The Study On Residual Stress Distribution Of 32mm-thick Ti-6Al-4VELI Pressure Spherical Shell After Welding And Heat Treatment

The Underwater equipment provides exploration platform for Marine scientific research,among which,deep-sea submersible is the frontier field of underwater equipment,and the pressure performance is the first design requirement of deep-sea submersible development.Spherical pressure hull is the most commonly used pressure-resistant structure of deep-sea manned and unmanned submersibles.Because of its excellent properties,titanium alloy has been widely used in various applications and has become the preferred material for submersible spherical pressure hulls.In the process of making spherical pressure hulls,several large melon-shaped petals or two hemispherical hulls are generally welded into a complete spherical hull.The welding residual stress which affects the bearing capacity of the hull will be generated at the welds.The post-welding heat treatment is an effective means to reduce the welding residual stress.It is of great significance to evaluate the magnitude and distribution of the final residual stress for ensuring the service safety of the pressure spherical hull.In this paper,a Ti-6Al-4V ELI(800MPa class)pressure spherical hull is taken as the research object,after welding and heat treatment of residual stress distribution.First,the 32mm-thick Ti-6Al-4V ELI plate was welded by electron beam and the residual stress after welding was measured.Secondly,the DFLUX subroutine of welding heat source for numerical simulation analysis was written using computer programming language Fortran,and the electron beam welding process of 32mm-thick Ti-6Al-4V ELI plate was simulated numerically based on the thermal elastoplastic simulation method of Abaqus finite element simulation software.The distribution of temperature field and residual stress field at the weld are obtained.Then,the effectiveness of nonlinear finite element simulation is verified by comparing the measured residual stress with the calculated stress.Therefore,three-dimensional nonlinear numerical simulation was carried out on the position of the equatorial weld of 32mm-thick Ti-6Al-4V ELI spherical pressure hull.The welding temperature field and welding residual stress field of the equatorial weld of spherical pressure hull were calculated,and the distribution characteristics and variation rules of welding residual stress at the electron beam weld of the spherical pressure hull were studied.Finally,the residual stress field of32mm-thick Ti-6Al-4V ELI spherical pressure hull was analyzed by thermal coupling heat treatment,and the stress redistribution law was obtained.According to the research content of this paper,the following conclusions can be drawn:(1)Three-dimensional nonlinear numerical simulation of temperature field and stress field of 32mm-thick Ti-6Al-4V ELI plate during vacuum electron beam welding is carried out.The results show that the residual stress in the plate is mainly longitudinal residual stress,and the longitudinal tensile residual stress concentrates on both sides of the weld center line and near the weld.The magnitude and distribution of residual stress are basically consistent with the test results.During the welding process,the high temperature area was concentrated near the weld center line,and the weld pool was pinhole shape.The simulated weld morphology was basically consistent with the test results.The comparison results show that the numerical simulation method is reasonable and can be used to analyze the welding process of Ti-6Al-4V ELI spherical pressure hull.(2)The numerical simulation of the temperature distribution in the welding process of 32 mm thick Ti-6Al-4V ELI pressure spherical shell was carried out,and the results were consistent with the temperature distribution law of electron beam welding with composite heat source.The temperature field of the hull is mainly concentrated within30 mm of the welding center line,in which the temperature changes obviously and the welding temperature gradient is large.The rest of the hull is cooler and has a smaller temperature gradient.The longitudinal and transverse residual stresses perpendicular to the weld in the 32 mm thick Ti-6Al-4V ELI pressure spherical hull are tensile stresses,while the transverse residual stresses in the hull are compressive stresses.The longitudinal tensile residual stress is mostly greater than the transverse stress and the maximum longitudinal tensile residual stress is almost equal to the yield strength of the material.The welding residual stress is mainly distributed in a small area near the weld.The welding stress gradient and stress of 32mm-thick Ti-6Al-4V ELI pressure spherical hull decreased with the increase of distance weld.The peak appears near the starting point and the residual stress level increases.(3)During the post-welding heat treatment,the transverse and longitudinal residual stresses of the 32mm-thick Ti-6Al-4V ELI pressure spherical hull continue to decrease in the heating stage,and the residual stresses at and near the equatorial weld obviously decrease,the difference decreases,and the height becomes uniform.Global residual stress redistribution of 32mm-thick Ti-6Al-4V ELI pressure spherical hull.For the weld on the outer surface of spherical pressure hull,the longitudinal residual stress is tensile state and the transverse residual stress is compression state.Both the longitudinal and transverse residual stresses in the weld zone of the spherical pressure hull are tensile,and the longitudinal tensile residual stress is greater than the transverse tensile residual stress.(4)During heat treatment of 32mm-thick Ti-6Al-4V ELI spherical pressure shell,the welding residual stress is fully eliminated when the thermal treatment reaches the creep activation temperature,and the heating rate and holding time have little effect on the removal of welding residual stress.Therefore,in the finite element simulation,in order to improve the computational efficiency,the heating rate can be increased and the holding time can be reduced.In the actual manufacturing,the holding time can be appropriately shortened and the heating rate and cooling rate can be improved under the condition of ensuring the stability of the structural components caused by heat treatment.