Flux Pastes – Part 2

Go to Flux Pastes – Part 1

Properties

NOCOLOK® flux and brazing pastes offer numerous advantages that distinguish them from other products.

1. Solvent system
Use of systems miscible with water and glycols

  • Equipment and facilities used for paste application can be easily cleaned with water
  • If required, the setting or adjustment of viscosities is possible with certain glycols

 

2. Variable viscosity
Depending on requirements, the pastes can be produced in a wide viscosity range and with different solids contents.

  • Flux pastes
    Possible viscosity range 500 – 50,000 mPa·s
    Flux content 5 – 60 %
    Variable flux content at constant viscosity 15 – 30 %
  • Brazing pastes
    Possible viscosity range 1,000 – 80,000 mPa·s
    Flux content 15 – 40 %
    Plummet content 15 – 45 %

 

3. Minimum precipitation of the pastes

  • Low settling behaviour of the contained solids even after several weeks of storage
  • Simple agitation, homogenisation is – if necessary – possible

 

4. Very good adhesion

  • Marginal running during application of the paste, even on vertical surfaces
  • By use in multi-chamber tubes, there is no leakage during transport, storage or processing

Comparison of vertical adhesion of brazing pastes

comparison-of-brazing-pastes
Above is a conventional brazing paste; Below NOCOLOK® brazing paste after 15, 45 and 75 minutes

5. Residue-free solvent system

  • Evaporation and removal of the glycol carrier system from the surfaces takes place at below 200 °C
  • The complete solvent content of the pastes comes off in the first third of the brazing cycle
  • Consequently, the removal of the glycol carrier system is possible in the drying phase or in the degreasing furnace before the brazing process

The resulting emissions are thus removed in good time before the brazing process by appropriate channelling of the waste gases. Therefore, the actual brazing process is not affected.

Differental Thermo Analysis (DTA) of NOCOLOK® Flux Paste – Representative Sample

differental-thermo-analysis-of-nocolok
At just under 200 °C, the organic solvent have decomposed without residue.

Packaging

Available packaging forms

  • Plastic container 1 kg
  • Plastic buckets 5, 10, 15 or 20 kg
  • Optional: Plastic drums 60 or 200 kg

Go back to Flux Pastes – Part 1

For more information please download the brochure

 

 

 

 

 

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Flux Pastes – Part 1

Applications

  • Flux Pastes are mainly used inside B-tubes and folded tubes, in order to provide a line of flux on a cladded surface. These paste formulations are available in FG (fine grade) version, the N version (“new” – i.e. with adjusted rheology and re-mixing characteristics) and UV version (ultraviolet sensitive pigments for special application monitoring).
  • Metalized Flux Pastes (Brazing Pastes) are often used manifolds/tubes or blocks/manifolds or header/tubes or in any place there is need for joint formation with additional filler metal (usually used to compensate for challenging design situations or for larger tollerances on stamped parts).
  • Ultra Flux Paste is used inside B-tubes and folded tubes, in order to provide a line of flux on a cladded surface (more “sticky” than glycol family “028”).

NOCOLOK® Flux Pastes

NOCOLOK® flux and brazing pastes command a maximum variety of options in flux and brazing alloy powder applications.

Consequently, NOCOLOK® flux pastes can be individually adapted according to respective technical requirements and the brazing processes used.

Possible NOCOLOK<sup>®</sup> Flux brazing paste variations:

  • NOCOLOK® Flux Standard
  • NOCOLOK® Cs Flux
  • NOCOLOK® Li Flux

Alloy powder:

In combination with the various flux powders, NOCOLOK® brazing pastes can contain different brazing alloy according to the application requirements:

AlSi12     AL104 (DIN EN 1044)     AA 4047

AlSi10     AL103 (DIN EN 1044)     AA 4045

AlSi7,5     AL102 (DIN EN 1044)    AA 4343

The grain size of the brazing powder can be adapted to all corresponding applications.

Application Areas for NOCOLOK® Flux and Brazing Pastes

  • Production of multi-chamber tubes
  • Use as B-pipe flux paste
  • Furnace brazing
  • Flame brazing

nocolok-paste

Go to Flux Pastes – Part 2

For more information please download the brochure

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Myths about Aluminium Brazing Fluxes – Part 1

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Myth – Fluxes With a Lower Melting Range are Superior

There are claims that a lower melting point flux is better for brazing (i.e. melting between 550 and 560°C – approximately 10 – 15°C below conventional fluxes). The idea here is to try to fool the engineer by illustrating the merits of “early” flux melting, and thus “prolonged” flux action. However, the facts are very different.

