAG specimens could be removed after brazing by pulling. Only a broken inner and outer fractured seam line was left – as can be seen below in picture 1 a.
flux_residus_part3_1
flux_residus_part3_2
flux_residus_part3_3

Picture 1: a) Photos, b) and c) SEM/EDX of NOCOLK® Cs Flux brazed sample (load 15 g/m²) Coupon 0.68% Mg, angle 0.68% Mg – angle removed by pulling

From the SEM analysis it is evident that a proper met-allurgical joint between base and angle has not been formed.

flux_residus_part3_4
flux_residus_part3_5

Picture 2: SEM/EDX analysis of aluminium ‘angle on coupon‘ brazed with KAlF4/BaF2 blend

There is flux residue present in the pulled apart fillet. This indicates that the liquid filler alloy was not capa-ble of pushing out completely the flux of the joint and it could be an explanation for the weakness of the fillet.

However, in case of the blend MD001212 LiCs24 with load 15g/m2 the joint structure is thorough as can be seen in picture 3 a).

flux_residus_part3_6

Picture 3: Microstructures of the brazed joints
a) Flux MD001212, load 15g/m2
b) Flux MD001223, load 15g/m2

It is worth mentioning when connecting blocks are brazed to condenser manifolds often a high load of manually applied flux is used in order to overcome the high Mg content in the block material. For such a case using the mixture MD001212 would allow for having quite high Mg content in the block material, which can be required by the designers of condens-ers.

The total concentration of 1.36% Mg (joint interface) is probably too high, because for most brazing applica-tions, a flux load of 15g/m2 is impractical. Thus, we decided to reduce the level of Mg in our samples to half – i.e. to 0.68% – by switching to an AA1050 (Al 99.5%) angle. We also reduced the flux load to a more process-typical level of 5g/m². The results are listed in table 2:

Flux Type Load Fillet visual validation Comment
MD001212 LiCs24 5 g/m2 100% good seam
NOCOLOK® Cs 5 g/m2 87% small joint

Table 2: Brazing tests AMAG coupon (0.68% Mg)/Al99.5 angle

The structure of the joint cross section below (picture 4) obtained with flux MD001212 LiCs24 shows good quality.

flux_residus_part3_7

Picture 4:Joint cross sections of alloys containing 0.68% Mg brazed with MD001212 LiCs24, load 5g/m2

To be continued…


  1. P Garcia et al, Solubility Characteristics of Potassium Fluoroaluminate Flux and Residues, 2nd Int. Alum. Congress HVAC&R, Dusseldorf (2011)
  2. P Garcia et al., Solubility and Hydrolysis of Fluoroaluminates in Post-Braze Flux Residue, 13th AFC Holcroft Invitational Aluminum Brazing Seminar, Novi (2008)
  3. J Garcia et al, Brazeability of Aluminium Alloys Containing Magnesium by CAB Process Using Cs Flux, VTMS5, 2001-01-1763 (2001)
  4. H Johannson et al, Controlled Atmosphere Brazing of Heat Treatable Alloys With Cesium Flux, VTMS6 C599/03/2003 (2003)
  5. Handbook of Chemistry and Physics; Ref. BaSO4: 0.0025 g/l
  6. U Seseke, Structure and Effect – Mechanism of Flux Containing Cesium, 2nd Int. Alum. Brazing Con., Düsseldorf (2002)

Approach to non-corrosive fluxes for further reduced residue solubility and improved magnesium tolerance
Technical Information by Ulrich Seseke-Koyro, Hans-Walter Swidersky, Leszek Orman, Andreas Becker, Alfred Ottmann
We split the article in four parts:

  1. Abstract and Basic Experimental Laboratory Procedures
  2. Reduced Flux Residue Solubility
  3. Improved Magnesium Tolerance
  4. Summary and Outlook

Reduced Flux Residue Solubility

The water solubility of standard NOCOLOK® Flux is 4.5 g/l, whereas for post-braze flux residue (pbr) it is 2.7 g/l. Post-braze residue of NOCOLOK® Li Flux shows a solubility of 2.2 g/l [1].

In the periodic table of chemical elements the group I fluorides have a reasonable low solubility (LiF: 2.7g/l [20°C]), but their Al-F-complexes much lower (Li3AlF6: 1.1g/l , K2LiAlF6: 0.3g/l with about 183 mg F-/l, K3AlF6: 2g/l), the group II fluorides (Alkaline Earth Fluorides “AEF”) show very low solubility (MgF2: 0.13g/l, CaF2: 0.016g/l, SrF2: 0.12g/l [25°C], BaF2: 0.12g/l [25°C]) [5]. Based on the facts of the dissolution behaviour of NOCOLOK® Li and the much lower solubility of the AEFs, we started investigating combinations of potas-sium fluoroaluminate fluxes with selected AEFs to combine the brazing characteristics of NOCOLOK® type flux with the very low solubility of AEF.

NOCOLOK® Flux consists of potassium fluoroalumi-nates with a specific ratio of KAlF4 and K2AlF5. Each of these compounds has different solubility. The combination of the (pure) compounds with different AEFs was of our main interest. We melted and pulverized the flux blends, dissolved them in a defined amount of DI-water and analyzed for K, Al and F.

The data achieved form these experiments is illus-trated in figure 1:

Flux-Residue-dia-1

Fig. 1: Solubility of flux blends – melted and pulverized
(lines are used to illustrate differences of the blends)

Considering minor statistical variations, the results look quite reasonable, with the blend of NOCOLOK® Li/BaF2 showing the lowest K value. This observation can be explained by the low solubility of NOCOLOK® Li Flux. Of more relevance is the actual post-braze solubility (flux residue) on brazed Al surfaces. Interactions of base material and molten filler metal may have a more complex chemical impact on the solubility behaviour

The results from coupon brazing under laboratory conditions and the solubility of the flux residue can be seen in figure 2.

