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

For added strength and machineability, certain alloys contain Mg. Most notably are the 6XXX series alloys (up to 1% Mg) that are used for fittings and machined components and the so-called long life brazing sheet alloys (up to 0.3% Mg in the core). There is a limit to the amount of Mg tolerated in NOCOLOK^® Flux brazing. Up to 0.5% Mg can be tolerated in furnace brazing while around 1% Mg is tolerable for flame brazing.
When an Al alloy containing Mg is heated, the Mg diffuses to the surface and reacts with the surface oxide to form MgO and spinels of MgO:Al₂O₃. The diffusion is time-temperature dependent and is rapid above 425°C. These spinel oxides have reduced solubility in the molten flux. Furthermore, Mg and/or MgO can react with the flux forming compounds such as MgF₂, KMgF₃ and K₂MgF₄. All of these serve to poison the flux and significantly reduce its effectiveness.
In flame brazing, higher Mg concentrations can be tolerated since the faster heating rates do not allow the diffusing Mg enough time to appreciably decrease the beneficial effects of the flux. Flame brazing components containing > 1% Mg may be possible under some circumstances with increased flux loadings and very fast heating rates (<20 second braze cycle).
It should be noted that when one speaks of the brazing tolerance to Mg, it is the total sum of the Mg concentrations in both components:

[Mg] _{component 1} + [Mg] _{component 2} = [Mg] _total

The figure below shows the effect of Mg on fillet size and geometry:

0.1% Mg

0.4% Mg

If the user is experiencing difficulties brazing and suspects elevated Mg levels as the cause, there are a couple of ways to be sure. First, check with the supplier of the alloys or perform a chemical composition analysis on the suspect alloys. This is the most certain way. Secondly, look for a golden hue on the brazed product. This is an indication that Mg alloys are being used and the color is a result of the increased oxide thickness. Furthermore, there may be a very light, almost fluffy residue on the brazed component that can literally be blown off by mouth. These visual indicators can most certainly be traced back to poor brazing results due to the presence of Mg.

Improving brazeability
There are a few ways in which the brazeability of Mg containing alloys can be improved:

1. Increasing the flux loading. A substantial improvement is gained when increasing the flux loading up to 10 g/m² or more in furnace brazing. In cases where there is just one component containing Mg such as in a fitting, extra flux can be brushed around the area of the joint.
2. Increasing the heating rate. Slow heating rates allow more Mg to diffuse to the surface thereby hindering brazeability. For furnace brazing Mg containing alloys, the fastest possible heating rates achievable without sacrificing temperature uniformity will increase the tolerance to Mg.
3. Combining increased flux loadings and faster heating rates.
4. Maintaining proper gap tolerances and joint designs.
5. Increasing the nitrogen flow rate to minimize furnace atmosphere contaminants that also compete to reduce brazeability.

Tip: NOCOLOK^® Cs Flux

Better results are reported when using cesium containing fluxes for aluminum alloys containing Mg up to 0.6 – 0.8% Mg. Fewer leaks are observed when compared with standard flux and less porosity is noted in the joint areas. Furthermore, standard flux loads and braze cycles can be used with Cs containing fluxes.

NOCOLOK^® Cs Flux is a flux of the general formula K_xCs_yAlF_z where Cs is chemically bound. It has a melting range of 558°C – 566°C. The maximum Cs content is limited to 2% to keep the cost of the flux down. Increasing the Cs content does not increase brazeability as shown below:

Cesium reacts as a chemical buffer for Mg by forming CsMgF₃ and/or Cs₄Mg₃F₁₀. The flux inhibiting factors of Mg are therefore reduced.

This increase in efficiency means the same refrigerant capacity can be produced with smaller exchange surfaces at the condenser and evaporator, with an associated reduction in piping volume, i.e. a higher heat exchange efficiency means smaller systems and lower refrigerant charge. Important given that third generation HFC refrigerant blends such as R 410 A are much more expensive than R 22 which they are now replacing.

Greater reliability, easy recycling and lower weight Aluminum alloys offer high heat conductivity but also high resistance to corrosion. Brazed heat exchangers also boast higher mechanical resistance, especially in the fin connection, so that even incorrect handling or accidental collisions cause less deterioration with time. Moreover, microchannel heat exchangers are single-alloy system components which means easy and efficient recycling. And, although aluminum brazed heat exchangers have a similar performance to all copper units of similar size, they are about three times lighter.

Yorck heat exchanger

Aluminum alloys are classified according to their alloying elements. The Aluminum Association designations are listed in the table below:

Designation System for wrought aluminum alloys
Alloys series   Description or major alloying element
1xxx                  99.00% minimum Aluminum
2xxx                 Copper
3xxx                 Manganese
4xxx                 Silicon
5xxx                 Magnesium
6xxx                 Magnesium and Silicon
7xxx                 Zinc
8xxx                 Other Element
9xxx                 Unused Series

The chemical composition of each AA alloy is registered by the Aluminum Association and a few examples are listed:

Example of aluminum alloy composition limits in weight percent*

*Maximum, unless shown as a range

How to measure?

In the case of heat exchangers, the surface area being fluxed must first be determined. For ease of calculation, the louvers on the fin can be ignored. The radius on the fin can also be ignored.

