Flux Characteristics and Transfer Systems in Electrostatic Application – Part 1

Sep 29
2017

flux-characteristics-and-transfer-systems-in-electrostatic-application

 

Introduction:

This article summarizes some experimental results of a project on electrostatic application of non-corrosive fluxes for aluminum brazing. The objective is to qualify and quantify flux powder properties and equipment parameters with positive effect for dry flux technology.

For more than 30 years, controlled atmosphere brazing (CAB) [NOCOLOK ® Flux brazing] has been the leading technology for the manufacture of aluminum heat exchangers for the automotive industry.

The most common flux application method is by spraying an aqueous suspension. Constantly agitated flux slurries with concentrations of approximately 10 – 35% solids are pumped from tanks to fluxing booths. All aluminum surfaces involved in the brazing process are coated with the slurry, resulting in a uniform flux layer. Excess flux slurry is removed with a high-volume air blow; the excess is then collected, recycled and reused in the fluxing booth. Before going into the furnace, the heat exchangers are pre-dried in a separate drying oven to remove residual moisture.

In wet flux application, the following are critical factors and need specific observation by the user:

  • Flux slurry concentration
  • Consistency and uniformity of applied flux
  • Flux loading on heat exchangers
  • Drying step

Depending on the particular brazing operation, flux slurries may become contaminated with dust, metal particles, rust and organic compounds. The used slurry also contains the soluble portion of the flux (i.e., small levels of potassium, fluoride and aluminum), and must therefore be treated and then disposed of in accordance with environmental regulations.

Over the past five years, some users of NOCOLOK brazing technology have implemented dry flux application methods. Based on the principles of powder paint technology, an alternative application technique was introduced in the brazing industry.

The benefits of electrostatic application are directly related to the problems of wet application:

  • No need to mix slurries
  • No need to monitor slurry concentration
  • No need for a surface wettability concept (i.e., surface treatment or wetting agent)
  • No separate drying step required to remove moisture
  • No waste water effluent

Particularly when dry fluxing is used in connection with evaporative oils and lubricants, the objective is to eliminate or significantly reduce water consumption during the process.

To be continued …

Folded Radiator Tubes (B-Tubes)

Jul 31
2017

Brazing Issues

Folded tubes (or B-type tubes) for radiators have been developed several years ago. There are slightly different designs patented by most of the heat exchanger manufacturers.

Below illustrations show three different B tube designs:
B-TUBES

The folded tubes are produced from brazing sheet coils by a multi-step roll forming process – bringing the sheet gradually into a “B” shape. B-tubes have certain advantages – particularly regarding strength. The folded ends of the tube sheet are brazed inside the tube, which creates a very robust bridge between the walls. This results in higher burst pressure resistance.

The B tube roll forming process replaces the tube seam welding procedure. Standard flat radiator tubes are seam welded on one side – while folded tubes are joined during brazing. It is a general observation that folded tubes (“B-tubes”) are more difficult to braze (when compared with seam welded tubes).
Some of the problems are related to flux coverage. Other difficulties are related to erosion effects due to the fact that there is more filler metal available from folded tubes than from welded tubes. One common feature of all folded tubes designs is the presence of a triangular gap or “delta” between the flat exterior portion of the tube wall surface and the fin convolution. This delta is created by the presence of the folds in the tube. This is a common failure site at the tube to header joint because there is not enough filler metal at the joint to allow for proper fillet formation.

In order to avoid troublesome inner tube fluxing and to minimize the level of flux residue in the cooling loop it has become more and more common to flux the tube folds (the so called leg) at the folding machine using a needle like dispenser which places a thin bead of flux paste along where the tube leg touches the tube inner surface in the subsequent folding operation. A specially formulated flux paste is required for such operation. Solvay offers for this purpose several formulations with different flux concentrations, viscosity ranges, and additives.

Another common feature with folded tube designs is that the fin-to-tube-joints are consistently larger on the folded tube side than on the non-folded side of the tube. This is attributed to the path created by the fold in the tube which allows the filler metal to flow from the header, up the tube and from the tube fold panels up into the fin to tube joint.

The dominant phenomenon present in brazing is capillarity, i.e. the force which draws the filler metal into the joints. A heat exchanger core may be considered as a complex matrix of capillary sites. Now, in a non-folded tube design, all the joints are considered separate and autonomous, that is none of the joints are connected. For the most part then, filler metal is drawn into the fin to tube and tube to header joints from the immediate area surrounding the joint.

When a folded tube is added, many of the previously separated joints become connected and inter-dependent. The fin to tube joints on the side of the tube fold are now in direct contact with the seam along the fold of the tube, which is also in contact with the tube to header joint at the header slot. Now, the heat exchanger has an extended zone to draw filler metal from, as the available clad for the fin to tube joint now extends throughout the entire length of the tube seam and even includes the header. During brazing, the center of the core will heat up faster (lighter weight compared to the heavier thermal mass of the headers and side supports) and creates a temperature gradient. The filler metal from the header is able to travel down the seam throughout the length of the seam, depleting the area around the tube to header joint of valuable filler metal. The result is smaller tube to header joints with a greater risk of failure and large tube to fin joints on the folded side of the tube.

One way to reduce the flow of filler metal along the seam resulting in saturation of the tube to fin joints is by putting Mg in the fin. This goes back to the principle of competing joints, where the joint with more Mg will draw less filler metal. By adding a small percentage of Mg in the fin yet keeping it brazeable, the wettability of the fin to tube joint is slightly reduced. The fin-to-tube-joints neither draw up all of the available clad from the headers nor from the tube seams. The clad no longer runs up the tube. Reduced wettability forces the clad to stay in the tube seam and in the tube to header joints.
If problems are observed with excessively large tube to fin joints on the folded tube side and/or if the tube to header joints are small and commonly fail due to a lack of filler metal, it may be considered to use fins with more Mg – to take advantage of the features mentioned above.

