Methods for Improving Thermal Fatigue Resistance of Copper: History
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Limin Jin

Thermal fatigue is the fatigue failure phenomenon caused by the thermal stress (or thermal strain) cycle caused by the temperature gradient cycle. 

  • thermal fatigue
  • crack length
  • plastic strain
  • synchrotron radiation

1. Structure Optimization

The optimization of the cooling structure includes many methods, such as reducing the slope of the beam receiving surface (which needs to consider the limitation of the installation space) and modifying the smooth beam receiving surface to the concave and convex block structure. The principle of the above-mentioned methods is to effectively avoid the high energy beam directly acting on the beam receiving surface, and the bumpy block structure also serves to disperse the stress and strain. The high concentrated stress at the corner can be reduced by designing the arc of the window on the surface of the absorber element [1][2].
APS incorporates vertical fins on the absorber surface to divide the beam footprint into two parts. The first part is intercepted by the fins, while the second part is intercepted by the grooves between the fins. At an incidence angle of 11°, the intercept distance between the two beams is 8 mm, while the water channel is only about 5 mm from the surface. Inner fins are also used in the water channel of the absorber to enhance heat transfer. The principle of this method is to increase the light contact area to avoid excessive temperature in a certain area while increasing the cooling contact area to remove heat faster and lower the temperature[3] .
The crotch absorber of the Spanish synchrotron light source (ALBA) consists of two jaws. In addition, according to the applicable performance is divided into two; one is suitable for medium power size. The width of the teeth on this absorber is 10 mm, and the inclination of each tooth (concerning the radiation plane) is 8.8°, which reduces the normal incident power density by about 85%. The other one is the main (critical) absorber; this type of absorber will absorb radiation from the dipole installed inside it or behind it (near the radiation source point), where the power is the highest relative to the other types. To reduce the total strain and increase the number of cycles, this absorber has a tooth thickness of 6 mm, a tooth inclination of 6.6° and eight pin holes introduced in the absorber. Radii (rounded corners) were introduced to reduce stress and strain concentrations at the edges of the tooth tips. In addition, the ends of the pin holes were geometrically optimized to improve the heat transfer efficiency at this location [4]. The principle of this method is to reduce the concentrated thermal stress by digging grooves in the surface of the element to increase the light area.
To cope with the new radiation mode of the SPring-8 soft X-ray beamline station, a new metal mask is developed using volume heating technology. This technique involves brazing together beryllium and oxy-free copper (OFHC) using a metal surface heat load at a low z depth, such as beryllium or graphite. Thermal analysis simulations were performed after physical experiments were conducted to determine that the brazed joint had sufficient mechanical strength. The results showed that the ability of the matching metal material to absorb high thermal loads was improved by approximately 4 kW/0.2 m. The principle of this method is that OFHC has multiple cooling channels capable of absorbing and cooling the high-energy portion of the X-ray beam passing through the beryllium block and eliminating the heat load absorbed by the beryllium block through brief cooling. The thermal analysis results show that the heat absorbed by beryllium accounts for about 82.5% of the total heat absorbed by the element [5]. Otherwise, the structure has the advantage that the total required length in the beam axis direction is shorter than the past technology of the pre-slit.

2. Improvement of Film Coefficient

1. Increase the flow rate/velocity of cooling fluid [6].
For the convective heat transfer method, the key issue is to improve the cooling effect by increasing the film coefficient at the cooling inner wall. The film coefficient can be increased by appropriately increasing the fluid velocity and flow rate or reducing the diameter of the circular cooling tube. In addition, the physical properties of cooling fluid, such as density, specific heat, thermal conductivity, viscosity, etc., will also affect the heat transfer effect.
2.
Insert the wire coil into the cooling tube.
There is currently an enhanced cooling method that is used to insert the wire coil into the cooling tube [7]. The enhanced cooling mechanisms are [8]:
(1)
Increasing the cooling area
(2)
Thinning or destruction of the boundary layer
(3)
Promoting the formation of turbulence
In recent years, this method has also been applied and promoted at some synchrotron facilities. The wire coil is shaped similarly to a spring and is generally made of copper.

