<p>Friction stir welding (FSW) is an energy-efficient and eco-friendly metal joining process that is widely used to weld aluminium alloys. In the growing trend of replacing steel with high-strength, lightweight aluminium alloys, FSW has excellent potential. Numerical modelling of FSW is a powerful tool for designing suitable process parameters to produce defect-free welds. However, its capabilities are not fully utilized because of improper modelling approaches. This work has explored how a commonly used technique, named mass scaling, which is used to reduce computational time in explicit numerical analysis, affects simulation results. As there is no general rule for selecting the mass scaling factor, this work has performed the numerical analysis of FSW with different mass scaling factors to find the deviation in results. The deviation in temperature and normal force predictions were less than 7%, even after adopting a mass scaling factor of 10⁶. However, the deviation in equivalent plastic strain was 31%, and material velocity was 73%, even for a mass scaling factor of 100. The underlying governing equations were analyzed to find the reasons for the discrepancies. This work recommends not to use the mass scaling technique when the prime focus of the study is the precise metal flow behaviour. Adopting such techniques limits the scope of numerical models in effectively designing the process parameters for commercial applications.</p> Graphical abstract <p></p>