research papers\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoJOURNAL OF
SYNCHROTRON
RADIATION
ISSN: 1600-5775

Vacuum joints of CuCrZr alloy for high-heat-load photon absorber

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aShanghai Synchrotron Radiation Facility, Zhangjiang Laboratory, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, People's Republic of China
*Correspondence e-mail: liyj77@163.com, zhuwanqian@zjlab.org.cn

Edited by M. Wang, Paul Scherrer Institute, Switzerland (Received 3 September 2021; accepted 30 November 2021; online 7 February 2022)

A photon absorber, as a critical component of a synchrotron front-end, is mainly used to handle high-heat-load synchrotron radiation. It is mostly made of dispersion strengthened copper or CuCrZr which can retain high performance at elevated temperatures. Joining processes for vacuum, including tungsten inert gas welding (TIG) and electron beam welding (EBW), are novel ways to make a long photon absorber from two short ones and reduce power density. The mechanical properties of TIG joints and EBW joints of CuCrZr to the same material are obtained by tensile tests at 20°C, 100°C, 200°C, 300°C and 400°C. Testing results indicate that the tensile strength and yield strength of both vacuum joints decline as temperature increases. Compared with TIG joints, EBW joints have higher strength, better ductility and a more stable performance. An engineering conservative acceptance criteria of the vacuum joints is created by the polynomial fitting method. A novel welded photon absorber with a total length of 600 mm has been successfully designed and manufactured. Finite-element analysis by ANSYS shows that the maximum temperature, equivalent stress and strain are only 31.5%, 36.2% and 1.3%, respectively, of the corresponding thresholds. The welded photon absorbers with EBW joints will be applicable in the highest-heat-load front-end in the Shanghai Synchrotron Radiation Facility Phase-II beamline project.

1. Introduction

Shanghai Synchrotron Radiation Facility (SSRF) is an advanced third-generation synchrotron radiation light source consisting of a 150 MeV electron linear accelerator, a full energy booster and a 3.5 GeV storage ring (He & Zhao, 2014[He, J. H. & Zhao, Z. T. (2014). Natl. Sci. Rev. 1, 171-172.]; Zhao et al., 2013[Zhao, Z. T., Yin, L. X., Leng, Y. B., Zhang, W. Z., Jiang, B. C. & Tian, S. Q. (2013). Proceedings of the 4th International Particle Accelerator Conference (IPAC2013), 12-17 May 2013, Shanghai, China, pp. 178-180.]). Insertion devices, including undulators and wigglers, are always employed to generate high-quality synchrotron light. The power output of an insertion device is up to several tens of kilowatts and its peak power density can be larger than 100 kW mrad−2. The photon absorber, as a critical component of a synchrotron front-end, is an ultrahigh-vacuum unit primarily for handling the high heat load from insertion devices or bending magnets (Sakurai et al., 1998[Sakurai, Y., Oura, M., Takahashi, S., Hayashi, Y., Aoyagi, H., Shiwaku, H., Kudo, T., Mochizuki, T., Oikawa, Y., Takahasi, M., Yoshii, K. & Kitamura, H. (1998). J. Synchrotron Rad. 5, 1195-1198.]; Shu & Ramanathan, 2002[Shu, D. M. & Ramanathan, M. (2002). Rev. Sci. Instrum. 73, 1580-1582.]). High-heat-load photon absorbers are mostly made of dispersion strengthened copper or CuCrZr alloy at SSRF and many other synchrotron radiation facilities. However, due to the technical requirements of high-dimensional accuracy and the low surface roughness of photon absorbers, only short ones whose lengths are no more than 300 mm could be made by low-speed wire-cut electrical discharge machining in China, which were widely used in the SSRF Phase-I project. Joining processes for vacuum, including tungsten inert gas welding (TIG) and electron beam welding (EBW), are novel ways to make a longer photon absorber by joining two shorter ones and to reduce power density.

