Fundamentals Of Bulging Process And Its Application For GH4169 And TC4 Alloy Rings

Forged rings are one of the most important components in aero-engine.Their dimensional accuracy and stability in the machining process and service are important guarantees to ensure high efficiency,high reliability and long life of aero-engine,and are also key technical problems in the field of hot processing and manufacturing of rings.However,the components would be distorted,bented or strentched and thus out of shape during machining or under service,which is contributed mostly by the residual stresses which are produced,transfered and accelarated after several sequencies of hot processing of rings.Since 1990s,many advanced manufacturers abroad have begun to explore the application of bulging technology in the field of ring rolling,in order to improve the dimensional accuracy and regulate the residual stress in the ring.However,up to now,the research on bulging technology was still the blank in our country.There are still many prominent problems for manufacturing of rings such as large fluctuation of dimension accuracy,unidentification of residual stress in rings and out of shape during machining.In this paper,the technical fundamentals of bulging for GH4169 and TC4 alloy aeroengine rings most commonly used in aeroengine are systematically studied,which is expected to resolve the problems of dimensional precision control and residual stress regulation in ring production.The main research conclusions are summarized as follows:The stress and residual stress distribution in the ring were analyzed and fomulated during elastic-plastic and plastic deformation stages based on the thick-walled cylinder theory.The curves of stress and residual stress in the ring along the wall thickness at different deformation stages are obtained.The effects of bulging ratio and die velocity on distribution and evolution of the strain and stress in the rings were clarified by simulation for the rectangle and three types of profile rings.The strain and stress are periodicly distributed along the hoop direction of the rings with rectangular and profile sections.In the sections of these rings,the gradients of strain and stress are presented and the values of strain and stress decrease from inner to outer of the rings along radial direction.The increase of die velocity is benifit to reduce the strain and stress range.Both the stain and the stress are affected greatly by the bulging ratio and reduced with the increase of bulging ratio.Different from the rectangle ring,the strain and stress in the profile rings are sorted by the real bulging ratio calculated considering the slope.Three groups of experiments on bulging processes of GH4169 alloy rings were carried out.The effects of bulging sequence,temperature and ratio on microstructures and properties were analyzed.For the rings of GH4169 alloy with 920℃and bulging ratio 2.6%,either before or after solution,the microstructures are unchanged obviously as well as the mechanical properties satisfy the standard specification.Additionally,the hardness and the tensile strength in the ring by bulging after solution are better than that by bulging before solution.Comparing to the rings without bulging,for the rings of GH4169 alloy under 1010℃or 960℃and bulging ratio beyond 3.7%,the grain size becomes a bit bigger.The tensile and yield strength are greater,but plasticity is worse when the ring bulged at 960℃than at 1010℃.As well as,the tensile strength with bulging at 1010℃is not good as the ring without bulging.Tensile strength increases first and then decreases,and yield strength increases after bulging at 960℃.Comparing with the non-bulging ring,the mechanical properties of GH4169 alloy rings with bulging ratio of 0-4.75%are little changed,but the mechanical properties show fluctuation change.And there are peaks near 2.78%for the strength of GH4169 alloy at room and high temperature.Three groups of experiments on bulging processes of TC4 alloy rings were carried out.For the rings of TC4 alloy under 750℃and bulging ratio 2.6%,no obvious change is found for microstructure by just changing the order of bulging and annealing.However,for the ring after bulging,the tensile and yield strength at room temperature tend to increase,and the hardness is slightly higher than that of non-bulging ring.It shows that there are no obvious effects on strength and plasticity at room temperature andonly that plasticity at high temperature tends to increase by bulging after annealing with bulging ratio 1.85%.Compared with by bulging after rolling,the mechanical properties of the ring by bulging after anealing have a downward trend.There is still no abvious change on strength and plasticity by adding an extra bulging with ratio 1.85%after rolling and 1.85%bulging.However,the mechanical properties are obviously low for the ring of 14.8%bulging,which leads a great negative impact on the mechanical properties.The effects of bulging on the mechanical properties of GH4169 and TC4alloy rings was revealed.It shows that,comparing with the non-bulging ring,the mechanical properties of TC4 alloy rings with bulging ratio of 0-4.75%are little changed,but the mechanical properties show fluctuation change with bulging ratio.And there are peaks near 2.78%for the strength of TC4 alloy at room and high temperature.The effects of combination of bulging and heat treatment on the hoop and axial/radial residual stresses and their distribution of GH4169 alloy ring were found.Due to the lack of one heating process during hot bulging of ring of GH4169 alloy after rolling,the mechanical stress produced by bulging can not be effectively released and relaxed,which makes the residual stress on the three test surfaces larger.It is found that the residual stress of TC4 alloy ring can be regulated by 2.5%bulging at 730℃inlucding reducing the residual stress especially the hoop stress and improve the distribution uniformity of residual stress.It shows that the mean hoop stress decreases by 24.8%and 20.2%in the inner and top surface of the ring and the internal angle standard deviation,standard deviation and range of the ring are lowered.For typical aeroengine rings of GH4169 alloy and TC4 alloy,the process of ring rolling and bulging have been developed and the rings were manufactured.And both the microstructure and the mechanical properties satisfy the standard specification.For GH4169 alloy ring,the average values of residual stressesσ_θandσ_r are in range of-209.5～-47.9MPa and-174.0～-43.6MPa,the internal angle standard deviations are32.3°～54.8°and 32.5°～56.2°.For TC4 alloy rings,the average values of residual stressesσ_θandσ_r are in range of-274.3～-95.0MPa and-174.0～-43.6MPa,the internal angle standard deviations are 15.3°～38.9°and 16.3°～33.5°.