NOCOLOK^® Sil Flux brazing is a technique, which eliminates the need for clad brazing sheet or conventional Al-Si filler metal. Sil flux brazing uses filler metal generated in-situ to effect brazing. The mechanism for creating this filler metal in-situ is described below:

1. One of the surfaces to be joined is coated with a mixture of NOCOLOK^® Flux and metallic Si powder. The coated assembly is then heated in the same fashion as in conventional furnace or flame brazing techniques.
2. As the temperature rises, the flux melts at 565°C, dissolving the oxides on both the Al substrates and the Si particles.
3. The bare Al surface is now in contact with metallic Si, and in the absence of oxides, allows solid-state inter-diffusion of Al and Si. Very quickly the composition near a Si particle reaches that of the Al-Si eutectic (Al-12.6% Si).
4. As the temperature increases beyond the eutectic reaction temperature of 577°C, the formation of a liquid pool is established. The formation of the liquid leads to rapid dissolution of the remaining Si through liquid diffusion. The pool of liquid continues to grow, consuming Al, until all of the Si is consumed in the melt. In the presence of a joint, the liquid pool is drawn to the joint by capillary action.
5. Upon cooling, the liquid layer solidifies to form a metallurgical bond between the components.

When brazing aluminum to stainless steel using:
a) NOCOLOK^® Flux and Al-Si filler alloys are suitable
or
b) alternatively CsAlF-Complex flux (melting range between 420 and 480°C) and Zn-Al filler alloys.

Regarding a): Brazing of aluminum to stainless steel works both with NOCOLOK^® Flux + Al-Si filler alloy and with NOCOLOK^® Sil Flux. After the flux melts and the oxides are removed, there is a reaction between Al and Fe, forming a thin intermetallic layer of FeAl₃. This layer forms the metallurgical bond between the Fe and Al components. FeAl₃ is very brittle and thus the thickness of this layer should be minimized, otherwise the joint can easily fracture.

From a metallurgraphic point of view, there is a multi-layer system (microscopic structures). First, there is the stainless steel, then the layer of FeAl₃, then the Al/Si filler metal, and finally the aluminum base material. The thickness of the brittle FeAl₃ layer is a function of brazing time and temperature; – consequently the need for a short brazing cycle with fast heat-up and very short holding time at maximum temperature. Too high brazing temperatures must be strictly avoided. Only with a short brazing cycle, successful joining of aluminum to steel is possible.

Joining of Al to steel using NOCOLOK^® Flux is done on large scale commercially for the production of pots and pans (stainless steel pots with aluminum ‘compensation base plates’) – mostly in induction brazing. It is also used for the production of heating elements (steel heating plates with aluminum base plates and tubes for the electrical heating wires). Another application for aluminum to steel joining is brazing of large aluminum-plated steel tubes – up to 11 meters long – with aluminum fins for power plant cooling modules.

In the manufacturing of pots and pans where there is a large surface area between the Al base plate and the pot, a mixture of filler metal powder and flux is often used. This circumvents the use of filler metal shim stock which is said to be costly and difficult to implement. In Al tube to steel or stainless steel tube joining, conventional flame brazing techniques can be used. Filler metal wire, either pre-placed or fed into the joint must be used. In the production of power plant cooling modules (with aluminum-plated steel tubes), the filler alloy is provided by clad fin material.

1) What is the NOCOLOK^® Sil Flux quantity (per cm²) required for sandwich brazing
or pressure cookers (stainless steel to aluminium)?
The recommended load for NOCOLOK^® Sil Flux is approximately 15 to 25 g/m². Brazing aluminium to stainless steel requires rapid processing, i.e. very fast heating ramp and short time at brazing temperature. Usually, this can only be accomplished with induction brazing.

When brazing aluminium to stainless steel using NOCOLOK^® Sil Flux, the Sil Flux first forms the filler metal from the aluminium component. The filler metal then reacts with the stainless steel to form a thin layer of FeAl₃.
From a metallurgraphic point of view, there is a multi-layer system (microscopic structures). First, there is the stainless steel, then the layer of FeAl₃, then the Al/Si filler metal, and finally the aluminium substrate. The FeAl₃ layer is very brittle, and so it is important that this layer is kept as thin as possible. The thickness of this layer is a function of time and temperature,- consequently the need for a short brazing cycle.

2) To prepare a NOCOLOK^® Sil Flux slurry or paste:
What is the exact mixing ratio (flux to solvent) required?
The mixing ratio for NOCOLOK^® Sil Flux slurries or pasts depends on the application method on site. In some cases, the main focus is a specific viscosity for an automated fluxing system. In other cases, only small flux quantities are prepared for immediate consumption.
NOCOLOK^® Sil Flux can be prepared with alcohol (ethanol or isopropyl alcohol) or alcohol/ water mixtures (70% alcohol content) in any ratio from 20 to 60 wt% (solids). As mentioned earlier, the actual slurry concentration will depend on the application procedure. The objective is to achieve 15 to 25 g/m² surface area.
If the NOCOLOK^® Sil Flux slurry is not completely consumed within one or two days, we recommend to use pure alcohol as carrier to avoid any chemical reaction between the solvent and the metal powder (silicon). Due to hydrolysis of the silicon powder, water should not be used to prepare NOCOLOK^® Sil Flux paste. Brushing, dipping or spraying can be utilised to apply the flux. Uniformity of the applied flux coating is very important.

