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                          Overview of composition design and development of TC4 titanium alloy

                          The elastic deformation capacity of metal materials is affected by the yield strength and elastic modulus, and the tensile linear elastic limit (ε0.2) is mostly less than 1%. The strength of traditional titanium alloy is in the range of 400-1500 MPa depending on the alloy grade, the modulus of elasticity is between 50-120 GPa, much lower than that of steel (about 210 GPa), and the elastic deformation capacity is about twice that of steel. Titanium alloy's high strength and low elastic modulus endow it with excellent elastic deformation ability, and it is widely used in the aerospace field as a structural and functional integrated material.

                          In the 1950s, the United States first used Ti-6Al-4V titanium alloy bolts on the B-52 bomber, which opened the application of titanium alloy fasteners in the aerospace field. With the continuous lightweight requirements of aerospace and weaponry, light-weight, high-strength and high-elasticity titanium alloys have gradually replaced the traditional 30CrMoSiA steel in fasteners, improving the safety and reliability of equipment. The tensile strength of commonly used α+β and β-type titanium alloys is basically 1000 MPa, such as Ti-6Al-4V, Ti-3Al-5Mo-4.5V, Ti-5Mo-5V-8Cr-3Al and Ti-15Mo -3Al-2.7Nb-0.3Si (β 21S) and so on.

                          Since the 1970s, McDonnell Douglas began to use Ti-13V-11Cr-3Al to manufacture springs for civil aircraft, replacing spring steel to reduce weight by 70%. Subsequently, Lockheed, Boeing, and Airbus began to use beta titanium alloy materials to make spring parts such as landing gear locks, hydraulic return and aircraft controls. Representative alloys are Ti-15V-3Cr-3Al-3Sn and Ti-3Al- 8V-6Cr-4Mo-4Zr (β-C), its elastic modulus is about 104 GPa, and its tensile strength is 1300~1450 MPa.

                          Typical grades used in China are TB2, TB3 and TB5. At present, the α+β and β titanium alloys used in springs and fasteners generally adopt α+β two-phase structure to obtain high strength. At the same time, the elastic modulus (90~120 GPa) is also high, resulting in low elastic properties. It is difficult to meet the demand of advanced aircraft for the use of high-strength and high-elastic materials. As a special material for rivets, β-type Ti-45Nb alloy has been used in aerospace products at home and abroad. The alloy has the advantages of low elastic modulus, good plasticity and cold working formability, but the strength, especially the yield strength, is low, and the match between strength and elastic properties is poor.

                          Since the 1990s, in order to reduce the elastic modulus of medical titanium alloys, a series of low elastic modulus metastable β-type titanium alloys have been developed, such as Ti-29Nb-13Ta-4.6Zr and Ti-35Nb-5Ta-7Zr And so on, it has obtained better elastic properties, but this type of titanium alloy is developed for the medical field and has low strength, which is difficult to meet the high strength and high elasticity requirements of titanium alloys for aviation fasteners and springs. In 2003, Toyota Central Research Institute of Japan developed a multi-functional titanium alloy (rubber metal) with excellent comprehensive properties. The typical composition is Ti-23Nb-0.7Ta-2Zr-1.2O (atomic fraction%). The alloy is 90% cold rolled After deformation, the strength can reach 1200 MPa, the elastic modulus is 55 GPa, and the elastic limit is up to about 2.5%. It shows excellent matching of high strength and high elasticity, and the alloy has constant elasticity in a wide temperature range.

                          The metastable β-type alloy Ti-24Nb-4Zr-8Sn (Ti-2448) developed by the Institute of Metal Materials of the Chinese Academy of Sciences also shows excellent elastic properties, with an elastic modulus as low as 42 GPa and an elastic strain as high as 3.3%. It also has excellent high strength and high elasticity matching after solution aging treatment. Rubber metal and Ti-2448 are typical representatives of advanced high-strength and high-elasticity titanium alloys. It indicates that titanium alloys can achieve high-strength and high-elasticity matching. The composition of the alloy fundamentally determines the performance of the alloy. With the current requirements for material performance With the increasing improvement of TC4 alloy and the change of material design concepts, people have carried out different levels of composition optimization and redesign for TC4 alloy in order to meet different needs.

                          There are many optimized redesigns for the composition of TC4, such as the early TImetaL 62S (Ti-6Al-2Fe-0.1Si), Ti8LC (Ti-Al-Fe-Mo), Ti12LC (Ti-Al-Fe-Mo) and so on. In recent years, TImetaL CL4 (Ti-5Al-3V-0.6Fe-0.1O), ATI 425 (Ti-4Al-2.5V-1.5Fe-0.25O), Ti575 (Ti-5.3Al-7.7) have also been developed in succession. V-0.5Si), Ti-54M (Ti-5Al-4V-0.75Mo-0.5Fe), Ti407(Ti-0.85Al-3.7V-0.25Fe-0.25Si) and a series of TC4 modified titanium alloys.

                          There are two main purposes for TC4 modification, one is for performance considerations, especially its dynamic mechanical properties. For example, Ti575 alloy, compared with TC4, the alloy has reduced Al equivalent, increased Mo equivalent, and added a small amount of Si to improve the strength of the alloy. Its tensile strength and yield strength are both higher than TC4, and when the tensile strength is 1200 MPa, the elongation can reach 10.5%, which is 8% higher than the strength of TC4. And compared to TC4 alloy, this alloy has better fatigue properties and forgeability.

                          Another purpose of the TC4 modification is to reduce costs. TImetaL CL4 and ATI 425 both reduce the content of Al and V on the basis of TC4 and add a certain amount of Fe and O elements. While ensuring the strength, it improves the cold workability of the TC4 alloy, thereby reducing the cost. Ti407 alloy has a lower Al equivalent and improves its processing performance by sacrificing the strength of the material. The design purpose of this alloy is mainly to partially replace TC4 and reduce the cost of commercial aircraft materials. Compared with TC4, Ti-54M contains a lower Al equivalent, and a small amount of Mo and Fe are added to lower the β transformation temperature. Compared with TC4, this alloy has better processability and formability, and its superplastic forming ability is better than that of TC4 material, which can significantly reduce processing costs. Its excellent performance depends on ingenious component design and suitable preparation process.

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