Study On Forming Rules Of 21-6-9 High-strength Stainless Steel Tubes In NC Rotary Draw Bending Process

With the rapid development of modern industrial technology, the bent tube parts become more and more complex, and the requirements of forming quality and forming limit become higher in aviation, aerospace and other high technology fields, which is an urgent needs to research and develop of numerical control(NC) precision rotary draw bending technologies of tube. 21-6-9 high-strength stainless steel tube(HSST) has been widely used in pipeline systems of aviation, aerospace and other high technology fields owing to their high strength, excellent resistance to corrosion and high pressure, and good high temperature oxidation resistance. The 21-6-9 HSST has some special properties such as high yield strength and tensile strength,low elongation at room temperature, high ratio of yield strength to elastic modulus, and so on, which lead to the great forming difficulty and bad forming quality. In this dissertation, a systematical and in-deep research on the key problems, such as wall thickness variation, cross section distortion and springback under multi-die constraints and with multi-factors coupling effects in the 21-6-9 HSST NC rotary draw bending have been carried out. The main research contents and achievements are as follows:Approximate formulas of stresses in different directions, curvature radius of neutral layer, bending moment, wall thickness variation ratio and cross section distortion ratio are derived base on in-plane strain assumption and exponential hardening law during tube NC rotary draw bending. An analytical model of springback of tube NC rotary draw bending is established by using principle of virtual work. The model has been verified to be the closest to the experimental values comparing with any previous analytical model, which can be used to estimate preliminary springback in tube NC rotary draw bending process.A three dimensional(3D) elastic plastic finite element(FE) model for the whole process of the 21-6-9 HSST NC rotary draw bending and unloading springback is established based on the FE platform of ABAQUS. Key technologies involved in the modeling process are solved, and the model is verified to be stable and reliable through meshing size, mass scaling factor, energy curve and experimental results. By using the model, the effects of no-mandrel bending and mandrel bending(the mandrel types including the cylindrical mandrel, hemispherical mandrel and ball-and-socket mandrel) on wall thickness variation and cross section distortion in 21-6-9 HSST NC rotary draw bending are deeply studied. The results show that applied mandrel is necessary conditions to obtain qualified bent tube parts during NC rotary draw bending of 21-6-9 HSST, and the suitable mandrel type is hemispherical mandrel.The effects of geometrical parameters, material parameters and process parameters on wall thickness variation and cross section distortion in the 21-6-9 HSST NC rotary draw bending process are investigated systematically under the condition of the hemispherical mandrel. The results show that: 1) The wall thickness variation ratio and cross section distortion ratio first increase and then tend to be stable with the increase of bending angle, and the wall thinning ratio is greater than wall thickening ratio under the same bending angles. The wall thickness variation ratio and cross section distortion ratio increase with the decrease of relative bending radius, and the relative bending radius must be greater than two to get qualified bent tube. 2) Increasing elastic modulus, hardening exponent or decreasing strength coefficient, yield strength can reduce cross section distortion ratio. Increasing strength coefficient, hardening exponent or decreasing yield strength can reduce wall thickness variation ratio, while elastic modulus hardly has an effect on wall thickness variation ratio. Poisson′s ratio has no obvious effect on wall thickness variation ratio and cross section distortion ratio. 3) Increasing the clearance between tube and wiper die, the clearance between tube and mandrel or reducing the clearance between tube and bending die, the clearance between tube and pressure die can reduce wall thinning ratio. Reducing the clearance between tube and mandrel or increasing the clearance between tube and pressure die can reducing wall thickening ratio, while other clearances between tube and dies have no significant impact on wall thickening ratio. Reducing the clearance between tube and bending die, the clearance between tube and pressure die, the clearance between tube and mandrel or increasing the clearance between tube and wiper die can reduce the cross section distortion ratio. 4) Reducing the friction coefficient on tube-mandrel can reduce wall thinning ratio, while other friction coefficients on tube-dies have no obvious impact on wall thinning ratio. Increasing the friction coefficient on tube-bending die, the friction coefficient on tube-wiper die, the friction coefficient on tube-mandrel can reduce wall thickening ratio, while the friction coefficient on tube-pressure die has no significant effect on wall thickening ratio. Increasing the friction coefficient on tube-bending die, the friction coefficient on tube-wiper die or reducing the friction coefficient on tube-mandrel can reduce cross section distortion ratio, while the friction coefficient on tube-pressure die hardly has an effect on cross section distortion ratio. 5) With the increase of the mandrel extension length, wall thinning ratio increases, cross section distortion ratio decreases, while the wall thickening ratio has no obvious variation. 6) Bending speed and push assistant speed of pressure die have no obvious effect on wall thickness variation ratio and cross section distortion ratio.The deformation behaviors of the whole process and the laws of springback for 21-6-9 HSST NC rotary draw bending are deeply studied. The results show that the stress of bent tube has been unloaded during retracting mandrel and springback, and a reverse loading phenomenon in tangent stress occurs on the inner and outer sides of the bent tube. The springback angel increases almost linearly with the increase of the bending angle, while the springback radius decreases with the increase of the bending angle, and it meets Allometric function relationship. The methods of getting springback angle and springback radius are put forward by solving the intersection angle between two linear equations before and after springback, and fitting the inside crest line circular arc radius of bent tube before and after springback, respectively.The effects of material parameters and process parameters on springback behaviors during 21-6-9 HSST NC rotary draw bending are systematically researched. The results show that: 1)The springback increases with the decrease of elastic modulus or increase of strength coefficient, yield strength, while hardening exponent and poisson′s ratio have little effect on springback. 2)The springback increases with the decrease of mandrel extension length, the clearance between tube and pressure die or increase of the clearance between tube and bending die, the clearance between tube and mandrel, the friction coefficient on tube-bending die, the friction coefficient on tube-wiper die and the friction coefficient on tube-mandrel. 3) Bending speed influences springback angle significantly, while has little effect on springback radius. With the increase of the clearance between tube and wiper die, the springback first decreases and then increases. The push assistant speed of pressure die and the friction coefficient on tube-pressure die have no obvious effect on springback. 4) The sensitivity of springback on material parameters is greater than that of process parameters, and the sensitivity of springback on material parameters from high to low are strength coefficient, elastic modulus, yield strength, hardening exponent and poisson′s ratio. The most sensitive process parameter for the springback is mandrel extension length, while bending speed, the clearance between tube and wiper die is the least sensitive parameter for the springback radius, springback angel, respectively. 5) A springback compensation method is proposed to achieve the precision bending in terms of both springback angel and springback radius, which is verified to be effective via an example study.The above research results have important value of theory and application to overcome the forming difficulty and improve the bending forming performance and forming quality of 21-6-9 HSST, which can provide theoretical and technical support in 21-6-9 HSST NC rotary draw bending for the pipeline systems of aviation aircraft.