The main application areas of titanium alloy on ships include pressure resistant shells, seawater pipeline systems, heat exchangers, coolers, various pipe joints, engine components, lifting devices, and launch devices. Russia and the United States were the earliest countries to engage in research on titanium alloys for ships, and formed their own titanium alloy systems for ships. Russia is at the forefront of the world in the development and practical application of titanium for ships, with different strength levels of titanium alloys for ships, and has classified these titanium alloys according to their uses. It is currently the only country with all titanium submarines. China began developing titanium alloys for ships in the 1960s, and has now formed a series of titanium alloys for ships with a strength range of 320-1250 MPa. The main grades include low strength alloys such as TA2 and Ti31, medium strength alloys such as Ti70, Ti75, and Ti91, and high-strength alloys such as TC4, Ti80, TC11, Ti62A, Ti-B19, and Ti-B25. From the perspective of alloy types, low strength and medium strength titanium alloys for ships are usually alpha and near alpha titanium alloys, while high-strength titanium alloys for ships are alpha+beta or near beta titanium alloys. Low strength titanium alloy has the characteristics of high plasticity and good weldability, making it easy to process into thin-walled tubes and suitable for the preparation of various heat exchangers, coolers, and other pipe materials; Medium strength titanium alloy has good comprehensive performance matching and is suitable for large thick section components, sea pipelines, etc; High strength titanium alloy has the characteristics of high strength and low plasticity, and is suitable for pressure resistant shells, high-pressure vessels, special ship components, etc.
For ordinary marine titanium alloy structural components, considering the matching of material strength and toughness, stress corrosion fracture toughness, weldability, etc., the strength level of the material should not be too high, and mature near alpha titanium alloys should be selected as much as possible. However, for structural components with special strength requirements, high-strength titanium alloys must be selected. With the development of marine equipment towards deep blue, higher requirements have been put forward for the performance of titanium materials used in pressure resistant structures such as deep-sea submersibles and deep space stations, promoting the development of high-strength titanium alloys for marine use. Improving the strength of materials can reduce the cross-sectional thickness of components and the weight of pressure resistant structures. However, increasing strength often sacrifices the toughness of materials. Therefore, maintaining high strength while having good toughness is the key to the application of high-strength titanium alloys for ships. High strength and toughness titanium alloys have also become a research hotspot for various research institutes and titanium enterprises in recent years. The research approach is carried out from two aspects. On the one hand, in response to the urgent needs of major national projects, design units tend to choose more mature titanium alloy materials. By optimizing the alloy composition and component preparation process, the performance potential of materials can be explored, and the strength toughness matching of alloys can be improved. Many studies have focused on optimizing the design of mature TC4 and Ti80 alloys. On the other hand, we draw on the development concept of aerospace high-strength and tough titanium alloys to develop new types of high-strength and tough titanium alloys for marine engineering.
During the 13th Five Year Plan period, the Northwest Nonferrous Metals Research Institute (Northwest Institute) conducted research on alloy composition optimization design based on Ti80 alloy, aiming to improve the toughness of the alloy while maintaining its high strength. The influence of α - stable elements, β - stable elements, and interstitial elements on the strength and toughness of Ti80 alloy was systematically studied using a combination of Yu Rui theory calculations and experiments. The micro mechanism of element influence on alloy strength and toughness was revealed through Yu Rui theory calculations. In depth research was conducted on the changes in strength and toughness of Ti-6Al alloy after adding Mo and Nb elements. It was found that Mo and Nb elements have little effect on the room temperature tensile properties of the alloy, but can significantly improve the impact toughness of the alloy. This is mainly attributed to the addition of β - stabilizing elements changing the phase composition in the microstructure, exciting more dislocations and deformation twins under impact load, consuming more impact load, thereby improving the alloy's ability to resist crack propagation and achieving higher impact performance. The influence of O element content on the impact performance of Ti80 alloy bars with different microstructure was studied, and it was found that the impact performance is more sensitive to the O element content in the alloy. By adjusting the content of each element and the heat treatment system, it was found that Ti80 alloy has the best strength toughness matching in the annealed state. Its microstructure is a bimodal structure composed of equiaxed primary alpha phase and beta transition phase, as shown in Figure 1.
