The tensile strength of direct AA5754/Ti6Al4V joints performed by high speed Yb:YAG laser welding is found to be determined by morphology and phase content of dissimilar interface formed between contacting Al-rich and Ti-rich melted zones. Three types of contact interfaces were observed: (1) thin (<20μm thick) interface composed mostly by TiAl and formed under 0.2mm beam shift to Al side and linear energy of welding ≥37.5kJ/m; (2) cracked interface (190–300μm thick) composed by Al3Ti and other Al–Ti intermetallics and formed under beam shift at 0.2mm to Ti side and linear energy ≥37.5kJ/m; (3) malaxated interface composed of layers and isles of Ti3Al and TiAl forming in other tested conditions and favored by welding speed >10m/min.Maximal linear tensile force (220N/mm for 2mm thick weld) can be attained when thin contact interface is formed. In this case, the fracture starts in intermetallics-rich zone but propagates mainly in Al-rich melted zone, when in other cases it occurs in brittle intermetallic layers.