Understanding Diffusion Welding: A Comprehensive Guide

Ti(C,N) based metal ceramics, known for their high strength, hardness, wear resistance, and heat resistance, are widely used in the manufacture of cutting tool blades. However, due to the brittle nature of metal ceramics, their higher melting point compared to metals, and the significant difference in their linear expansion coefficient, these ceramics show poor compatibility with metals, making conventional welding methods ineffective.

Alternatively, high-temperature vacuum diffusion welding can reliably connect metal ceramic blades with steel handles. Tests of high-temperature vacuum diffusion welding of metal ceramic cutting edges and 40Cr knife bodies show good joint performance after welding with 40Cr. By tempering 40Cr, the tensile strength at the welding interface can reach 650MPa, and the shear strength can reach 550MPa.

Principles and Characteristics of Diffusion Welding

Diffusion Welding (DFW) is a solid-state welding method that involves placing tightly contacted weldments in a vacuum or protective atmosphere and maintaining them at a certain temperature and pressure for a period of time. This allows the atoms at the contact interface to diffuse into each other, achieving a reliable connection.

Diffusion welding is particularly suitable for joining dissimilar metals, heat-resistant alloys, ceramics, intermetallic compounds, composites, and other new materials. Especially for materials that are difficult to weld using fusion welding methods, diffusion welding has significant advantages and is attracting increasing attention.

Currently, diffusion welding is widely used in the aerospace, aeronautics, instrumentation, and electronics sectors of the defense industry, and is gradually expanding into machinery, chemical, and automobile manufacturing fields.

Principles of Diffusion Welding

During diffusion welding, two or more weldments are tightly pressed together and placed in a vacuum or protective atmosphere. They are heated to a temperature below the parent material’s melting point and then pressure is applied. This shatters the oxide film on the surface and the microscopic protrusions on the surface undergo plastic deformation and high-temperature creep, achieving close contact.

This activates the diffusion between the atoms at the interface, achieving bonding in several small areas. After maintaining the temperature for a certain period of time, these areas further expand through atomic diffusion. When the entire connection interface forms a metallic bond, the diffusion welding process is completed.

In reality, the surface to be welded always has microscopic unevenness, gas adsorption layers, oxide films, etc., and the crystal orientation of the surface of the weldment is different for different materials. These factors hinder the formation of metallic bonds between atoms at the contact points, affecting the stable progress of the diffusion welding process.

Therefore, appropriate process measures must be taken to address these issues during diffusion welding. Temperature, pressure, time, protective atmosphere, and vacuum conditions create conditions for atomic diffusion between metals and metallic bond formation.

The formation process of the diffusion weld seam is shown in Figure 3-1. For the sake of analysis and research, it is typically divided into three stages: physical contact, mutual diffusion and reaction, and growth of the joint layer.

Figure 3.1 Formation Process of Diffusion Weld Seam

a) Initial contact with unevenness
b) Physical contact stage
c) Mutual diffusion and reaction of elements stage
d) Joint layer growth stage

Before diffusion welding, materials are typically machined, ground, polished, and cleaned. However, no matter how the materials are processed, the surface of the processed materials is still rough on a microscopic level, often covered with an oxide film. When such solid surfaces contact each other without pressure at room temperature, the bonding surface is limited to a few protruding points, as shown in Figure 3-1a.

The first stage of diffusion welding is the physical contact stage. Under high temperature, pressure is applied to the weldments, causing plastic deformation at the contact points of the microscopic protrusions on the material surface.

This shatters the surface oxide film, increasing the contact area and flattening it. Metallic bonds are formed at the clean contact points, while the remaining uncontacted parts form micropores left on the interface, as shown in Figure 3-1b.

The second stage of diffusion welding is the mutual diffusion and reaction stage. Under high temperature, the microscopically uneven surface, under external pressure, experiences continuous atomic diffusion on the closely contacted interface, causing many micropores on the interface to disappear.

At the same time, the grain boundaries at the interface migrate away from the original interface, but many small micropores are still left within the grains, as shown in Figure 3-1c.

The third stage of diffusion welding is the growth stage of the joint layer. Atomic diffusion develops in depth, and the interface and micropores finally disappear to form new grain boundaries, achieving metallurgical bonding. The composition in the joint area finally becomes uniform, forming a reliable welding joint, as shown in Figure 3-1d.

During the welding process, the surface oxide film is not only damaged by plastic deformation but also removed or thinned by dissolution and spheroidizing aggregation.

