CN115178852A

CN115178852A - Diffusion bonding method for connecting tungsten and stainless steel

Info

Publication number: CN115178852A
Application number: CN202210877901.3A
Authority: CN
Inventors: 吴玉程; 涂清波; 谭晓月; 罗来马
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2022-07-25
Filing date: 2022-07-25
Publication date: 2022-10-14
Anticipated expiration: 2042-07-25
Also published as: CN115178852B

Abstract

The invention discloses a diffusion bonding method for bonding tungsten and stainless steel. The tungsten layer is a tungsten fiber reinforced tungsten (Wf/W) composite material, and by virtue of excellent fracture toughness, the fracture resistance is improved, and brittle cracking of a tungsten matrix is inhibited; the V foil is used as an intermediate layer material, the thermal expansion coefficient of V is between that of tungsten and steel, and the V foil, the tungsten and the steel can form a continuous solid solution, and the V foil is used as a tungsten-steel connecting intermediate layer to relieve the residual stress of a connecting part, so that a brittle intermetallic compound is prevented from being formed when the tungsten and the steel are in diffusion connection, and the mechanical property of the connecting part is improved. The invention realizes the diffusion connection of the tungsten-vanadium-steel system by using the discharge plasma diffusion connection technology, the components are tightly combined, no new phase is generated on the interface, no defects such as obvious pores and cracks exist, and the shearing strength of the joint reaches 243MPa.

Description

Diffusion bonding method for connecting tungsten and stainless steel

Technical Field

The invention belongs to the field of diffusion welding connection of layered metal materials, and particularly relates to a diffusion connection method for connecting tungsten and stainless steel.

Background

Tungsten has the advantages of low activity, high melting point, high thermal conductivity and the like, is one of the most promising plasma-oriented materials, steel is an important structural material, and a tungsten-steel connecting component has important application value in the fields of national defense, military industry, nuclear industry and the like.

According to related research reports at home and abroad, the tungsten-steel connection mainly adopts brazing, plasma spraying and diffusion welding technologies, wherein the diffusion welding technology becomes an effective method for tungsten/steel connection due to the advantages of low connection temperature, high joint use temperature and the like. Due to the inherent brittleness of tungsten, cracks are easily generated in the cooling process of the tungsten substrate after connection, and the joint fails. Conventional intrinsic toughening methods, such as dispersion strengthening or grain refinement, have limitations in high temperature joining processes and long term high temperature applications. The intrinsic toughening mechanism will gradually fail under the effect of severe internal damage caused by grain growth during joining or neutron irradiation in fusion applications. To overcome this problem, tungsten fiber reinforced tungsten-based (Wf/W) composites have been proposed. The fracture resistance is improved through energy dissipation mechanisms such as ductile deformation of the fiber, pulling out of the fiber, crack bridging and deflection at the interface and the like. Research shows that the Wf/W composite material has excellent fracture toughness compared with pure tungsten.

Because of the mismatch in the coefficients of thermal expansion of tungsten and steel, high residual thermal stresses along the interface are generated upon cooling from the joining temperature to room temperature, and the joint is susceptible to failure due to excessive thermal stresses. Moreover, the tungsten-steel is directly connected, and the interface is easy to react to generate FeW and Fe ₂ Intermetallic compounds such as W and carbides such as tungsten carbide, and the generation of these intermetallic compounds and hard brittle phases rapidly deteriorates the mechanical properties of the joint, and seriously affects the service life of the joint. In order to obtain a high-quality steel-tungsten connecting joint, a large amount of research is carried out by domestic and foreign scholars, the residual stress of the joint is relieved by adding an intermediate layer and controlling connecting process parameters, the formation of harmful substances such as brittle intermetallic compounds and metal carbides is inhibited, and systematic research is carried out on the aspects of the influence of the material of the intermediate layer on the tissue performance of the connecting joint.

