Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) techniques. Here microstructure formation and residual stress state in dissimilar steel to aluminum CMT welds are investigated. The intermetallic phase seam between the filler and the steel is only a few micrometers thick. Residual stress analyses reveal the formation of the typical residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses exist in the steel plate parent material, tensile residual stresses are present in the heat affected zone of the steel and the aluminum alloy. The area containing tensile residual stresses is larger in the aluminum alloy due to its higher heat conductivity than in the steel. Due to the symmetry in the patented voestalpine welding geometry and the welding from bottom and face side of the weld, the residual stress distributions at the top and at the bottom side of the weld are very similar.

Joints of zinc coated steel (DX54D+Z200) to aluminium alloys (AW6181‐T4 and AW5182‐H111) were produced by the Cold Metal Transfer (CMT) technique, a modified metal inert gas (MIG) joining process. The aluminium alloy sheets and the AISi3Mn1 filler material are welded, while brazing occurs between the AISi3Mn1 filler and the steel sheet. At the interface between the Al‐based filler and the steel sheet, aluminium‐rich intermetallic FexAly‐phases are formed. The comparatively low heat input of the CMT process and the choice of filler composition limit the thickness of the intermetallic phase seam (IMP) to a few micrometers. The structure of the intermetallic phase and its morphology are strongly influenced by the alloying elements (Mn, Si) of the filler. Tensile tests and crash tests of the steel to aluminium alloy joints revealed good mechanical properties.

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The microstructure and composition of interlayers in ultrasonically welded Al/Al2O3 compounds are investigated by high voltage and high resolution transmission electron microscopy including X‐ray microanalysis. The joining is mainly effected via a 2 to 20 nm thin amorphous interlayer of Al and O, which probably is an oxide with an excess of metal. Oxygen has diffused from alumina toward the metal, with the interlayer probably being a residue of the former oxide layer on the metal or a reaction product caused by the welding process. Numerous crystal defects, unbonded porous regions and microcracks in the ceramic owing to friction and thermal mismatch are observed as well as indications of local transient melting. Different orientations and pre‐treatments do not lead to major changes in the observed interlayer defect structure which possibly governs the quality of the welding procedure and thus the interlayer shear strength.

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