<p>This study presents the first investigation on employing microwaves for processing and joining magnesium alloy, AZ31B, without using filler material, further integrating both experimental and 3D multiphysics modelling approaches. Selective microwave hybrid heating was utilized to harness the distinct microwave–material interactions, where graphite rods with a high loss tangent efficiently converted electromagnetic energy within a low-loss casket, enabling rapid and clean magnesium processing. A comprehensive three-dimensional FEM-based multiphysics framework was developed to simulate the electromagnetic field distribution and resulting temperature effects during heating. The model’s analytical predictions were validated experimentally, showing less than ± 4.25% deviation between theoretical and practical results. This close agreement confirms the accuracy of the developed model. The study provides significant insights into the coupling of electromagnetic and thermal fields in magnesium processing, demonstrating the feasibility and sustainability of microwave-based techniques for lightweight alloys. The findings are highly relevant for automotive and aerospace manufacturing applications.</p> Graphical abstract <p></p>