Rail transit is a safe, comfortable, environmentally friendly and energy-saving green transportation, is an important part of China's public transportation. The scale of rail transit construction is expanding year by year, the operation network is increasing, the energy consumption is increasing dramatically, the traction energy consumption in rail transit accounts for about 30% of the total power consumption, if the vehicle weight is reduced by 10%, the energy consumption can be reduced by 6% to 8%.
With the domestic vigorously promote the construction of rail transit, "14th Five-Year Plan" period of rail transit equipment industry is also in the rapid growth of development opportunities. Rail transportation equipment in the new materials, new technologies and new techniques, especially in the lightweight equipment, spectrum, high-speed heavy-duty and green intelligent direction, the development of its more urgent needs. Titanium alloy, because of its low density, high specific strength, good weldability and good corrosion resistance, has been the focus of the rail transportation industry, and gradually carried out the feasibility study of titanium alloying of related products and on-board applications.
On April 21, 2022, China's independent research and development of the new Fuxing high-speed integrated inspection train successfully achieved a relative rendezvous speed of 870 km per hour, creating a world record of high-speed rail trains with bright-line rendezvous speed, marking the national "14th Five-Year Plan" planning of the "CR450 Science and Technology Innovation Project The CR450 Science and Technology Innovation Project of the National "14th Five-Year Plan" has been in full swing. From CR400 to CR450, the speed is increased by 50 km/h, which puts forward higher requirements on the safety, strength and lightweight of materials.
1 Titanium Alloy Characteristics
Titanium and titanium alloys have excellent comprehensive mechanical properties, in aerospace, naval weaponry, chemical industry and marine construction and other aspects of a wide range of applications, known as the "metal of the air", "marine metal", "the rise of the third metal". The third metal".
The performance advantages of titanium and titanium alloys are as follows.
(1) low density, high specific strength (strength and density ratio). The density of titanium alloy is about 4.5 g/cm3, and the specific strength is located in the first metal. The higher specific strength means that when the corresponding structural strength is met, the lighter the mass of the material is required, which enables the structural design to be more compact and the weight of the structure to be significantly reduced, thus improving the safety of the equipment.
2) Good weldability. Titanium alloy is suitable for TIG welding, laser welding and electron beam welding and other welding methods, the strength of the weld can reach more than 90% of the strength of the substrate, such as welding defects, can be repaired by secondary welding.
3) Good corrosion resistance. Titanium and titanium alloy surface is easy to form an oxide film and is not corrosive, and the membrane is broken after the healing ability. Working in the humid atmosphere and seawater media, its corrosion resistance is far better than stainless steel. Therefore, the use of titanium alloy can be protected without coating.
(4) excellent fatigue resistance. U.S. strategic reconnaissance aircraft SR-7l using titanium alloy structure of the airframe, flight altitude of 30,000 m, the maximum speed of 3.5 times the speed of sound, began service in 1966, to the 1998 permanent decommissioning of the airframe in service within 32 years of no damage.
(5) Good compatibility with composite materials, preferentially used as carbon fiber pre-embedded parts. With the development of the aviation industry, due to the high specific strength of titanium alloy, corrosion resistance, while carbon fiber reinforced composites (CFRP) has a small specific gravity, good rigidity and strength, etc., so these 2 materials have been widely used in the aviation industry. With the large number of applications of composite materials in the aviation industry, the amount of titanium alloy is also gradually increasing. Compared with other metals, titanium alloys are more compatible with composites and have gradually replaced steel and aluminum alloys in some parts of the aircraft.
With the increasing requirement of lightweight of rail vehicles, the application of carbon fiber composites in rail transportation equipment is gradually increasing, such as car body, bogie frame, driver's room and equipment compartment, etc. are using carbon fiber composites for trial production. The commonly used materials for pre-buried parts are aluminum alloy, titanium alloy and iron-nickel alloy. Considering the strength, light weight, thermal stability and electrochemical corrosion caused by the potential difference with carbon fiber of the pre-buried metal structural parts, titanium alloy should be preferred as the pre-buried parts.
2 Titanium alloy in rail transit vehicles in the current state of research
2.1 Titanium alloy bogie frame
Bogie is one of the most important components of rail vehicles, realizing the function of rail vehicle walking, which is directly related to the quality of vehicle operation, power performance and driving safety. Bogie frame is the carrier for the assembly of bogie parts, generally including side beams, cross beams, and suspension mounts required for the installation of related equipment, etc. The use of titanium alloy bogie frame can realize the function of railcar traveling. The use of titanium alloy frame can realize high-strength, lightweight bogie structure, reduce the unsprung mass and the inter-spring mass, and then improve the force between the wheels and rails, and improve the safety and operational reliability of the bogie structure.
