Numerical Simulation Study Of Tube NC Bending Forming Under Mulit-die Constraints

With the development of modern industrial technology, tube numerical control(NC)bending technology is heading in the direction of high precision, high efficiency and low cost. But tube NC bending forming is a complex process under mulit-die constraints, the unreasonable forming parameters is easy to cause various forming defects. As a common pipe used in the field of aviation and aerospace, 0Cr21Ni6Mn9 N stainless steel tube is facing with some problems, such as the great forming difficulty and bad forming quality, owing to its high strength, low elongation at room temperature and high ratio of yield strength to elastic strength, and so on. Thus, it has important guiding significance for practical production to carry out the reach of the influence law of different die sets, process parameters and material parameters on forming quality in0Cr21Ni6Mn9 N stainless steel tube NC bending. The forming limitation of tube NC bending was also obtained. The main research contents and achievements are as follows:A three-dimensional elastic plastic finite element model of 0Cr21Ni6Mn9 N stainless steel tube NC bending was established based on ABAQUS/Explicit platform.Using the established finite element model, the influence laws of different die sets on stress and strain distribution, wall thickness change and cross section distortion in tube NC bending were analysed. And the optimal die sets was determined, including bending die, clamp die, pressure die and mandrel.The influence of processing parameters on forming quality of 0Cr21Ni6Mn9 N stainless steel tube was studied based on the optimal die sets. The results show as follows:(1) With the increasing of the clearance between tube and mandrel Cm, friction coefficient between bend die and tube fb, friction coefficient between pressure die and tube fp, push assistant speed of pressure die vp or the decreasing of clearance between tube and pressure die Cp, friction coefficient between mandrel and tube fm, mandrel extension length e, wall thinning ratio decreases. With the increasing of the clearance between tube and bending die Cb, wall thinning ratio decreases firstly and then increases.(2)Wall thickening ratio increases with the increasing of the clearance between tube and mandrel Cm, friction coefficient between pressure die and tube fp and push assistant speed of pressure die vp or the decreasing of clearance between tube and pressure die Cp.With the increasing of the clearance between tube and bending die Cb, wall thickening ratio decreases firstly and then increases. friction coefficient between bend die and tube fb, friction coefficient between mandrel and tube fm and mandrel extension length e have no significant effect on wall thickening.(3) With the increasing of the clearance between tube and mandrel Cm, the clearance between tube and bending die Cb, friction coefficient between mandrel and tube fm and the decreasing of friction coefficient between bend die and tube fb, friction coefficient between pressure die and tube fp,mandrel extension length e, and push assistant speed of pressure die vp, the cross section distortion ratio increases.(4) Considering the influence of process parameters on wall thickness change and cross section distortion of tube NC bending comprehensively, the reasonable values of Cm、Cb、Cp、fm、fb、fp、vp and e are 0.075mm、0.1mm、0.05mm、0.05、0.25、0.25、1.0vp and 1.0mm.The influence of material parameters on forming quality of 0Cr21Ni6Mn9 N stainless steel tube was studied based on the optimal die sets and reasonable process parameter values. The results show as follows:(1)Wall thinning ratio decreases with the increasing of anisotropy exponent r and strength coefficient K or the decreasing of hardening exponent n and poisson’s ratio v. Yield stresss? have no obvious impact on wall thinning.(2)Wall thickening ratio decreases with the increasing of anisotropy exponent r and strength coefficient K or the decreasing of hardening exponent n, but have on obvious change with the increasing of poisson’s ratio v and yield stresss?.(3)The cross section distortion ratio decreases firstly and then increases with the increasing of anisotropy exponent r, strength coefficient K and hardening exponent n, and increases with the increasing of yield stresss?,but it hardly change with the variation of poisson’s ratio v.The method to determine forming limitation of tube NC bending is presented. And the minimum bending radius of ? 6.35 mm ? 0.41 mm 0Cr21Ni6Mn9N stainless steel bent tube is 2.0 based on the optimal die set and reasonable process parameter values.