Bonding of Aluminium to Low Carbon Steel using an Overcasting Process

To reduce the weight of a vehicle and improve its fuel efficiency, it is preferable to produce its body in aluminium in place of steel. However, designs may require hybrid structures of aluminium and steel, and there remains a challenge to make a sound bond between the two metals due to their differences in physical, mechanical and metallurgical properties. In the present work, the effect of a zinc coating on the diffusion behaviour between liquid aluminium and a steel substrate was studied. A cost-effective overcasting process was used to cast commercially pure aluminium around uncoated mild steel and 20 µm thick zinc coated steel samples. A reaction layer of similar composition and crystal structure was found between the aluminium and the steel in both cases. SEM-EDS characterisation showed that the reaction layer consisted of 73 at.% aluminium and 28 at.% iron. Moreover, EBSD analysis confirmed that the reaction layer corresponded to the Al13Fe4 phase. The thickness of the reaction layer increased with the presence of the zinc coating as compared to uncoated steel sample. The reason for enhanced reaction layer thickness is believed to be caused by improved wetting of molten aluminium on the zinc coated steel.