Diffusion Bonding is a metalworking technique in which two metals are joined together by applying dynamic load and heat in controlled conditions. The diffusion of atoms at the interface of two joining metals takes place within these conditions which lead to uniform grain structure joint closing the interfacial voids leading to solid-state bonding.
Why Diffusion Bonding?
Diffusion bonding, used to join two similar metals, is a preferred choice over other metalworking techniques due to its superior bonded surface finish, especially when intricate structures with fine boundaries are involved. Some of the common techniques used in industry and their comparison with diffusion bonding include:
|Technique||Principle Method||Profile Modification|
|Welding||High-temperature melting and diffusion of two joining metals using filler materials||Yes, melting deforms weld joint surface leading to profile modification.|
|Brazing||Similar to welding but fusion between two joint metals does not take place only filler material is melted between the joining metals to form a bond.||Yes, heat and melted filler material will modify the surface profile at joining faces.|
|Soldering||Joining of two metals by using third low melting point material which holds two metals together after hardening.||Yes, the melted filler can seep in between and over two joining faces.|
|Mechanical Fastening||Riveting or Nut/Bolt fastening is used where permanent joints are not needed but require a spacious design to accommodate the holes for rivets or bolts. The holes are also stress raisers.||No, Joint faces do not undergo any modification.|
|Adhesive Bonding||Adhesives are used to glue metal together, these glues could be epoxy or plastic agents and when applied between two metals that are then held under pressure the glue cures to develop a strong bond.||Yes, Glues are prone to creep into uneven surfaces and can’t be used for intricate joints with the varying surface feature.|
|Diffusion Bonding||Joint faces are kept under pressure and heated to diffuse the atoms across the boundaries of joining faces and hence result in strong bonding.||No, Metal surfaces with intricate features can be joined this way and hence can be used for the precision joining of metals with intricate features/flow paths.|
In diffusion bonding two metals are joined without liquid fusion, melting, filler materials, or loss of weight and hence the characteristic metallurgical features of the parent metal do not change and the bond so developed is stronger. Diffusion bonding is extremely useful for manufacturing those components that have intricate internal structure or flow paths that will be difficult to machine using conventional manufacturing process when made from single-parent metal block. The component is designed in individual layers that can be etched separately, and in some cases combined with machined layers. These layers are then stacked to be diffusion bonded, creating a mutidemetional component.
Diffusion Bonding Process
As mentioned above, diffusion bonding is solid-state bonding that means two surfaces that are to be bonded together do not undergo liquid state transition in the process of fusion and hence the process steps must be followed meticulously and when done so the bond is of high quality. The surface preparation, the pressures to be applied, the temperature to be maintained, the hermetic seal requirements and the environment in which to carry out the entire process are critical factors. The simplified steps of diffusion bonding are:
- Surface Preparation- Ensure that the two surfaces that are being joined are smooth, debris-free, flat, and de-greased. Good surface preparation is essential.
- Pressure Loading- The two surfaces are then put together and pressurized to predetermined levels depending on the requirements of the bonded component, and the metallurgical limitations of the metals. This is done in an environment-controlled chamber, in which the process initiates by creating a vacuum to purify the process environment.
- Dynamic Heating- The metals are then heated to predetermined levels (lower than the melting point of parent metals) to accelerate the atoms fusion across the boundaries of joining surfaces. The uniformity of temperature is crucial to ensure fusion equilibrium over the whole of the bonding surfaces.
When the process variables are precisely controlled the resultant bond’s strength and ductility matches that of parent metals and when put under tensile test the component should fail at parent metal and not at the bonded joint. The good diffusion bonded component cross-section should be structurally indistinguishable from the bond line and should be void-free.
Stages of Diffusion Bonding
During the entire controlled process of diffusion bonding, the bonding faces undergo the following four key stages:
- Initial contact of parent metals.
- Plastic deformation of asperities under pressure load due to creep.
- Voids disappear as grain boundary diffusion under high temperatures. Grain diffusion plays a major role in plastic deformation.
- Elimination of pores by volumetric diffusion.
Application of Diffusion Bonding
Diffusion bonding is used across multiple industries for its ease in manufacturing precision components with complicated internal features. Diffusion bonding can be used primarily with similar metals, metals of different thicknesses, and reactive metals. Common applications are in manufacturing of:
- Miniature Heat Exchangers
- Fuel Cells
- Medical Devices
- Electrical Components
- Fluid Mixing Valve
The high precision offered by diffusion bonding is of the value +/-0.002” which enables its application in the manufacturing of critical components for various industrial applications such as:
- Aerospace (Avionics/Space)
- Electrical Equipment
- Scientific Equipment
- Marine (Sensors/Equipment)
- Electronic Fibers
VACCO offers a complete solution to all these industries from photo etching to diffusion bonding with its expert team of engineers and R&D team that works closely with clients to ensure a high-quality product. VACCO’s photo etching and diffusion bonding capabilities can handle thickness that ranges from 0.001” through 10”, a variety of materials including titanium and iron-nickel alloy. The use of ceramic or molybdenum materials to make fixtures ensures a high-quality finish of the product. The in-house welding and liquid penetrant inspection processes comply with the Performance Review Institute and national Aerospace and Defense Contractors Accreditation Program. VACCO’s Precision Etched Parts group is equipped with an in-house vacuum furnace to handle all your bonding needs. The Vacuum furnace consists of a 12” wide, 12” high and 12” deep hot zone capable of handling temperatures up to 1315℃ . For more information and queries related to any of your bonding needs please reach out to us!