Lap joints of aluminum alloy A6061-T6 and AZ31B magnesium alloy were produced by cold metal transfer welding with Al-5 %Si filler metal. Four heat inputs designated as A (175 J/mm), B (185 J/mm), C (195 J/mm) and D (205 J/mm) were used during the process and the joints made were subjected to analysis of microstructure, mechanical properties and corrosion behaviour. The thickness of the fusion line (diffusion layer) varied from 3 to 12 µm depending on the heat input. It was also found that the joints made using the heat input of 205 J/mm exhibited highest tensile strength of 360 N/mm, least tensile stress in the weld and better pitting corrosion resistance. Electron microscopy study of the weld revealed the presence of β′-Mg2Si, Al6Mn and β-Al3Mg2 particles. X-ray diffraction study in the weld revealed the formation of γ-Al12Mg17 and β-Al3Mg2 phase with Mg2Si strengthening precipitates. Tensile failure occurred at the fusion line near magnesium. © 2016, The Indian Institute of Metals - IIM.