Research On Crack Behavior During Hot Power Backward Spinning For 7075 Aluminium Alloy Cylinder

The 7075 high-strength aluminum(7075HSAC) cylinder has been widely used in the fields of aerospace and weapons for the merits of high strength, light weight, strong corrosion resistance, high pressure resistance, etc. The hot power backward spinning is main way to manufacture this type of cylinder(HPBS). Crack, however, one of common defects in the HPBS of the cylinder, is an important factor acting on the quality and limit forming, especially micro-cracks generated during the forming mostly invisible to bare eyes. In this study, a systematical and thorough investigation on cracking behavior of 7075 HSAC in HPBS has been carried out using finite element(FE) numerical simulation method combined with theoretical analysis, modified by Lemaitre damage model(LDM). A principal introduction to the project and its main achievements obtained are as follows.Given that no taking the difference under tension and compression and the effect in temperature coupled with strain rate into consideration, the Lemaitre criterion has been improved and obtained the modified LDM. The user material subroutine(VUMAT) of ABAQUS/Explicit code and the FE mode of 7075 HSAC in HPBS has been developed, which based on the modified LDM and validated experiments.The thermodynamic behavior of the cylinder in HPBS processing are investigated systematically. The temperature of annular and ladder distributed in the cylinder, and the maximum temperature located at the contact area of rollers and blank. The maximum equivalent stress, which located in the contact area, are increased during processing. The value of PEEQ behind the contact area is greater than in the contact zone. In axial direction, the value of PEEQ in the middle zone of cylinder is greater than the blank in the two ends, and the annular inhomogeneous PEEQ appears also in thickness direction.The crack behavior of the cylinder in HPBS processing are investigated systematically. The inhomogeneous damage has been distributed in axial direction, annular direction and the thickness direction. In axial direction, the damage value in the middle zone of cylinder is greater than in the two ends of the blank. The max-damage outside the cylinder appeared at juncture of the stability region and the unpinning region, and the max-damage inside the cylinder appeared at juncture of the stability region and the top region. The damage of cylinder presents a wave distribution in annular direction. The increasing rate of damage inside of cylinder is greater than outside. Cracks get more easily in the top and roots of the inside of the cylindrical parts. Root damage value is non-linear development. The increasing rate of max damage during the initial of the rollers feeding is faster and then it becomes relatively stable.Based on the prediction model, the influence regularities of processing parameters for damage in cylinder are investigated systematically. Results show that damage of cylinder can be improved when the preheat temperature of blank, feed rate of roller and friction coefficient between blank and dies are properly increased, but the violent damage will been appeared when the value of those parameters. With the increase of press amount of roller, the max-damage of cylinder presents a wave distribution. When processing parameters of HPBS processing of 7075 HSAC were 300 ℃, 20~30% 0.6~0.7mm/r and 0.3, respectively, the max-damage of cylinder had the lowest. This study has large valuable for the determination and optimization of process parameters in HPBS processing of 7075 HSAC.