Diffusion-welding:

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Diffusion-welding, under the nonstandard name of Diffusion Bonding, was anticipated in a short note in section 2 of Issue 10 of Practical Welding Letter (PWL#010) for June 2004.

The term was reminded again in section 6 of Issue 36 (PWL#036) for August 2006.

Click on the Links to see them.

In this dedicated page we intend to explore the process in more depth, presenting additional information to provide a better understanding of its uses and applications.

Diffusion-welding

Diffusion-welding is a solid state process controlled by diffusion, that produces a weld between the intended members by the long time application of considerable pressure at elevated temperatures.

The characteristic features of this process are that it does not introduce macroscopic deformations or relative motion in either of the welded parts and that it does not melt base metals.

Advantages:

  • This solid state process avoids pitfalls of fusion welding
  • Dissimilar materials welds are possible
  • Properties and microstructures remain similar to those of base metals
  • Multiple welds can be made in one setup at the same time
  • Produces a product finished to size and causes minimal deformation
  • Presents less shrinkage and stresses compared to other welding processes
  • Highly automated process does not need skillful workforce

Limitations:

  • Costly equipment especially for large weldments
  • Costly preparation with smooth surface finish and exceptional cleanliness
  • Protective atmosphere or vacuum required
  • Long time to completion
  • Not suited to high production rates
  • Difference in thermal expansion of members may need special attention
  • Limited nondestructive inspection methods available


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Process

The Diffusion-welding process consists in bringing together the smoothed surfaces to be diffusion welded after having eliminated all contaminants and surface oxides.

Then pressure is gradually applied and temperature is elevated to permit diffusion at the atomic level.

Local deformation at the contact points by yield and creep permits increasingly larger areas to touch.

Then diffusion causes the interface to disappear slowly while the remaining voids between the original surfaces shrink or are absorbed within the grains.

Finally the interface cannot be seen any more (in a metallographic section) and residual voids, if any, result no larger or frequent than those of the base metals.

Variables

Temperature is the most important variable in Diffusion-welding.

It should be selected and controlled so as not to interfere with metallurgical changes or transformations that may occur in the materials.

Time needed to perform atomic diffusion is temperature dependent.

Longer times become less and less effective. The time needed cannot be determined simply but has to be found experimentally.

Once welding is performed, longer times will not add to the properties.

Pressure directly affects the outcome of Diffusion-welding and its importance is great, especially in the initial stages of the process.

It can be linked to the yield point of the metals involved but it is difficult to deal in theory as a predetermined value.

Although local deformation is introduced at the contact point as an essential stage in the process, macroscopic deformations are avoided.

Pressure is generally limited to the minimum required to get good results, because of the high equipment costs associated with high compression.

Pressure and temperature are practically selected so that they will permit the performance of suitable welds in acceptable time.

Although filler metal is in principle not required for Diffusion-welding, it has been found that a foil of suitable materials placed at the interface can sometimes facilitate the process.

The reasons are for providing a soft layer to maximize surface contact in the first stage, or to avoid the formation of brittle compounds, or to promote diffusivity, or to scavenge impurities.

This may result in the opportunity to reduce one or more of the three essential variables (Pressure, Temperature, Time), with consequent economic gain.

Equipment

Most of the equipment, especially tooling, has to be built specially for the items to be welded.

Presses or autoclaves should be adapted for providing the required atmosphere and the needed heat source to the parts, sometimes embedded in ceramic molds.

Materials

Can be Diffusion-welded: Titanium alloys, nickel alloys, aluminum alloys, as well as different combinations of materials not easily joined by traditional means.

Steels are preferably welded by alternative more economic methods.

But large, flat surfaces of low carbon steel have been Diffusion-welded without filler metal under the proper conditions.

An Article on A new method of brazing stainless steel parts was published (7)in Issue 151 of Pracctical Welding Letter for March 2016.
Click on PWL#151.

An Article on Field Assisted Sintering was published (7) in Issue 155 of Pracctical Welding Letter for July 2016.
Click on PWL#155.

Watch the following Video

Diffusion Bonding Process Illustrated

http://www.youtube.com/watch?v=5EhMYMr834o

For further information you may wish to watch a Lecture about diffusion bonding, by Dr Amir Shirzadi at
University of Cambridge.

See also images in:
Diffusion bonding
Material.

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When requirements are high, limitations are tough, and cost is not an obstacle, Diffusion-welding can be the best process for the application.