Typical Applications of Diffusion Welding Explained

Due to the high and stable quality of diffusion welding joints, and its wide range of applicable materials, it’s especially suitable for welding brittle materials and special structures. In fields like aerospace, electronics, and nuclear industries, many components operate in harsh environments, with special structural requirements for products.

Designers have no choice but to use specialty materials, such as adopting hollow structures to reduce weight, and requiring the joints to match the composition and properties of the parent material. In these situations, where welding quality is more critical, even though the production cost of diffusion welding is slightly higher, it’s still the preferred welding method.

Diffusion Welding of Similar Materials

Diffusion Welding of Titanium Alloys

Titanium and its alloys are high-performance materials with high specific strength, corrosion resistance, and high-temperature resistance. They are suitable for manufacturing lightweight, reliable structures, and are now widely used in aerospace industries. They are commonly used to manufacture pressure vessels, storage tanks, engine casings, satellite casings, frames, engine jet pipe extensions, etc.

When titanium alloys are diffusion welded, their joint performance is better than conventional fusion welding. Titanium alloys don’t require special preparation and control of the workpiece surface during diffusion welding.

The oxide film on the surface of titanium can dissolve in the parent material at high temperatures, and under a pressure of 5MPa, up to 30% of TiO2 can be dissolved, so the oxide film does not hinder diffusion welding. There are no traces of the original interface in the joint structure of diffusion welded titanium alloys of the same composition.

Titanium alloys can absorb large amounts of O2, H2, and N2 gases, so they should not be diffusion welded in H2 and N2 atmospheres, but should be performed under vacuum conditions or with Ar gas protection. The most common welding method for titanium alloys is Superplastic Forming Diffusion Bonding (SPF/DB).

The chosen temperature is basically the same as the temperature usually used for diffusion welding, but it should be noted that pressure and time must be chosen to match. The original grain size of titanium alloys also affects the quality of diffusion welding. The finer the original grains, the shorter the time required for good diffusion welding, and the lower the pressure applied.

Therefore, the SPF/DB welding process requires that the titanium alloy parent material must have a fine grain structure. Common welding parameters for titanium alloys are: heating temperature 1123~1273℃; holding time 60~240min; pressure 2~5MPa; vacuum 1.33×10-3Pa or above or welding under Ar gas protection.

For large-area titanium alloy diffusion welding, an interlayer can be added for diffusion brazing. The interlayer mainly uses Ag-based brazing materials, Ag-Cu brazing materials, and Ti-based brazing materials. Cu-based brazing materials and Ni-based brazing materials tend to react with Ti, forming intermetallic compounds, and are generally not used as interlayers or brazing materials.

Diffusion Welding of Nickel Alloys

Nickel alloys have excellent high-temperature resistance, corrosion resistance, and wear resistance. Their weldability is poor during fusion welding, and the toughness of the joint is much lower than the parent material, so diffusion welding is more often used for connection.

Because of the high-temperature strength of nickel alloys and the large deformation resistance, the welding temperature must be increased or the welding pressure must be increased during welding; the surface of nickel alloys contains oxide films of Ti and Al, and Ni also easily forms NiO at high temperatures.

These oxide films are quite stable, so the workpiece surface must be carefully prepared; during the welding process, the atmosphere must be strictly controlled to prevent surface contamination, and pure nickel is generally required as an interlayer.

During diffusion welding of nickel alloys, the direct diffusion welding method, adding interlayer diffusion welding method, or liquid phase diffusion welding method can be selected according to different alloy types and structural forms. The parameters for diffusion welding of nickel alloys are: heating temperature 1093~1204℃; holding time 10~120min; pressure 2.5~15MPa; vacuum 1.33×10-2Pa or above.

The actual welding parameters are related to the geometry of the component. To achieve satisfactory welding quality, it should be determined based on the results of the test.

Welding of High-Temperature Alloys

High-temperature alloys have high thermal strength and difficult deformation. They are also sensitive to overheating, so welding parameters must be strictly controlled to obtain welded joints that match the performance of the parent material.

