DVS Paper summary of Cesium-free brazing flux with improved magnesium tolerance

Dr. Alexander Rehmer, SOLVAY GmbH, Germany

In the automotive industry, light weighting continues to be a key focus due to its direct impact on performance efficiency and environmental sustainability. Lowering the overall vehicle weight is important, especially as current new energy vehicles are 10–15% heavier compared to traditional internal combustion engine vehicles. Weight reduction is made possible by use of aluminum with lower sheet thickness, but sufficient mechanical stability. Magnesium is therefore used as a common alloying element. A significant disadvantage of magnesium during brazing is its interaction with potassium fluoroaluminate fluxes, which can results in poor joint formation. This effect, known as “flux poisoning”, is caused by the formation of high melting chemical compounds. The addition of cesium to the flux has proven to suppress this poising effect. In terms of sustainability, the use of recycled scrap aluminum with high magnesium content and the limited availability of cesium in the world, a cesium-free brazing flux was investigated.

The brazeability of standard NOCOLOK^® Cs flux and cesium-free flux with Mg containing alloys with concentration of 0.68% Mg and 0.72% Mg is shown in figure 1. The brazeability was determined by the angle-on-coupon method where the legs of the angle are raised using stainless steel wire. Brazeability is thus measured as a function of the length of the fillet formed. In case of clad-less AlMg1 coupon, the brazeability of cesium-free flux is improved when compared with NOCOLOK^® Cs flux. This can be explained by the higher flux activity of cesium-free flux compared to NOCOLOK^® Cs flux. Another reason is that the “flux poisoning” is reduced by a similar brazing mechanism as for cesium flux. The only difference is that the reaction with magnesium and cesium-free flux during brazing generates a Mg compound with a higher melting point compared to a Cs-Mg compound. However, the melting point of such Mg compound from cesium-free flux is lower compared to typical high melting chemical compounds such as MgF₂ and KMgF₃. Thus, the high flux activity and the low melting Mg compound during brazing ensure the brazeability for low and high Mg containing alloys, especially for clad-free Mg containing alloys like AlMg1 base material.

The cesium-free flux is promising solution to improve brazing of higher Mg containing aluminum alloys and may allows the usage of recycled scrap aluminum in the automotive industry. This could be a potential alternative for NOCOLOK^® Cs flux in the future. A complete study about the characterization and brazing performance of such cesium-free brazing flux is available as scientific article in the DVS International Congress and Exhibition on Aluminum Brazing featuring: ‘Conference on Aluminum Heat Exchanger Technologies for Heating, Ventilation, Air Conditioning and Refrigeration (HVAC&R)’ in May 2025.