3) How fast after applying NOCOLOK^® Sil Flux, the components should be 
processed for best results?
Before the part is heated up, the NOCOLOK^® Sil Flux slurry or past coating on the component surfaces should be thoroughly dried or allowed to evaporate. If alcohol is used as a carrier, the evaporation will only take a few seconds (with 15 to 25 g/m² flux load). NOCOLOK^® Sil Flux is non hygroscopic (i.e. the flux does not attract and absorb moisture) and non-corrosive under normal conditions (i.e. there is no reaction between the flux and the metal surfaces at room temperature). If a water mixture is used as the flux carrier, the components should be dried after flux application to avoid water-based corrosion effects.

4) What is the grain/particle size distribution of the silicon metal powder
in NOCOLOK^® Sil Flux?
The silicon particles in NOCOLOK^® Sil Flux show a particle size distribution curve with most of the grains within a range of 10 to 45μm. The Solvay specification for NOCOLOK^® Sil Flux coarse grade (which is used for brazing pots and pans) is as follows:
< 5μm: < 25%
10 to 20μm: 50%
> 35μm: < 10%

5) What are the key points regarding product fit-up for good joint formation
in NOCOLOK^® Sil Flux technology?
During the brazing process it is important that the components to be joint are in intermediate contact with each other. There must be firm pressure applied on all the surfaces of the plates throughout the brazing cycle to avoid large voids and gaps. Filler metal can only fill gaps up to a certain width (i.e. approximately 0.10 to 0.15 mm).

All heat exchanger manufacturers currently using zinc-coated tubes or zinc-coated brazing sheet and extrusions for corrosion protection are invited to consider NOCOLOK^® Zn Flux as an alternative.

How does NOCOLOK^® Zn Flux work?
The flux – a fine white powder with the chemical formula KZnF₃ – is manufactured by Solvay Fluor similarly to standard NOCOLOK^® Flux in a water-based batch process. Thus this material has a distinguished and uniquely homogeneous composition. Coating methods for the new product include slurry-based spraying, dipping, and painting, or electrostatic spraying for aluminium surfaces where zinc diffusion layers are required.

NOCOLOK^® Zn Flux is a so-called “reactive” flux, i.e. the compound becomes active only at brazing temperature when it is in contact with aluminium. Pure KZnF₃ without contact to aluminium decomposes at temperatures above 700°C. But when in contact with an aluminium substrate, this material initiates a series of reactions beginning at around 565°C. At first, a NOCOLOK^®-type flux is liberated together with elemental zinc from the KZnF₃ and the aluminium surface according to the following equation:

6 KZnF₃ + 4 Al 3 KAlF₄ + K₃AlF₆ + 6 Zn

The flux then dissolves the oxides present on the aluminium.

Once the Al/Si eutectic melting point is reached at 577°C, the filler alloy (supplied from a clad layer by one of the base components) starts to gradually liquefy until the maximum brazing temperature is reached. During this entire period the elemental zinc, which is formed by the NOCOLOK^® Zn Flux, diffuses into the surface of the base component, thus creating a zinc diffusion layer – as is also known from zinc sprayed surfaces.

The coating process for NOCOLOK^® Zn Flux on aluminium components can be adjusted to provide exactly the same diffusion profiles as in zinc flame spraying. Furthermore, this material allows the modification of precise zinc levels in a much more controlled manner, because the handling of a fine powder provides increased flexibility.

Tests confirm the good results
Successful brazing is feasible with a NOCOLOK^® Zn Flux load of just 3 – 5 g/m² – Another step to reduce overall flux consumption – because this material provides flux and zinc at the same time. Well defined physical and chemical product properties are essential performance factors. The melting behaviour and compatibility with appropriate coating binder systems were researched by Solvay and further developed and optimised in joint projects with our customers.
Currently, the main fields of application for NOCOLOK^® Zn Flux are the production of automotive condensers and next generation evaporators. The compound therefore is used together with cladded brazing sheet.

Watch the brazing process as an animated video:

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.

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:

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.

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.

NOCOLOK® Cs is a special mixture that is intended for brazing Al alloys with a higher Mg content. By adding Mg to the alloy (types 6000 or 3000) higher material- and pressure resistance as well as better machine processing is now given, compared with NOCOLOK® standard Flux which could tolerate only low Mg contents.

During brazing, Mg diffuses to the Al surface and joins the flux. These chemical compositions have a higher melting point than the filler metal, so removal of the oxides by the flux is inhibited.

In this situation Cs can function as a buffer while the original potassium-based flux remains unchanged in its composition. So alloys with a Mg content of up to 0.8% can be brazed in a CAB furnace.

While flame brazing, the brazeability rate is even higher (1.5 % Mg) due to quicker heating.

The synthetic mixture of only 2% Cs represents an optimized ratio with respect to efficiency and profitability whereas the physical characteristics of NOCOLOK® standard grade such as melting point and granulometry remain nearly unchanged. The following benefits are also very interesting:

• Lower leakage rates and less porosity in production
• Better finish of the brazed work piece after brazing
• No change with respect to standard flux loading
• No change in brazing cycle.

The market trends are clearly obvious. Enormous global cost pressure requires reduction in costs for basic Al material (decrease in wall thickness) and lowering of production costs. NOCOLOK® Cs can help in this and make headers of heat exchangers or fittings more easily brazeable.