When high-temperature alloys are diffusion welded, higher welding temperatures and pressures are required, with the welding temperature about 0.8~0.85Tm (Tm is the melting temperature of the alloy). The welding pressure is usually slightly lower than the yield stress of the alloy at the corresponding temperature.

When other parameters are unchanged, the larger the welding pressure, the larger the interface deformation, the larger the effective contact area, and the better the joint performance; but too high welding pressure will make the equipment structure complex and the cost expensive.

When the welding temperature is higher, the joint performance improves, but too high a welding temperature will cause grain growth and reduce plasticity. During solid-state diffusion welding of precipitation-strengthened high-temperature alloys containing a high amount of aluminum and titanium, precipitates such as Ti(CN) and Ni-TiO3 will form on the interface, thereby reducing the performance of the joint.

If a thin Ni-35%Co (mass fraction) interlayer alloy is added, a joint with uniform structure and performance can be obtained, and the effect of welding parameter changes on joint quality can be reduced. All types of high-temperature alloys, such as mechanized high-temperature alloys, cast high-temperature alloys containing high Al or Ti, etc., can almost all be solid-phase diffusion welded.

High-temperature alloys contain elements such as Cr and Al, and the surface oxide film is very stable and difficult to remove. Strict machining and cleaning must be performed before welding, and even surface plating is required before solid-phase diffusion welding.

In actual production, the determination of welding parameters should be based on the joint performance obtained from welding tests to select an optimal value or optimal range. The welding parameters commonly used for diffusion welding of the same material are shown in Table 3-7. The welding parameters for diffusion welding of the same material with an added interlayer are shown in Table 3-8.

Table 3-7 Welding Parameters Commonly Used for Diffusion Welding of Similar Materials

Serial NumberMaterials to be WeldedHeating Temperature
/℃
Holding Time
/min
Pressure
/MPa
Vacuum Degree
/Pa (or Protective Atmosphere)
12AI4 Aluminum Alloy5401804
2TC4 Titanium Alloy900 ~93060 ~ 901 ~21.33 × 10 -3
3Ti3Al Alloy960 ~980608 ~101.33 ×10 -5
4Copper800206.9Reducing Atmosphere
5H72 Brass75058
6Molybdenum1050516 ~ 401.33 ×10-2
7Niobium120018070 ~1001.33 ×10-3
8Nickel127310151.33 × 10-2
9GH304414736201.33 × 10-2
10GH4037134820201.33 ×10-2
11GH2130127310201.33 × 10-2

Table 3-8 Welding Parameters for Diffusion Welding of the Same Material with an Intermediate Layer

Serial NumberMaterials to be WeldedIntermediate LayerHeating Temperature
/°C
Holding Time
/min
Pressure
/MPa
Vacuum Degree
/Pa (or Protective Gas)
15A06 Aluminum Alloy5A025006035 ×10 -3
2AlSi58019.8
3H62 BrassAg + Au400 ~50020 ~300.5
412Cr18Ni9TiNi100060 ~ 9017.31.33 ×10 -2
5K18Ni-based High-Temperature AlloyNi-Cr-B-Mo1100120Vacuum
6GH141Ni-Fe117812010.3
7GH22Ni11582400.7~3.5
8GH188 Cobalt-Based Alloy97 Ni-3Be11003010
9Al2O3Pt15501000.03Air
1095 CeramicCu10201014 ~ 165 ×10 -3
11SiCNb1123 ~ 17906007.26Vacuum
12MoTi90010 ~2068 ~ 86
13MoTa9152068.6
14WNb9152070

Diffusion Welding of Dissimilar Materials

In actual production, to achieve certain usage properties or to reduce the weight of components, different metal materials often need to be welded together. Due to the significant differences in physical and chemical properties between different materials, it can be difficult to achieve high-quality joints using fusion welding.

Currently, diffusion welding is chosen based on the potential for mutual diffusion between the two different metal materials.

Diffusion Welding of Steel with Aluminum and Aluminum Alloys

The main issue when welding steel with aluminum and aluminum alloys is that Fe-Al intermetallic compounds easily form near the welding interface, reducing the strength of the joint. To achieve a good diffusion-welded joint, it is suggested to add an intermediary transition layer to get a firm joint. This transition layer can be a thin layer of metal, electroplated using various methods.

