Copper and copper alloys can be joined by a variety of techniques, including mechanical coupling, weld­ing, brazing, and diffusion bonding. A comprehensive overview of joining techniques for copper and copper alloys can be found in the reference.118 The welding techniques commonly used for copper and copper alloys include arc welding, resistance welding, oxy — fuel welding, and electron beam welding. Welding is generally not recommended for joining high-strength copper alloys. PH copper alloys lose their mechanical strength because of the dissolution of precipitates

(b)

(d)

Figure 20 Examples of cleared channels formed in the OFHC-Cu irradiated (to 0.3 dpa) and tested at 323 K to different strain levels: (a) before yield, (b) before yield, (c) 1.5%, and (d) 14.5%. Note that at 14.5% strain level the grain is subdivided by numerous channels formed on different slip planes. All images shown in this figure were taken in the STEM bright field mode. Reproduced from Edwards, D. J.; Singh, B. N.; Bilde-Sorensen, J. B. J. Nucl. Mater. 2005, 342, 164.

during the welding process. The welded component must be resolution annealed and aged to recover some of the initial strength in the joint. Recrystallization in the melt layer degrades the mechanical property of the weldment. DS copper alloys cannot be welded by conventional welding processes because ofthe loss of oxide particles and recrystallization in the weld zone.

Brazing is the most common method for joining copper alloys. All conventional brazing techniques can be used to join copper and copper alloys, includ­ing furnace brazing, torch brazing, induction brazing, resistance brazing, and dip brazing. A wide range of filler metals are available, and the most common brazing filler metals are Cu-Zn, Cu—P, Cu-Ag-P, and Ag — and Au-based alloys.118 Ag — and Au-based filler metals are unacceptable in fusion reactor envir­onments because of concerns of high radioactivity from neutron-induced transmutation.1

Copper alloys are typically brazed at tempera­tures between 600 and 950 °C with hold times at the brazing temperature ranging from 10 s (torch, resistance, or induction brazing) to 10 min (furnace
brazing).2 The brazing process can significantly soften PH copper alloys as a result of the adverse precipita­tion process. To reduce the softening effect, a fast induction brazing technique has been developed to minimize the holding time at high temperature to retain sufficient mechanical properties.120 Alterna­tively, the brazed component can be aged following furnace brazing to restore part of its initial strength. Complete recovery of high strength after furnace brazing by heat treatment in PH alloys is rather diffi­cult in practice as the component must be heated to a temperature greater than typical brazing tempera­tures and rapidly quenched to create a supersatura­tion of solute prior to aging. Oxide DS copper has been successfully joined using torch, furnace, resis­tance, and induction brazing.2 Softening is not a seri­ous concern for the base metal of DS copper alloys because of their high recrystallization temperature. The brazed copper joints show good fatigue proper­ties and relatively low ductility.2

Diffusion bonding is a viable technique to produce joints with high mechanical strength for DS copper alloys, but cannot be used to produce high-strength
joints in PH alloys because of significant softening of the base metal during high-temperature exposure. The DS CuAl15 and CuAl25 alloys can be joined by diffusion bonding with acceptable bond strengths under the diffusion bonding conditions similar to the normal HIPing conditions.121

Techniques for joining copper alloys to beryllium or austenitic stainless steels have been developed for the ITER plasma-facing components. A review of the joining technology was given by Odegard and Kalin.1 9 Recent work has focused on small — and medium-scale mock-ups and full-scale prototypes of the ITER first wall panels.122 The first wall panels of the ITER blanket are composed of a composite Cu alloy/316L(N) SS water-cooled heat sink structure with Be tile clad. A number of joining techniques have been explored for joining copper alloys to aus­tenitic stainless steel, 316L(N), including diffusion bonding, brazing, roll bonding, explosive bonding, friction welding, and HIP.123 HIP joining is by far the most desirable technique. For the PH CuCrZr alloy, the heat treatment must be integrated with the bonding cycle, and a high cooling rate (>^50 °C min j is required to obtain good mechanical proper­ties of CuCrZr after subsequent aging treatments. Two alternative processes are recommended12 : the HIP cycle (1040 ° C and 140 MPa for 2 h) followed by quenching in the HIP vessel, or a normal HIP cycle with a subsequent heat treatment in a furnace with fast cooling. Gervash et al}[7] studied alternative SS/Cu alloy joining methods, for example, casting, fast brazing, and explosion bonding. Cast SS/CuCrZr joint may be suitable for some ITER applications.

