Titanium and titanium alloys become an excellent ship structural materials because of their high specific strength, corrosion resistance to seawater and other media, low temperature resistance, and non-magnetic, sound transmittance, shock and vibration resistance. The use of titanium and titanium alloy in ships greatly extends the service life of the equipment, reduces the 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 quality requirements of welded joints are very high for titanium alloy materials used on ships. Especially for titanium alloy thick plate, the general welding technology is low efficiency and welding quality is difficult to guarantee. With the increasing scale of national defense equipment, the welding problems of thick plate and super thick plate are becoming more and more prominent. Vacuum electron beam welding has the advantages of large energy density, strong penetration, small heat input, fast welding speed, small deformation, and high efficiency when welding thick plate, making it very suitable for welding titanium alloys for ships, especially with a large weld depth to width ratio, making it unique in the welding processing of thick plate titanium alloys.
Electron beam welding (EBW) is a new welding technology that uses extremely dense high-speed electron flow to heat, melt, cool and crystallize the welded metal to form a weld. The high energy density of the electron beam occupies the first place in the various welding heat sources actually used now, and has many technical advantages that the traditional welding process can not match:
(1) The weld depth to width ratio is large. High power density electron beams can form welds with large depth to width ratios. Generally, the depth to width ratio of arc welding is less than 2∶1, while electron beam welding can reach 20∶1, and pulsed 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 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) The workpiece deformation is small. Due to the energy concentration, the welding speed is fast, the heat input to the workpiece is small, the depth to width ratio is large, and the welding heat affected zone is small, so the workpiece deformation is small.
(4) Good physical properties of the weld. Electron beam welding speed is fast, can effectively avoid grain growth, increase the ductility of the joint. At the same time, because the heat input is small, the high temperature action time is short, and the alloying elements are less precipitated, the weld has good corrosion resistance. The 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, repeatability and reproducibility is good.
(6) The stirring effect of the vacuum electron beam breaks the dendrites, makes the orientation of the grain in the weld zone non-directional, and increases the number of crystal nuclei, thus refining the grain, making the performance of the welded joint significantly improved.
It is precisely because of the above characteristics of electron beam welding that it is very suitable for the welding of titanium alloys with strong activity to achieve long service life reliability. The experimental results show that the fracture toughness and fatigue crack propagation resistance of TC4-DT titanium alloy vacuum electron beam welded joint are better than that of the base material. In addition, the study on vacuum electron beam welding of TB13 forgings with thickness of 130 mm found that the welding coefficients of all welds were greater than 0.9, and the KIC value of welds increased with the increase of welding depth. However, the toughness of the upper weld and the heat affected zone is lower than that of other layers, which is because the larger thickness is easy to produce uneven structure after welding, resulting in complex residual stress. The test shows that the residual stress of weld can be improved and the quality of weld can be improved significantly by local heat treatment with vacuum electron beam after welding.