The composition of the intermediary layer can be chosen based on the alloy phase diagram and the potential new phases that may form at the interface, typically Cu and Ni are used. As Cu and Ni can form unlimited solid solutions, and Ni with Fe and Ni with Al can form continuous solid solutions, this can prevent the appearance of Fe-Al intermetallic compounds at the interface, thus improving the strength of the joint.

The welding parameters for diffusion welding of carbon steel, stainless steel with aluminum, and aluminum alloys are shown in Table 3-9.

Table 3-9 Welding Parameters for Diffusion Welding of Carbon Steel, Stainless Steel with Aluminum, and Aluminum Alloys

Dissimilar Metals Intermediary Layer Heating Temperature
/°C 
Insulation Time
/min
Pressure
/MPa
Vacuum Degree
/Pa
3A21 + Q235 Steel Electroplated Cu, Ni 5502 ~ 2013.71.33 × 10-4
1035 + Q235 Pot Ni 5502 ~ 1512.31.33 × 10-4
1071 + Q235 Steel Ni 350 ~ 450 5 ~ 152.2 ~ 9.81.33 × 10-3
1071 + Q235 Steel Cu450 ~ 500 15 ~ 2019.5 ~ 29.41.33 × 10-3
1035 + 12Cr18Ni9Ti50020 ~ 3017.56.66 × 10-4

Diffusion Welding of Steel and Titanium

When diffusion welding steel to titanium and titanium alloys, an intermediary layer or composite filler material should be added. Typically, intermediate layer materials are V, Nb, Ta, Mo, Cu, etc., and composite filler materials are V+Cu, Cu+Ni, V+Cu+Ni, as well as Ta and bronze. The welding parameters for diffusion welding between stainless steel and pure titanium TA7 can be found in Table 3-10.

Table 3-10: Welding Parameters for Diffusion Welding of Stainless Steel to Pure Titanium TA7

Dissimilar MetalsIntermediary Layer MaterialHeating Temperature
/℃
Hold Time
/min
Pressure
/MPa
Vacuum Level
/Pa
Notes
Cr25Ni15 + TA7700106.861.33 × 10-4At the interface of steel and titanium, there is an alpha phase.
Ta900108.821.33 × 10-4The tensile strength of the joint Rm=292.4MPa.
Ta11001011.071.33 × 10-4There are TaFe2 and NiTa compounds.
12Cr18Ni10Ti + TA7900150.981.33 × 10-5Rm = 274 ~ 323MPa.
V900150.981.33 × 10-5Rm = 274 ~ 323MPa.
V+ Cu900150.981.33 × 10-5Intermetallic compounds are present.
V + Cu + Ni100010~154.91.33 × 10-5Intermetallic compounds are present.
Cu + Ni100010~154.91.33 × 10-5Intermetallic compounds are present.

Diffusion Welding of Steel and Copper

Diffusion Welding of Steel and Copper

During the diffusion welding of steel to copper, when welding at a temperature of 750°C and holding for 20~30min, the presence of eutectic in the diffusion weld joint can be observed through metallographic analysis.

Hence, when diffusion welding steel to copper, the welding parameters like temperature and time must be strictly controlled to limit the thickness of the eutectic brittle phase at the interface to no more than 2~3μm, otherwise, the entire welding interface will become brittle.

The welding parameters for diffusion welding of steel to copper are as follows: heating temperature 900°C; insulation time 20min; pressure 5MPa; vacuum degree 1.33×10-3~1.33×10-2 Pa. To enhance the strength of the diffusion weld joint between steel and copper and their alloys, Ni can be used as an intermediary transition layer. Ni can form an unlimited continuous solid solution with Fe and Cu.

When the heating temperature is above 900°C and the holding time is more than 15min, a diffusion weld joint with the same strength as copper can be formed. The specific welding parameters for diffusion welding of steel to copper can be found in Table 3-11.

