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In non-corrosive flux brazing the flux contains a eutectic mixture of K3AIF6, and KAIF4, which starts to melt at around 560°C. The melted flux disrupts the existing oxide film on aluminium alloys and suppresses further oxidation thus maintaining fluidity of the AI-Si based brazing alloy. To facilitate downgauging there is a drive to develop higher strength core alloys. Using alloys with an increased level of magnesium will readily achieve significant improvements in post-brazed strength, however, the brazeability of alloys containing high levels of magnesium deteriorates as a result of magnesium reacting with the flux to form a higher melting point compound, which inhibits fluidity of the molten cladding alloy. This paper discusses the influence of brazing conditions to increase tolerance to magnesium in core alloys.

The use of aluminium alloys in automotive heat exchangers, including both engine cooling and air conditioning systems, is now well established for vacuum, and more recently, non corrosive flux brazing. There is a growing trend towards greater weight saving and, thus, increasing the mechanical, thermal and corrosion performance of the heat exchanger. This has placed greater demand on improving material properties for tubestock, platestock and finstock for radiators and other types of heat exchanger units. This paper discusses some of the key material developments within Alcan Rolled Products UK to meet these growing demands from the automotive heat exchanger industry and, in particular the need to select material systems to meet the requirements of individual applications.

To determine the true galvanic compatibility of radiator components a test has been developed using a zero resistance ammeter (ZRA) technique, which measures the magnitude of the galvanic current between different materials, thus allowing specific corrosion rates to be calculated. It is believed that the use of the ZRA technique will help provide a better balance between sacrificial behaviour and thermal performance of fin alloys. In particular, it will be demonstrated that it is not necessary to make additions of zinc to the fin alloys to attain a sacrificial effect, which in the longer term may compromise the recyclability of radiator units.

The use of aluminium alloys for automotive heat exchangers has increased considerably in the last 15-20 years and, in parallel, new alloys have been developed to meet the increased demand for strength and improved corrosion resistance. A non-heat treatable Al-Mn alloy, X800, has been developed by Alcan to significantly increase the corrosion resistance of radiator tubes when subjected to typical service environments. The alloy development employed considerable microstructural understanding to provide heat exchanger manufacturers with an improved product that readily attained enhanced performance during any brazing cycle. A similar philosophy has been adopted to address the issue of increased mechanical performance, higher intrinsic sheet strength, both during and after brazing, provides the opportunity for sheet downgauging and thus lightweighting of components.