The dissolution of the oxide is achieved through the diffusion of interstitial atoms into the metal parent material, and the spheroidizing aggregation of the oxide is realized through the diffusion caused by the excessive surface energy of the oxide film. Both must be completed under certain temperature and time conditions.

The three stages of the diffusion welding process do not have clear boundaries but are intertwined, even with some local overlap. It is difficult to accurately determine their start and end times. The welding area undergoes creep, diffusion, recrystallization, and other processes to finally form a solid-state metallurgical bond.

It can form solid solutions and eutectics, and sometimes it may produce intermetallic compounds, thus forming a reliable diffusion weld joint.

Characteristics of Diffusion Welding

Compared with other welding methods, diffusion welding has the following advantages:

1) During diffusion welding, the base material does not overheat or melt, making it possible to weld almost all metals or non-metals without reducing the properties of the welded material. It is particularly suitable for materials that are difficult to weld with fusion and other methods, such as reactive metals, heat-resistant alloys, ceramics, and composite materials.

For materials of the same kind with poor plasticity or high melting points, and dissimilar materials that do not dissolve or form brittle intermetallic compounds during fusion welding, diffusion welding is a suitable welding method.

2) Diffusion welding joints are of high quality, with microstructures and properties similar or identical to those of the parent material. There are no welding defects in the seam, nor is there an overheated structure and heat-affected zone. Welding parameters are easy to control precisely, and the quality and performance of the joint are stable in mass production.

3) The precision of the weldment is high, and the deformation is small. Because the pressure applied during welding is small and the workpiece is generally heated as a whole and cooled with the furnace, the overall plastic deformation of the weldment is very small, and the workpiece generally does not undergo mechanical processing after welding.

4) Large-section workpieces can be welded. Because the required welding pressure is not large, the tonnage of the equipment required for large-section welding is not high, which is easy to achieve.

5) It can weld workpieces with complex structures, joints that are not easy to approach, and workpieces with large differences in thickness. It can implement welding on many joints in the assembly at the same time.

The disadvantages of diffusion welding are:

1) The preparation and assembly quality of the weldment surface are required to be high, especially for the joint surface.

2) The welding thermal cycle time is long, and the production rate is low. Each welding takes a few minutes to several tens of hours. It can cause grain growth in some metals.

3) The initial investment in equipment is large, and the size of the welded workpiece is limited by the equipment, so it is impossible to carry out continuous mass production.

The comparison of diffusion welding with fusion welding and brazing in terms of process is shown in Table 3-1.

Table 3-1 Comparison of Diffusion Welding with Fusion Welding and Brazing in Terms of Process

Conditions/MethodsDiffusion WeldingFusion WeldingBrazing
Heating RangeWhole/PartialPartialWhole/Partial
Temperature60%~80% of the Parent Material’s Melting PointParent Material’s Melting PointAbove Brazing Material’s Melting Point
Surface PreparationImportantNot StrictImportant
AssemblyPreciseNot StrictNot Strict
Weldment MaterialsMetals, Alloys, Non-metalsMetals, AlloysMetals, Alloys, Non-metals
Dissimilar Materials ConnectionUnrestrictedLimitedUnrestricted
Tendency for CracksNonePresentWeak
Tendency for PorosityNonePresentPresent
Joint StrengthClose to Parent MaterialClose to Parent MaterialDepends on Brazing Material Strength
Joint Corrosion ResistanceGoodSensitivePoor

Types and Applications of Diffusion Welding

Types of Diffusion Welding

Based on the combination and pressurization methods of the welded materials, diffusion welding can be classified into same-material diffusion welding, different-material diffusion welding, diffusion welding with an intermediate layer, transition liquid-phase diffusion welding, superplastic forming diffusion welding, and hot isostatic pressure diffusion welding, etc. The main types and process characteristics of diffusion welding are shown in Table 3-2.