Table 3-11 Welding Parameters for Diffusion Welding of Steel and Copper

Serial NumberWelding MaterialsInterlayerHeating Temperature
/°C
Holding Time
/min
Pressure
/MPa
Vacuum Degree
/Pa
1Cu + Low Carbon Steel850104.9
2Kovar Alloy + Copper850 ~ 950104.9 ~ 6.81.33 × 10-3
3Stainless Steel + Copper9702013.7
4Cu + Cr18-Ni13 Stainless SteelCu98220
5QCr0.8 + High Cr-Ni Alloy900101
6QSn10-10 + Low Carbon Steel720104.9

Precision components such as friction pairs and stop disks in airplane engines require tin bronze and steel to be welded together. This type of material tends to form porosity when fused and brazing methods can lower the corrosion resistance of the joint, hence diffusion welding is often used.

Copper and Aluminum Diffusion Welding

Key factors influencing the quality and stability of copper and aluminum diffusion welding are heating temperature, welding pressure, holding time, vacuum degree, and the preparation state of the workpiece surface. Before welding, the workpiece surface must be finely processed, leveled and degreased.

The aluminum oxide film on the surface of the aluminum material must be removed to ensure the surface is as clean and free of impurities as possible. Since aluminum has a low melting point, the heating temperature during welding cannot be too high, otherwise the joint’s toughness will decrease due to grain growth in the base material. Influenced by the thermal properties of aluminum, the pressure cannot be too high.

When below 540℃, the strength of the Cu/Al diffusion welded joint increases with the rise in heating temperature. However, if the temperature continues to rise, the joint’s toughness decreases as an eutectic of Al and Cu forms near 565℃. A diffusion welding pressure of 11.5MPa for Cu/Al can prevent interface diffusion porosity.

With constant heating temperature and pressure, extending the holding time to 25~30 minutes significantly improves joint strength. If the holding time is too short, the Cu and Al atoms won’t have sufficient time to diffuse fully, preventing the formation of a firmly bonded diffusion welding joint.

However, a lengthy holding time can cause grain growth in the Cu/Al interface transition zone and increase the thickness of intermetallic compounds, reducing joint toughness. At a heating temperature of 510~530℃, a diffusion time of 40~60 minutes, and a pressure of 11.5MPa, the joint interface bonds well.

Based on the microhardness test results of the Cu/Al diffusion welding joint, intermetallic compounds may form in the copper-side transition zone. At high temperatures, Al and Cu can form various brittle intermetallic compounds.

At 150℃, CuAl2 forms at the onset of reaction diffusion; at 350℃, an additional layer of Cu9Al4 compound appears; at 400℃, a CuAl layer appears between CuAl2 and Cu9Al4. When the thickness of the intermetallic compound layer reaches 3~5μm, the strength of the diffusion welding joint decreases significantly.

The welding parameters for copper and aluminum diffusion welding should be determined based on the actual conditions. For vacuum electrical components, the welding parameters are: heating temperature of 500~520℃; holding time of 10~15min; pressure of 6.8~9.8MPa; vacuum degree of 6.66×10-3Pa. At a pressure of 9.8MPa, the interface bonding rate of the diffusion welding joint can reach 100%.

Copper and Titanium Diffusion Welding

Copper and titanium diffusion welding can use direct diffusion welding or diffusion welding with an intermediate layer. The former has low joint strength, while the latter has high strength and some plasticity. When directly diffusion welding copper and titanium without an intermediate layer, the welding process should be completed in a short time to avoid the formation of intermetallic compounds.

The welding parameters for direct diffusion welding of copper and pure titanium TA2 are: heating temperature of 850℃; holding time of 10 minutes; pressure of 4.9MPa; vacuum degree of 1.33×10-5Pa. Although this temperature is lower than that for forming eutectic, the joint strength is not high and is lower than the strength of copper.

Surface cleanliness greatly influences the quality of diffusion welding. Before welding, copper parts should be cleaned with trichloroethylene to remove grease, then etched in a 10% H2SO4 solution for 1 minute, followed by rinsing with distilled water. Subsequent annealing treatment should be performed at 820~830℃ for 10 minutes.