The selection principle of the intermediate layer material is that the intermediate layer material has small difference with the physical and chemical properties of the parent metal, does not generate a hard brittle phase and a eutectic phase with the parent metal, is easy to plastically deform, reduces the internal stress of a joint as much as possible, and improves the metallurgical bonding of a tungsten-steel interface. Various materials have been studied at home and abroad as interlayers of tungsten-steel diffusion bonding, such as Cu, ni, nb, zr, ti, V and combinations thereof.

Disclosure of Invention

The invention mainly solves the technical problem of providing a diffusion connection method for connecting tungsten and stainless steel, and solves the problems that because the thermal expansion coefficients of tungsten and steel are not matched, the generation of harmful substances such as brittle intermetallic compounds and metal carbides for connecting tungsten and steel is inhibited, and the poor mechanical property of a connecting piece caused by the concentration of thermal stress is avoided. The vanadium foil is used as the intermediate layer for tungsten-steel connection, and as the thermal expansion coefficient of vanadium is between that of tungsten and steel, and can form a continuous solid solution with tungsten and steel, the vanadium foil is used as the intermediate layer for tungsten-steel connection to relieve the residual stress of the joint.

In view of the high melting point of tungsten, the connection time is too long by adopting diffusion welding technologies such as hot pressing, hot isostatic pressing and the like, a large amount of brittle carbides are generated on a vanadium-steel interface, and the mechanical property of the joint connection part is greatly influenced. The invention takes vanadium foil as the intermediate layer and adopts the discharge plasma diffusion bonding technology for bonding, thereby effectively shortening the bonding time and inhibiting the generation of brittle carbides. And Boron Nitride (BN) is sprayed on the inner wall of the female die, so that current does not pass through the female die, heat is concentrated on a pressure head, a sample is heated, the connection temperature is controlled at a specific temperature, and the heat preservation time is 15min. The cooling rate was controlled at 10 deg.c/min during the initial cooling stage and held for 5 minutes while cooling to 700 deg.c to reduce residual thermal stress.

The tungsten block adopted by the invention is a Wf/W composite material large sample, the mass fraction of the fiber is 40%, and the porosity and pore distribution in the prepared large sample tissue can be accurately regulated and controlled. The weak connection between the fiber and the matrix is realized by utilizing the controllable porosity of the fine-scale matrix, so that cracks from the matrix cannot penetrate into the fiber, the damage tolerance is improved, and the internal cracking failure of the tungsten layer in the connection process or practical application is avoided. The porous Wf/W block adopted by the invention has the relative density of about 85.9 percent, uniform internal pore distribution, excellent mechanical property and bending strength of 250MPa. Having an average coefficient of thermal expansion of (5. + -. 0.24). Times.10 at room temperature to 400 ℃ of ^-6 K, compared to pure tungsten (coefficient of thermal expansion 4.5X 10) ^-6 K) in contact with the V foil (coefficient of thermal expansion 8.4X 10) ^-6 and/K) the residual thermal stress of the joint is lower during diffusion bonding, and a high-quality bonding interface can be formed.

Tungsten block size of

V foil size of

The stainless steel is 316L stainless steel with the size of

Specifically, the diffusion bonding method of the present invention comprises the steps of:

step one, surface treatment: grinding and polishing the upper and lower surfaces of a tungsten block and a stainless steel sample, wherein the surface roughness Ra is less than or equal to 5 mu m, the parallelism is less than or equal to 0.02mm, and the tungsten block and the stainless steel sample are ultrasonically cleaned in acetone solution or alcohol for 15min before connection;

secondly, sample loading: spraying a layer of BN on the inner wall of the graphite female die, enabling current to only flow through a pressure head and a sample part, sequentially loading a tungsten block, a vanadium foil and 316L stainless steel from bottom to top, and separating the sample from the graphite die by using a Mo foil;

thirdly, connecting the samples in the mould by using a discharge plasma connecting device: the applied pressure is 30MPa, the connection temperature is 870 ℃, the temperature is kept for 15min, then the temperature is reduced to 700 ℃ at the speed of 10 ℃/min, the temperature is kept for 5min, and then the furnace is cooled.

