Titanium and titanium alloys have become an excellent ship structure material due to their advantages of high specific strength, resistance to seawater and other medium corrosion, low temperature resistance, and other characteristics such as non-magnetic, sound-permeable, and shock-resistant vibration. The use of titanium and titanium alloys in ships greatly extends the service life of the equipment, reduces weight, and improves the technical performance of the equipment and the entire ship. Due to the complexity and particularity of the ship’s use environment, the titanium alloy materials used on the ship have high requirements for the quality of the welded joints; especially for titanium alloy thick plates, the general welding technology is inefficient and the welding quality is difficult to guarantee. With the increasing size of national defense equipment, the welding problem of thick plates and ultra-thick plates has become increasingly prominent. Vacuum electron beam welding has the advantages of large energy density, strong penetration ability, small heat input, fast welding speed, small deformation, and high efficiency when welding thick plates, making it very suitable for welding titanium alloys for ships, especially The large aspect ratio of the welding seam makes it unique in the welding process of thick titanium alloy.
Electron beam welding is a new type of welding technology that uses a very dense high-speed electron flow to hit the welded metal to heat, melt, cool and crystallize it to form a weld. The high energy density possessed by the electron beam ranks first among the various welding heat sources currently used, and has many technical advantages that are unmatched by traditional welding processes:
(1) The weld aspect ratio is large. The high power density electron beam can form a weld with a large aspect ratio. Generally, the depth-to-width ratio of the arc welding seam is less than 2: 1, while the electron beam welding seam can reach 20: 1, and the pulse electron beam welding can even reach 50: 1.
(2) High welding efficiency. Due to the concentration of energy, the melting and solidification processes are greatly accelerated, so the welding speed is accelerated. When welding large thickness parts, the deep penetration ability of the electron beam plays an irreplaceable role in improving the welding efficiency. While maintaining high efficiency, the quality accuracy of the joint is also relatively high.
(3) Workpiece deformation is small. Due to the concentrated energy, the welding speed is fast, the heat input to the workpiece is small, the aspect ratio is large, and the welding heat affected zone is small, so the deformation of the workpiece is small.
(4) The physical properties of the weld are good. Electron beam welding is fast, which can effectively avoid grain growth and increase joint ductility. At the same time, since the heat input is small, the high-temperature action time is short, and the alloy element precipitation is small, the weld seam has good corrosion resistance. Vacuum has a good protective effect on the weld, avoiding the pollution of the weld metal by the environment and impure substances.
(5) Welding process parameters are easy to adjust, process adaptability is strong, and repeatability and reproducibility are good.
(6) The stirring effect of the vacuum electron beam breaks the dendrites, making the orientation of the grains in the weld zone non-directional, and increasing the number of crystal nuclei, thereby refining the grains and making the weld joints have significant performance improve.
Because of the above-mentioned characteristics of electron beam welding, it is very suitable for welding titanium alloy with strong activity and achieves long service life reliability. Experiments show that the fracture toughness and fatigue crack propagation resistance of TC4-DT titanium alloy vacuum electron beam welded joints are better than those of the base metal. In addition, the vacuum electron beam welding of 130 mm thick TB13 forgings found that the welding coefficient of the total weld thickness is greater than 0.9, and the KIC value of the weld increases with the increase of the welding depth. However, the toughness of the upper weld and heat-affected zone is somewhat lower than that of other layers. This is due to the large thickness, which tends to produce uneven structure after welding, resulting in complex residual stress. The test proves that the post-weld vacuum electron beam local heat treatment can improve the residual stress of the weld and significantly improve the quality of the weld.