CuCrZr, as a precipitation hardening copper alloy with excellent thermal conductivity, strength retention, good ductility and microstructural stability at elevated temperatures, developed in recent years, is widely applied in aerospace, fusion application, synchrotron radiation etc. (Huang et al., 2019[Huang, Y. C., Li, M., Ma, C. Q., Xiao, Z. B. & Liu, Y. (2019). J. Cent. S. Univ. 26, 1550-1562.]; Shueh et al., 2017[Shueh, C., Chan, C. K., Chang, C. C. & Sheng, I. C. (2017). Nucl. Instrum. Methods Phys. Res. A, 841, 1-4.]; Zinkle, 2016[Zinkle, S. J. (2016). Phys. Scr. T167, 014004.]). Compared with dispersion strengthened copper, CuCrZr alloy is much cheaper and available even in our local supply vendors in China (Jin et al., 2021[Jin, L. M., Li, Y. J., Zhu, W. Q., Xue, S., Wang, N. X. & Xu, Z. M. (2021). Nucl. Instrum. Methods Phys. Res. A, 989, 164955.]; Sheng et al., 2016[Sheng, I. C., Chang, C. C., Chan, C. K., Shueh, C. & Wu, L. H. (2016). Proceedings of 9th International Workshop on Mechanical Engineering Design of Synchrotron Radiation Equipment and Instrumentation (MEDSI2016), 11-16 September 2016, Barcelona, Spain, pp. 77-79.]). Extensive studies on the mechanical properties of CuCrZr alloy have been conducted (DeGroh et al., 2008[Groh, H. C. III de, Ellis, D. L. & Loewenthal, W. S. (2008). J. Mater. Eng. Perform. 17, 594-606.]; Edwards et al., 2007[Edwards, D. J., Singh, B. N. & Tähtinen, S. (2007). J. Nucl. Mater. 367-370, 904-909.]; Nomura et al., 2002[Nomura, Y., Suzuki, R. & Saito, M. (2002). J. Nucl. Mater. 307-311, 681-685.]). Performances of the joints of CuCrZr alloy to other alloys have also been studied by various groups (Gillia et al., 2009[Gillia, O., Briottet, L., Chu, I., Lemoine, P., Rigal, E. & Peacock, A. (2009). J. Nucl. Mater. 386-388, 830-833.]; Sahlot et al., 2018[Sahlot, P., Nene, S. S., Frank, M., Mishra, R. S. & Arora, A. (2018). Sci. Technol. Weld. Joining, 23, 715-720.]; Wei et al., 2018[Wei, R., Li, Q., Wang, W. J., Wang, J. C., Wang, X. L., Xie, C. Y. & Luo, G. N. (2018). Fusion Eng. Des. 128, 47-52.]).

In this paper, to meet the project requirements, CuCrZr was chosen to make tensile specimens by TIG and EBW. The mechanical properties of the vacuum joints of CuCrZr to the same material at 20°C, 100°C, 200°C, 300°C and 400°C were obtained by tensile tests. An engineering conservative acceptance criteria of the vacuum joints is created by the polynomial fitting method. A novel high-heat-load photon absorber with a total length of 600 mm has been successfully designed and manufactured. Finite-element analysis by ANSYS shows that the maximum temperature, equivalent stress and strain on the EBW joint of the photon absorber are much lower than the corresponding thresholds, which illustrates that welded photon absorbers will be applicable in the highest-heat-load front-end in the SSRF Phase-II beamline project.

