Fasteners mainly use three types of materials: the first type is low Mo equivalent α-β two-phase alloy, such as Ti-6Al-4V; the second type is metastable β alloy, including βIII, Ti-44.5Nb, Ti-15-3 in the United States and TB2, TB3 and TB8 in my country; the third type is subcritical composition α-β two-phase alloy, such as BT16l in Russia.

Ti-6Al-4V is a low Mo equivalent α-β two-phase alloy. Among the three types of alloys, it has the lowest β stability coefficient (only 0.27) and the highest aluminum equivalent (reaching 6). Therefore, the β phase content in the annealed state is only 7% (volume fraction). Its advantages are the lowest density, the best strength and fatigue performance, the simplest composition, and the lowest cost of semi-finished products. However, since the room temperature plasticity is not high enough, induction heating is required for hot upsetting, as well as vacuum solution treatment and aging treatment when processing fasteners, and the processing cost is relatively high.

The second category is β alloy (such as TB2, TB3, TB5, TB8, etc.), which is completely different from α-β alloy. The β stability coefficient is very high, ranging from 1.15 to 1.97, while the aluminum equivalent is reduced to about 3. Therefore, a single β phase can be obtained during solution treatment, so that bolts and rivets can be cold-forged at room temperature, and the processing cost is low. The disadvantage is that the density is high. Although the strength is comparable to Ti-6Al-4V, the fatigue performance is not as good as Ti-6Al-4v, and the composition is complex, and the cost of semi-finished products is high. Since vacuum aging treatment is also required, the cost of finished fasteners is still higher than that of Ti-6Al-4V, and the use temperature is also lower than that of Ti-6Al-4V.

 

The density of BT16 alloy is slightly higher than that of Ti-6Al-4V, but significantly lower than that of β alloy. The β stability coefficient of BT16 alloy is 0.83, which is between the above two categories and close to the critical composition (β stability coefficient is 1). In the binary alloy composed of β-stabilizing elements and Ti, as the content of β-stabilizing elements increases, the grain size gradually decreases. Near the critical concentration, the number of α phase and β phase is equal, and the grain size reaches the minimum. When the stabilizing elements are further increased, the grain size increases. Smaller p grains and up to 25% (volume fraction) of β phase content in the annealed state determine that BT16 alloy has excellent room temperature T-process plasticity. Therefore, BT16 alloy has the conditions for rapid upsetting of the fastener head at room temperature, that is, cold upsetting.

 

2 Application of fastener titanium alloy Ti-6A1-4V is a medium-strength α-β type two-phase titanium alloy with excellent comprehensive properties. The semi-finished product specifications are complete, including bars, forgings, thick plates, thin plates, profiles and wires. The alloy has a long-term working temperature of up to 400℃ and has been widely used in the aviation and aerospace industries. It is the main fastener material used in the aviation and aerospace sectors in the United States and Western European countries. Russian titanium alloy fasteners mainly use BT16 titanium alloy. BT16 alloy belongs to Ti-Al-Mo-4V series α-β type high-strength titanium alloy. The main semi-finished products are hot-rolled bars and polished bars and wires for cold heading. They are mainly used to manufacture fasteners such as bolts, screws, nuts and rivets. The maximum working temperature is 350℃. The strength of this alloy in the solution-aged state is slightly lower than that of Ti-6Al-4V alloy. The main advantage is that it can be cold-headed in the annealed state, which significantly improves production efficiency. Fasteners manufactured by cold deformation are widely used in Russia's machinery manufacturing industry. They are also the main standard parts material used by Russia's aviation and aerospace departments, and are also used in some models of aircraft in the country. This alloy has two usage states: cold deformation strengthening without heat treatment and hot upsetting plus solution-aged treatment.

 

 

 

βIII alloy was included in the AMS4977 specification as a fastener material in 1969 and has some applications in aircraft, but in 1987 AMS4977B announced that the aerospace materials department recommended that β11I alloy no longer be used as a standard part material for future new designs. According to recent reports, the alloy has been discontinued. Ti-44.5Nb was included in the AMS4982 specification as a special material for rivets in 1974, and was revised to AMS4982C in 2002. It is still in use today, but only a small section is welded on the head of the Ti-6Al-4V rivet to make it cold riveted. Ti-15-3 (TB5) was first included in the AMS4914 specification as a thin plate in 1984. TB5 and TB8 are used in China as supporting rivets and screws for drag parachute beams and wind guide covers (high temperature use) for certain aircraft models. TB2 and TB3 are β alloys developed by my country. TB2 was used for sheet parts in the early days, and later used as rivets on certain models. TB3 was developed as a material for bolts from the beginning, and has also been used on certain models.

6 Conclusion Titanium is an important structural material for my country's development of national defense, aviation, high technology and other fields, and has important strategic significance. At present, my country's research and development level, production capacity and output of sponge titanium and titanium processed materials have ranked among the best in the world. The future development direction should focus on application needs and combine with international development trends. It is urgent to research and develop higher-performance alloys, improve the technical level of the titanium production industry, and strive to move from a major titanium industry country to a strong titanium industry country.