Research of annealing bimetallic brass-aluminium strips in a draw furnace

The aim of the research was to study the effect of annealing on the structure and properties of a thin bimetallic strip - brass L90-aluminium (magnalium) AMg2-brass L90 after cladding (welding) by cold rolling. During annealing of the test composition, intermetallics appear on the boundary of aluminium-brass contact layers, and their quantity, structure and chemical composition depend on the annealing time and temperature. Annealing at a temperature of 550 °C in the soaking zone of the furnace with the exposure duration of more than 180 sec. followed by air cooling is accompanied by the formation of defects, such as blistering of the cladding layer which is separated from the base by the brittle fracture of the adjacent part of the diffusion layer. This is explained by increased levels of thermal stresses in the presence of brittle intermetallics formed during the exposure in the diffusion zone. The formation of intermetallic compounds in the transition zone reduces layer adhesion. Decreasing the annealing temperature up to 500 °C permits to reduce the thickness of the intermetallics diffusion layer and thermal stresses, and thereby to avoid the formation of blisters on the package surface. Thus, the annealing of the brass-aluminium-brass bimetallic strip after cladding (welding) is to be carried out at a temperature of 500 °C in the soaking zone of the draw furnace for 160-260 sec.