Joining of high-strength aluminium and titanium alloys gives new opportunities for cost- and energy-efficient designs especially in the transportation industry. In the present study, friction stir lap joining of AA7050 and Ti6Al4V was conducted, investigating the thermal cycle in the interface via variation of energy input by the spindle rotation rate. Temperature measurements reveal that the interface temperature exceeds the solidus temperature of the aluminium alloy and reached up to 570 °C, so that the process should be classified as semi-solid-state. Additionally, no indication for intermetallic compounds was found. Lap-shear strength of the joint is found to be limited by low diffusion kinetics at low temperatures and liquation cracking at high temperatures, revealing clear indications for the limitations of this specific material combination for the first time.