After cleaning the titanium base material with trichloroethylene, it should be etched in a water solution of HF (2% by mass) and HNO3 (50% by mass) with ultrasonic vibration for 4 minutes to remove the oxide film, then cleaned thoroughly with water and alcohol.

Adding Mo and Nb intermediate layers between copper (T2) and titanium (TC2) can inhibit interfacial reactions between the welded metals, preventing the formation of low-melting-point eutectics and brittle intermetallic compounds. This greatly improves joint performance. The welding parameters and tensile strength of the copper and titanium diffusion welding joint are shown in Table 3-12.

Copper and Nickel Diffusion Welding

Diffusion welding of copper and nickel is widely used in vacuum device manufacturing. Copper and nickel and their alloys have good plasticity and can form continuous solid solutions during the diffusion process, improving the quality of the welded joint. The welding parameters for vacuum diffusion welding of copper and nickel and nickel alloys are shown in Table 3-13.

Table 3-12 Welding Parameters and Tensile Strength of the Copper (T2) and Titanium (TC2) Diffusion Welding Joint

Intermediate LayerWelding Temperature
/℃
Holding Time
/min
Pressure
/MPa
Vacuum Level
/Pa
Tensile Strength
/MPa
800304.91.33 × 10-462.7
8003003.41.33 × 10-4144.1 ~ 156.8
Mo (Sprayed)950304.91.33 × 10-478.4 ~ 112.7
9803003.41.33 × 10-4186.2 ~ 215.6
Nb (Sprayed)950304.91.33 × 10-470.6 ~ 102.9
9803003.41.33 × 10-4186.2 ~ 215.6
Nb (0.1mm Foil)950304.91.33 × 10-494.2
98030
0
3.41.33 × 10-4215.6 ~ 266.6

Table 3-13 Welding Parameters for Vacuum Diffusion Welding of Copper to Nickel and Nickel Alloys

Materials to be WeldedJoint FormProcess Parameters
Welding Temperature
/℃
Holding Time
/min
Pressure
/MPa
Vacuum Level
/Pa
Cu + NiButt Joint400209.81.33 x 10-4
Butt Joint90020 – 3012.7 – 14.76.67 x 10-5
Cu + Nickel AlloyButt Joint90015 – 2011.761.33 x 10-5
Cu + Machinable AlloyButt Joint950101.9 – 6.91.33 x 10-4

Diffusion Welding of Ceramic Materials

Diffusion welding technology is extensively used in the welding of ceramic materials. The primary advantage of diffusion welding of ceramics is its high welding strength, easy control of size, and suitability for welding different materials. Its drawbacks, however, include high welding temperatures, long welding durations, and the need for a vacuum environment, which leads to higher costs and size and shape limitations of the test pieces.

The most commonly used ceramic materials are alumina and zirconia ceramics, which are welded to metals like copper (oxygen-free copper) and titanium (TAl). Ceramics such as Al2O3, SiC, Si3N4, and WC have been studied and developed for welding earlier, while ceramics like AlN and ZrO2 have been developed relatively late.

Diffusion welding of ceramic materials can occur in the following situations:

1) Direct welding of the same type of ceramic material.

2) Welding the same type of ceramic material using another thin-layer material.

3) Direct welding of different types of ceramic materials.

4) Welding different types of ceramic materials using a third thin-layer material.

Main Issues in Diffusion Welding of Ceramic and Metal Materials

a) Significant thermal stress at the interface

During the welding of ceramic and metal materials, due to the significant difference in thermal expansion coefficients of ceramics and metals, thermal stress inevitably occurs during the diffusion welding process. This uneven distribution of thermal stress causes stress concentration at the joint interface, thereby reducing the load-bearing performance of the joint.

