Solutions for brazing challenges in battery coolers – Part 2
Leszek Orman, Sebastian Gutmann, Solvay Fluor GmbH, Germany
3. Not-brazed dikes/dimples
Not-brazed dikes/dimples is the most common brazing problem. The battery coolers must have a perfectly flat surface from one side due to the fact that the batteries are positioned on that surface. When some of the inner dimples are not properly brazed, a small deformation (bulge) of the initially flat surface are formed during pressure leak test. Such parts are therefore scrapped.
Case 1
A cut out from a plate battery cooler with bulged surface (not-brazed dikes) has been analysed by optical microscopy and scanning electron microscopy (SEM).
Figure 5. Cut out from a large cooler plate with slightly bulged surface
During investigations a few metallographic cross sections were prepared both across flat surface of the plate and across the bulged area.
Figure 6. Cross section through slightly bulged area
Figure 7. Cross section through slightly bulged area; high magnification
The above pictures allowed for elimination of joint geometry (gap size) and temperature as possible reason for the braze failure. The plate material was clad with 4343 alloy. Observation of the joint at high magnification reveals presence of whitish ribbons, which are free of precipitates. This is a witness mark of the 4343 clad, which must have been liquid during the brazing cycle (wavy like character of the surface). No silicon precipitates are seen since it must have diffused into the bulk of the material during the brazing cycle. This observation suggests that the brazing temperature must have been correct during brazing, but the liquid clad did not wet the opposite side of the joint. Thus possible reason for the braze failure are: lack of flux, bad furnace atmosphere and/or cleanliness. Those parameters can be investigated by SEM.
Figure 8. Surface morphology of the not-brazed joint. Case 1
Figure 9. EDX diagram from the surface of not-brazed joint
Observation of the surface morphology of the not-brazed joint showed presence of only few crystals of flux residue. The observed crystals have rectangular character (no high level of moisture – correct furnace atmosphere [8]). This observation were confirmed by the energy dispersive x-ray (EDX) diagram, which showed very weak signals from flux (fluorine and potassium), very low level of oxygen and also no traces of contaminants (carbon) were found. Thus one can conclude low level of flux deposit was responsible for the failure of brazing. It is not within the scope of this work to discuss the precise method of fluxing, as this is confidential information by the manufacturer.
Case 2
A similar design as in Case 1 was investigated. The difference was such that a much wider area of not-brazed dikes appeared in this case. Similarly as in Case 1, a not-brazed surface of the dike was observed under SEM.
Figure 10. Surface morphology of the not-brazed joint. Case 2
The not-brazed surface is characterized by presence of hexagonal flux crystals. This indicates the presence of moisture in the furnace atmosphere during the brazing cycle [8]. This situation was caused either by insufficient removal of organic binder from the flux paint and/or by incomplete drying of the plates.
Case 3
Similar cut outs as discussed in Case 1 and 2 were analysed by optical microscopy and SEM; however the inner channels are not formed by dikes but by single dimples. Mechanically opened up not-brazed dimples are shown in figure 11.
Figure 11. Not-brazed dimples. Plate mechanically opened up after the brazing
When analysing under optical microscope a contact between the flat plate and a spherical cone like dimple, one must realize that the distance between these two surfaces is a function of the observation plane location. Only when the observation plane runs exactly through the apex of the dimple, definite conclusions about the joint gap can be drawn. Unfortunately, determining the exact location of the observation plane in the relation to the dimple apex is quite difficult and during a standard optical observation it is not usually performed. However one rule should be always observed; the observation plane should be parallel to the line determined by the dimples tips. In this way when observing different gap size at the dimples located next one to each other one can conclude that the plate were not perfectly flat. In the performed investigation all the observation planes were parallel to the dimples tips.
Figure 12 present two dimples located one next to the other where one is brazed and the other one is not. The distance between the dimple surface and the flat tube is about the same for both cases. Thus it is rather not justified to ascertain that the lack of brazed is caused by lack of contact. However, in the vicinity of the not-brazed joint one can observe an agglomeration of the filler alloy shown in Figure 13 at higher magnification. This would suggest that for some reason the flow of the filler was somewhat restricted.
