New High Strength, Long-Life Aluminium Alloys with Excellent Sagging Resistance for Heat Exchanger Tube Applications

A new family of high strength brazing sheet core alloys with extended long life corrosion performance and high sagging resistance have been developed for use in the tubes of CAB brazed automotive heat exchangers. Normal mill production practices were used to produce the materials. Direct chill casting was followed by scalping, cladding, pre-heating, hot and cold rolling to 0.27 mm gauge with final partial annealing to the -H24 temper. The core was braze clad with AA4343 and the opposite side was clad with FA6815 (a proprietary Sapa sacrificial alloy) for specific high waterside corrosion purposes. The new Sapa alloys denoted FA7872 and FA7873 contain high amounts of manganese, copper and also some zirconium, all of which contribute to the high strength achieved after brazing. The post-brazed strength primarily results from high amounts of alloying elements retained in solid solution. Additionally, as sheet gauges continue to decrease the sagging resistance of tube stock becomes increasingly important. These alloys exhibit almost twice the resistance to sagging during brazing compared to normal long-life tube stock. This is due to the large recrystallised grain size which is characteristic of these alloys. The addition of Mg to the core provides a further strength increase without any negative impact on the sagging resistance.

Corrosion studies show that the braze simulated sheet survives more than 40 days in SWAAT without any perforations. This long-life corrosion behavior is due to the formation during brazing of a very protective sacrificial surface layer. This can be seen in the SWAAT tested sample sections and in the corrosion potential depth profiles. The extremely good long-life corrosion behavior is facilitated by the low silicon, high manganese and high copper contents of the core alloys.