WO2015092317A1

WO2015092317A1 - Process for assembling at least two elements by brazing, assembly comprising at least two elements and a brazing joint obtained by said process

Info

Publication number: WO2015092317A1
Application number: PCT/FR2014/053452
Authority: WO
Inventors: Olivier Mailliart
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2013-12-20
Filing date: 2014-12-19
Publication date: 2015-06-25
Also published as: FR3015319B1; FR3015319A1

Abstract

Process for assembling at least two elements by brazing, the first element being made of yttriated hafnia or made of yttriated thorina, the second element being made of yttriated hafnia, made of yttriated thorina or made of a metal alloy, said process comprising the following successive steps: - bringing the two elements into contact with an FeNiTi brazing alloy, - melting the FeNiTi brazing alloy in order to form a brazing joint.

Description

A method of assembling at least two elements by brazing, an assembly comprising at least two elements and a solder joint obtained by said method.

Technical field of the invention

The invention relates to a method of assembling at least two elements by soldering, and to an assembly comprising at least two elements and a solder joint obtained by said method.

State of the art Liquid sodium is an excellent heat transfer fluid, it is used in particular in fast neutron nuclear reactors.

However, the presence of oxygen in metal structures containing sodium greatly increases their corrosion kinetics. It is therefore necessary to measure the dissolved oxygen content in the molten sodium baths.

Ytrtria thoria and yttria are good ionic conductors and can be used as electrolytes for oxygen probes to equip liquid sodium cooled fast neutron reactors.

One of the challenges is therefore to integrate the functional ceramic part (electrolyte) of the probe into a metal structure. This involves in particular to assemble one or more pieces made of materials containing yttrie hafnia or thorium ytrtriée them or with one or more metal parts.

It is essential that the ceramic and metal elements retain their integrity at the end of the assembly operation and the junction made must be perfectly hermetic. Brazing is an assembly technique to ensure a good seal at the assembly. This technique consists of melting a metal or solder alloy, the solder, between the parts to be assembled. After cooling, the solidified solder ensures the cohesion of the assembly.

Patent FR2325928 (1976) describes, for example, the production of an electrochemical device for measuring the oxygen concentration of a liquid alkali metal. The electrode is made by assembling thorium ytria with a tube of stainless steel or nickel by soldering. The brazing material consists of a mixture of 70% by weight of gallium and 30% by weight of nickel. However, the assembly obtained is sensitive to thermal shocks and a significant portion of the assembled probes breaks after a very short time of use given the expected life of this type of equipment.

The article "Oxygen determination in liquid sodium with a continuous electrochemical measuring probe" by J. Jung (Journal of Nuclear Materials, 56, 213-220 (1975)) also describes the assembly of a steel tube with thorine yttria. The solder joint is made using a Cu-2Ni alloy which requires pre-metallization of the ceramic with a Mo / Mn process followed by nickel metallization.

The article "Development of electrochemical oxygen meter for liquid sodium" by D. Jakes (Solid State Electronics, 13, 165-173 (1984)) shows that this metallization layer, used to promote the wetting of another solder (Nicrobraz), has been infiltrated in depth causing degradation of the electrolyte.

"Studies in Solid State Electronics" (Report - Rez, January 1987) by D. Jakes and J. Rosenkranz describes an oxygen probe made by a brazing process between ytrtria thoria and a Ni-Ni-Fe-Cn alloy. The solder used is a non-silicate glass of formula CaO.Al ₂ O ₃ .BaO + 1% by weight of TiO ₂ . However, the use of glass seals is discouraged because of their great fragility in particular vis-à-vis the thermal cycling and poor resistance to the chemical attack of liquid sodium.

Object of the invention

The object of the invention is to overcome the drawbacks of the prior art and, in particular, to propose a brazing process which makes it possible to obtain a junction having good durability in liquid sodium.

This object is approached by the appended claims.

Brief description of the drawings

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of non-limiting example and represented in the accompanying drawings, in which:

- Figures 1 and 2 show, schematically, in section, different steps of a reactive brazing assembly process in a configuration called "sandwich",

FIGS. 3 and 4 show schematically, in section, various stages of a reactive brazing assembly process in the so-called capillary configuration,

FIG. 5 schematically shows, in three dimensions, two cylinders arranged to be assembled by reactive brazing in a so-called capillary configuration;

FIGS. 6 and 7 represent clichés, obtained by scanning electron microscopy, at a solder joint of a yttrie / FeNi alloy hafnium assembly. Description of a preferred embodiment of the invention

The method is a method of assembling at least two elements by brazing.