Flux-Residue-dia-2

Fig. 2: Post-braze fluoride solubility of selected flux/ AEF combinations on Al coupons
(lines are used to illustrate differences of the blends)

Among the combination of NOCOLOK® type fluxes with diverse AEF additions, KAlF4/BaF2 shows the lowest residue F– solubility, i.e. 4mg/l. All our laboratory brazing tests with the samples showed the same good results like with standard NOCOLOK® Flux.

Corrosion comparison tests will be subject for future investigations.

To be continued…


  1. P Garcia et al, Solubility Characteristics of Potassium Fluoroaluminate Flux and Residues, 2nd Int. Alum. Congress HVAC&R, Dusseldorf (2011)
  2. P Garcia et al., Solubility and Hydrolysis of Fluoroaluminates in Post-Braze Flux Residue, 13th AFC Holcroft Invitational Aluminum Brazing Seminar, Novi (2008)
  3. J Garcia et al, Brazeability of Aluminium Alloys Containing Magnesium by CAB Process Using Cs Flux, VTMS5, 2001-01-1763 (2001)
  4. H Johannson et al, Controlled Atmosphere Brazing of Heat Treatable Alloys With Cesium Flux, VTMS6 C599/03/2003 (2003)
  5. Handbook of Chemistry and Physics; Ref. BaSO4: 0.0025 g/l
  6. U Seseke, Structure and Effect – Mechanism of Flux Containing Cesium, 2nd Int. Alum. Brazing Con., Düsseldorf (2002)

Approach to non-corrosive fluxes for further reduced residue solubility and improved magnesium tolerance

Technical Information by Ulrich Seseke-Koyro, Hans-Walter Swidersky, Leszek Orman, Andreas Becker, Alfred Ottmann

We split the article in four parts:

  1. Abstract and Basic Experimental Laboratory Procedures
  2. Reduced Flux Residue Solubility
  3. Improved Magnesium Tolerance
  4. Summary and Outlook

Abstract

 

For more than 30 years, potassium fluoroaluminates (NOCOLOK®) fluxes are already successfully used in controlled atmosphere brazing (CAB) of aluminium heat exchangers. Residues of these so-called non-corrosive fluxes have very low – but evident – solubility in water [1] [2]. In the discussion about corrosion of CAB produced aluminium heat exchangers, the flux residue solubility is an important parameter. There are concerns that – in addition to several other factors – fluoride ions (F–) potentially released from dissolved residue play a role in aluminium corrosion.

A theoretical option to address this point is the development of virtually insoluble flux. More realistic, however, will be fluxes with less soluble residues than the current compositions.

Some commercialised NOCOLOK® derivates, like NOCOLOK® Li Flux show already reduced solubility when compared to the standard product [1]. While investigating the chemical possibilities for further minimising the residue solubility and the release of F- ions, we have developed NOCOLOK® variants in combination with selected inorganic fluorides.

During this R&D project we also looked closely at the brazing properties of the new fluxes – with a focus on their performance for brazing of aluminium alloys with higher magnesium level. The current maximum magnesium range suitable for CAB with standard NOCOLOK® Flux is approximately 0.3%. Some improvement can be seen when using caesium-containing NOCOLOK® formulations (up to 0.5% Mg) [3] [4]. Some of the new fluxes we developed for further reduced residue solubility surprisingly show higher magnesium tolerance. This article summarizes the results of our laboratory work related to the development of fluxes with further reduced residue fluorides solubility and improved magnesium tolerance.

Basic experimental laboratory procedures

 

1. Lab brazing and alloy specimen setup
For experimental lab furnace brazing we used standard CAB brazing profile and 25 by 25 mm clad sheet coupons (single side) with angle on top. In case of the Mg topic an AMAG (Austria Metal AG) clad alloy (6951/4343) was brazed with an AMAG clad-less angle. Fluxing was done manually (flux load weight on precision scale, drops of isopropanol and homogenous spreading).

Test coupon

2. Solubility data generation
Coupon (3003/4343) with Al angle (Al 99.5%) were manually coated with a dedicated amount of flux blend and brazed as described in point 1. Brazed samples were placed in PET bottles and a defined quantity of demineralised water was added. Daily visual control and air exposure (by opening and closing the lid) was done.

PET-Flasche_klar

To be continued…


  1. P Garcia et al, Solubility Characteristics of Potassium Fluoroaluminate Flux and Residues, 2nd Int. Alum. Congress HVAC&R, Dusseldorf (2011)
  2. P Garcia et al., Solubility and Hydrolysis of Fluoroaluminates in Post-Braze Flux Residue, 13th AFC Holcroft Invitational Aluminum Brazing Seminar, Novi (2008)
  3. J Garcia et al, Brazeability of Aluminium Alloys Containing Magnesium by CAB Process Using Cs Flux, VTMS5, 2001-01-1763 (2001)
  4. H Johannson et al, Controlled Atmosphere Brazing of Heat Treatable Alloys With Cesium Flux, VTMS6 C599/03/2003 (2003)
  5. Handbook of Chemistry and Physics; Ref. BaSO4: 0.0025 g/l
  6. U Seseke, Structure and Effect – Mechanism of Flux Containing Cesium, 2nd Int. Alum. Brazing Con., Düsseldorf (2002)