Diffusion Brazing

Diffusion brazing or bonding is a joining processor which joins a wide range of similar or dissimilar metals, alloys, ceramics & non-metals ultimately producing small and large components. The form of pressurization, the use of inter layers & the formation of a transient liquid phase are the variants on which the brazing method is dependent upon. Accordingly, for joining the varied materials range & geometries, these find their specific application.

Furthermore, this particular brazing method can even function as a viable joining method for more components of aerospace structures wherein the researches are going on to determine its usefulness & functional aspects.

Parameters for Diffusion Brazing
Diffusion brazing do require certain parameters which determines its execution. These are primarily categorized into :

• Time
• Applied pressure
• Bonding temperature
• Heat application method

Diffusion Brazing & Diffusion Bonding
As its already clear, diffusion brazing is also known as diffusion bonding. But certain differences are there which further clears their characteristics.

• Solid Phase Diffusion Bonding
  Bonding done in a solid phase is known as Diffusion Bonding. This is solid in form, since, its execution is done in a vacuum or a protective atmosphere. By means of radiant, induction, direct or indirect resistance that the heat is applied. Using solid phase to join dissimilar materials such as metal to ceramic joints, the bonding process is aided by single or multiple inter layers of other materials apart from modifying post-bond stress distribution.
  
  The solid phase requires certain amount of pressure which can be applied either uniaxially or isostatically. Uniaxially require a low pressure of about 3-10MPa to prevent any macro deformation of the parts, therefore, it is essential to provide a good surface finish on the mating surfaces. The surfaces in such a situation should be clean to minimize surface contamination. The contribution provided by plastic yielding to bonding gets restricted. The pressure is much higher in case of hot isostatic pressing.
  
  Solid phase process has the advantage to make use of uniform gas pressurization which allows the bonding of complex geometries.
  
  
• Liquid Phase Diffusion Brazing
  Diffusion brazing are basically in a liquid phase format. Functional only to dissimilar material combinations or alike materials, a dissimilar metal insert is basically applied. The bonding temperature is selected at the point where the solid phase melts when there is a change of composition at the bond interface by the sold diffusional process. With the alternate insertion of dissimilar metal, it melts at a lower temperature than the parent material thereby creating a thin layer of liquid spreads along the interface which leads to the formation of a joint at a lower temperature than the melting point of either of the parent materials. By holding at that specific temperature, this phase can be diffused away into the parent material when there is a reduction in the bonding temperature leading to the solidification of the melt. For bonding materials like aluminum alloys where eutectics is formed with copper, silver or zinc which assist in the break-up of the stable aluminum surface oxide, such a technique is applied.

Alloy Systems for Diffusion Brazing : Characteristics
There are certain characteristics & features which an alloy system should be including in order to be perfect for diffusion brazing. These are basically categorized into the following pointers :

• The joint design & joining process should be kept quiet simple by basing it on a simple alloy binary or ternary system.
  
  
• There should be a comparatively low melting point eutectic reaction in the phase construction. It helps in initiating the melting process.
  
  
• There should be a few inter metallic compounds & the ones which exist should be stable over a wide range of composition. This melts at a low temperature & therefore reduces the tendency for diffusion barriers towards becoming established that would obstruct the solidification process & the advance growth of brittle phase in the joint.
  
  
• There should be a vast range of solid solubility of other constituents in the terminal primary metal phase.

Working Procedure of Diffusion Brazing
In a diffusion brazing method there are several methods to be followed for carrying out the process in a successful way.

• Atleast two materials are pressed together most probably in a vacuum at a specific bonding pressure using a bonding temperature for a particular holding time.
  
  
• Bonding temperature
  • Requires melting temperature of 50%-70% of the most fusible metal in the composition.
  • The inter diffusion of atoms across the joint face is aided by the increased temperature.
  
  
• Bonding pressure
  • The contact between the edges of two materials to be joined are closed with forces close.
  • To fill the voids within the weld zone, it deforms the surface asperities.
  • The clean surface provided after dispersing the oxide films on the materials helps the diffusion and coalescence of the joint.
  
  
• Holding time
  • It is always minimized which has :
    • Time minimization lessens the physical force on the machinery.
    • Lessens the diffusion bonding process cost.
    • Voids might be created in the weld zone if there is a long holding time. It might even change the metal's chemical composition or when joining of dissimilar metals or alloys, it may lead to the formation of brittle inter metallic phases
  
  
• A ceramic to metal diffusion bonding sequence
  • Contacting of hard ceramic and soft metal edges.
  • High local stresses lead to the yielding of metal surface.
  • Void shrinkage due to continuation of deformation in the metal.
  • Formation of the bond.

Advantages of Diffusion Brazing
The plus points of diffusion brazing have been highlighted here :

• Parent materials' properties remains unchanged generally.
• Similar or dissimilar bonding of metal & non metals by this process.
• High quality formation of joints by diffusion bonding.
• Natural leads towards automation.
• No production of dangerous gases, ultraviolet radiation, metal spatter or fine dusts.
• No requirements for expensive solders, special grades of wires or electrodes, fluxes or shielding gases.