2. Experimental procedures

2.1. Material

CuCrZr is a precipitation hardened alloy with about 0.5–1.5% chrome and 0.05–0.25% zirconium. Glidcop®AL-15, a kind of Al2O3 dispersion strengthened copper produced by North American Höganäs High Alloys LLC in the USA, was utilized in the SSRF Phase-I front-end. Table 1[link] shows the physical properties of CuCrZr and Glidcop®AL-15 at 20°C. The mechanical properties of CuCrZr and Glidcop®AL-15 at different temperatures are shown in Table 2[link] (DeGroh et al., 2008[Groh, H. C. III de, Ellis, D. L. & Loewenthal, W. S. (2008). J. Mater. Eng. Perform. 17, 594-606.]; Glidcop®, 2021[Glidcop® (2021). North American Höganäs High Alloys LLC, https://www.hoganas.com/.]). Compared with Glidcop®AL-15, the elongation of CuCrZr is worse at some temperatures, while the tensile stress and yield stress of CuCrZr are much better at all listed temperatures. In this paper, CuCrZr was chosen to make tensile specimens.

Table 1
Physical properties of CuCrZr and Glidcop®AL-15

Material Melting point (°C) Density (g cm−3) Elastic modulus (GPa) Poisson's ratio Thermal conductivity (W m−1 °C−1) Thermal expansion coefficient (°C−1), 20°C
CuCrZr 1075 8.9 128 0.33 330 1.70 × 10−5
Glidcop®AL-15 1083 8.9 130 0.33 365 1.66 × 10−5

Table 2
Mechanical properties of CuCrZr and Glidcop®AL-15

Material Mechanical properties 20°C 100°C 200°C 300°C 400°C
CuCrZr Tensile stress (MPa) 564 532 487 425 358
Yield stress (MPa) 550 508 452 396 339
Elongation (%) 11.1 14 17.2 18.7 19.2
Glidcop®AL-15 Tensile stress (MPa) 420 380 330 292 263
Yield stress (MPa) 355 332 304 277 248
Elongation (%) 27 25 21 16.5 12

2.2. Welding technology

TIG welding is the most versatile welding process employed in vacuum fields where a high degree of quality and accuracy is required (Manohar et al., 2018[Manohar, M. S., Rao, Y. R. & Sandilya, N. (2018). J. Sci. Ind. Res. 77, 533-536.]). A welding speed of 50 mm min−1 and a gas flow of 9 L min−1 were chosen to join two pieces of CuCrZr specimens using an WSE-500 argon arc welder. The test welding is done by keeping voltage at about 15 V and current at about 160 A throughout the welding process.

EBW is a fusion welding process under vacuum as it is of high energy density, low heat input and rapid cooling rate (Zeng, 1993[Zeng, L. (1993). Modern Welding Handbook. Shanghai Science and Technology Press.]). An EBW700 electron beam welder was employed in the experiment and pumped to a vacuum degree of 4.5 × 10−3 Pa. A voltage of 150 kV and a beam current of 9 mA were precisely controlled to produce a beam of high-velocity electrons. With a welding speed of 10 mm s−1, two pieces of CuCrZr specimens melted and flowed together on the joint.

2.3. Testing procedures

After welding, tensile specimens were machined according to GB/T 2651-2008 (ISO 4136: 2001, IDT), as shown in Fig. 1[link]. Tensile tests at room temperature were conducted using a Zwick Z100TEW electronic universal testing machine based on GB/T 228.1-2010 (ISO 6892-1: 2009, MOD); a Zwick Z250 electronic universal testing machine was used at elevated temperatures adopting GB/T 4338-2006 (ISO 783: 1999, MOD).

[Figure 1]
Figure 1
Tensile specimen (units: mm).

3. Results and discussions

3.1. Testing results of TIG joints

Thirty TIG tensile specimens were separated into five groups randomly and mechanical properties at five different temperatures were obtained. The results are shown in Tables 3[link], 4[link] and 5[link], where A is elongation after fracture, [\bar A] is the mean of A, [A_{\rm{s}}] = [[\textstyle\sum_{i\,=\,1}^6(A_i-{\bar A})^2/5]^{1/2}] (i = 1–6) is the standard deviation of A by the Bessel formula; Rm is the tensile strength, [\overline {{R_{\rm{m}}}}] is the mean of Rm, Rms is the standard deviation of Rm; Rp is the yield strength, [\overline{{R_{\rm{p}}}}] is the mean of Rp, and Rps is the standard deviation of Rp. Fracture surfaces of all specimens occurred on the joints.

