Effect of the Ni plating on Al–Cu dissimilar metal laser welded joint

In this study, the joint strength and interfacial microstructure of Al–Cu laser welded joints are investigated to understand the effect of Ni plating. Cu without plating and Cu plated with 1 μm and 4 μm of Ni were laser welded with Al, respectively, and their differences in joint strength and microstructure were comparatively analyzed. Macrostructure observations showed that the plating had no effect on the change of weld pool shape. Lap shear tests revealed that there was a distinctive improvement in joint strength with the Ni plating. Under a 1.4 kW laser power condition, it was noted that the sample with 4 μm Ni plating showed a 49% increase in load and a 100% increase in elongation compared to the sample without plating. Fracture surface observations revealed that Ni addition has changed the crack propagation path resulting in the increased joint strength. Microstructural analysis showed that the Ni is occupied in intermetallic compounds (IMCs) such as Al₃Cu₂, AlCu, and Al₂Cu₃ developed at the interface between Cu and Al₂Cu which were known as the primary crack propagation phase in Al–Cu laser welded joint. Additionally, increases in their thickness and changes in their shapes to irregular wavy forms were observed, potentially reducing load concentration and increasing joint surface area, facilitating stable joint formation. Density functional theory calculations showed that the IMCs with Ni undergo contraction in the cell volume suggesting solid solution strengthening of IMCs by Ni addition.