Figure 12. Brazed and not-brazed dimples located one next to each other
Figure 13. Agglomeration of the filler alloy on the not-brazed dimple surface. Spot mark with red arrow in Figure 12
The flow of the molten filler on the joint surface can be supressed by oxidation caused by bad furnace atmosphere, lack of flux and/or contamination of the substrate surface. To determine which of those potential root causes were responsible in this particular case the joint surface must be analysed under SEM.
Figure 14. EDX diagram of the not-brazed dimple surface. Case 3
The EDX diagram reveals that the level of flux was quite low and at the same time, there is a higher level of carbon and oxygen. Most likely carbon and oxygen comes from not removed binder at lower temperature (fluxing was done with paint flux, which means that the flux slurry contained organic binder).
It should be worth mentioning that the design of the single dimple is extremely unfavourable for securing the contact between the brazed surfaces. Since the contact is done only in one point even a slight deviation from flatness will cause that some dimples are not touching the opposite surface, which results in lack of braze. This situation can be somewhat improved by applying a dead weight on the surface of the part providing the weight is uniformly distributed over the whole surface of the part.
4. Fluxing methods
For brazing of aluminium in a protective atmosphere furnace, surfaces to be brazed need to be covered with flux, which role is to remove the protective aluminium oxide layer from the joint surfaces. There are various flux application methods; however the important point is that regardless of application method it is the final load of flux and its uniformity which decides about success or failure of brazing. The goal is to obtain a uniform flux layer of minimum load (3 to 5 g/m2) or deposit flux only in the actual joints to be brazed [9].
The most common method of flux application is using flux in form of liquid slurries which can be applied to the parts by spraying, dipping or atomized spraying. This method secures reliable flux coverage and usually provides high capacity of the process; however the flux coating does not adhere well to the surface and it can be easily removed by simply touching, which could be a problem during handling the parts before brazing. The problem of weak adhesion is usually solved by using slurries with organic binders (Paint Flux), but the organic binder must be removed before brazing (evaporation at higher temperatures) which sometimes could be troublesome particularly for enclosed spaces.
Flux can also be applied using thick pastes. In this process, flux is applied only to the spots to be brazed, but if it is used for larger number of fluxing points, it may not have the required capacity. This method is recommended when low level of Post Brazed Flux Residue [PBFR] is required [10].
When talking about flux application methods, one should also mention fluxing methods which secure excellent adhesion, but do not involve any organic binders. One of the technologies is called Trillium, where the flux is embedded into the filler alloy clad already at the stage of ingot casting. Brazing sheet obtained by such methods yields excellent brazing results [11], but the drawback of this technology might be connected to a higher cost.
A second fluxing method which secures very good adhesion is based on a spray of flux particles where the flux grains hit the coated aluminium surface with a supersonic speed. This method has not yet been industrialized, though laboratory brazing experiment proved to be quite successful [12].
5. Conclusions
Major brazing problems for flat battery coolers are not-brazed internal dikes/dimples. This issue is most often caused by mistakes done in fluxing and by design of the inner turbulators. The convex design (single dimples) is extremely unfavourable for securing the contact between the brazed surfaces. Since the contact is done only in one point even a slight deviation from flatness will cause that some dimples are not touching the opposite surface which results in lack of braze.
There is a variety of fluxing coating methods, however it is the final flux load that is important for the final brazing result regardless the application method. In some cases, brazing problems could be caused by insufficient removal of the organic binder used for securing adhesion of the flux coating. Therefore, choosing a fluxing method should always be considered individually dependently on particular design, volume of production, existing equipment and acceptable structural cost of manufacturing.
Literature
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[8] H.Swidersky “Fundamentals of Aluminium Brazing”, 12th Annual International Aluminium Brazing Seminar, Wixom MI, USA (2007)
[9] https://www.aluminium-brazing.com/sponsor/nocolok/Files/PDFs/NOCOLOK-Encyclopedia-2013.pdf (accessed 2023-03-01)
[10] H. Pawera. “Selective fluxing. Flux mixtures for selective fluxing” Solvay International Brazing Seminar 2023, Hannover, Germany
[11] C. Martín-Callizo, Gränges, 6th Int. Congress Aluminum HEX technologies for HVAC&R, Düsseldorf, 2019
[12] T. Wojdat, “A new approach to flux deposition for brazing aluminum by low pressure cold spraying”, Materials Science Poland, 2023, to be printed
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