Investigation on low hydrostatic stress extrusion technology for forming of large thin-walled components with high ribs

An increase in hydrostatic stress improves the plasticity of metallic materials. However, for some special components, such as large thin-walled components with high ribs, excessive hydrostatic stress can cause several problems that make it impossible to prepare such components using plastic forming methods. In this study, the effect of hydrostatic stress on the deformation behavior of large thin-walled components with high ribs was investigated using a combination of numerical simulations and theoretical derivations. The results indicated that excessive hydrostatic stress leads to die failure, causing metal flow difficulties, uneven deformation, inconsistent mechanical properties, and reduced forming accuracy. Therefore, a low-hydrostatic-stress extrusion method was proposed by adjusting the contact friction, metal flow direction and displacement, and force boundary conditions to reduce the hydrostatic compressive stress. These principles are mainly reflected in the following three aspects: First, the size of the difficult deformation zone was reduced by changing the friction conditions from dry friction to fluid friction or boundary friction to reduce the frictional resistance. Second, the high-stress zone was eliminated by changing the metal flow direction, adjusting the strain state from unidirectional to multi-directional flow, shortening the metal flow path, and reducing the metal flow resistance. Third, the strong compressive stress state in the three directions was weakened by regulating the force boundary conditions and changing the loading method from one-way extrusion to multi-directional loading. Based on these principles, a series of new forming technologies have been developed, such as actively counteracting frictional resistance, slotting, and ditching on dies for long-lasting lubrication, multi-directional short-range metal flow, drawing-assisted extrusion, and multi-directional active loading. The adoption of these technologies realizes the labor-efficient formation of large thin-walled components with high ribs and provides the formed members with high precision and uniform mechanical properties.