Quality Control Of Thick TC4ELI Titanium Alloy EBW And Spherical Shell Deformation Prediction

In this paper, 60 mm thick TC4 ELI titanium plates were welded with EBW, to study the effect of the different process parameters on microstructure and mechanical properties. Then developed the appropriate controlling and repair technology for the possible welding defects when welding the large-size titanium ball with EBW, and based on the finite element method, predicted stress fields and deformation after welding of TC4 ELI titanium spherical shell circumferential weld.After reaching a certain weld heat input, the full penetration joints cross-section of thick TC4 ELI titanium alloy electron beam welding was cone shaped, with good weld forming sides. The weld zone is composed with a coarse columnar crystals, intragranular organization showing a "basket-shaped" distribution needle α’ martensite phase, and it is different from the α + β phase morphology of TC4 titanium alloy base material. HAZ contains α+β+α’ phase, with a continuous distribution characteristic. On the penetration direction, since the weld heat input decrease from top to bottom, the columnar grain size decreases, intragranular α’ phase decreases on the same. Since the concentrated welding energy and less welding heat input, the Ti, Al, V content of the upper and lower part of weld is roughly equal to the base metal.Under different process parameters, the joint tensile specimen of upper, middle and lower parts fractures in the base metal, tensile strength reaches 905 MPa,and the fracture presents a porous aggregating fracture characteristics. For a non-penetration weld, since existing the weld root defects, the tensile strength of the region is only 300 MPa. The average indoor temperature impact toughness of full penetration welds reaches 53.8J/cm2, roughly equal to base material along welding line, this indicates that electron beam welding has a good processing performance for large thickness TC4 titanium alloy. Relative to the upper weld, the martensitic transformation is more thorough in the lower portion, so the lateral bending strength is higher and the bending angle is smaller, after using a larger indenter, the joint realizes 180° lateral bending.Using an intermediate frequency 120 Hz electron beam scanning and reducing the descent gradient in convergent beam segment, the internal weld porosity and surface defects can be controlled effectively. Under a suitable remelting process, the undercut, porosity and other defects off the beam pit can repaired effectively, after repair welding, the surface forms a good morphology. In the weld zone, the size of columnar grain increases, and the size and morphological of intragranular α’ phase change. The joint tensile specimen fracture remains in the base material and weld strength decreases slightly, but this does not affect the joint strength,this suggests that repairing defects in the welding process using electron beam welding method is entirely feasible.After cooling, the weld position of spherical shell shows a tensile stress due to welding heat cycle, the maximum transverse tensile stress reaches 750 MPa, and a wide compressive stress zone appears on both sides of the weld. The shrinkage deformation occurs on the vicinity zone of weld, the maximum amount of deformation reaches 0.91 mm, and a swell deformation occurs on both sides of the weld area. After stress-relief annealing, the residual stress of the ball shell structure is weakened, and the stress distribution becomes uniform, then the amount of deformation after welding is also reduced. These shows stress-relief annealing can effectively reduce the residual stress of ball shell structure, and improve the performance of the structure.