Surface Cleaning before Brazing
Dust and dirt, condensates, lubricants and oils must be thoroughly removed. If the metal work pieces are poorly prepared, the flux will not spread evenly and the flow of filler alloy will be haphazard: it will either not spread properly or will discolour. The consequence would be an incomplete joint.
The first step is therefore: always clean the components of all oil and grease. The surfaces can be cleaned using either chemical, water-based or thermal cleaning techniques and substances.
Aqueous or water based cleaning is a quite efficient and robust process, but still generates some waste water.
Aqueous cleaning starts off with a concentrated metal cleaning agent, which is subsequently diluted with water to 1% to 5% (v/v). The composition of a supplier’s cleaning solution is proprietary, but usually contains a mixture of surfactants, detergents and active ingredients such as sodium carbonate that serves to elevate the pH. Once diluted, the cleaning solution will typically have an elevated pH in the range of pH 9 to 12. There are acid based solutions, but appear to be less common.
The best water-based cleaners contain water, tensides, cleaning agent and active ingredients such as carbonates.
The cleaning solution works best at higher temperatures and is usually recommended to operate at 50°C to 80°C. Cleaning action is quicker at higher solution temperatures.
Thermal degreasing works by elevating the temperature of the work piece so that lubricants present on the surfaces will be evaporated. This procedure only works with special types of lubricants known as evaporative or vanishing oils. Vanishing oils are light duty lubricants used mostly for the fabrication of heat exchanger fins, although they are now finding uses in the stamping and forming of other heat exchanger components. Lubricants not designed for thermal degreasing must not be used. These could leave behind thermal decomposition products and carbonaceous residues which at higher level prevent brazing and have the potential to degrade product appearance and accelerate corrosion.
At my work place we are using Thermal Degreasing. You mentioned that temperature range is 180 to 250 Deg Centigrade. I think it is also dependent on the cutting/forming oil used at Fin Forming operation. Also it is function of time. How long should a core kept in oven for perfect degreasing. We observe that higher temperature and long duration produces some assembly/fixture problems such as loosening/dropping of fins
The most thermal degreaser units work at the mentioned temperature levels till max 250 degrees. Higher temperature can cause reoxidazation of the aluminium and people need more flux again which influences negatively the finish of the core. Of course you are correct regarding the lubricant properties themselves. The oil producer should know in which timie the most oil evaporates. Some rest might be tolerable but if too much organic still is on the surface you need more flux again, you contaminate more and more the slurry and increase the failure rates.
The mentioned fin trop issue is not caused by insufficient degreasing but more because of the longer brazing time and the fixturing. The gap is too big after the clad part started to melt.
we produce cooling systems using a batch furnace. the application of flux is by the conventional dunking system.
The products is degreased before brazing and all sub assemblies are cleaned as per standard norms.
The brazed cores do not have a bright shine and is dull. this is causing some final finish/ aesthetic issues.
How can we improve the look of the brazed product – extra brightness and shine
Important factors for good post braze surface appearance are:
a) surface cleanliness prior to fluxing and brazing
b) uniformity of flux application
c) flux load
d) furnace atmosphere conditions (oxygen level, humidity level, potential contaminations)
All these points need thorough review for improvement.
In many cases, there is a combination of several factors resulting in dull and grayish surfaces. Too high flux load with non-uniform coverage and poor furnace atmosphere are the most common.
Flux load is recommended at 5 g/m2 – many brazing operations use only 3 g/m2. Oxygen levels must be below 100ppm – better below 50ppm – and humidity levels must be kept below -40°C in the brazing zone.
A review of the relevant process data about flux load and furnace atmosphere conditions should help to identify possible route causes.
thank you for the your inputs.
Thank you for posting this, it has been very informative.
Our operation does not degrease before fluxing and assumes that degreasing & drying will happen together in the drying oven. This seems like it would NOT be a best practice as the flux would avoid flowing into areas where oil was present, yes?
i want to know working of furnace in heat exchanger (CAC & radiator). can you share full detail of brazing process.
I want to know what is the surface cleanliness required (in terms of mg/squre meter) for brazing process using CAB (Closed Atmosphere brazing) process.