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Diffusion Bonding Welding |
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Diffusion Bonding Welding - Diffusion * bonding or welding is a solid state process wherein coalescence of the faying surfaces is produced by the application of pressure and elevated temperatures to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion.
The process does not involve macroscopic deformation or relative motion of the parts. The process can join either like or dissimilar metals with or without the use of another material between.
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The of Diffusion Welding
Diffusion welding process involves two steps or stages:
1. Any surface to be diffusion welded or otherwise is never extremely smooth. It has a number of peak points and valleys. Moreover, this surface may have,
(i) An oxidized layer,
(iii) Absorbed gas, moisture or both.
The first stage is to achieve intimate metal to metal contact between the two pieces to be diffusion welded. This is done by the application of pressure that deforms the substrate roughness and disrupts and disperses the above mentioned surface layers and contaminants. The pressure applied in diffusion welding ranges from 350 to 700 kg/cm2.
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2. The second stage involves diffusion and grain growth to complete the weld and ultimately eliminate the interface formed in the previous stage. The second stage induces complete metallic bonding across the area of contact.
In order to increase diffusion rate, moderate heating temperatures (usually below 1100°C) are used.
Without applying heat if it takes many hours to perform a certain bonding, with the application of heat, the time element will be cut to a few hours or minutes.
(a) Two typical workpiece surfaces to be diffusion welded can be seen.
(b) The individual peaks and valleys (or asperities) which make up the roughness are deformed by the application of increasing pressure.
(c) At places where the surfaces move together under shear, the (oxide) mms * are disrupted and metal to metal contact takes place.
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The oxide films dissolve in the metals such as titanium, tantalum, columbium, zirconium etc. In aluminium they do not do so and thus the disruption process for the trapped films is spheroidization. This process leaves a few oxide particles along the weld line.
Both dissolution and spheroidization require diffusion to complete.
(d) After metal to metal contact is established, the atoms are within the attractive force fields of each other (and hence a high strength joint is generated). The joint resembles a grain boundary.
(e) A planer interfacial boundary being thermodynamically unstable, tends to migrate to a more stable configuration if conditions permit.
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