The first element is in yttried hafnia or in ytered thorium.

The second element is in yttried hafnia, in yttria thorium or in metallic alloy.

Several configurations are possible:

- a yttrie hafnie element can be assembled with a yttrie hafnie element, ytrtria thorina or metal alloy,

a yttria thoria element may be assembled with a yttria hafnia, ytrtria thorium or metal alloy element. By "element is in" yttrie hafnie means a material composed of at least 50% by weight of HfO ₂ -Y ₂ O ₃ , and preferably at least 90% by weight of HfO ₂ -Y ₂ O ₃ and, even more preferably, at least 95% of HfO ₂ -Y ₂ O ₃ . By "element is in" ytered thorium is meant a material composed of at least 50% by weight of ThO ₂ -Y ₂ O ₃ , and preferably at least 90% by weight of ThO ₂ -Y ₂ O ₃ and also more preferably, at least 95% of ThO ₂ -Y ₂ O ₃ . The remaining percentages may correspond to one or more other oxides used in the composition of the ceramic. These elements make it possible to modify the properties of ceramics (ionic conductivity, mechanical strength, etc.). The ceramic may also be reinforced by particles of other materials, for example by Al ₂ O ₃ fibers.

Preferably, the yttria hafnia comprises from 0.5% to 15% by mass of yttrium oxide.

Preferably, the yttria thorium comprises from 0.5% to 15% by mass of yttrium oxide. In the case of an assembly between two elements ytrtried thorium or between two yttrie hafnie elements, they may contain different percentages of yttrium oxide different or identical. The metal alloy may be a binary alloy.

Preferably, the binary metal alloy comprises:

- 40% to 70% iron mass,

- 30% to 60% by weight of nickel. The metal alloy can be a ternary alloy.

Preferably, the ternary metal alloy comprises:

- 40% to 70% iron mass,

20% to 60% by weight of nickel,

a third element chosen from Ti, Co, Cr, Mo, Mn, V, Si, Nb, S, C or Al.

The third element makes it possible to modify certain properties of the material (mechanical resistance, coefficient of thermal expansion, etc.).

The sum of the mass percentages of these various components in the alloy is equal to 100%. The presence of impurity in the alloy is conceivable.

Preferably, the alloy is a Fe _{5 5} Ni ₄ 5 alloy (in percentage by mass). The method of assembling at least two elements previously described by soldering comprises the following successive steps:

bringing the two elements into contact with a FeNiTi brazing alloy,

melting the FeNiTi solder alloy by heating it to a brazing temperature above the melting temperature of the solder alloy to form a solder joint. By FeNiTi brazing alloy is meant a ternary alloy composed of iron, nickel and titanium, also called Fe-Ni-Ti alloy.

Hafnia and thoria are particularly stable ceramics and very difficult to reduce in comparison with other ceramics such as AI ₂ O ₃ or ZrO ₂ . The Applicant has unexpectedly discovered that titanium reacts with these ceramics to form a layer of hooked titanium oxide. The layer of hooked, also called reaction layer, is formed at the interface between the solder and the hafnie or the thorium and provides a solder joint forming a mechanically strong junction between the two elements to be assembled.

By mechanically strong junction is meant an assembly whose mechanical strength is of the same order of magnitude as that of the assembled bulk materials. The rupture of such an assembly will occur within the assembled materials, the rupture is said to be cohesive, and not at the level of the interfaces, in the case of a so-called adhesive rupture.

The melting point of the FeNiTi eutectic used is 11 ° C., for example.

The use of the FeNiTi alloy makes it possible to have a relatively low melting point, due to the existence of a eutectic, and to limit the reactivity with respect to the FeNi alloy with which the ceramic is assembled. .

Brazing at a moderate temperature makes it possible to limit the thermomechanical stresses to cooling after the assembly cycle.

The assembly can be used up to temperatures of the order of 1060 ° C.