The invention has the beneficial effects that:

(1) The invention provides a diffusion bonding method for bonding tungsten of a large sample and 316L stainless steel, wherein the melting point of metal tungsten is very high, tungsten-steel generally needs to be bonded for a long time under the conditions of high temperature and high pressure, a V foil is used as a bonding intermediate layer, and a large amount of brittle carbides are easily generated on a vanadium-steel interface; the invention controls the cooling rate in the cooling stage, reduces the temperature to 700 ℃, and preserves the temperature for 5min, thereby reducing the residual thermal stress of the connecting piece.

(2) The invention relates to a diffusion bonding method for bonding a large sample of tungsten and 316L stainless steel by using a discharge plasma diffusion bonding technology, which is characterized in that BN is sprayed on the inner wall of a female die, so that current only flows through a pressure head and a sample without passing through the female die, and joule heat generated by the graphite pressure head, the sample and the like is concentrated around the sample to promote the diffusion bonding of a sample interface. This approach avoids the consumption of heat in other ways.

(3) The method has the advantages of simple operation, low cost, no use and generation of polluting and harmful substances in the process of preparing the materials, and is an environment-friendly technology for preparing the materials.

(4) According to the invention, excellent mechanical properties and high damage tolerance are obtained by controlling the porosity and pore distribution of the tungsten block; the optimized tungsten block has good toughness, and cracks can be prevented from being generated in a connecting interface and the matrix. Furthermore, the residual thermal stress of the joint is lower when diffusion bonding is performed using porous Wf/W as compared to pure tungsten, and a high quality joint can be obtained.

(5) The joint prepared by the invention is tightly combined, and has no obvious cracks and pores; the connection interface has no brittle phase generation, the mechanical property of the joint is excellent, and the shear strength reaches 243MPa.

(6) The invention reduces the current density when the discharge plasma is in diffusion connection by enlarging the size of the sample, and avoids the excessive diffusion of elements at the interface to form brittle phases such as carbide and the like.

Drawings

FIG. 1 is a diagram of a sample object prepared according to the present invention;

FIG. 2 is a schematic view of a discharge plasma diffusion bonding apparatus;

FIG. 3 shows the present invention with the selected size of

The section appearance of a sample prepared from vanadium foil;

FIG. 4 is a crack deflection phenomenon of a Wf/W composite having a fiber mass fraction of 40%;

FIG. 5 is a fracture morphology before connection of the Wf/W composite blocks with 40% fiber mass fraction according to the present invention;

FIG. 6 is a fracture morphology diagram after connection of the Wf/W composite material blocks with the fiber mass fraction of 40% according to the invention;

FIG. 7 shows selected dimensions

A vanadium-steel connection interface of the vanadium foil under the pressure of 30 MPa;

FIG. 8 shows selected dimensions

A vanadium-steel connection interface under the pressure of 10MPa of vanadium foil;

FIG. 9 shows selected dimensions

FIG. 10 shows the selected size of

A connection interface of a sample prepared from vanadium foil, which is not subjected to heat preservation at 700 ℃;

FIG. 11 is a cross-sectional view of a mass of a Wf/W composite having a fiber mass fraction of 60%.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Example 1:

firstly, selecting a large sample of a Wf/W composite material with the fiber mass fraction of 40 percent, wherein the size of a tungsten block body is

Has a relative density of about85.9%, the internal pores are uniformly distributed.

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel;

fourthly, grinding and polishing the upper surface and the lower surface of a tungsten block and a stainless steel sample, wherein the surface roughness Ra is less than or equal to 5 mu m, the parallelism is less than or equal to 0.02mm, and the tungsten block and the stainless steel sample are ultrasonically cleaned in acetone solution for 15min before connection;

fifthly, spraying a layer of BN on the inner wall of the graphite female die, enabling current to only flow through a pressure head and a sample part, sequentially loading a tungsten block, a vanadium foil and 316L stainless steel from bottom to top, and separating the sample from the graphite die by using a Mo foil;

and sixthly, connecting the samples in the die by using discharge plasma connecting equipment, wherein the applied pressure is 30MPa, the connecting temperature is 870 ℃, after heat preservation is carried out for 15min, the temperature is reduced to 700 ℃ at the speed of 10 ℃/min, the heat preservation is carried out for 5min, and then furnace cooling is carried out.

