B. Kalin ^a_, V. Fedotov ^a, 0. Sevryukov ^a, A. Plyuschev ^a, I. Mazul ^b, A. Gervash ^b, R. Giniatulm ^b

a Moscow State Engineering Physics Institute, Kashirskoe sh. 31, Moscow 115409, RF

Brazing is a process used to join materials by introducing an interfacing metal that has a melting point higher than 450 ^oС, but lower than melting point of the materials to be brazed [1]. The brazing is the most-used joining technique of ITER materials (CFC- composites, metal-ceramic assembly, SiC-composites, dispersion-strength alloys, beryllium, refractory metals et al.). The most of analyzed references are devoted to join of beryllium: Be-Cu [2-7,17-21], Be-Be [2,8], Be-SS [8], next is joining of Cu: Cu-CFC [9-11], Cu-Cu [7,12,13], Cu-SS [14]. Sometimes, diffusion barriers are used in a brazing process to eliminate the formation of intermetallic between the filler metal (FM) and the base materials and a spacer to regulate difference of thermal expansion coefficient. The general requirements for FMs in brazing of ITER materials must be following:

• the ability to wet the substrate materials;
• the compatibility with base materials and resistance to operating conditions;
• the providing reliable mechanical and heat contact;
• the ability to alloy or react with base materials without forming brittle intermetallic compounds;
• the good flow characteristics at the brazing temperature.

Selecting FM it is necessary to minimize the high temperature brazing cycle; FM must have narrow melting and solidification ranges. Analysis of references on the brazing technique for ITER materials is shown: as FMs are used Ag-alloys, Cu-alloys, Ni-alloys, elements (Al, Mg, Ti, Si). As this take place, technical solutions is “installating” to commercial FMs. The exeption is investigations of the feasibility of rapidly quenched amorphous FM for ITER materials [6,7,15], but about it will be say below. A new FMs in form of ductile thin foil may be of interest. These foils are manufactured by rapid solidification and they posses some useful properties, for example:

• extremely high chemical and phase homogeneity;
• more high diffusion [16] and capillary [1,17] activity than crystalline analogues;
• more narrow melting and solidification ranges;
• ”instantaneous” melting across the whole width and thickness of foil [1];
• outstanding ductility: amorphous foils bends 180° without fracturing to comply with complex joint geometric to easy fixturing; ductile enough to be mechanically performed and shaped to three-dimensional configurations.

A wetting may be increased by doping of amorphous FMs with active elements: Mn, Ti, Zr, Nb et al. By now it is gained some experience of application of rapidly quenched FMs STEMET for joining of dissimilar PFC-materials: Cu-graphite, Mo-graphite, V-graphite, Be-Cu, V-Be, SS-Be [ 2, 6, 7, 15, 17 ].

This paper discusses the problems connected to the brazing of Be with CuCrZr copper alloy using the new ribbon-type rapidly solidified Cu-Sn-In-Ni FM (STEMET 1108) in combination with a fast heating at brazing.

The characteristics of rapidly solidified ribbon and cast alloy of STEMET 1108 composition are shown in table 1.

Besides the same FM was presented itself in a good light for brazing of Cu with SS [16]. This fact is demonstrated a possibility of manufacturing of three-metal structure Be-Cu-SS during one brazing cycle.

It is necessary to point the traditional brazing technique of ITER components would be change to increasing of heat and cool rates of brazing assembly. It will be useful for preservation of base material properties (for example, dispersion-strength Cu-alloys).

Metallographic analysis of all previous Be/Cu tested samples [ 21 ] clearly revealed a very thick (~100 m m) brittle intermetallic layer in the bonding seam. We believe that one of the main reason for relatively low number of thermal cycles before failure in preceding testing campaigns is a thick intermetallic formation. It was shown [ 21 ] that all considered previous joining technologies use significantly long brazing (heating-soaking-cooling down) cycle. Such a long cycle takes place because of using the traditional “slow” resistor furnace. At the same time in accordance with practically all references (see for ex. [ 20 ] ) the best results of thermal cyclic testing were achieved when the fast heating like induction furnace was used for brazing. That is why it was decided to use the TSEFEY’s electron-beam heating for the fast brazing procedure.

