How does the "Weld" work

[1oldwelder]

Forgive my ignorance, I have been welding for to many decades. AWS 3g 4g 5g 6g certs so I know how to weld. My question is, How does the actual bead bond to the base metal? Simply phrased, I know every thing melts and fuses together. but at that microscopic edge of the puddle between molten metal and solid metal, the only thing I can picture in my head is a microscopic mechanical connection. I have seen many a coupon cut and bent, saw the cut edge where the weld fades to the base metal. I just don't get it.

 

[MikeHalloran]

I think it's the same atomic force that holds the base metal together.

There's been some progress lately, but I don't believe anyone really understands it yet.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

Great question. It made me curious so I googled and found this as an explanation of what happens between weld and base metal:

http://books.google.com/books?id=vK...v=onepage&q=weld base metal interface&f=false

Ted

 

[metengr]

How does the actual bead bond to the base metal? Simply phrased, I know every thing melts and fuses together. but at that microscopic edge of the puddle between molten metal and solid metal, the only thing I can picture in my head is a microscopic mechanical connection.

When you fusion weld even the leading edge of the weld puddle wets the base metal for some microscopic distance. During solidification of this weld edge, it is the same atomic forces that bind metal atoms together in base materials that have not been welded that bind this new cast metal to existing metal. Maybe think of this approach, metals are molten and solidify into a crystal structure. Think of the welding as casting of metal except it is controlled and localized, the outcome is solidified metal with mixing of weld and base metal elements.

 

[1oldwelder]

Thank You all for the responses,

I have read several times what hydtools linked to. The way metengr explained it is somewhat understandable. As my name implies, I have been welding for a long time. I am not a engineer (always a dream of mine) but I do have the ability to see things in my minds eye, as to how they are supposed to work.

I have to study on this info for a bit to comprehend how the metal is reacting right at that point between liquid and solid in the crystal forming zone.

Again, Thank you for your time in answering this perplexing (to me) question. Mark

 

[Ron]

Metengr's explanation is good. A weld bead is a casting. To give you a further example, if you deposit weld metal onto ceramic backing, it doesn't bond to the backing(except sometimes you'll get a little mechanical bond where the ceramic melts and little and creates fissures).

The weld metal in contact with base metal creates a molten interface between the two that becomes a solid solution as it cools. The reason you can see the weld so clearly in a macroetch specimen is that the grain size of the solidified weld material is usually smaller than that of the base metal. What you end up with is base metal, mixed metal, and weld metal, all being slightly different metallurgically, but capable of the noted atomic bond.

If you heat treat the welded section, you can cause the grain size to become similar; however, the grain orientation will still likely be different. Doesn't make much difference except that it looks different under a microscope.

 

[0707]

This is what happen during welding


luis