Factors influencing the thermal stress of the joint include material factors such as linear expansion coefficient, elastic modulus, Poisson’s ratio, porosity, yield strength, and work hardening coefficient, as well as structural factors such as thickness, width, length, number of welding material layers, layer arrangement order, interface shape, and surface roughness, and temperature distribution factors such as heating method, temperature, and cooling rate.

b) Tendency to form brittle compounds

Due to the significant difference in physical and chemical properties between ceramics and metals, various chemical reactions can easily occur during welding, resulting in the formation of carbides, nitrides, silicides, oxides, and other complex compounds at the interface. These compounds, with their high hardness and brittleness, are the main reasons for the brittle fracture of the joint.

c) Difficulty in quantitative analysis of interface compounds

When determining the interface compounds, the quantitative analysis of light elements like C, N, B, etc., introduces a large error, requiring the preparation of various standard specimens for calibration. The determination of the phase structure of complex compounds is generally done by comparing with the X-ray diffraction standard spectrum, but the lack of standards for some new compound phases makes it difficult to determine the phase.

Diffusion Welding Process of Ceramic and Metal Materials

In the diffusion welding of ceramic and metal materials, the surfaces to be welded must be very smooth and clean. The welding can be done in a vacuum or in a H2 atmosphere. The metal surface is more likely to chemically react with the ceramic/metal when there is an oxide film on the surface.

Therefore, a reducing active medium must be introduced in the vacuum chamber to maintain a thin oxide film on the metal surface to give the diffusion weld joint higher strength. Because ceramics are hard and strong, and not easily deformed, pressure must be applied during diffusion welding.

The pressure range is 0.1~15MPa; the welding temperature is also higher, usually 90% of the metal’s melting point; the welding time is longer than other welding methods.

The strength of the joint in the diffusion welding of ceramic and metal primarily depends on the heating temperature, holding time, applied pressure, environmental medium, surface condition of the welded interface, and the chemical reaction and physical properties (such as linear expansion coefficient) matching between the materials being welded.

a) Heating Temperature

The heating temperature has the most significant impact on the diffusion process. When welding ceramics and metals, the temperature usually reaches 90% of the metal’s melting point. During solid-phase diffusion welding, the chemical reaction layer caused by the mutual diffusion of elements can promote the formation of interface bonding.

When welding steel and alumina ceramics with a 0.5mm thick aluminum intermediate layer, the tensile strength of the diffusion welded joint increases with the increase in heating temperature. However, too high a welding temperature can alter the properties of the ceramics or result in brittle phases near the interface, thus reducing the joint’s performance.

The tensile strength of the ceramic and metal diffusion welded joint is related to the melting point of the metal. When diffusion welding alumina ceramics and metal, as the metal’s melting point increases, the joint’s tensile strength also increases.

b) Holding Time

Holding time is one of the significant factors affecting the strength of the diffusion welded joint. The relationship between tensile strength (Rm) and holding time (t) is Rm=B0t1/2, where B0 is a constant. However, at a certain test temperature, there exists an optimal value for holding time. In Al2O3/Al joints, the effect of holding time on the joint’s tensile strength is shown in Figure 3-11.

Figure 3-11: Impact of Holding Time on the Tensile Strength of the Joint

When using Nb as an intermediate layer for diffusion welding SiC/18-8 stainless steel, excessive holding time results in the appearance of Nb-Si2 phase, which has a significantly different linear expansion coefficient from SiC, thus reducing the joint’s shear strength, as shown in Figure 3-12.

Figure 3-12: Impact of Holding Time on the Shear Strength of the SiC/Nb/18-8 Stainless Steel Joint

When using V as an intermediate layer for welding AIN, if the holding time is too long, the appearance of the brittle phase V5Al8 will also reduce the joint’s shear strength.

c) Welding Pressure

The pressure applied during the diffusion welding process is to induce plastic deformation at the contact interface, reduce surface unevenness, and break the surface oxide film, thereby increasing the surface contact area to facilitate atomic diffusion. To prevent significant deformation of the components, the pressure applied during the diffusion welding of ceramics and metals is generally small, ranging from 0.1~20MPa.

This pressure range is usually sufficient to reduce surface unevenness, break the surface oxide film, and increase the surface contact area. When the pressure is low, increasing the pressure can improve the joint strength, as shown in Figure 3-13 for Cu or Ag welding Al2O3 ceramics, and Al welding SiC.