Meet us at the 5th International Congress on
“Aluminium Heat Exchanger Technologies for HVAC&R”

May 12
2017

May 16 – 17, 2017
 in Düsseldorf, Germany

 

The 5th International Congress and Exhibition on Aluminium Heat Exchanger Technologies for HVAC&R, organised by the DVS German Welding Society, will take place from 16 – 17 May, 2017 in Düsseldorf/Germany in the Radisson Blu Scandinavia Hotel.

Stringent environmental requirements for increasing energy efficiency and reducing emissions are the greatest challenges the heating, ventilation, air conditioning and refrigeration industry faces.

The 5th International Congress on “Aluminium Heat Exchanger Technologies for HVAC&R” is dedicated to these challenges.

This is a must-attend event for producers and users in the field of air conditioning and refrigeration.

 

DVS-2017

Programme and Registrationform

Download the invitation, programme and registration form

Myths about Aluminium Brazing Fluxes – Part 2

Sep 23
2016

In Myths about Aluminium Brazing Fluxes Part 1 we reported about the rumor that fluxes with a lower melting range are superior. Now we take a look to another Myths about Aluminium Brazing:

Myth – A Flux with Smaller Particle Size is More “ACTIVE” and Leaves Less Flux Residue

Some rumours have been spread that a flux with a smaller particle size leads to better brazing and results in a more pleasing post-braze appearance.

But the facts are very different. It is true that a flux with a smaller particle size covers the surfaces of the work-piece more completely. Smaller flux grains will also adhere better to those surfaces, assuming that the two fluxes to be compared indeed show a difference in particle distribution.

DSC Scans

Figure 2

As particle size decreases, the total surface area of the flux increases. This allows a higher surface area of flux to be in contact with the work-piece. During heat up, there may be more efficient energy transfer to the flux with the smaller particle size. The net result is that this would affect the kinetics of melting – how quickly the flux melts – but it does not affect the melting temperature range. This is analogous to crushed ice melting quicker than a block of ice, but both melt at the same temperature. The above figure shows the melting action of various particle size fluxes using Differential Scanning Calorimetry or DSC. As expected, all particle sizes melt at precisely the same temperature.

It is also speculated that the ability of the flux to melt and spread (activity) increases as particle size decreases. In fact, the activity of the flux is related to chemistry and phase composition, not particle size. Spreading is simply a liquid phase reaction unrelated to the particle size distribution of the solid phase.

A practical example showing how flux particle size is unrelated to brazing results is comparing NOCOLOK® Flux (X50: 2 – 6µm) used for wet fluxing with NOCOLOK® Flux Drystatic (X50: 3.5 – 25µm) used for electrostatic fluxing. Heat exchangers brazed by wet fluxing can be brazed with the same results using dry fluxing, and the only difference is the particle size of the flux. The success simply depends on applying the flux uniformly.

Finally, the appearance of the post-braze surface is only related to the initial flux loading, not particle size. Once the flux melts, it is completely liquid. In its molten state, the flux has no particles – neither large nor fine. Once the flux is liquid, it immediately spreads out and wets the surfaces. Upon cooling and solidification, the amount of flux residue and its distribution on the surface of the work-piece is related entirely to the initial flux loading, and not particle size.

It is true that particle size distribution of a flux affects slurry characteristics. A finer powder will stay longer suspended (i.e. it settles slower) than a coarser product. Material with larger grains seems to build-up more rapidly on inside surfaces of slurry tanks and spraying equipment. Regardless of the specific particle distribution of a flux, continuous agitation is necessary to prevent settling and build-up. Regular maintenance is the only way to avoid the formation of solidified material residues.

A realistic comparison of particle size distribution can only be done by measuring samples on the same equipment under exactly the same conditions.

Furthermore, we have electron microscope comparison pictures of several flux powders – showing that the grain morphology is quite similar in all products.

SEM Picture NOCOLOK© Flux Quzhou 1,000/4,000/16,000

SEM Picture NOCOLOK© Flux Quzhou 1,000/4,000/16,000

SEM Picture NOCOLOK© Flux Bad Wimpfen 1,000/4,000/16,000

SEM Picture NOCOLOK© Flux Bad Wimpfen 1,000/4,000/16,000

SEM Picture Competitor 1 1,000/4,000/16,000

SEM Picture Competitor 1 1,000/4,000/16,000

SEM Picture Competitor 2 1,000/4,000/16,000

SEM Picture Competitor 2 1,000/4,000/16,000

Myths about Aluminium Brazing Fluxes – Part 1

Aug 18
2016

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…

SECO/WARWICK Webinar:
Low Volume Cab Systems/Vacuum Purging

Mar 04
2016

Looking for alternatives of a standard aluminium brazing process with a lower level of nitrogen consumption or you are involved in the process of heat exchangers manufacturing, especially in the automotive and aftermarket, don’t miss the Webinar. SECO/WARWICK guarantee a high quality of brazing process of elements, especially for unique types of aluminium heat exchangers.

Webinar date: 3PM CET, March 22, 2016

Visit the Website of SECO/WARWICK for details

Flame Brazing Technology – Part 4

Feb 29
2016

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.

Flame Brazing Technology – Part 3

Jan 29
2016

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…

Flame Brazing Technology – Part 2

Dec 21
2015

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…

Flame Brazing Technology – Part 1

Nov 30
2015

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…

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