Mechanism and control of wall thickness evolution in mandrel-free drawing of capillary tubes

Capillary tubes, valued for their large specific surface area and exceptional heat transfer efficiency, are widely used in aerospace, biomedical devices and chemical engineering. Mandrel-free drawing is essential for producing high-performance capillary tubes. In this process, through-thickness deformation is governed by external constraints on the outer surface, whereas the inner surface maintains a free boundary. This creates a radially asymmetric stress field, resulting in uneven deformation through the thickness. When superimposed with process parameter fluctuation, tube blank dimension change, and size effect, the uneven deformation was further exacerbated, making precise control of wall thickness in capillary tube fabrication more challenging. In this work, taking the mandrel-free drawing of GH4169 capillary tubes as the study case, through a series of well-designed simulations and experiments, the coupling effects of process parameters (section reduction, friction coefficient, die angle and sizing band length), tube blank dimension (D/t, the ratio of tube diameter to wall thickness), and size effect factor (t/d, the ratio of wall thickness to grain size) on wall thickness evolution were systemically investigated and revealed. The main findings include: 1) The section reduction, friction coefficient, die angle, and D/t exhibit significant influence on wall thickness evolution of capillary tubes during the mandrel-free drawing, while the sizing band length and t/d exert relatively minor effects. 2) The increase in friction coefficient and die angle raises the deformation gradient and axial stress, making the circumferential compressive strain more easily transform into the axial tensile strain to coordinate deformation, which decreases radial strain and alleviates wall thickness thickening. Conversely, the large section reduction and D/t decrease the deformation gradient and axial stress, resulting in an increase in wall thickness. 3) Due to the separation of the tube from the sizing band, the sizing band length has little influence on the evolution of wall thickness. As the t/d increases, the ratio of axial to radial deformation resistance remains constant, making the evolution of wall thickness independent of the t/d. 4) Based on the above insights, a novel wall thickness control strategy, employing increased friction coefficients (f = 0.12) and large die angles (α = 24°), was proposed to actively regulate the wall thickness of capillary tubes during the mandrel-free drawing process. The drawing experiments indicate that the absolute error of wall thickness was decreased from 0.041 mm to 0.013 mm, achieving a 68.29% improvement in forming accuracy. The developed method in this work will contribute to the high-precision manufacturing of high-performance capillary tubes.