Table 3
Elongation of TIG joints

Temperature A (%)
A1 A2 A3 A4 A5 A6 [\bar A] As
20°C 1.8 2 3 2.5 1.5 1.6 2.1 0.6
100°C 1.9 2.5 2 1.6 2.6 3.5 2.4 0.7
200°C 2.5 2 1.8 3.5 2.1 1.7 2.3 0.7
300°C 1.5 3.4 2.9 2.5 2.1 2 2.4 0.7
400°C 3 1.6 2.9 1.5 2.1 2.2 2.2 0.6

Table 4
Tensile strength of TIG joints

Temperature Rm (MPa)
Rm1 Rm2 Rm3 Rm4 Rm5 Rm6 [\overline {{R_{\rm{m}}}}] Rms
20°C 91.5 97.5 174.5 108 136 118 120.9 30.6
100°C 129 83.5 139 146 103.5 105 117.7 24.1
200°C 93.5 161 85.5 102 118 105 110.8 26.9
300°C 73.5 89.5 76 49 86 76.5 75.1 14.2
400°C 32.5 52.5 36.5 93 127 39.5 63.5 38.2

Table 5
Yield strength of TIG joints

Temperature Rp (MPa)
Rp1 Rp2 Rp3 Rp4 Rp5 Rp6 [\overline {{R_{\rm{p}}}}] Rps
20°C 87.5 82 113.5 95 101 88 94.5 11.4
100°C 75 75 106 87 97 77.5 86.3 12.9
200°C 62.5 121.5 83 68 90 89 85.7 20.8
300°C 70 80.5 55.5 43.5 85.5 74.5 68.3 15.9
400°C 31 49.5 36 76 94 34.5 53.5 25.8

All TIG joints have yielding processes with poor consistency, demonstrating that TIG joints have poor ductility. From [\overline {{R_{\rm{m}}}}] and [\overline {{R_{\rm{p}}}}], tensile strength and yield strength decline with the rising of temperature. Meanwhile, tensile strengths and yield strengths vary a lot at the same temperature. Take the tensile strength at 400°C, for example: the maximum is nearly four times the minimum, and the biggest standard deviation is 38.2 MPa, which is even larger than half of [\overline {{R_{\rm{m}}}}] at 400°C. It is indicated that the TIG joints by the welding process have low stability and are not recommended to be applied in those cases requiring high strengths.

3.2. Testing results of EBW joints

Similar to Section 3.1[link], mechanical properties of 30 EBW tensile specimens at five different temperatures are acquired. The results are shown in Tables 6[link], 7[link] and 8[link]. Fracture surfaces of all specimens also occurred on the joints.

Table 6
Elongation of EBW joints

Temperature A (%)
A7 A8 A9 A10 A11 A12 [\bar A] As
20°C 2.1 2.5 2.4 3 2.6 2.5 2.5 0.3
100°C 3.4 3 2.5 2.6 3.1 2.5 2.9 0.4
200°C 2.8 3.5 3 2.5 2.7 3.6 3.0 0.4
300°C 2.7 3 3.1 3.5 3 3.3 3.1 0.3
400°C 2.5 2.4 2.9 3.4 3.1 2.6 2.8 0.4

Table 7
Tensile strength of EBW joints

Temperature Rm (MPa)
Rm7 Rm8 Rm9 Rm10 Rm11 Rm12 [\overline {{R_{\rm{m}}}}] Rms
20°C 199 201 219 235 223 243 220 17.7
100°C 142 178 184 172 188 145 168.2 19.9
200°C 155 139 141 128 129 127 136.5 10.8
300°C 123 136 118 119 135 118 124.8 8.5
400°C 131 106 88 119 98 104 107.7 15.3