3. Selection of Materials

Copper metal materials with high heat conduction, high-temperature stability, and small photon absorption coefficient are usually selected to manufacture the high heat load components at the synchrotron front end. The total power of the first or second-generation synchrotron radiation devices is relatively low, and the heat load on the beam receiving surface of components cannot even cause the yielding of the materials (generally high conductivity oxygen-free copper, OFHC); therefore, no special means are needed to deal with the high heat load issue [9]. The third-generation synchrotron radiation light source has greatly improved the total power and peak power density, which can reach tens of kW and hundreds of W/mm2, respectively. Under this condition, if no corresponding measures are taken, the components are very vulnerable to thermal damage. According to the actual situation, considering the thermodynamic performance and cost of the specific material, for the aperture with low thermal power density, the applied material is generally oxygen-free high-conductivity copper (OFHC) or chromium zirconium copper (CuCrZr). Glidcop® AL-15, with better mechanical properties, is generally used for fixed maskers and photon shutters, which bear a high-power density [1][10][11][12].
Specifically, the purity of copper in OFHC is greater than 99.95%. Copper has high thermal conductivity, good weldability, excellent plasticity and ductility, excellent cold workability, and non-magnetism, so the dispersed OFHC overcomes the shortcomings of low yield strength after annealing and poor creep resistance at high temperatures. Therefore, it has the characteristics of high-temperature resistance, high strength, and high thermal conductivity [13]. The chemical composition of chromium zirconium copper (CuCrZr) is (mass fraction %, Cu: 0.4–0.77, Cr: 0.15–0.35, Zr: 0.08–0.25). It has high strength, hardness and thermal conductivity, as well as good wear resistance. After the aging treatment, its hardness, strength, and thermal conductivity are significantly improved, and it is easy to weld [14]. Glidcop®Al-15 is an oxide dispersion-strengthened (ODS) copper that is widely used in high-heat load components around the world due to its high strength, especially at high temperatures.
Among the three materials, thermal conductivity and yield stress are two important indicators. The thermal conductivity of OFHC is 391 W/(mK), 1.18 times that of Glidcop® AL-15, and CuCrZr, respectively. Additionally, the yield stress of Glidcop® AL-15 is 420 MPa, 1.24 times and 1.2 times higher than that of OFHC and CuCrZr, respectively. Of course, the commercial costs of the applied materials should also be considered. Among the three materials, OFHC has the lowest price, about 70 RMB/kg, followed by CuCrZr, and the price of Glidcop® AL-15 is the highest, which is more than ten times that of OFHC  [6].
From the above content, researchers can conclude that when the thermodynamic performance parameters of OFHC can fully meet the actual working conditions, researchers choose the most economical OFHC. Although Glidcop® AL-15 has a slightly lower thermal conductivity than OFHC, it has higher strength at higher temperatures; when the temperature and thermal stress under working conditions are too high, the priority principle is Glidcop® AL-15. CuCrZr is not generally used unless specifically required.
It is worth pointing out that in addition to the selection of materials with good performance, special alloying elements can also be added to metal materials or non-metallic particles (nitride, oxide, etc.) as additives to achieve the purpose of improving its heat-resistant fatigue performance. For example, CuCrZrAg with stronger thermal fatigue resistance was formed by adding a trace silver element to CuCrZr alloy. The principle is that the Ag element can promote the formation of dispersed nanoparticles, thus improving the grain size and distribution of the alloy. These nanoparticles prevent the movement and diffusion of grain boundaries, thus improving the strength and stability of the alloy. The addition of silver elements can also improve the thermal conductivity, which is conducive to improving the thermal fatigue resistance of the alloy [15][16].
In addition, strengthening copper by mechanical alloying with some granular materials, such as silicate glass particles [17], silicon carbide [18], alumina [19] or nano-diamond [20], can keep the copper matrix stable at high temperatures and improve the thermal fatigue resistance.

