Friction-stir welding (FSW) is a relatively new solid-state process, ‘solid state’ in that it joins two metal components without melting them. The components are heated and softened using friction provided by a rotating cylindrical part passing over the joint between two pieces of clamped metal. The parts are then fused together using pressure, sort of like pinching together two pieces of clay. FSW is considered to be one of the most important recent developments in metal joining, producing high-strength, fracture-resistant welds particularly in soft metals such as aluminium and its alloys. In fact, it has been adopted by the National Air and Space Administration (NASA) for the External Tank project of the Space Shuttle. In addition to its mechanical and cost benefits, FSW is an extremely ‘green’ process, consuming less energy than conventional methods and using environmentally friendly materials and techniques. The FSW process produces highly unusual microstructural characteristics and various microstructural zones. Detailed understanding of the FSW process has lagged behind its actual implementation and a virtual design tool would be extremely valuable to the design and manufacturing sectors. European researchers set out to develop a software tool for simulating the FSW process with funding for the ‘Detailed multi-physics modelling of friction stir welding’ (Deepweld) project. The main goals were to include multiple scales, from micro- to macro-scale modelling, and to obtain accurate predictions of part behaviours, weld properties and tool loads. Deepweld scientists successfully developed a multi-physics, multi-scale numerical simulation tool for the FSW process. The tool integrated a novel material flow solver at the smallest scale with industrially available finite element modules to model all phases of the FSW process. Researchers incorporated novel metallurgy modules into the small-scale thermo-mechanical flow solver to account for microstructure changes during stirring and cooling of the metal. The Deepweld solver was applied to actual aeronautical components. It accurately computed macroscopic properties of joints such as residual stress and deformation as validated in experiments on welded panels. Deepweld FSW simulation software provides an important tool to designers and manufacturers of metal components. It has the potential to significantly reduce time and cost associated with product design and manufacture while enhancing reliability and quality of finished products.