Difference Between TLP Diffusion Welding and Brazing 1

[JonnyK]

Hello,

I am looking for clarification between Transient Liquid Phase (TLP) Diffusion Welding (DFW) and Brazing. The sources I have found so far have a lot of contradictions about the definition and terminologies involved in these processes. Some call TLP Diffusion Welding a form of Brazing other say it erroneous to classify them together.

My current [right/wrong] understanding:

Brazing as I understand it basically “bonds” two work parts together with a filler. The filler remains between the faying surfaces after the process completion.

DFW involves the migration of atoms between the two work parts creating a metallurgically homogenous part. When two dissimilar parts are used a low melting temperature filler material can be used (TLP DFW).

Someone how this filler material must instigate diffusion between the dissimilar materials otherwise it would just be Brazing. Yet I do not understand how the molten filler material can drive diffusion between the dissimilar materials at a lower temperature.

What am I missing?

Thanks

JonnyK

 

[kenvlach]

Brazing & Transient Liquid Phase Welding are both performed below the substrate melting point.

Braze filler metal only needs to wet the substrates to form metal-to-metal bonds, (significant) interdiffusion or alloying is not required, and the braze metal only solifies when cooled from the brazing temperature. The filler alloy contains different elements and is lower melting than the substrates. If necessary, a brazed joint can be separated by reheating to brazing temperature.

Transient Liquid Phase Welding involves a filler which is normally one of the base metals, plus some alloying to form a eutectic liquid (other types of liquid phases can also be used). While holding at constant temperature, the filler wets the substrates and with time, solidifies due to interdiffusion. To better understand this requires knowledge of the binary or ternary or higher phase diagrams involved. A simple example is the TLP welding of Ni-based or Ni-compatible alloys, e.g., Inconel, stainless steels) using electroless nickel plating as the TLP-forming filler. The electroless nickel (EN) plating used is actually a nickel-phosphorus alloy containing 6-12 wt% P in a supersaturated solid solution. The EN can be plated onto one or both substrates. When heated above 1116^oC, a eutectic forms between the Ni phase & the Ni₃P phases; see Ni-P phase diagram:

http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Ni-P.jpg&dir=SGTE

While holding above the eutectic temperature, the EN melts, wets and alloys with the 2 substrates. The liquid metal phase solidifies over time, holding at constant T, as P atoms diffuse into the 2 substrates (of course, Ni and other atoms interdiffuse as well). This increases the fraction of solid in the joint while still at temperature. When [P] decreases at all points below ~0.3 at%, the joint is completely solid, with Ni-Ni bonds existing throughout. Additional holding time further reduces the spike in the [P] profile across the joint.
A TLP welded joint can be difficult to detect by metallography since perhaps an fcc-Ni matrix throughout, and cannot be separated by reheating to the original welding temperature.

There is info on the Internet for you to search through. The lower 1/3 of the following paper gives a straightforward description of TLP welding:
"Diffusion Bonding," by Amir Shirzadi of Cambridge Univ.

http://www.msm.cam.ac.uk/phase-trans/2005/Amir/bond.html

 

[JonnyK]

kenvlach,

Thank you for the informative reply, I think it makes sense now.


It is interesting that you brought up the link to "Diffusion Bonding," by Amir Shirzadi of Cambridge Univ.
[ul]
[li]This is one of the sources I was confused by because it refers to this process as “Diffusion Bonding”, and makes a point that this is not a Brazing process. [/li]

[li]A second source refers TLP Diffusion as “Diffusion Brazing”.[/li]

[li]A third sources emphasis that Brazing is not welding, and uses the description of “Diffusion Welding”.[/li]
[/ul]

It is this inconsistency in terminology that let to my confusion.

Again thanks for the help.

JonnyK

 

[kenvlach]

I agree that the terminology can be confusing.
Here's an example for comparison with the prior example of TLP welding of Ni-base alloys using electroless nickel filler (Ni-P):

Aluminum brazing using a lower melting aluminum alloy filler (more alloyed, to give a lower eutectic temperature than the Al base alloys). E.g., Al 3003 (Al-1.2% Mn), melting range 1190-1210 F, can be brazed at 1100-1140 F using Al-7.5% Si. [you can compare the AL-rich end of the Al-Mn phase diagram with that of Al-Si, which has a significantly lower eutectic:

http://www.crct.polymtl.ca/fact/documentation/SGTE/SGTE_Figs.htm

]
The Al filler alloy can be foil placed between the 2 base parts, or it can be clad onto sheet material. During brazing, the filler melts, wets the the base pieces* (& of course some interdiffusion occurs, even a little melting of base material with interalloying). But for the most part, the base remains solid, and the filler stays liquid until cooled -- whereas in TLP the filler alloys & solidifies at the joining T.

*wetting via capillarity is a key feature of brazing (& soldering). I described Al brazing as done in furnace or molten salt bath. But, there is also fillet braze welding. So that I don't create more confusion, I suggest reading the encyclopedia article on Brazing:{/b]

http://en.wikipedia.org/wiki/Brazing