Characterization of DP600-Al6061 dissimilar welds using an innovative FSW-based process to join Al to steel

Aluminium alloys and advanced high strength steels have acquired a major role in the lightening of vehicle structures to reduce greenhouse gas emissions. Nevertheless, transport industry still lacks of efficient methods to join aluminium and steel in a dissimilar weld. Several efforts have been made to adapt friction stir welding (FSW) to join these two materials; however, the low advancing speeds achieved and the high wear of the tool remain major issues. In this framework, an innovative process, based on FSW and named as ‘friction melt bonding' (FMB), has been conceived for a lap-joint configuration and relies on the highly different melting temperatures of Al and steel and the formation of Fe/Al intermetallics, characteristics that have traditionally been drawbacks for other welding processes such as gas tungsten arc welding. For this process, the sheets to be welded are stacked and clamped together, placing the material with higher melting temperature (steel) on the top and the one with lower melting temperature (aluminium) at the bottom. A pinless rotating cylindrical tool is pressed against the steel surface, generating heat as done in FSW, by both friction and plastic deformation of the steel under the tool. Subsequently, heat is transmitted through the steel to the aluminium,partially melting it at the interface. The molten aluminium reacts with the steel creating a controlled intermetallic layer (IML) establishing the mechanical bond. FMB shows as main advantages the simplicity of the tool and the high advancing speeds that can be reached, up to 1 m/min. Firsts campaigns of tests performed on nearly-pure grades (ultra-low-carbon steel and Al1050)confirmed the strength of the bonding and intermetallic thicknesses that go down to 2 μm. The process is now adapted to two common grades of the automotive industry, dual phase steel and the age-hardenable Al6061. The effect of the process parameters on the thermomechanical cycles, and, therefore, on the joint formation and the microstructure evolution is assessed. Welds were carried out at different rotational and advancing speeds with different back plates. During the welding, temperatures were measured placing thermocouples around the tool path in both advancing and retreating sides. SEM observations revealed that the IML layer thickness decreases as the advancing speed increases. Optical observations on the DP steel combined with micro-hardness measurements showed the evolution of the microstructure. In the heat affected zone the steel was tempered,while in the thermo-mechanically affected zone, the annealing temperatures and fast cooling rates lead to microstructures mainly dominated by martensite and bainite. Aluminium shows columnar dendritic microstructures and segregation in the fusion zone as well as some porosity for higher advancing speeds. Lap-shear and tensile tests were also performed in order to assess the mechanical behaviour of the welds.