Numerical simulation and experimental study on mechanism and characteristics of tube free-bending forming process

Free-bending is a new flexible bending forming technology, which has the great bending potential and can achieve complex bending geometries without changing the die and re-clamping the tube. In our research, the FEA model for tube free-bending was established and the bending ratio of down to R/D 0 ≈ 2.5 of brass tube was realized in the finite element simulation. With the simulation model, the motion trace scope of bending die and stress variation in free bending process were identified and it was found that the equivalent stress and the displacement of the strain neutral layer increased with the decrease of bending radius. Meanwhile, the absolute value of the maximum tangential strain in extrados (PE11=1.779) is much less than that in extrados (PE=-2.337) when the bending ratio is about 2.5. The finite element simulation results also confirmed that the axial boosting force during free-bending process led to a displacement of 0.3 mm of the strain neutral layer from the inner side of the bend into the direction of the outer bend and a maximum wall thickness reduction of 6% in the extrados when the bending radius is minimal. From the bending tests, the minimum bending ratio of R/D 0 ≈ 2.8 of brass tube were realized, and the bent tube also has very small wall thickness reduction and the resulting bending geometry shows only small cross section deformations and rupture since the bend die holds the required roundness. Both finite element analysis and bending test verify the above-mentioned characteristics of free-bending.