Imagine then the fin pulled out of the heat exchanger and straightened out to form one long strip. Similarly, the surface area of the slots in the header can also be ignored.

Remember that in calculating the surface area of the heat exchanger, there are 2 sides to every tube, 2 sides to every fin and 2 sides to the headers. The total surface area is then expressed in m2: All dimensions are in meters (m) to yield a surface area in square meters.

Header

Assuming it is a cylindrical (condenser) header:

SA (m2) = (2 x 3.14 x radius of header(m)) x length of header (m) x 2 headers

Assuming it is a radiator header:

SA (m2) = length of header (m) x width of header (m) x 2 (sides/header) x 2 (headers)

Tubes

SA(m2) = width of tube(m) x length of tube (m) x 2 (sides/tube)

Fins

Ignore the louvers in the fins

SA (m2) = width of fin (m) x (fin height (m) x number of fin legs/tube) x 2 ( sides/fin) x total number of fins

Total Surface area in m2 = SA headers + SA tubes + SA fins

To determine the flux loading, a degreased and thoroughly dry heat exchanger is weighed. The heat exchanger is then run through the fluxer, blow-off and dry-off section of the furnace. The heat exchanger is removed just prior to entering the brazing furnace and weighed again.

The flux coating weight is then determined using the following formula:

Weight of unit fluxed and dried (g) – weight of unit un-fluxed (g) x Surface area (m2)

To make sure that the flux loading was determined on a completely dry unit, run it through the dry-off section a second time and re-weigh.

Brazing also offers the chance to change the design of heat exchangers by substituting round tubes with flat channels (microchannels) which offer improved heat transfer on both refrigerant and air sides for two reasons: better section/surface ratios, which affect the efficiency of heat exchange on the air and the refrigerant side; smaller surfaces in the air stream shadow where heat transfer is inefficient and lots of noise is generated. Brazed connections between fins and tubes are also rigid structures producing less mechanical noise in the presence of air turbulence.
More efficient heat exchange means lower air flows to exchange the desired heat, and microchannel technology already offers lower resistance to the air flow – flat is therefore better than round: reducing resistance by up to a factor of 3 under typical operating conditions (see figures below)!

Round Tubes – Air-Side Effects

Flat Tubes – Air-Side Effects

Flame brazing aluminium to copper is common in the refrigeration industry where copper tubes are brazed to aluminium roll-bond panels or tubes. Brazing copper to aluminium is very similar to brazing aluminium to aluminium, but some precautions are necessary.
There is a eutectic between copper and aluminium at 548°C. When the flux melts and the oxides are removed, there is rapid and unavoidable inter-diffusion of aluminium and copper. This means that at brazing temperature, the aluminium and copper materials are rapidly consumed to form the eutectic metal. Management of time and temperature is critical to minimize the inter-diffusion and metal consumption. There is an advantage however. Since filler metal is created in-situ, there is no need to supply filler metal to the joint. The only requirement is that the design of the joint allows metal consumption without sacrificing joint integrity.
In flame brazing the inter-diffusion of copper and aluminium can be halted rapidly by simply removing the heat source – in this case simply removing the flame is sufficient to allow the joint to cool quickly. This is next to impossible in furnace brazing. There are no conventional furnace designs which will cool quickly enough to halt the continual formation of the aluminium-copper eutectic. For this reason, brazing copper to aluminium in a furnace is not practiced.

One of the largest potentials to increase efficiency of heat exchangers lies within the heat-transfers: reducing condensing temperatures by 3 °K will improve overall system efficiency by approx. 10 % for a standard R 410 A air conditioning cycle. A minimization of the temperature difference between the air flows and the phase change temperatures of the refrigerants can be achieved by improving the heat transfer efficiency of the heat exchangers. Brazed microchannel heat exchangers have already proven that they are a cost effective solution for the utilization of this optimization potential – as well as boasting a number of other benefits (see below). Brazed microchannel heat exchangers have been the technology of choice in the automotive industry for the past 10 to 15 years, and are already making inroads into the stationary HVAC&R industry for the following convincing reasons.

Poor contacts between fins and tubes account for approximately 5 – 10 % of heat transfer resistance in a standard heat exchanger manufactured by mechanical or hydraulic expansion of the round tubes because this always leaves imperfect connections between the parts. The microscopic image shows the small gaps between fins and tubes responsible for contact resistance that reduces heat transfer performance.

Fig. 1: Small gaps between fins and tubes reduce heat transfer performance in mechanically or hydraulically manufactured heat exchangers.

Brazed connections are much better because they metallurgically bond the fins and tubes in a single conductive material, eliminating all potential sources of contact resistance.

Fig. 2 Excellant heat transfer performance because no gaps in brazed connections.

NOCOLOK® Flux is the world’s most widely used flux for aluminium brazing in a controlled atmosphere. Well-proven in the automotive industry, NOCOLOK® Flux is also increasingly used for brazing aluminium coolers for air conditioning and refrigeration systems. In the well-known standard applications NOCOLOK®Flux is not corrosive. To improve the positive properties under extreme conditions even further, Solvay Fluor, has developed a new brazing agent for the markets: NOCOLOK® Li Flux.

This new flux builds a very smooth surface residue. The new physical-chemical properties present an optimization of the compatibility in hydrous environments. NOCOLOK® Li Flux has passed several test series with good results and is meanwhile in the testing phase in many companies.