TA18(grade 9) titanium alloy is a low aluminum equivalent, nearly α-type titanium alloy evolved from Grade 5 titanium alloy, and its nominal composition is Ti-3Al-2.5V. TA18 titanium alloy is 20-50% stronger than pure titanium at room temperature and high temperature. It not only has good room temperature and high temperature mechanical properties and corrosion resistance, but also has excellent cold and hot processing process plasticity, formability and welding properties. The alloy is notch-insensitive and has good corrosion resistance, making it the material of choice for aerospace piping systems.
The strengthening method of TA18 titanium alloy is mainly through cold working deformation. After cold working, the pipes can reach different strength levels through different heat treatment regimes. The most commonly used are two typical representatives: the medium-strength grade in the annealed state (Rp0.2≥620MPa) and the high-strength grade in the stress-relieved annealed state (Rp0.2≥860MPa). Among them, annealed medium-strength pipes are mainly used in engine piping systems, and stress-relieved annealed high-strength pipes are mainly used in aircraft hydraulic and other piping systems.
The piping system of an aerospace aircraft is the lifeline of the aircraft, and mainly includes fuel pipelines, hydraulic oil pipelines, lubricating oil pipelines, air source pipelines, compressed air pipelines, injection water pipelines, etc. They are numerous in number, have complex structures and shapes, and the processing quality directly affects the accuracy and stability of their connections. Various defects have always appeared in pipe bending and forming, such as: bend cross-section deformation, wrinkling, bend thinning, bending rebound, etc. This not only affects product quality and production and assembly progress, but also enables aircraft to achieve system functions. Bring safety risks.
The reliability and durability of the piping system are important factors in meeting airworthiness requirements, ensuring flight safety and reducing maintenance costs. Therefore, improving the bending and forming technology of pipes is very important to improve aircraft performance.
Current situation of foreign applications
TA18 high-strength titanium pipe is used in hydraulic piping systems. It can not only meet higher strength requirements, but also meet the needs for lightweight components in aerospace and other fields. Therefore, it has long been valued and widely used abroad. In the mid-20th century, the United States developed TA18 titanium alloy for aircraft pipeline hydraulic systems. Pipes made of this alloy have been used in hydraulic and fuel pipes of F-14A, F-15, B-1 and Boeing 747, 757, 767 and other aircraft. be promoted and applied in road systems. The world’s major airlines, such as Boeing, General Motors, McDonnell Douglas, Pratt & Whitney, Rolls-Royce, Airbus, etc., also use a large number of TA18 titanium alloy pipes on their military aircraft and commercial passenger aircraft and other transportation vehicles. At present, TA18 titanium alloy has been widely used in pipe fittings in military projects in many Western countries (including helicopters), becoming the first choice material for military and civil aircraft piping systems.
Domestic application status
In the mid-1970s, relevant units headed by the Northwest Nonferrous Metals Research Institute began to conduct in-depth research on medium-strength TA18 titanium alloy, and it was widely used in aerospace aircraft piping systems. However, the development of high-strength TA18 titanium alloy pipes Development is still in its early stages. At present, because titanium alloy pipe bending and pipe joint technologies are not yet fully mature, titanium alloy pipes have not been widely used in both civil and military aircraft. In terms of pipe bending and forming, due to the lack of corresponding equipment, currently only pure titanium (such as TA1, TA2) and some low-strength titanium alloy pipes (such as TA16M) and medium-strength titanium alloy pipes (such as TA18M) can be achieved relatively stably. ) CNC cold bending forming.
Research progress in pipe bending and forming technology
Pipe bending and forming methods
The most common pipe bending and forming methods include press bending, push bending, roll bending and winding bending.
Press bending is the earliest bending method used for pipes. The bending process is completed by applying a load to the pipe through a bending die. This method has high production efficiency, simple mold adjustment, and is widely used in elbow processing. However, the load applied during bending is concentrated between the two support rollers, and cross-sectional distortion is easily formed at the initial contact between the tube blank and the bending die, affecting the bending quality.
Push bending is generally used to form elbows. A load is applied in the axial direction of the pipe, and the tube blank is pushed into the cavity to achieve bending. During the push-bending process, the outer wall thickness of the pipe will not be excessively thinned, but the mandrel needs to be reasonably designed and the tube blank and mold cavity should be lubricated, otherwise instability and wrinkling will easily occur.
