Use of Pulsed CO2 Laser in Laser Roll Bonding of A5052 Aluminium Alloy and Low Carbon Steel

Abstract

Use of aluminium in automobile vehicles is increasing to reduce weight and thereby improve fuel efficiency. Therefore, the problem of joining steel to aluminium alloys arises in body structure assembly. Dissimilar metal joining of steel to aluminium is faced with the problem of formation of a brittle intermetallic compound layer at the interface. In the present work, a new process, laser roll bonding, is suggested for joining sheets of low carbon steel (SPCC steel) and aluminium alloy A5052. With steel sheet placed overlapped on aluminium sheet, laser heating is performed so that the interface temperature increases rapidly above 1 200 °C by conduction. The rolling operation is immediately followed to cause intimate contact. The aluminium melts, and spreads over the bottom surface of the steel sheet. Rapid diffusion of iron into molten aluminium, and molten aluminium into solid iron takes place. Rapid cooling to below 200 °C leads to form ductile intermetallic compounds on the steel side of the interface layer, in addition to brittle intermetallic compounds on aluminium side. Due to the very short diffusion time a thin interface layer is formed, which helps in improving joint strength. Laser roll bonding was carried out with both continuous wave and a pulsed CO2 laser beam. The process parameters like laser power, roll pressure, and travel speed were varied. Their effect on the interface layer and their composition was investigated. The laser roll bonded specimens were subjected to a tensile shear test. It was found that use of a pulsed laser beam improves the shear strength of the joint beyond 50 MPa by increasing the percentage of ductile intermetallic compounds in the interface layer in the range of 5 % to 50 %. The failure occurred in the low carbon steel base metal in tensile mode indicating a stronger joint. The interface layer was uniform, and the spiked boundary was seen on the steel side. Kirkendall voids were also observed in it. Formability of pulsed laser roll bonded joints — measured in the Erichsen test — was between 5.0 to 7.5 mm, which is less than that of A5052 aluminium alloy sheet.