Coupling theoretical analysis and FE framework for revealing the size effect on the deformation characteristics of 304 stainless steel microtubes manufactured via free-bending forming technology

The current study on free bending focuses on traditional macroscopic tubes and profile components. Nevertheless, as the geometric size of the tube decreases, the resulting size effect affects the free bending deformation behavior of the microtube in the less constrained state. In this investigation, a constitutive model of microtubes considering the surface layer model was established. The corresponding model parameters were determined by fitting the flow curves of surface single crystals and internal polycrystals. According to the principle of free bending, the springback internal bending moment of the microtube was derived to explain the phenomenon whereby the bending radius of the microtube decreases with decreasing size factor. With decreasing size factor, the influence of the surface layer grains on the bending behavior of the microtube becomes more intense. Furthermore, by comparing the results of the microscale free bending experiment and simulation, it can be concluded that the material properties of microtubes in the finite element (FE) simulation should be defined based on the surface layer and inner layer. With decreasing microtubule size factor, the bending radius of the tube decreases, and the wall thickness changes more significantly. The cross-sectional distortion of the microtubes decreases with increasing grain size and wall thickness. This study explored the feasibility of the free bending process to achieve bending of microtubes and revealed that the size effect on the deformation behavior of microtubes should be considered in the microscale free bending process.