Research On Crack Of Damage During Multi-pass Hot Power Backward Spinning Process For 7075 Casting Aluminium Alloy Tube

Thin-walled 7075 casting aluminum alloy tube undergoes high-temperature and complicated deformation with multi-factors coupling interactive effects in The multi-pass hot power backward spinning(MHPBS)process.Because of the low plasticity and uneven plastic deformation,the crack phenomenon is easy to produced in the process,which will seriously restrain the production efficiency and quality of the workpiece.Based on modified of Lemaitre damage model(LDM),a systematical and thorough investigation on cracking behavior of thin-walled 7075 casting aluminum alloy tube in MHPBS has been carried out by finite element(FE) numerical simulation method combined with experimental study analysis in this study.A principal introduction to the project and its main achievements obtained are as follows.Based on the isothermal constant strain rate compression tests, the effects of deformation temperature and strain rate on the flow stress are revealed during hot deformation process of 7075 casting aluminum alloy.The rheological stress constitutive equation is established, which add the softening factor and provide a reliable basis for the establishment of the damage and fracture criterion of 7075 casting aluminum alloy.Considered 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 and the FE mode of thin-walled 7075 casting aluminum alloy tube in MHPBS has been developed,based on the modified LDM and validated experiments.Based on the PE model, the occurrence, development and evolution characteristics of the crack of thin-walled 7075 casting aluminum alloy tube in MHPBS are systematically analyzed.The results show that the axial, circumferential and radial damage distribution of the cylinder are uneven and the outer damage value is greater than that in the tube inner.The difference of damage value between the tube inner and outer gradually decrease with spinning pass increasing.The damage degree of the tube is reduced by properly increasing the preheating temperature of the cylinder, but the damage value is increased sharply due to the excessively high preheat temperature.The damage value can be decreased by increasing the feed rate in first,second,and passes.However,that of the workpiece increases with the increase of the feed rate in the fourth passes.The appropriate increase in the frictioncoefficient can reduce the damage of the tube in the first, second and third passes,but the degree of damage will increase with the increase of friction coefficient in the fourth time.The damage degree of the tube can be reduced by properly increase of press amount of roller, but increased amount of damage value will be happened when it be too much.Thus,selecting proper process parameters of 7075 casting aluminum alloy thin-walled tube tube in MHPBS can reduce the damage degree of the workpiece.