Table 8
Yield strength of EBW joints

Temperature Rp (MPa)
Rp7 Rp8 Rp9 Rp10 Rp11 Rp12 [\overline {{R_{\rm{p}}}}] Rps
20°C 176 156 149 153 152 168 159 10.6
100°C 125 108 149 115 140 117 126 15.8
200°C 125 106 86 98 122 115 108.7 15
300°C 98 88 91 102 122 103 100.7 12.0
400°C 90 94 76 97 85 90 88.7 7.4

All EBW joints have yielding with good consistency, demonstrating that EBW joints have better ductility. Tensile strength and yield strength decline as temperature is increased. Meanwhile, tensile strengths and yield strengths vary a little at the same temperature with a maximum difference of 46 MPa and all standard deviations are less than 20 MPa, indicating that the EBW joints by the welding process have high stability and will be preferred in practical applications.

3.3. Welded joints versus bulk CuCrZr

Mechanical properties of both welded joints are much lower than that of bulk CuCrZr, as shown in Fig. 2[link]. The tensile strength of TIG joints is about 1/5 of the base material and 1/3 for EBW joints. The yield strength of TIG and EBW joints is only 1/6 and 1/4 of the base material, respectively. Elongation of EBW joints is approximately 1/5 of CuCrZr, while that of TIG joints is about 1/8. Changing trends of tensile strength and yield strength of both joints are consistent with that of the base material.

[Figure 2]
Figure 2
Welded joints versus bulk CuCrZr.

3.4. Acceptance criteria

To ensure that the high-heat-load photon absorbers can work safely during a long service period, the failure of vacuum joints due to the excessive thermal stress and strain should be avoided. It is necessary to establish the acceptance criteria when performing finite-element analysis. On the basis of the engineering conservativeness, the temperature on the joints should be no more than 250°C; thermal stress and strain should be no more than the yield strength and elongation at the temperature (Thomas et al., 2016[Thomas, F., Biasci, J. C., Coulon, D., Dabin, Y., Ducoing, T., Ewald, F., Gagliardini, E. & Marion, P. (2016). Proceedings of the 9th International Workshop on Mechanical Engineering Design of Synchrotron Radiation Equipment and Instrumentation (MEDSI2016), 11-16 September 2016, Barcelona, Spain, pp. 257-261.]; Zhang et al., 2002[Zhang, L., Biasci, J. C. & Plan, B. (2002). Proceedings of the 2nd International Workshop on Mechanical Engineering Design of Synchrotron Radiation Equipment and Instrumentation (MEDSI2002), 5-6 September 2002, Argonne, USA, pp. 377-389.]).

From the above testing results, formulas of yield strength and elongation based on the temperature can be obtained by the polynomial fitting method. For TIG joints, that is

[R_{{\rm{p}}-{\rm{TIG}}} = 93.5 - 0.014T + 0.0002{T^{\,2}}, \eqno(1)]

[A_{\rm{TIG}} = 2.08 + 0.0028T + 0.000006{T^{\,2}}. \eqno(2)]

For EBW joints, that is

[R_{{\rm{p}}-{\rm{EBW}}} = 162.9 - 0.359T + 0.0004{T^{\,2}}, \eqno(3)]

[A_{\rm{EBW}} = 2.41 + 0.0056T + 0.000011{T^{\,2}}, \eqno(4)]

where T (°C) is the temperature on the joints and should be no more than 250°C. Rp–TIG (MPa) is the yield strength of TIG joints on T, Rp–EBW (MPa) is the yield strength of EBW joints on T, and ATIG and AEBW (%) are the elongation of the TIG joints and EBW joints at T, respectively.

The temperature, thermal stress and strain thresholds corresponding to both vacuum joints are listed in Table 9[link].