Roll bending is commonly used for bending thick-walled tubes. Roll bending is to place the tube blank between the rollers under the action of three or more driving rollers, and realize the bending and forming of the tube by controlling the spacing and driving mode of the rollers. Since there is no mandrel support in roll bending, the cross-sectional quality of the pipe is difficult to control. However, the use of multi-roller roll bending can improve the bending accuracy of the pipe, reduce the cross-sectional distortion, and improve the uniformity of the pipe bending deformation.
Bending forming is currently the most commonly used pipe bending method. This process is combined with modern CNC technology to form CNC pipe bending forming technology. When bending, one end of the tube blank is placed in the clamping die, and the bending die is driven by the CNC program to bend the tube. CNC bending can not only improve the forming quality of pipe bending, but also realize continuous bending through program control to complete complex space bending.
Research progress
Pipe CNC bending forming technology is a very complex process, which is affected by many factors such as material properties, pipe wall thickness factors, bending processes, and mold design and manufacturing levels. Pipe bending and forming are prone to various defects, such as thinning of the outer wall thickness, thickening of the inner wall thickness, interface distortion, wrinkling and cracking, etc., which seriously restricts the development of my country’s aerospace piping system. Regarding the current pipe bending and forming technology, researchers have done a lot of work mainly from the following aspects and achieved certain results.
In terms of theoretical analysis and research, researchers have deduced a large number of theoretical formulas mainly from various aspects such as pipe cross-sectional shrinkage, wall thickness thinning rate, neutral layer offset, pipe feeding length, bending moment and section distortion rate. And the model of the characterization formula is established, and the theoretical analysis results are in good agreement with the experimental verification results. However, in the process of theoretical analysis, a large number of assumptions and formula simplifications were adopted, and the influence of some factors was ignored. Therefore, the accuracy and application scope of the formula are subject to certain limitations.
In terms of numerical simulation analysis, researchers used finite element analysis software to model the pipe bending forming process and conducted secondary development of algorithms to varying degrees. Through finite element simulation analysis, the material flow rules, stress and strain distribution states, changes in pipe wall thickness, bend rebound rules, generation mechanisms and change rules of bending forming defects during the pipe bending and forming process were studied, and the bending forming process parameters were revealed. , such as mandrel parameters, friction parameters, bending speed and pipe mold gap, etc., have a regular influence on the pipe forming process.
In terms of experimental research, current experiments are mainly focused on the experimental verification of theoretical analysis and numerical simulation results in universities and scientific research institutes. Researchers have studied the impact of bending process parameters and heat treatment systems on the organizational morphology and mechanical properties of pipes. Certain research has been carried out, but there are few reports on the process and uniformity control of mass production.
Problems
Although my country’s research on the bending and forming of high-strength TA18 titanium alloy pipes has made great progress, it is still in its early stages, and batch production processes and control methods have not yet been formed, and it still faces new challenges.
First of all, theoretical research and numerical simulation analysis are out of touch with production test research. Due to limitations of equipment and production test conditions in universities and other research institutions, a large amount of research mainly focuses on theoretical analysis and numerical simulation. Production units have certain experimental and trial production conditions, but Due to the existing operating mechanism, a lot of research work has not yet been invested.
Secondly, in terms of process technology, the cold bending process of TA18 high-strength titanium alloy pipes requires large tonnage equipment, low angle control accuracy, and defects such as springback and wrinkles are prone to occur; during hot calibration, mold closing is prone to occur The “undercut” defect, and long-term heating is prone to high-temperature creep, making it difficult to control the roundness of the product cross-section.
Finally, the two-step cold bending + hot shaping currently used has a long forming cycle and high cost.
TA18 titanium alloy pipes have become the preferred material for aerospace piping systems and are widely used abroad, but they are still in their infancy in China. Due to the current demand for lean aviation products, future research on high-efficiency, high-precision, high-yield, and low-cost forming technology for titanium alloy pipes is urgent. In order to achieve precise CNC bending forming of titanium alloy pipes, it is urgent to establish a relatively comprehensive bending forming process system and numerical simulation system for titanium alloy pipes.
