Study on heat treatment and strengthening process of titanium alloy spring for railway vehicle

3.1. Chemical constituents of three kinds of high-strength titanium alloys

Chemical constituents of three kinds of materials TB2, TB3 and TB6 are shown in Tables 2-4 respectively.

3.2. Properties of three kinds of high-strength titanium alloys

Three kinds of high-strength titanium alloys are coiled into springs by hot treatment, and samples are destructively taken from both ends of each spring. The tensile and impact properties of the samples are tested and evaluated, and the numerical values are shown in Table 5.

3.3. Microstructures of three kinds of high-strength titanium alloys

In order to grope the heat treatment method suitable for the titanium alloy spring material, two comparison programs are proposed: program 1 and program 2. Program 1: Coiling spring at the heating temperature of 760 ℃ and the aging temperature of 480 ℃. Program 2: Coiling spring at the heating temperature of 800 ℃ and the aging temperature of 510 ℃. The details is seen in Table 1.

After treated with program 1 and 2, the microstructures of three kinds of high-strength $\beta$ type titanium alloys TB2, TB3 and TB6 are shown in Fig. 1-3 respectively.

Fig. 1(a) and Fig. 1(b) are the microstructures sampled from the spring bodies after the heat treatment of program 1 and program 2 respectively. In the pictures, it’s clear that two programs both have a certain number of strengthening phase dissolved into the matrix, clear grain boundary, and large grain size. The features above lead to high material strength and poor plasticity of spring body, so that the spring is easy to fragment when it’s compressed. Therefore, the TB2 material is not suitable for spring production with heat treatment processes program 1 and 2.

Fig. 1Spring microstructure of TB2 material

a) Program 1 Spring 200 times microstructure of TB2 material

b) Program 2 Spring 200 times microstructure of TB2 material

Fig. 2 is the microstructure of TB3 spring, magnified 200 times. Precipitation strength is one of the methods to strengthen $\beta$ alloy. When $\beta$ stable element content in alloy is high, due to the small phase transformation driving force, aging at low temperature often takes a relatively long time to reach the peak of enhancement. From Fig. 2(a) and Fig. 2(b), it’s apparent that material composition is not uniform, resulting in inhomogeneous material properties, which will eventually result in shortening the service life of the product. Therefore, spring material requires not only good surface quality, but also uniform distribution of structure inside the material.

Fig. 2Spring microstructure of TB3 material

a) Program 1 Spring 200 times microstructure of TB3 material

b) Program 2 Spring 200 times microstructure of TB3 material

In Fig. 3, supersaturated solid solution of TB6 alloy is obtained by solid solution treatment, and then the strength properties of alloy are improved by aging strengthening. Aging is the process of the second phase precipitating from supersaturated solid solution. And it’s also a kind of solid phase transition with formation and growth of the new phase nucleation. During aging at 480 ℃ and 510℃, $\beta \to \beta +\alpha$ transition and $\alpha$ phase induced nucleation occur. As long as a secondary ${\alpha }_s$ slice forms at dislocation, grain boundary or other grain defect, the nucleation probability of ${\alpha }_s$ phase can greatly increase in the $\beta$ matrix near the ${\alpha }_s$ slice. It’s clear in Fig. 3(a) and Fig. 3(b) that the organization of the TB6 spring material is uniform, indicating that TB6 material is suitable for spring production with treatments program 1 and program 2.

Fig. 3Spring microstructure of TB6 material

a) Program 1 Spring 200 times microstructure of TB6 material

b) Program 2 Spring 200 times microstructure of TB6 material

3.4. Impact fracture morphology analysis

The impact fracture morphologies of three kinds of high–strength β type titanium alloys TB2, TB3 and TB6 after heat treatment with program 2 are shown in Fig. 4-6 respectively.

Fig. 4 shows the macro and micro features of fracture toughness specimens of TB2 spring. Fig. 4(a) is the macro fracture morphology. It can be seen that fracture fiber region has more dimples. Fiber and radial region are obviously. It shows the fracture mechanism is ductile fracture and the plasticity of alloy is good. Since the fracture is in plane stress state, fiber region is formed in the crack propagation, and impact fracture has fiber region with small area, as shown in Fig. 4(b) and Fig. 4(c). Due to shear stress, shear lip region shows dimple morphology, as shown in Fig. 4(d).

