Highest Cooling rate ( Centre of the weld or Fusion Line) 1

[waqasmalik]

Hello all. Hope this post finds you well.

My question is about general trend of cooling rate observed in welding. Which area of fusion zone experiences highest cooling rate? Weld centre or Fusion line at the end of fusion zone?

Thanks all in advance.

Best regards.

 

[EdStainless]

The edge of the fusion zone. It has the shortest distance for heat transfer and the steepest temp gradient.


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P.E. Metallurgy

 

[waqasmalik]

Thanx for response Edstainless. I learned and understood from the book this way.
Lets say G is temperature gradient and R is the growth rate of solid liquid interface. According to Welding metallurgy book by Sindo Kou, GR defines the cooling rate.

At the fusion line, G is maximum but R is less. At centre G is less but R is very very high compared to R at fusion line. This gives us higher valur of GR at weld centre. Now higher value of GR should give us higher cooling rate at centre.

If you have book please see its page 202. I have attached snapshot as well.

Am i misunderstanding something here.

 

[MagBen]

The edge of the fusion zone is the coolest area but may not be necessarily with the highest cool rate. majority of heat is conducted through fusion zone, fusion line, HAZ, and base metal. It is hard to tell which areas have the highest temp gradient. Toward to end of welding, and if melting temperature is high, even the base metal is heated up, the highest cool rate may be exist at border between melting weld and fusion zone.

 

[EdStainless]

I open with the fact that it has been 40 years since I worked on the models and math behind this.
There are three distinct things going on in the weld, liquid is cooling, solidification is taking place, and hot metal is cooling.
The cooling rates will vary by location and time.
This also depends on alloys, what is the solidification band, are there phase changes, are secondary phases forming/growing/dissolving?
So it comes down to 'what is the question'? Often the cooling in immediate post-solidification range that is most important with regards to the microstructural development. And that cooling rate is definitely slower at the center-line of the weld, and in most cases it will fastest at some intermediate position. But that is a different story.



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P.E. Metallurgy

 

[waqasmalik]

I was reading a research article, then the welding metallurgy book by sindo kou. That why i asked because according to my understanding, i found disparity.

Please find attached herewith snapshot of research article.

 

[MagBen]

So you are trying to correlate 2nd dendrite spacing structure to cooling rate?! I agree with what you read in the research article. fusion line area was the first solidified, at that time the base metal is coolest, and the cooling rate can be the highest, and so the spacing is smallest.

In the book, G is temp gradient, R is growth rate. mathematically GR is cooling rate, while metallurgically, GR cannot probably be defined as the cooling rate. One cannot say the higher the growth rate, the higher the cooling rate.

No "STAR", no further comment

 

[waqasmalik]

Here is another snapshot from sindo kou book.

 

A professor I had in the previous millennium, Tom North of U of Toronto, wrote a text that covers this subject exhaustively. Unfortunately the title escapes me and I doubt you could find it anyway.
Nowadays researchers probably go straight to computer modeling to answer your question. Alternatively, stick a thermocouple into the weld pool.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[gtaw]

I think we have to agree on terminology before we can go too far with this.

I suspect the highest thermal gradient would be between the weld (liquid with some superheat) and the unaffected base metal (below the lower transition temperature if working with carbon steels and high strength low alloy steels).

Where solidification is likely to begin? The weld interface, i.e., epitaxial grain growth, where the existing lattice of the HAZ is extended as latent heat of fusion is liberated and the atoms align with the existing lattice. The center of the weld bead toward the weld face is the last to solidify as equiaxial grains. Solute rejection would tend to push the low melting point constituents toward the centerline of the weld bead.


Best regards - Al

 

[waqasmalik]

Thanx for comments GTAW and ironic metallurgist.

What i know is this.

Once the weld pool is formed then solidification occurs in the back half of the pool, opposite to the direction of welding. New crystals will appear on the fusion boundary which is all along in a shape of moon...Not full moon... Inverted C shape...It occurs at the fusion boundary all along.. Weld pools are ovel or tear drop shaped... Along this C shape fusion boundary we have a fusion line and the weld centre line..

Please see the image attached.. Solidification is not occuring at the geometric centre of weld pool. It is occuring along the fusion boundary which is an inverted C shape which we call mushy zone and i was taught recently by my mentor (AM) and as GTAW pointed out about epitaxial growth where already existing lattice extends... So We have two fusion lines, upper and lower and a weld centre line.

Solidification will start along the fusion boundary because heterogeneous nucleation is always favored having less energy barrier and small group of atoms happen to gather by chance can form a stable nuclei upon an already existing surafce and grow towards the geometric centre of pool. Geomteric centre although experienced the temperature much higher than the liquidus but the discussion of cooling rate at geometric centre is completely irrelevant (from solidification begining point of view) in my personal opinion because even if it experienced the highest cooling rate then solidification will not start homogeneously.. What i have learnt is that homogeneous nucleation is very very difficult because it requires to overcome very high critical energy barrier and the critical size of nucleus is also very high before we can call it stable nuclei and able to grow..

What i am confused is about the cooling rate variations along that Inverted C shape fusion boundary/mushy zone at the back half of weld pool. Sindo Kou says the Fusion line experiences highest cooling rate because according to him GR is defined as cooling rate and GR is minimum at fusion line, both upper and lower... At weld centre line, GR is maximum so cooling rate is maximum. While this seems to be true mathematically becauuse units of GR are same as cooling rate but the confusion is how this can be explained metallurgically? Also will this kind of GR cooling rate explanations be valid only during the solidification? Because no weld pool is there after solidification..

Sindo kou relates this GR cooling rate to dendrite arm spacing and presents that dendrite arm spacing will be higher at fusion line and lesser or we can say finer at weld centre line...

Please comment and edifice about my misconceptions and lack of knowledge ..

Best regards
Waqas

 

[waqasmalik]

Anyone could guide please..