The Role of Interlayer Materials in Diffusion Welding

Role and Performance Characteristics of the Interlayer

(1) Role of the Interlayer

To facilitate the diffusion welding process, reduce the diffusion welding temperature, shorten the diffusion welding time, reduce the pressure, and improve the joint performance, an interlayer can be added between the materials to be welded.

Adding an interlayer between the workpieces is an effective means of diffusion welding of dissimilar materials, especially for materials with significant atomic structure differences.

The interlayer can improve the contact state of the material surface, lower the preparation requirements for the welding surface, reduce the diffusion welding temperature and pressure, shorten the diffusion welding time, and avoid or reduce the tendency to form brittle intermetallic compounds, as well as other metallurgical problems caused by significant physical or chemical differences between the materials being welded.

(2) Performance Characteristics of the Interlayer

The interlayer is a pure metal with a lower melting point and better plasticity, such as Cu, Ni, Al, Ag, etc., or an alloy with a composition, physical or chemical properties close to the parent material and containing a small amount of low melting point elements that are easy to diffuse. The interlayer should contain elements that accelerate diffusion, such as Be, B, Si, etc.

The interlayer should not react with the parent material to produce brittle phases or eutectic phases that cause adverse metallurgical reactions and should not cause electrochemical corrosion problems at the joint.

Principles of Interlayer Selection

The interlayer can take various forms such as foil, powder, plating, ion sputtering, and spray coating. The thickness of the interlayer is generally tens of micrometers to shorten the time for homogenizing diffusion. An overly thick interlayer will remain in the interface area in a layered manner after welding, affecting the physical, chemical, and mechanical properties of the joint.

When the thickness of the interlayer is 30~100μm, it should be sandwiched between the surfaces to be welded in the form of a foil. Materials for the interlayer that cannot be rolled into a foil can be directly coated on the surface of the material to be welded by methods such as electroplating, vacuum vapor deposition, plasma spraying, etc. The thickness of the coating is only a few micrometers.

The thickness of the interlayer can be calculated and preliminarily selected based on the final composition, and then determined by trial correction. The interlayer material is a modified material with a lower alloy content than the parent metal, and pure metals are more commonly used.

In solid phase diffusion welding, soft pure metal materials are often used as the interlayer, and the commonly used materials include Ti, Ni, Cu, Al, Ag, Au, and stainless steel, etc. For example, Ni foil is used as the interlayer when diffusion welding Ti-based superalloys, and Ti foil is used when diffusion welding Ti-based alloys.

During liquid phase diffusion welding, in addition to the requirements for the interlayer to have the above properties, the interlayer should also have good wetting with the parent material, short solidification time, and contain elements that accelerate diffusion. For Ti-based alloys, a Ti-based interlayer containing Cu, Ni, Zr, etc. can be used.

For aluminum and its alloys, an Al-based interlayer containing Cu, Si, Mg, etc. can be used. For Ni-based substrates, the interlayer must contain B, Si, P, etc. In the diffusion welding of ceramics and metals, active metal interlayers such as V, Ti, Nb, Zr, Ni-Cr, Cu-Ti, etc. can be selected.

Anti-soldering Agents

During diffusion welding, to prevent the pressure head from sticking together with the workpieces or some areas between the workpieces, a sheet or powder anti-soldering agent needs to be added.

For example, when steel is diffusion welded with steel, synthetic mica sheets can be used to isolate the pressure head; when titanium is diffusion welded with titanium, a layer of boron nitride or yttrium oxide powder can be applied. The anti-soldering agent should have the following properties.

1) The melting point or softening point should be higher than the welding temperature.

2) It should have good high-temperature chemical stability and not react chemically with the workpiece, fixture, or pressure head at high temperatures.

3) It should not release harmful gases that contaminate the nearby surfaces to be welded and should not destroy the protective atmosphere or vacuum level.

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