Preferably, the brazing alloy comprises:

- 20% to 70% atomic iron,

- 15% to 75% by weight of titanium,

- 10% to 65% atomic nickel Even more preferably, the brazing alloy comprises:

- 33% to 44% atomic iron,

18% to 28% by weight of titanium,

- 33% to 44% atomic nickel

The solder alloy comprises:

- 38.5 atomic% of iron (± 2 atomic%),

23 atomic% of titanium (± 2 atomic%),

- 38.5 atomic% of nickel (± 2 atomic%).

The brazing alloy may be in the form of a powder, a wire, a sheet or a stack of sheets.

The brazing alloy, in powder form, can also be mixed with a binder to form a paste.

The method of assembling the two elements by soldering comprises a step for bringing the two elements into contact with the solder alloy FeNiTi. For example, the various elements can be positioned in the so-called sandwich configuration.

As shown in Figures 1 and 2, in sandwich configuration, the faces of the first element 1 and the second element 2 to be assembled are placed face to face. The solder composition 3 is placed between the faces of the elements 1 and 2. The faces to be brazed are then brought into contact.

The heat treatment, carried out above the melting point of the solder composition, melts the latter which after cooling forms a solder joint 4 mechanically bonding the two elements 1 and 2. The elements can also be brought into contact with each other. in so-called capillary configuration. As shown in FIGS. 3 and 4, in the capillary configuration, the elements 1 and 2 to be assembled are brought into contact without having put a solder composition between them.

The solder composition 3 may be disposed in the form of a reservoir at the periphery of the solder joint.

The heat treatment, carried out above the melting point of the solder composition, melts the solder leading to the infiltration of the solder into the joint by capillarity and thus to the formation of the solder joint 4. Brazing used in this process is reactive brazing.

The assembly, formed of the parts to be assembled and the solder composition, is subjected to a thermal cycle. The thermal cycle consists of a rise in temperature up to the brazing temperature, a bearing at the brazing temperature of the order of ten minutes (5 - 15 minutes) and then a cooling ramp to a temperature below the melting temperature of the solder. Preferably, the cooling is carried out to room temperature.

By ambient temperature is meant a temperature of the order of 20-25 ° C. The brazing temperature is lower than the melting temperatures of the materials to be assembled and is greater than the melting temperature of the solder alloy.

In the case of a FeNiTi system, the brazing temperature is between 1120 ° C. and 1400 ° C.

During the thermal cycle, a reaction layer, for example of TiO _x type, is formed at the interface between the solder composition and the elements to be assembled, thus creating a strong connection between the parts to be assembled.

The brazing is carried out under secondary vacuum or under neutral gas.

The assembly process will now be described by means of the example following, given, of course, by way of illustration and not limitation.

Example: Assembly of a HfO ₂ -Y ₂ O ₃ element and a FeNi element by reactive brazing in a capillary configuration. Brazing alloy: Fe-Ni-Ti. Brazing FeNiTi system.

As shown in FIGS. 6 and 7, in this example, a tethered hafnit tube 1 was brazed to a FeNi alloy tube (FessNUs) 2.

The scale of FIG. 6 corresponds to 100 μm and that of FIG. 7 to 20 μm.

The composition of the target solder is the eutectic of the following composition

The solder was made from pure iron, nickel and titanium which was melted in a secondary vacuum alumina crucible at 1200 ° C. The ingot obtained is Fe-Ni-Ti. The composition of the brazing alloy developed was checked by scanning electron microscopy (SEM or Scanning Electron Microscopy SEM) and more particularly by energy dispersive analysis (or EDX for Energy Dispersive X-ray analysis).

The melting point of the eutectic is 11 ° to 13 ° C.

The ingot was milled to obtain a powder which was mixed with a binder to obtain a solder paste. The solder paste thus obtained is disposed at the zone 5 of FIG.

The brazing temperature is of the order of 1200 ° C. The whole is subjected to this temperature for a period of 20 minutes.

At the outlet of the oven, the elements 1 and 2 are well assembled via the solder joint 4.

During the thermal cycle, a reaction layer 6 (FIG. 7), in TiO _x , is formed at the interface between the solder composition and the element 1 made of yarn, thus creating a strong connection between the various parts.

The helium tightness of the assembly thus produced was tested.

