Basic research has been done on diffusion-welded joints with the overall aim of an adjusted interface design. The phase formation of aluminium alloy/magnesium alloy bonds has been revealed by X-ray diffraction, energy-dispersive spectroscopy and electron back-scatter diffraction. The presence of Al12Mg17 and Al3Mg2 phases in a bi-layer has been confirmed. The distribution and microstructure of the formed intermetallics and the arrangement of the precipitates originating from the alloys have been determined. Precipitates originating from the alloys agglomerate in the bi-layered welding zone. Their arrangement in combination with the grain boundary evolution of the intermetallics and the change of the aluminium base solid solution in the interface region has a significant influence on the mechanical properties of the joint.