Brazing and diffusion bonding have been consid­ered for joining the beryllium armor to a copper alloy heat sink. The Be/DS copper alloy joints can be made by high-temperature HIPing and furnace brazing.126 Results from shear tests on small-scale specimens and from high heat flux tests of the first wall mock-ups showed good performance of joints brazed with STEMET 1108 alloy at ^780 °C for less than 5 min.122 The Be/Cu-Al25 solid HIPing (e. g., 730 °C and 140 MPa for 1 h) showed good performance from shear tests, high heat flux tests, and neutron irradiation.122

The development of joining techniques for PH CuCrCr alloy must consider the loss of mechanical strength because of overaging at high temperatures. The HIPing temperature must be reduced to be as close as possible to the aging temperature. The best results obtained so far is for HIPing at 580 °C and 140 MPa for 2h.126 A fast induction brazing tech­nique has also been developed to minimize the holding time at high temperature. Diffusion bonding of Be/CuCrZr joints gives much better high heat flux performance than brazing, and has been selected as the reference method for the European Union ITER components.120 A low-temperature Be/Cu alloy bond­ing process has also been developed that is compatible with both DS and PH copper alloys.124,127 In the United States, several different joint assemblies for diffusion bonding a beryllium armor tile to a copper alloy heat sink have been evaluated.12 To prevent formation ofintermetallic compounds and promoting a good diffusion bond between the two substrates, aluminum or an aluminum-beryllium composite (AlBeMet-150) has been used as the interfacial mate­rial. Explosive bonding was used to bond a layer of Al or AlBeMet-150 to the copper substrate that was subsequently HIP diffusion bonded to an Al-coated beryllium tile. A thin Ti diffusion barrier (0.25 mm) was used as a diffusion barrier between the copper and aluminum to prevent the formation of Cu-Al intermetallic phases. The Be/Cu alloy joints showed good strength and failure resistance.

4.20.3 Summary

High heat flux applications for fusion energy systems require high-strength, high-conductivity materials. Selection of materials for high heat flux applica­tions must consider thermal conductivity, strength and tensile ductility, fracture toughness, fatigue and creep-fatigue, and radiation resistance. Pure copper has excellent conductivity but poor strength. PH and DS copper alloys have superior strength and suffi­cient conductivity, and are prime candidates for high heat flux applications in fusion reactors. These two classes of alloys have their own advantages and dis­advantages with regard to fabrication, joining, and in­service performance.

PH copper alloys, such as CuCrZr, are heat — treatable alloys. Their properties are strongly depen­dent on the thermomechanical treatments. They possess high strength and high conductivity in the prime-aged condition, and good fracture toughness and fatigue properties in both nonirradiated and irradiated conditions. However, this class of alloys is susceptible to softening at high temperatures because of precipitate overaging and recrystallization. Their properties can be significantly degraded during large component fabrication because of their inability to achieve rapid quenching rates. DS copper alloys such as GlidCop Al25 have excellent thermal stability, and retain high strength up to temperatures near the melting point. The main disadvantages of this class of alloys are their relatively low fracture toughness and difficulty to join.

The effect of neutron irradiation in copper alloys depends largely on the irradiation temperature. At irradiation temperatures below ^300 °C, radiation hardening occurs along with loss of strain hardening capability and complete loss of uniform elongation. Radiation hardening saturates at about ~0.1 dpa in this temperature regime. At higher temperatures, radiation-induced softening can occur. Void swelling takes place between 180 and 500 °C, and the peak swelling temperature is ^300-325 °C for neutron irradiation at damage rates near 10-7 dpas-1. PH and DS copper alloys are more resistant to void swelling than pure copper. Irradiation slightly reduces the fracture toughness of copper alloys, and the effect is stronger in CuAl25 than in CuCrZr. Irradiation has no significant effect on fatigue and creep-fatigue perfor­mance. Transmutation products can significantly change the physical properties and swelling behavior in copper alloys.

Significant R&D efforts have been made to select and characterize copper alloys for high heat flux applications. The ITER Material Property Handbook provides a comprehensive database for pure copper, CuCrZr, and CuAl25. For the ITER first wall and divertor applications, CuCrZr has been selected as the prime candidate. Current focus is on fabrication, joining, and testing of large-scale components.