Electron beam welding of titanium alloy thick plates for ships
Titanium and titanium alloys have become excellent ship structural materials due to their high specific strength, corrosion resistance to sea water and other media, low temperature resistance, non magnetic sound transmission, impact vibration resistance and other characteristics. The use of titanium and titanium alloys in ships has greatly extended the service life of the equipment, reduced the weight, and improved the technical performance of the equipment and the whole ship. Due to the complexity and particularity of the marine service environment, the quality of welded joints of titanium alloy materials used on ships is required to be high; Especially for titanium alloy thick plates, the general welding technology efficiency is low and the welding quality is difficult to ensure. With the increasingly large-scale national defense equipment, the welding problem of thick plate and super thick plate has become increasingly prominent. Vacuum electron beam welding has the advantages of high energy density, strong penetration, small heat input, fast welding speed, small deformation, high efficiency when welding thick plates, which makes it very suitable for the welding of titanium alloys for ships, especially with a large weld depth to width ratio, making it unique in the welding process of thick titanium alloys.
Electron beam welding (EBW) is a new type of welding technology that uses extremely dense high-speed electron flow to hit the metal to be welded, so that it can be heated, melted, cooled and crystallized to form a weld. The high energy density of electron beam occupies the first place among all kinds of welding heat sources currently used, and it has many technical advantages that traditional welding processes cannot match: (1) The depth width ratio of weld is large. High power density electron beam can form a weld with a large depth to width ratio. Generally, the depth width ratio of arc welding seam is less than 2 ∶ 1, while that of electron beam welding seam can reach 20 ∶ 1, and that of pulsed electron beam welding can even reach 50 ∶ 1. (2) High welding efficiency. Because of the energy concentration, the melting and solidification processes are greatly accelerated, so the welding speed is accelerated. When welding thick parts, the deep penetration ability of electron beam plays an irreplaceable role in improving welding efficiency. While maintaining high efficiency, the joint quality accuracy is also relatively high. (3) The deformation of the workpiece is small. Due to concentrated energy, fast welding speed, small heat input to the workpiece, large depth width ratio, and small welding heat affected zone, the workpiece deformation is small. (4) The physical properties of welds are good. Electron beam welding is fast, which can effectively avoid grain growth and increase joint ductility. At the same time, due to small heat input, short high temperature action time and less alloy element precipitation, the weld has good corrosion resistance. Vacuum has a good protective effect on the weld, avoiding the pollution of the environment and impure substances on the weld metal. (5) The welding process parameters are easy to adjust, with strong process adaptability, good repeatability and reproducibility. (6) The stirring effect of the vacuum electron beam breaks the dendrites, which makes the orientation of the grains in the weld zone directionless, and increases the number of crystal nuclei, thus refining the grains and significantly improving the performance of the welded joint.
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, achieving long service life reliability. The experimental results show that the fracture toughness and fatigue crack growth resistance of TC4-DT titanium alloy vacuum electron beam welded joint are better than those of the base metal. In addition, the study on vacuum electron beam welding of TB13 forgings with 130 mm thickness found that the welding coefficients of all weld thicknesses were greater than 0.9, and the KIC value of the weld increased with the increase of welding depth. However, the toughness of the upper weld and heat affected zone is lower than that of other layers to a certain extent. This is due to the uneven structure after welding due to the large thickness, which causes complex residual stress. The test shows that the residual stress of the weld can be improved and the weld quality can be significantly improved by local post weld vacuum electron beam heat treatment.