The thickness of the middle layer vanadium foil is 0.3mm, the diffusion of connecting interface elements is sufficient, a solid solution diffusion layer with good metallurgical bonding can be formed, residual thermal stress caused by the mismatch of thermal expansion coefficients can be fully relieved, and the decisive effect is played for improving the mechanical property of the joint. The tungsten-steel joint consists of a tungsten-vanadium solid solution layer, an unreacted vanadium layer and a vanadium-steel diffusion layer. Wherein, the tungsten-vanadium and vanadium-steel interface is tightly combined, has no defects such as obvious pores, cracks and the like, and has no new phase generation, as shown in figure 3 and figure 7, and the shear strength of the joint reaches 243MPa.

The lower density of the tungsten block body enables the grain growth to be effectively inhibited in the connection process, and the grain size is increased from 6.5 mu m to 6.8 mu m in the diffusion connection process without obvious change, as shown in fig. 5 and fig. 6; the hardness of the tungsten block bodies before and after connection is about 510HV1, and the mechanical properties are not obviously changed.

Example 2:

Its relative density is about 85.9%, and its internal pore distribution is uniform.

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel;

fourthly, grinding and polishing the upper and lower surfaces of the tungsten block and the stainless steel sample, wherein the surface roughness Ra is less than or equal to 5 mu m, the parallelism is less than or equal to 0.02mm, and ultrasonically cleaning the upper and lower surfaces in an acetone solution for 15min before connection;

sixthly, connecting the samples in the die by using discharge plasma connecting equipment, wherein the applied pressure is 30MPa, the connecting temperature is 870 ℃, after heat preservation is carried out for 15min, the temperature is reduced to 700 ℃ at the speed of 10 ℃/min, heat preservation is carried out for 5min, and then furnace cooling is carried out.

Due to the low thermal conductivity of V (30.7 W.m) ^-1 ·K ^-1 ) In contrast to example 1, an excessively thick intermediate layer leads to a reduction in the heat transfer efficiency of the joint.

Example 3:

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel;

fifthly, spraying a layer of BN on the inner wall of the graphite female die, enabling current to only flow through a pressure head and a sample part, sequentially loading a tungsten block, a V foil and 316L stainless steel from bottom to top, and separating the sample from the graphite die by using a Mo foil;

and sixthly, connecting the samples in the die by using discharge plasma connecting equipment, wherein the applied pressure is 30MPa, the connecting temperature is 870 ℃, after heat preservation is carried out for 15min, the temperature is reduced to 700 ℃ at the speed of 10 ℃/min, heat preservation is carried out for 5min, and then furnace cooling is carried out.

Since the intermediate layer is thin as compared with example 1, residual thermal stress is concentrated at the interface of the intermediate layer and the base. The problems of stress concentration and matrix-interlayer deformation incompatibility are prominent with the reduction of the thickness of the interlayer, resulting in a reduction in joint performance.

Example 4:

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel;

Compared with the example 1, because the small-sized sample has larger current density in the discharge plasma diffusion connection process, the element diffusion of the interface between the middle layer and the steel is too sufficient, and a brittle carbide layer is formed at the interface, wherein the main component of the brittle carbide layer is V2C, and the brittle carbide layer is shown in figure 9; the generation of brittle phase causes the concentrated distribution of interface thermal stress, cracks are generated at the interface, the joint strength is greatly reduced, and the shear strength is 198MPa.

Example 5:

firstly, selecting a large Wf/W composite material sample with the fiber mass fraction of 40 percent, wherein the tungsten block has the size

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel;

and sixthly, connecting the samples in the die by using discharge plasma connecting equipment, wherein the applied pressure is 10MPa, the connecting temperature is 870 ℃, after heat preservation is carried out for 15min, the temperature is reduced to 700 ℃ at the speed of 10 ℃/min, heat preservation is carried out for 5min, and then furnace cooling is carried out.