In a titanium alloy bogie frame welding, the use of titanium alloys TA2 and TA18, in order to meet the existing frame strength on the basis of its total mass reduced by about 40%, as shown in Figure 1, Figure 2. In the development process of titanium alloy frame, the technical process problems of large deformation in the welding process of titanium alloy side beams and the inability of effective inert gas protection of some welded joints were solved, and the residual internal stress of welding was eliminated by vacuum heat treatment after welding, so that the titanium alloy frame meets the requirements of existing design indexes, and the basic data for the further structural optimization and design of the subsequent titanium alloy frames have been accumulated.
2.2 Titanium Brake Clamp
As a core component of the braking system, the performance and function of the brake caliper directly affects the operation status and quality of the braking system. The application of titanium alloy brake caliper can reduce the under-spring and inter-spring mass, improve the operation quality, and increase the rust and corrosion resistance; the structural strength performance is more stable under low temperature environment.
The developed titanium alloy three-point brake clamp is shown in Figure 3. The main loaded parts such as hanging, gate piece support, hanging seat, cylinder head, piston tube, cylinder head conduit, yoke and lever are made of TC4 titanium alloy, with a total weight reduction of 17.6 kg. The titanium alloy brake clamp unit was subjected to the strength test, low-pressure and high-pressure ambient temperature sealing test, ambient temperature sensitivity test, primary clearance adjustment amount test, maximum clearance adjustment amount test and relief clearance test, respectively. The test results show that the titanium alloy brake clamp unit meets the functional index requirements, and at the same time, passes the 1 million times fatigue test and shock vibration test. In the -50 ℃ low temperature environment, after keeping 48 h, titanium alloy brake clamp unit all functions are normal, indicating that the titanium alloy brake clamp has strong low temperature resistance and is suitable for application in the alpine environment.
2.3 Titanium alloy transition hook
Transition hook is a kind of hook used to connect two different types of hooks, to ensure the shunting locomotive safely and smoothly transferring the vehicles to be overhauled, and at the same time, the transition hook needs to be frequently loaded and unloaded manually when it is in use. According to UIC660, the single weight of the transition hook cannot exceed 50 kg, however, the existing transition hook structure is bulky and heavy, it needs to be handled by many people at the same time during loading and unloading, and it will also cause personal injuries to the maintenance personnel if it falls off during the handling, therefore, under the premise of guaranteeing the safety of the transition hook, it is very necessary to carry out a lightweight design for the transition hook.
Design a lightweight titanium transition hook, based on the variable density method using ANSYSWorkbench in the Shape Optimization module of the transition hook for topology optimization, according to the topology optimization results of the titanium alloy transition hook for lightweight structural design, the lightweight titanium alloy transition hook weighs 42.15 kg, compared to the original E-class steel transition hook Compared with the original E-grade steel transition hook, the weight of the resulting lightweight titanium alloy transition hook weighs 42.15 kg, which is 57.98% less than the original E-grade steel transition hook.
A company in China Railway has developed a titanium alloy transition hook, as shown in Figures 4 and 5. The weight of a single module hook is about 20 kg, and the whole operation process can be completed by one person. During the 750 kN tensile load test and 850 kN compressive load test, the hook body did not fracture, as shown in Figure 6. After unloading, the hook body was tested and inspected as a whole, and there was no obvious deformation or damage to all parts of the titanium alloy 10-type and 13-type transition hooks. The test results show that the lightweight titanium alloy transition hook is light in weight, high in strength and high in operational efficiency, and meets the safety needs of the current transition hook operation, while there is also the feasibility of further lightweight.
In the production of titanium alloy subway transition cone, Shenyang Zhongtian Equipment Manufacturing Co., Ltd. adopts titanium plate die forging and rib plate welding molding process. Compared with the casting process of the original steel convex cone, this method has good molding, high efficiency, and good cone performance, which has been verified by tests to be able to meet the needs of use.
2.4 Traction rod
The center traction device mainly consists of the center traction pin, traction rod assembly (including the rod and its two ends of the rubber ball joint nodes) and connecting bolts and other components. Its main function is to realize the connection between the vehicle body and the bogie, and to realize the transmission of traction and braking force. Tie rod structure is simple, molding process is relatively simple, the use of titanium alloy material to replace not only to achieve the effect of weight reduction, while the use of die forging molding program can also improve the utilization rate of the material, the overall cost will not have a greater increase.
The titanium alloy traction tie rods jointly developed by Sifang and Titanium Equipment of China Railway Corporation are partially machined after adopting die forging molding, and the material utilization rate can reach more than 50%, and the overall weight is reduced by about 42%, which is a very obvious effect of weight reduction.