Fig. 4The macro and micro features of fracture toughness specimens of TB2 spring

a) Macroscopic micrographs

b) Microscopic cross region

c) Microscopic fatigue region

d) Microscopic expansion region

Fig. 5 shows the macro and micro features of fracture toughness specimens of TB3 spring. Fig. 5(a) is the macro fracture morphology. The macro fracture surface is rough, and the rough fracture increases the tortuosity of the crack propagation path, so that more energy can be absorbed during the crack propagation, which is beneficial to improve toughness. In Fig. 5(b) white points are strengthening $\alpha$ phases distributing along grain boundaries, and phases in the juncture is not significantly different from phases in the non-juncture, also there is a lot of tearing ridge. The crack propagation and fracture are sensitive to microstructure, and fracture characteristics in the pre-cracked region are micro-region facet and tear ridge. Wide and deep secondary cracks can be seen in Fig. 5(c), making the organization have high fracture toughness. The microscopic morphological characteristic in propagation region is shallow tear dimple, as shown in Fig. 5(d), weakening the toughness fracture mechanism.

The macro and micro features of fracture toughness specimens of TB6 spring are shown in Fig. 6. Fig. 6(a) is the macro fracture morphology. It is clear that the macro fracture is relatively smooth, indicating that the fracture is little affected by the microstructure. Fig. 6(b) is the pre-cracked region, and the fracture surface is smooth, and basically shows a pattern of clouds seeing in Fig. 6(c). And the microscopic morphological characteristic in propagation region is dimple as shown in Fig. 6(d), which is the typical toughness fracture, and it’s also the root cause of high plasticity of TB6 spring.

Fig. 5The macro and micro features of fracture toughness specimens of TB3 spring

a) Macroscopic micrographs

b) Microscopic cross region

c) Microscopic fatigue region

d) Microscopic expansion region

Fig. 6The macro and micro features of fracture toughness specimens of TB6 spring

a) Macroscopic micrographs

b) Microscopic cross region

c) Microscopic fatigue region

d) Microscopic expansion region

3.5. Shot peening of titanium alloy spring

The primary spring of railway vehicle is installed on the axle box. It is exposed outside and suffers from erosion by wind, rain, snow, sunlight and even gravel impact. And the force between wheel and rail will be directly transmitted to the primary spring through axle and axle box, and the primary spring will directly affect the traffic safety. Therefore, the life of primary spring is strictly stipulated in railway transportation. At present, the trial of the primary spring on high-speed EMU in China is executed referring to EN13298 “Railway applications – Suspension components – Helical suspension springs, steel” UIC 822 “Technical specification for the supply of helical compression springs, hot or cold coiled, for tractive and trailing stock” and TJ/CL 285-2013 “EMU axle box spring” (Provisional). The stress of primary spring is large, especially the tensile stress, so the strength of the titanium alloy spring material should be more than 1250 MPa. And there is a clear requirement on impact toughness of the primary spring: 5 mm deep U-shape, in the room temperature of 20 ℃, KU ≥ 10 J; in the low temperature of –20 ℃, KU ≥ 10 J; in the low temperature of –40 ℃, KU ≥ 8 J. In addition, fatigue life requirement of primary spring is: when the dynamic coefficient is 0.5, fatigue life is 5×10⁵ times; when the dynamic coefficient is 0.3, fatigue life is 2.05×10⁷ times. After both two trials are completed, fatigue crack or break must not appear. Therefore, the surface strengthening treatment of titanium alloy spring is necessary to improve its fatigue life.

Surface strengthening treatment methods are usually classified into two categories: surface shot peening strengthening and surface roll strengthening. Because the spring is spiral, and there is not much space between the two adjacent circles, it is difficult to use surface roll strengthening, so surface shot peening strengthening is adopted.

Shot peening mechanism is forming a certain depth of residual compressive stress field by process. The residual compressive stress field can not only inhibit the initiation of fatigue crack, but also increase crack closure effect and reduce the propagation rate of short fatigue crack, even produce arrest phenomenon. Because the residual compressive stress produced by shot peening can counteract the effect of external tensile stress on material, and the concentration of stress around micro crack on the material surface is lower when tensile stress is smaller, and therefore the residual compressive stress can inhibit the formation and propagation of crack. The residual stress field formed by shot peening can transfer fatigue crack from surface layer to sub-surface layer, which can significantly improve the fatigue life of material and achieve the goal of improving the fatigue life of the spring.

Shot peening is beneficial to improve the fatigue life of titanium alloy spring. Large plastic deformation is formed on the surface layer of the metal material after shot peening, making the stress state of the surface layer change greatly; that is, large residual compressive stress and micro stress are produced. It is the existence of residual compressive stress and micro stress that counteract the effect of tensile stress on the spring surface, which has a beneficial effect on the mechanical properties of the spring.

Shot peening process parameters include the diameter of shot, jet velocity, shot flow, jet angle, distance between nozzle and spring surface and jet time. These parameters have a direct impact on quality and depth of the product surface strengthening layer after shot peening. In the actual shot peening process, shot peening process parameters affect the shot peening strength. If the shot peening strength is too small, it can’t meet the requirement of strengthening; while the shot peening strength is too large, it will produce defects or micro cracks on the sprayed surface. Only in the best shot peening strength can the spring obtain the best fatigue property. And this is one of the contents on which next study and trial lay emphasis.