Figure 3-13: Impact of Welding Pressure on the Shear Strength of the Joint

Like the effects of heating temperature and holding time, after increasing the pressure, there also exists an optimal pressure value or range for obtaining the best strength. For example, when using Al to weld Si3N4 ceramics, and Ni to weld Al2O3 ceramics, the optimal pressures are 4MPa and 15~20MPa, respectively.

(4) Interface Bonding Condition

The surface roughness significantly affects the strength of diffusion welded joints, as surface roughness can lead to localized stress concentration at the ceramic/metal interface, making it prone to brittle failure.

During solid-phase diffusion welding of ceramics and metals, the contact interface undergoes a reaction to form compounds. The types of compounds formed depend on the welding conditions, such as temperature, surface condition, and types and amounts of impurities. Different diffusion conditions yield different reaction products, resulting in significant variations in joint performance.

Generally, the strength of joints made by vacuum diffusion welding is higher than those welded in argon gas or air. The possible compounds that may occur in various ceramic/metal joints are shown in Table 3-14.

When Si3N4 ceramics are directly diffusion welded under high temperature conditions (1500℃), the high-temperature Si3N4 ceramics are prone to decomposition, forming pores. However, welding in a nitrogen atmosphere can limit the decomposition of ceramics, and when the nitrogen partial pressure is high, the joint’s flexural strength is higher.

The flexural strength of a joint welded in 1 MPa nitrogen is about 30% higher than that of a joint welded in 0.1 MPa nitrogen.

Table 3-14: Possible Compounds that May Occur in Various Ceramic/Metal Joints

Joint StructureInterface Reaction ProductsJoint CombinationsInterface Reaction Products
Al2O3/CuCuAlO2、CuAl2O4Si3N4 – AlAlN
Al2O3/TiNiO · Al2O3、NiO · SiAl2O3Si3N4 – NiNi3Si, Ni (Si)
SiC/NbNb5Si3、NbSi2、Nb2C、Nb5Si3Cx、NbCSi3N4 – Fe-Cr AlloyFe3Si, Fe4N, Cr2N, CrN, FexN
SiC/NiNi2SiAlN – VV(Al), V2N, V5Al8, V3Al
SiC/TiTi5Si3、Ti3SiC2、TiCZrO2 – Ni, ZrO2 – CuNo new phases were detected

(5) Selection of Intermediate Layer

When ceramics and metals are difficult to join directly using diffusion welding, an intermediate layer can be employed. The use of an intermediate layer in diffusion welding can lower the heating temperature, reduce pressure, and shorten the heat preservation time. This promotes diffusion and the removal of impurity elements while also reducing the residual stress at the interface.

The impact of the intermediate layer can sometimes be complex. If a reaction occurs at the interface, the effect of the intermediate layer can vary depending on the type and thickness of the reaction product.

The choice of the intermediate layer is crucial, and an inappropriate selection can lead to a deterioration in joint performance. For instance, the bending strength of the joint might be reduced due to the formation of brittle reaction products from intense chemical reactions, or the residual stress might increase due to a mismatch in the coefficient of linear expansion, or the corrosion resistance of the joint might decrease.

The intermediate layer can be applied directly using metal foil, or methods such as vacuum evaporation, ion sputtering, chemical vapor deposition (CVD), spraying, electroplating can be used. It can also be implemented using sintered metal powder, active metalization, metal powder, or brazing materials, all of which can achieve diffusion welding.

The welding parameters of Al2O3 ceramic diffusion welding and the tensile strength of the joint are shown in Table 3-15.

Table 3-15: Welding Parameters of Al2O3 Ceramic Diffusion Welding and Tensile Strength of the Joint

Intermediate Layer MaterialWelding Temperature
(°C)
Welding Pressure
(MPa)
Tensile Strength
(MPa)
Intermediate Layer MaterialWelding Temperature
(°C)
Welding Pressure
(MPa)
Tensile Strength
(MPa)
Al400200<1.0Ti1250200>65
Al450200<0.15Steel7502001.0 
Al5502019Steel8004001.0 
Al6301065Steel11005040
Al7005020 (Melted)Steel13001052
Al6002087Steel130050105
Al60020104Steel130050150
Ti10002002.0 Steel130050150

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