In contrast to example 1, insufficient pressure resulted in insufficient interface contact, resulting in discontinuity of the diffusion layer, presence of pores at the interface, and the like, see fig. 8. This causes the interface bonding to be loose and the joint performance to deteriorate.

Example 6:

firstly, selecting a large sample of a Wf/W composite material with the fiber mass fraction of 60 percent, wherein the size of a tungsten block body is

A second step of selecting the size of

Vanadium foil;

thirdly, selecting the size of

316L stainless steel of (1);

In contrast to example 1, when the mass fraction of the fibers is too high, the powder is not enough to fill all the voids between the fibers, the pores inside the Wf/W mass are not uniformly distributed, larger pores are present, and the connection process may cause voids at the connection interface, leading to poor joint performance and even connection failure, see fig. 11.

Example 7:

Its relative density is about 95% and its internal pores are uniformly distributed.

A second step of selecting the size of

A vanadium foil;

thirdly, selecting the size of

316L stainless steel of (1);

fifthly, spraying a layer of BN on the inner wall of the graphite female die, enabling current to only flow through a pressure head and a sample part, loading a tungsten block, a vanadium foil and 316L stainless steel from bottom to top in sequence, and separating the sample from the graphite die by using a Mo foil;

and sixthly, connecting the samples in the die by using discharge plasma connecting equipment, wherein the applied pressure is 30MPa, the connecting temperature is 870 ℃, and the samples are cooled along with the furnace after being kept for 15min.

In contrast to example 1, the thermal stress of the joint was too high due to the too fast cooling rate, and cracks were evident at both the tungsten-vanadium and vanadium-steel joint interfaces, see fig. 10. Cracks at the interface lead to deterioration of the joint performance.

The above description is only an embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications of equivalent structures and equivalent processes performed by the present specification and drawings, or directly or indirectly applied to other related technical fields, are included in the scope of the present invention.

Claims

1. A diffusion bonding method for joining tungsten and stainless steel, comprising the steps of:

(1) Grinding and polishing the surfaces to be connected of the tungsten block and the stainless steel for later use;

(2) Ultrasonically cleaning and drying the tungsten block and the stainless steel obtained in the step (1);

(3) Preparing a vanadium foil for later use;

(4) And (4) sequentially arranging and combining the tungsten block, the vanadium foil and the stainless steel in the steps (2) and (3), then placing the tungsten block, the vanadium foil and the stainless steel into a graphite die, and performing diffusion connection to obtain the tungsten-steel joint consisting of a tungsten-vanadium solid solution layer, an unreacted vanadium layer and a vanadium-steel diffusion layer.

2. A diffusion bonding method of joining tungsten and stainless steel according to claim 1, wherein:

in the step (1), the size of the tungsten block is

Stainless steel having a size of

3. A diffusion bonding method of joining tungsten and stainless steel according to claim 1, wherein:

in the step (1), the requirements of grinding and polishing are that the surface roughness Ra is less than or equal to 5 mu m and the parallelism is less than or equal to 0.02mm.

4. A diffusion bonding method of joining tungsten and stainless steel according to claim 1, wherein:

in the step (2), the solvent selected for ultrasonic cleaning is acetone or alcohol, and the ultrasonic cleaning time is 10-30 min.

5. A diffusion bonding method of joining tungsten and stainless steel according to claim 1, wherein:

in the step (3), the thickness of the vanadium foil is 0.1-0.5 mm.

6. A diffusion bonding method of joining tungsten and stainless steel according to claim 1, wherein:

and (4) the diffusion connection is discharge plasma diffusion connection.

7. A diffusion bonding method of joining tungsten and stainless steel according to claim 6, wherein:

the connection temperature in the discharge plasma diffusion connection process is 800-1100 ℃, the heat preservation time is 10-20 min, the connection pressure is 20-50 MPa, the vacuum degree is less than or equal to 50Pa, the temperature is reduced to 700 ℃ at the speed of 5-10 ℃/min, the heat preservation time is 5-10 min, and then the furnace is cooled to the room temperature.