This paper successfully proposes a novel model to predict nugget development during resistance spot welding (RSW) of binary Al-alloys. The model employs a coupled thermal-electrical-mechanical analysis, and also accounts for phase change and convective transport in weld pool. Faying surface contact area and its pressure distribution are simulated from coupled thermal-mechanical model using a finite element method. Temperature dependent thermal, electrical and mechanical properties are used. The proposed model can successfully calculate most of the RSW response in term of nugget diameter and thickness, the extent of heat affected zone, etc. The calculated nugget shape based on the thermal model agrees well with the experimental data. Convection effect due to the interactions between phases in the porous mushy zone and the buoyancy force arising from the temperature difference is determined to be not significant for the weld-nugget formation. The proposed model can be used to optimize RSW process parameters for industrial welding.