Table 9
Engineering conservative acceptance criteria of vacuum joints

  TIG joint EBW joint
Maximum temperature (°C) ≤250
Maximum equivalent stress (MPa) ≤93.5 − 0.014T + 0.0002T2 ≤162.9 − 0.359T + 0.0004T2
Maximum equivalent strain (%) ≤2.08 + 0.0028T + 0.000006T2 ≤2.41 + 0.0056T + 0.000011T2

4. Applications

A superconducting wiggler, as source of the ultra-hard X-ray applications beamline (BL12SW) in the SSRF Phase-II beamline project, will generate high-energy X-rays up to 150 keV and significantly strengthen the application potential of SSRF. The total radiant heat power received by the front-end of BL12SW is over 44.7 kW, and the peak power density is approximately 45 kW mrad−2, which is currently the highest heat load for a front-end at SSRF. The power density distribution of the superconducting wiggler is shown in Fig. 3[link].

[Figure 3]
Figure 3
Power density distribution of the superconducting wiggler.

Four photon absorbers made of Glidcop®AL-15 or CuCrZr are located in sequence to handle this radiation. Fig. 4[link](a) shows a photon absorber that was utilized in the SSRF Phase-I front-end and has only one absorber with a total length of about 300 mm. The novel photon absorber has two sub-absorbers welded to each other by EBW and a total length of up to 600 mm, as shown in Fig. 4[link](b). The inside of all the photon absorbers is in ultrahigh-vacuum while the outside is at atmospheric pressure.

[Figure 4]
Figure 4
Usual photon absorber (a) and welded photon absorber (b).

Finite-element analysis by ANSYS (Jin et al., 2021[Jin, L. M., Li, Y. J., Zhu, W. Q., Xue, S., Wang, N. X. & Xu, Z. M. (2021). Nucl. Instrum. Methods Phys. Res. A, 989, 164955.]; Xu et al., 2015[Xu, Z. M., Feng, X. K., Wang, N. X., Wu, G. Y., Zhang, M. & Wang, J. (2015). Nucl. Instrum. Methods Phys. Res. A, 774, 94-102.]) indicates that the maximum temperature on the joint is 78.7°C, maximum equivalent stress and strain are, respectively, 49.58 MPa and 0.039%, as shown in Fig. 5[link]. The yield strength and elongation of the EBW joint at 78.7°C can be calculated from equations (3)[link] and (4)[link] to be 137.1 MPa and 2.92%, respectively. Therefore, the practical maximum temperature, equivalent stress and strain are only 31.5%, 36.2% and 1.3%, respectively, of the corresponding thresholds, which indicates that the EBW joints will work safely in the highest-heat-load front-end in the SSRF Phase-II beamline project.

[Figure 5]
Figure 5
(a) Temperature, (b) equivalent stress and (c) equivalent strain distributions of the joint.

5. Conclusions

Mechanical properties of TIG joints and EBW joints of CuCrZr to the same material, which is utilized in the SSRF front-end, are obtained by tensile tests at 20°C, 100°C, 200°C, 300°C and 400°C. Testing results show that, in comparison with the base material, tensile strength and yield strength of both vacuum joints decline by 2/3 or 4/5 and elongation falls by 4/5 or more. Compared with TIG joints, EBW joints have higher strength, better ductility and more stable performance.

To ensure that the high-heat-load photon absorbers can work safely during a long service period, an engineering conservative acceptance criteria of vacuum joints is created by the polynomial fitting method. A novel welded photon absorber with a total length of 600 mm has been successfully designed and manufactured. Finite-element analysis by ANSYS shows that the maximum temperature, equivalent stress and strain are only 31.5%, 36.2% and 1.3%, respectively, of the corresponding thresholds.

In practical engineering applications, vacuum joints are subjected to low temperature, low stress and strain as are being kept out of direct high-power synchrotron radiation. Therefore, EBW joints of CuCrZr entirely satisfy the practical requirements and will be applicable in the highest-heat-load front-end in the SSRF Phase-II beamline project.

Funding information

The following funding is acknowledged: National Natural Science Foundation of China (award No. 11805262); Youth Innovation Promotion Association, Chinese Academy of Sciences (award No. 2020288).

References

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