As soon as the flux begins to melt, one of the components of the flux – KAlF4 – begins progressively evaporating, with a vapour pressure determined to be 0.08 mbar at 600°C. Evaporation of KAlF4 causes the flux melt to change composition, and it begins to dry out. Given enough time, it is possible for the flux melt to completely dry out before reaching the maximum peak brazing temperature.

A good brazing flux only needs to be available just before filler metal melting. The following table describes what happens at brazing temperature:

Myths on Brazing Flux

Table 1

As soon as the flux melts, it begins to dissolve the oxide layer, and this solvating process continues until the oxide is removed, even if the filler alloy has melted. The above table shows that even if the period of flux activity would be limited only to the time between complete flux melting and the lower brazing range of AA 4045, it is still adequate. The authors thus consider a flux melting range between 560 and 575°C as the most suitable for aluminium brazing with Al-Si filler alloys.

One should not completely dismiss the point made about “prolonged” fluxing action with lower melting point fluxes. However, once again, all the information must be examined. It has been shown that with an increase in the K2AlF5 content, the flux will start to melt at a lower temperature so that the flux will work at a lower temperature. However, even if KAlF4 evaporation is ignored increasing the K2AlF5 content eventually prevents the flux from spreading smoothly, and therefore affects the efficiency of the flux.

Merely lowering the melting point does not in itself create a better brazing flux.

Table 2

Table 2

 

Figure 1

Figure 1

 

To be continued…

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New NOCOLOK® Packaging Brochure now available!

This new brochure shows the different packaging types available for our NOCOLOK® product range including Powders, Pastes and Paints and Liquids.

Any special package request?
Don’t hesitate to contact us!

Download NOCOLOK Packaging

NOCOLOK Packaging

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Flame Brazing Technology – Part 4

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What is critical

Have a clean surface (free of dust, grease)
Heat the joint evenly to brazing Temperature
Choose the right brazing alloy for the job (Mg content !)
Select the appropriate flux to remove the oxide skin from the faying surfaces of the joint
Use a capillary gap of the appropriate size

ALUMINIUM BRAZING – Correct brazing temperature

Melting point of copper 1084°C
Copper‐phosphorus alloy

  • Elgalin Cu87 657°C ‐ 687°C
  • Elgalin Cu93 710°C – 820°C

Abbildung 4-1

No flux necessary
Brazing temperature is below the melting
point of base material

Melting point of aluminium 630‐660°C
Aluminium‐Silicon alloy

  • AlSi12 577°C – 585°C

Abbildung 4-2

Flux is necessary
Brazing temperature is very near the
melting point of base material

ALUMINIUM BRAZING – Correct brazing temperature ‐ flame

Acetylene + O2 3170°C
Propane + O2 2830°C
Natural gas + O2 2780°C

Abbildung 4-3

Acetylene + compressed air 2300°C
Propane + compressed air 1900°C
Natural gas + compressed air 1850°C

Abbildung 4-4

 

CAPILLARITY – right gap of the brazing joint

Capillary action works with gap between 0.05 and 0.2mm

Abbildung 4-5Preciseness is essential for Al brazing.

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Flame Brazing Technology – Part 3

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U‐shape brazing alloys with flux integrated into material

u-rings-picture

Advantages

  • Reduced labor cost
  • Reduced waste
  • No post-braze cleaning
  • Flexible design
  • Multiple applications
  • Ideal geometry for feeding
  • No hidden flux voids
  • Precise control of alloy and flux

 

Ideal for Preforms

  • Unlimited preform options
  • Flux flows unobstructed

u-rings-drawing

NON‐CORROSIVE FLUX

No post braze cleaning required, reducing the environmental impact associated with waste water

treatment

LESS CONTAMINATION

there are no powders leaching out to contaminate assembly equipment. The flux we deposit in the

channel stays in the channel.

Microsoft PowerPoint - HPa Flame brazing.pptx

Download the brochure.

To be continued…

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Flame Brazing Technology – Part 2

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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…

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Flux Paints and Pastes – Part 2

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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.

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Flux Paints and Pastes – Part 1

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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

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Flux Residue – Part 4

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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)
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