Brazing of Stainless Steel to Aluminium for Pots and Pans Production
The following article provides some answers on general questions regarding the use of NOCOLOK Sil Flux for manufacturing pots and pans.
What is the NOCOLOK Sil Flux quantity (per m²) 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.
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 using 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.
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 (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 must be dried after flux application to avoid water-based corrosion effects.
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 (fine grade) – which is used for brazing pots and pans – is as follows:
< 5μm: < 25%
10 to 20μm: 50%
> 35μm: < 10%
> 74μm: not detectable (by laser particle size analysis)
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 joined 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).
Dear expert, can we use our existing Radiation CAB Nocolok furnace for brazing of stainless steel with aluminium for production of pots and pans?
Dear Mr. Riaz,
Please allow me to start with some general comments on joining stainless steel with Al. 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 must 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 production of power plant cooling modules (with aluminum-plated steel tubes), the filler alloy is provided by clad fin material.
Now – coming back to your question about the manufacturing of pots and pans: As already mentioned, this is usually done by induction brazing. I don’t know of any pots and pans production in CAB furnaces. The induction brazing process time is much shorter when compared with furnace brazing. A short cycle reduces the risk of brittle intermetallics formation.
When joining stainless steel plates with Al plates, the surface areas are usually coated with a mixture of filler metal powder and flux. This circumvents the use of filler metal shim stock which is said to be more costly.
Thank you very much Sir for such a great piece of information.
Wow, it is very informative article.
Thanks for sharing.
What is the method for joining SS disc to Aluminium plate directly ?
Usually, this can only be accomplished with induction brazing.