In the automotive industry, developing multimaterial car bodies is seen as a way to lighten vehicles to reduce greenhouse gas emissions while keeping crashworthiness. The combination of aluminium and steel is of particular interest due to the lightness of aluminium and the strength of steel. In this context, a novel method to weld aluminium to steel has been developed at UCL, the Friction Melt Bonding (FMB) process. The weld is achieved by the formation of a bonding intermetallic layer (IML) at the interface between the base materials. Due to the temperature reached during the process, of the order of 700 °C, and the large difference of the coefficients of thermal expansion, residual stresses are induced. These ones have an impact in the measurement of the weld strength in general and the IML toughness in particular.Two welds performed at different welding speeds are studied in order to understand the building up of residual stresses. In particular, the transfer of stresses between the steel and the aluminium, the level of residual stresses with different heat affected and fusion zones and its impact on the mechanical properties on the IML interface will be considered.