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As part of the overall target in the automotive industry to decrease weight and production costs, the heat exchanger market has to develop more effective designs on an on-going basis. In turn this places demands on the material supplier to develop higher strength alloys, which enable down-gauging for lighter-weight structures, or the use of high pressure cycles, and/or an increased amount of fins for increased cooling efficiency. This increased performance must be achieved cost effectively and with brazing and corrosion performance equivalent to, or superior to, the existing material. In order to meet these demands Corus Aluminium has developed over the last few years new families of improved alloys for heat exchanger tubes and core plates, that are suitable for Vacuum and/or Controlled Atmosphere Brazing (CAB) [1]. To further complete this development a programme was started to develop a series of high strength alloys for the heat exchanger cooling fins.

Two newly developed brazing alloys suitable for vacuum brazing (VAC) and controlled atmosphere brazing (CAB), Hogal-3571 and Hogal-3572 respectively, will be presented in this paper. Together with high strength after brazing long life corrosion properties in SWAAT were achieved. Hogal-3571 and 3572 will be compared to other high strength alloys that are already delivered by Hoogovens Aluminium, i.e. Hogai-3570 as well as AA6063 and P.A6060. This paper describes the metallurgical development of the two new brazing alloys. The chemical compositions, the strengthening mechanism, the strength as a function of the cooling rate and the TTP-diagram of the Hogal-3571 alloy will be discussed.

The nature and level of the vacuum atmosphere together with the depth and composition of the oxide layer on brazing sheet can have a profound effect on the quality of the brazed joint. For example, magnesium is needed in the furnace atmosphere to obtain a good fillet. However, there is a lack of understanding of the interrelationship between oxide thickness and the amount of magnesium in the furnace atmosphere. This paper attempts to address this area. Samples of brazing sheet with modified oxide layers, some of which simulate storage or transport conditions were brazed with varying magnesium content in the furnace atmosphere. A correlation was made between these two parameters and the material's brazeability. The main conclusion of this study was that if sufficient magnesium is present in the furnace atmosphere the material can tolerate significant changes in oxide thickness and type without sacrificing brazeability.