Use of an aluminium composite material in a thermal joining method, said material consisting of at least one aluminium core alloy and at least one external brazing layer consisting of an aluminium brazing alloy provided on one or both sides of the aluminium core alloy, wherein the aluminium brazing layer has a pickled surface. Reduced costs and a lower environmental impact is achieved by using an aluminium composite material in which the pickled surface of the aluminium brazing layer had been pickled by pickling with an acid, aqueous pickling solution containing at least one mineral acid and at least one complex-forming agent or a complexing mineral acid, wherein the removal of material in the pickling is between 0.05 g/m2 and 6 g/m2, the aluminium composite material is used in a flux-free, thermal joining method, and the joining method is carried out in the presence of a protective gas.