To provide a relevant ITER cyclic thermal loading the TSEFEY electron-beam facility (St. Petersburg, Russia) was developed. This facility allows to create cyclic heat fluxes with required parameters of a cycle and energy density. The time of a cycle can be varied in the range of 0.1 sec. ... to stationary state. The energy density can be varied in the range 0.1 MW/m² ... 15 MW/m². We refer to [ 20 ] for the detailed description and features of the TSEFEY facility, but even from above mentioned parameters it looks suitable for the fast-heating brazing.

A unique brazing process was developed, which uses advanced STEMET 1108 in combination with a fast heating provided by TSEFEY’s electron-beam.

Chemical compositions of used Be (TGP-56) and Cu-alloy (CuCrZr) are shown in table 2. Main physical and mechanical properties of the base alloys are shown in table 3. The CuCrZr properties are strongly dependent on previous heat treatment. In our case the heat treatment was:

• Solution annealing (980°, 1 hour) + water quench
• Cold working ( ~ 40%)
• Aging (470°C, 4 hours)

Base metals	Chemical composition (wt. %)
CuCrZr	Cu - base Cr - 0.6 Zr - 0.08 Total others - 0.1 max.
TGP-56 beryllium	Be(min) - 97.8 BeO(max.) - 2.5 Al(max.) - 0.03 C - 0.12 Fe - 0.1-0.2 Si - 0.04 Ti - 0.05 Cr - 0.08 F - 0.005 Others - 0.08

Properties	CuCrZr	TGP-56 beryllium
Electrical conductivity	45 m/W mm2	-
Thermal conductivity	320 W/mK	180 W/mK
Young’s modules	130 GPa	297 GPa
Expansion coefficient	17.6 10-6/K	11 10-6/K
Density	8.92 g/cm3	1.85 g/cm3
Ult. tensile strength	480 MPa	380 MPa
0.2% Yield strength	350 Mpa	360 MPa
Total elongation	18 %	1,5 %
Hardness	140 HB	-

The microstructure of the brazed joint was investigated using optical microscopy. On the first stage of investigation the Be/CuCrZr joint sample (Sample 1) brazed in a traditional resistor furnace was compared with an identical Be/CuCrZr sample (Sample 2) but brazed using fast electron-beam heating in TSEFEY.

The photos of the brazed seams of samples are given in fig. 2. and fig. 3.

One can see at least two main differences:

• The width of the brazing seam for sample 1 which was brazed in the resistor furnace is ~ 420 m m but 20-30 m m only for the sample 2 which was brazed by fast electron-beam heating. This difference can be influenced by rather long period of brazing process for the sample 1.
• Thin (~ 10 m m) layer with a number of pores in Be near the pure Be/brazing FM boundary was formed for sample 1 but without pores for sample 2. This fact can be explained a by high rate of diffusion migration of Be into the brazing FM influenced by an elevated temperature and a long time of brazing. This information has to be checked more accurately. Significantly low shear strength of sample 1 (22 MPa) is due to cracking of the brazed joint through Be close to the brazing boundary. That fact can be considered as a confirmation of the presence of some pores in Be resulted from the Kirkandall diffusion effect. At the same time the shear strength of sample 2 produced by fast electron-beam heating was found to be 137 MPa.

After the fast electron-beam brazing in TSEFEY sample 2 was successfully tested during 3 x 10³ cycles under the thermal load ~ 1.5 MW/m². The brazed seam cross-section of sample 2 after the thermal cycling is presented in fig. 4.

In compare with the initial state (sample 2) the width of brazing seam has been insignificantly increased up to ~ 40 m m but pores or cracks were not found. The recent results of thermal cycling of the joint produced by TSEFEY fast-heating brazing, where the absorbed energy reached the value more than 8 MW/m², instil confidence [ 22 ].

The main conclusions of this work are:

1. The thick layer of brittle intermetallic compounds formed after Be/Cu joining using 'slow' heating in a resistor furnace is one of the main reasons of failure under cyclic thermal loading.

2. At the expense of reduction of a melting interval and small thickness, the STEMET 1108 rapidly solidified brazing FM permits carrying out the brazing process by fast heating without defects of seam filling and other defects of brazed joints.

3. The combination of fast brazing techniques (like TSEFEY used) with microcrystalline type of brazing alloys reduces significantly the width of the intermetallic zone.

4. Mechanical properties (shear strength) of Be/Cu joints produced by fast heating are significantly higher than heating in a resistor furnace.

5. Fast heating can avoid full annealing of the Cu alloy (in case of using CuCrZr). Recent thermal fatigue results obtained on the TSEFEY facility are evidence of the above mentioned logic.