Figure 2 shows the effect of O content on the yield strength and impact energy of Ti80 alloy with dual microstructure. It can be concluded that when the O content is 0.1% (mass fraction), the yield strength of the alloy reaches 800 MPa and the impact energy can reach 72 J (test standard GB/T229-2020). The pressure resistant shell of a deep-sea submersible is a typical representative of high-strength and tough titanium alloy used in deep-sea equipment, and the diving depth of the submersible is closely related to the specific strength of the material. The Alvin submersible in the United States has increased its maximum diving depth from 1868 to 4500 meters by replacing the pressure resistant shell material from steel to titanium. After further modification with titanium alloy, its design depth has been increased to 6000 meters. Looking at the material selection of pressure resistant shells for deep-sea submersibles in various countries, it can be seen that the main grades of titanium materials are Ti-6Al-4V (TC4) and Ti-6Al-4VELI (TC4ELI), and the diving depth of a three person submersible made of these two alloys is not greater than 7000 meters. In 2017, China independently developed and successfully built the TC4ELI alloy manned spherical shell and Ti80 alloy manned spherical shell, and successfully installed the TC4ELI manned spherical shell on the Deep Sea Warrior submersible, with a maximum diving depth of no more than 7000 meters. The maximum diving depth of the TC4ELI manned spherical shell, which is shaped like melon petals and imported from Russia, is 7000m. The "Striver" 3-person submersible made of Ti62A alloy can reach a diving depth of 10909m The alloy is a high strength and high toughness damage tolerant titanium alloy jointly developed by the Institute of Metals of the Chinese Academy of Sciences and Baoji Titanium Industry Co., Ltd. The strength of this alloy is greatly improved compared with TC4 alloy, while maintaining good toughness and weldability.
Jiti Industry Co., Ltd. and other units have conducted performance optimization research on Ti62A alloy and developed Ti542222 titanium alloy. The yield strength index of this titanium alloy is 1000MPa, and the impact energy is 40J. After double annealing treatment, it has the best strength plasticity toughness matching.
With the support of relevant national projects, Northwest Institute and the 725th Research Institute of China Shipbuilding Industry Corporation (CSIC) have successfully developed titanium alloys with yield strengths of 800900 and 1000MPa. Northwest Institute has independently developed a high-strength β - type titanium alloy Ti-B25, which has the characteristics of high strength and good cold working performance, and has been widely used in ship communication systems. The Institute of Metals of the Chinese Academy of Sciences has developed 1000 and 1200MPa high-strength and high toughness titanium alloys for titanium used in ocean engineering, and has prepared titanium alloy shells for the Abyss in-situ scientific experiment station and the Abyss glider in small batches, basically replacing Ti64 alloy.
In recent years, China has also introduced additive manufacturing technology into deep-sea equipment manufacturing. China Shipbuilding Industry Corporation Fenxi Heavy Industry Co., Ltd., in conjunction with Xi'an Bolite, has used laser melting deposition (LMD) technology to trial produce titanium alloy propellers, hollow shells, etc. The Institute of Metals of the Chinese Academy of Sciences, in conjunction with Shanghai University of Science and Technology, has developed a variety of deep-sea engineering titanium alloy components using additive manufacturing and powder hot isostatic pressing processes. Based on the design idea of high undercooling composition and the strengthening and toughening method of high-strength titanium alloys, a weak texture, equiaxed crystal titanium alloy composition system suitable for additive manufacturing processes has been developed, enabling titanium alloys manufactured with additives to achieve excellent strength, plastic matching and mechanical property isotropy.
During the 14th Five Year Plan period, Northwest Institute, relying on the sub project of the National Key R&D Program "Optimization and Preparation of High Strength and Tough Titanium Alloy Composition for Deep Sea Extreme Service Environment", developed ultra high strength titanium alloy Ti1300G for deep sea equipment and high strength and tough titanium alloy Ti5321G for deep sea equipment additive manufacturing based on high strength and tough titanium alloys Ti1300 and Ti5321. The yield strength of Ti1300G alloy pressure resistant shell can reach 1250MPa, elongation ≥ 9%, impact energy ≥ 24J, and fracture toughness ≥ 60MPa · m1/2; The yield strength of Ti5321G alloy additive manufactured components can reach 1050MPa, and the elongation rate is ≥ 9%. A pressure resistant shell component for deep-sea gliders was prepared using Ti1300G alloy, and a deep-sea ROV thruster propeller and experimental manipulator arm were prepared using Ti5321G alloy. Currently, the pressure resistant shell is waiting for testing after installation, and the ROV has successfully passed sea trials in the South China Sea.
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