TypeProcess Characteristics
Same-Material Diffusion WeldingSame-material diffusion welding involves direct contact of the workpieces without an intermediate layer. This method requires high-quality surface preparation and the application of substantial pressure during the welding process. The resulting joint structure is essentially identical to the parent material. Metals like titanium, copper, zirconium, and iron, which have a high solubility in oxygen, are easiest to weld, whereas metals prone to oxidation like aluminum and its alloys or iron- and cobalt-based alloys containing aluminum, titanium, and chromium are difficult to weld.
Different-Material Diffusion WeldingDifferent-material diffusion welding is implemented through direct contact between dissimilar metal materials or between metals and non-metals like ceramics or graphite. Due to the differences in the physical and chemical properties of the two materials, the following problems may arise during welding:
1. Thermal stress on the interface due to different linear expansion coefficients.
2. Formation of low melting point eutectics or brittle intermetallic compounds due to metallurgical reactions at the interface.
3. The development of diffusion voids in the joint due to different diffusion coefficients.
4. Potential electrochemical corrosion of the joint due to differing electrochemical properties.
Diffusion Welding with Intermediate LayerDiffusion welding with an intermediate layer involves introducing an intermediate layer material, typically in the form of a foil, electroplated layer, spray coating, or vapor-deposited layer, between the surfaces to be welded. This layer, typically less than 0.25mm thick, is suitable for welding dissimilar materials that are difficult to weld or metallurgically incompatible.
Transition Liquid-Phase Diffusion Welding (TLP Method)Transition liquid-phase diffusion welding, also known as diffusion brazing, involves placing an intermediate layer metal with a melting point lower than that of the parent material between the workpiece surfaces. The intermediate layer metal melts under heat and low pressure, wetting and filling the entire joint gap to form a transition liquid phase. Through diffusion and isothermal solidification followed by a period of diffusion homogenization, a welded joint is formed.
Superplastic Forming Diffusion Welding (PF-DB)Superplastic forming diffusion welding combines superplastic forming with diffusion welding. It is suitable for materials exhibiting phase transformation superplasticity, such as titanium and its alloys. Thin-walled components can be superplastically formed first and then welded, or the process can be carried out in reverse order, depending on the design of the component.
Hot Isostatic Pressure Welding (HIP)Hot isostatic pressure welding is a type of diffusion welding that utilizes hot isostatic pressing technology. The workpiece is placed in a sealed vacuum box, which is then placed in a heating furnace filled with high-pressure inert gas. The welding is completed under the combined action of high temperature and pressure through the heating of electric elements and the application of an isotropic pressure to the workpiece by the pressure difference between the high-pressure gas and the vacuum in the box. This method, due to its uniform pressurization, is less likely to damage the components and is suitable for brittle-material welding. It allows for precise control of the dimensions of the welded component.

Scope of Application for Diffusion Welding

Diffusion welding is suitable for welding special materials or unique structures, which are widely employed in industries such as aerospace, electronics, and nuclear technology, making diffusion welding extensively applicable in these sectors. Many components in aerospace and nuclear engineering operate under extremely harsh conditions, such as high-temperature resistance and radiation resistance.

These components often feature unique structural shapes, such as hollow lightweight honeycomb structures, and are predominantly made of dissimilar material combinations. Diffusion welding becomes the preferred choice for manufacturing these components. Titanium alloys, known for their corrosion resistance and high strength-to-weight ratio, are extensively used in the structure of aircraft, missiles, satellites, and other flying vehicles.

Figure 3-2 shows the superplastic diffusion welding of typical titanium alloy structures. Aluminum and its alloys, which exhibit excellent heat transfer and dissipation properties, are used to make aluminum heat exchangers, solar water heaters, and refrigerator evaporators through diffusion welding.

Figure 3-2 Superplastic Diffusion Welding of Typical Titanium Alloy Structures

a) Single-layer reinforced component
b) Double-layer reinforced structure
c) Multi-layer sandwich structure (three layers)
1 – Upper mold seal plate
2 – Superplastic forming blank
3 – Reinforcing plate
4 – Lower mold
5 – Superplastically formed part
6 – Outer superplastic forming blank
7 – Unconnected coating area (Yttrium-based or Boron Nitride)
8 – Inner layer blank
9 – Superplastically formed two-layer component
10 – Middle layer blank
11 – Superplastically formed three-layer component

Diffusion welding can be used to weld a variety of heat-resistant steels and alloys, making it possible to manufacture high-pressure combustion chambers, engine blades, guide blades, and rotors for highly efficient gas turbines.

Diffusion welding can join non-ferrous metals with iron and steel materials. For example, steam turbines can be made from Ti and CoCrWNi heat-resistant alloys, and channels for rocket engine combustion chambers can be made from high conductivity oxygen-free copper and stainless steel.

Diffusion welding can be used to join non-metals such as ceramics, graphite, quartz, and glass with metal materials. For example, sodium-ion conductive glass can be welded to aluminum foil or wire to create electronic components.

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