4. Process Design

4.1. Thermal Barrier Coating

Thermal barrier coating refers to a thermal protection technology that covers ceramic materials with high heat resistance, corrosion resistance, high thermal expansion coefficient, and low thermal conductivity on the surface of hot-end components in the form of coating, which can effectively reduce the temperature of metal components and, thus, extend the service life of hot-end components. It is widely used in gas turbines, aerospace engines, etc. [21][22]. A typical thermal barrier coating consists of a ceramic surface and a metal bonding layer. The main function of the ceramic surface layer is to isolate heat flow and corrosive media; The main function of the bonding layer is to alleviate the mismatch of thermal expansion between the ceramic layer and the metal substrate and prevent the metal substrate from oxidation at high temperatures [23][24][25].
A total of 6–8% Y2O3•ZrO2 is currently the most successful and widely used ceramic material for thermal barrier coating. The Thermodynamic properties of this material are shown in Table 1.
Table 1. Thermodynamic properties of 6–8% Y2O3•ZrO2 [26].
Material Melting Point (°C) Thermal
Expansivity (K−1)		 Density (g/cm3) Elasticity
Modulus (GPa)		 Thermal Conductivity
W/(m·k)
6–8% Y2O3•ZrO2 2700 10.7 × 10−6 6.4 50 2.5–4.0
Under the conditions of heating time and cooling time of 120 s and 300 s, the highest temperature of the experiment is 1323 K, the lowest temperature is 293 K, and the thermal fatigue test of gas cooling, the experiments of no pre-oxidation without high-temperature holding, no pre-oxidation with high-temperature holding, and pre-oxidation without high-temperature holding are carried out, respectively.
The experiment result shows that the fatigue life of the coating is the longest under the condition of no pre-oxidation and high-temperature maintenance. The fatigue life of the coating is the shortest when the coating is pre-oxidized for a long time without high temperature. For coatings, the typical failure modes are coating, spalling, and cracking, while oxidation usually occurs in the bond layer, which leads to oxidation playing an important role in coating failure [27]. In addition, thermal fatigue is easy to occur under the action of heat shock, which leads to the decrease of mechanical properties, which is also the main reason for the decrease in fatigue life.
The advantage of thermal barrier coating is that it is cheaper and faster to research than developing materials with better heat resistance. However, the thermal barrier coating is still limited in temperature; for example, 6–8% Y2O3•ZrO2 will undergo phase transformation and sintering after the temperature exceeds 1473 K, causing cracks or leading to coating failure. Therefore, more work needs to be conducted in the future.

4.2. Strengthening Process

Shot peening is also a common process method to improve the fatigue resistance of materials. The mechanism of this process is to spray high-speed shot flow onto the surface of parts to make them undergo plastic deformation and form a strengthened layer of a certain thickness. Residual compressive stress will be generated on the surface of treated materials, which can increase the closing force of microcracks and block crack growth, thus prolonging the fatigue life [28]. Tan et al. [29] proposed a laser shock treatment method for the surface of this reinforced material. The participating compressive stress generated by this method is more than ten times that of shot peening. The experimental results of Liu et al. [30] show that the residual compressive stress of the alloy material under this process is greatly increased, and the thermal fatigue life is increased by 120% compared with the as-cast state.
Deep cryogenic treatment (DCT) is to keep the material far below room temperature for a period and promote the microstructure transformation of the material through heat exchange and tissue reconstruction during the cooling process, to improve the mechanical properties of the material. As an effective supplement to conventional heat treatment, this method has the advantages of being clean, pollution-free, high efficiency, and economy [31]. Li et al. [32] found that DCT could refine grain and martensitic lath bundles, thus improving the mechanical properties of metal materials. Adem et al. [33] found that during the DCT, a large amount of residual austenite was transformed into martensite, and fine carbide precipitates out on the matrix, thus improving the mechanical properties and wear resistance of the material. Experiments by Zhang et al. [31] show that after DCT, many fine and dispersed carbide precipitates from metal materials, hindering dislocation movement and having a nailing effect on grain boundaries, delaying crack growth rate and surface hardness decline rate, thus improving the thermal fatigue performance of samples.
The thermal fatigue life of materials can be improved by using grain size control technology, such as high-pressure torsion [34] and other methods to control grain size and distribution. The principle of this method is that when the grain size is reduced, the dislocation density of the material increases, and the interaction between the stress field and dislocation at the grain boundary and within the grain is enhanced, thus increasing the strength and plasticity of the material. In addition, smaller grain sizes can limit grain boundary diffusion and reduce the deformation and damage of the material. Reducing grain size can also enhance the phase stability of the material, reduce the energy and diffusion of the grain boundary, and improve the thermal fatigue life of the material. After RK et al. [35] treated metal samples by high-pressure torsion (HTP), grain refinement is obvious, root mean square strain increases, and dynamic precipitation occurs. The thermal stability of the samples was significantly enhanced, the ultimate tensile strength was increased by 2.5 times, and the fatigue endurance limit was increased by 20%.

This entry is adapted from the peer-reviewed paper 10.3390/e25050714

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