The assembly was positioned on the metal tube side on a support plate for evacuating the sample and plugged on the ceramic tube side with a silicone plug. A pumping system makes it possible to reach the primary vacuum in the cavity of the assembly. Once the desired vacuum level is reached, helium is blown on the different parts of the assembly (tubes, solder joint, etc.). A helium detector mounted at the output of the device, under the support plate, makes it possible to measure the level of vacuum as well as the leakage rate at the assembly.

A vacuum level of 1 .10 ^"3 mbar is reached and a leak rate of ^{1.10" 10} mbar.l / s was measured.

The method makes it possible to obtain an assembly comprising:

at least two elements: the first element being made of ytriated hafnia or ytriated thorium and the second element being made of yttried hafnium, yttria thorium or metal alloy,

and a solder joint obtained according to the method described above.

The parts are mechanically secured to each other through the solder joint. The assembly method described above is particularly suitable for producing oxygen electrochemical probes, and more particularly for producing electrochemical oxygen probes for use in liquid sodium.

The elements to be assembled are, preferably, a first ceramic element selected from yttrie hafnia and ytria thorina and a second element metal alloy, yttrie hafnie or ytrtria thorina.

Yttried hafnia and yttria thoria show good ionic conductivity compared with other ceramics.

The assembly has, advantageously, a good seal at high temperature.

The electrochemical probe comprises for example:

an electrolyte in yttried hafnia or in ytriated thoria,

a metal alloy tube on which the electrolyte is brazed, the electrolyte being inserted into the metal alloy tube or placed in end-to-end configuration with the tube;

- A solder joint disposed between the electrolyte and the metal alloy tube, obtained by the assembly method.

The metal alloy on which the ceramic is brazed plays the role of interlayer can contribute to reducing some of the stresses on the ceramic, thus promoting the mechanical strength of the device. The brazing alloy used in the process has a good resistance to oxidation. The cost of the alloy is not excessive and the process is industrializable.

Claims

claims

1. A method of assembling at least two elements by brazing, the first element being yttrie hafnie or ytrtried thorina,

the second element being made of yttried hafnia, yttrie thorium or metal alloy,

said method comprising the following successive steps:

bringing the two elements into contact with a FeNiTi brazing alloy,

- Melt FeNiTi solder alloy to form a solder joint.

2. Method according to claim 1, characterized in that the brazing alloy comprises:

- 20% to 70% atomic iron,

- 15% to 75% by weight of titanium,

- 10% to 65% atomic nickel.

3. Method according to claim 2, characterized in that the brazing alloy comprises:

- 33% to 44% atomic iron,

18% to 28% by weight of titanium,

- 33% to 44% atomic nickel.

4. Method according to claim 3, characterized in that the brazing alloy comprises:

- 38,5 ± 2% atomic iron,

- 23 ± 2 atomic% of titanium,

- 38.5 ± 2 atomic% of nickel.

5. Method according to one of claims 1 to 4, characterized in that the yttrie hafnie comprises from 0.5% to 15% by weight of yttrium oxide.

6. Method according to one of claims 1 to 5, characterized in that the ytrtrium thorium comprises from 0.5% to 15% by weight of yttrium oxide.

7. Method according to one of claims 1 to 6, characterized in that the metal alloy comprises:

- 40% to 70% iron mass,

20% to 60% by weight of nickel,

a third element chosen from Ti, Co, Cr, Mo, Mn, V, Si, Nb, S, C or Al.

8. Method according to one of claims 1 to 6, characterized in that the metal alloy comprises:

- 40% to 70% iron mass,

- 30% to 60% by weight of nickel.

9. Process according to claim 8, characterized in that the metal alloy is a Fe _{5 5} Ni ₄ 5 alloy.

10. Method according to one of claims 1 to 9, characterized in that the brazing is carried out under secondary vacuum or under neutral gas.

11. Method according to one of claims 1 to 10, characterized in that the brazing temperature is between 1120 ° C and 1400 ° C.

12. Assembly comprising:

and a solder joint obtained according to the process of any one of claims 1 to 1 1.

13. Electrochemical probe comprising:

an electrolyte in yttried hafnia or in ytriated thoria,

a metal alloy tube in which the electrolyte is inserted,

a brazing joint disposed between the electrolyte and the metal alloy tube, obtained according to the method of any one of Claims 1 to 12.