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Dear Allan!
Now, as already "threatened" a "little" time ago, here am I again to write down a few of my own thoughts on Boon's post. It's the first time since long, that I have found time to return to the forum and I really don't want to know how many interesting posts I certainly have missed over this period.
However, it's a very good feeling to be back..!
This sentence here was originally not intended to be written, but since I have exceeded the maximum text length size (shame on me!), I am going to divide the original text corpus by "2" and will try to create two single replies of actually one entire one. I beg your understanding...
With the attachments please note that I will attach all the pictures of which I find they might be helpful to elucidate the matter within the # 2 of my response. I.e. all what you read in a oherence with "jpeg's" you will find in the second part of the post. I truly hope that this will work!
Please allow basically to express that the subsequent considerations are my humble try to explain myself what kind of coherences are responsible for the sometimes different behavior the material shows in a direct relation to what Boon has stated to be his application in his initial post.
I am honest, I personally guess that this mentioned post is one of the very most interesting ones I have read on the forum.
Since I have considered long about all the different items, perhaps being in charge, I found out that it was as usual. The more one considers the more he is going to find out and the less he appears to know finally.
However, what I have done - of course under simplifying everything as best as I could due to the restriction of space in the forum - is, that I have tried to bring both what fascinates me personally (Arc- and Solid State Physics) together in a way that it could make sense finally for treating the particular topic - Autogenous GTA Welding of AISI 304.
First off however, I must agree with you and Dave! Al's explanation is again and as usually one outstanding good one.
When I have read what Boon has initially posted, my very first reaction was to say "Hmm, this should only be hardly possible!" But nonetheless, I am careful by knowing myself. When I am asking a question and somebody says "No, this is not possible!" something awakes in me and driving me to find out the pros and cons making something either "possible" or "impossible".
I must thank you for according me the time for this late, or better very late response. You know I was quite busy and thus the time for visiting the forums was extremely restricted, unfortunately!
Meanwhile however I had time enough to pay one of the scrap boxes of our company's welding training center a visit, trying to find some pieces of ~ 304 sheet metals GTA- a/o eventually Plasma-welded to see how the welding seam appearances were. I had luck by having found some "pieces of gold" :-).
Well, here we go..!
First question I asked myself after having read what Boon has posted was: "How can it be as far as it can even be?".
Next thought was going towards separating all the "things" being involved into the procedure making a weld seam even a weld seam. There we can state:
1. (GT) Arc
2. Base Material
3. Filler Material
Since we know that no filler should be used in the case Boon has mentioned we can cancel item 3 again. This should simplify the entire affair appropriately (?).
So in fact what do we have? Actually merely two different things and - and I guess this is one of the most intricate but as well interesting process in physics - the interaction between the plasma (energy) and the solid state matter.
I have tried to continue my considerations on the basis of analyzing (surely quite basically):
1. How is it possible to handle a Gast Tungsten Shielded Arc and its influence on the solid base material in a simple way?
2. What are the results of the Arc's energy in particular terms of the used 304 base material
For (very partially) replying the question 1 let us have a short look upon the structure of a TIG Arc maintained under Argon as the used shielding gas. It has - normally - a bell shaped form with a brighter (i.e. "hotter") core and a lighter (i.e. "colder") zone around the core, see also Picture 1. Both criteria are specific and the results of indeed interesting but nonetheless very complex plasma-physically interactions. As an extract of both one can say that the shielding gas properties are responsible for the later shape and consistency of the arc plasma. Due to the specific properties in terms of thermal conductivity Argon as the shielding gas has a relatively low ability of conducting the heat through the arc volume which results again in a "hot" inner core and a "cold" outer area. What does "hot" and "cold" mean when we are talking of temperatures high enough or far beyond those ones able to melt metals. I hope you may agree when I say that this topic to treat herein could be a hard to realize undertaking, what's the reason for that I would like to avoid it furthermore. But - at least from my personal viewpoint - it is even unnecessary. Because what is much more important for having a possibility to deal with the application Boon has stated in his post, is the fact, that we should treat the arc plasma at our particular purpose, and as mentioned extremely simplified, as consisting of different electrically neutral and electrically charged particles being roughly mixed by pushing a current through a gap between the work and the electrode and therefore using the voltage. The actual major carriers of the electrical charge within the arc plasma are the electrons. The higher the concentration of electrons (i.e. the higher the current density) the higher again the temperature ("hot" core) and - and this is crucial - the higher the p r e s s u r e the arc causes upon the base material, or better, the weld pool.
Pressure..?
Of course. The arc causes a pressure upon the weld pool and all of you know that of course, since everyone of you has observed the fascinating spectacle a Gas Shielded Tungsten Arc is carrying out on the weld pool's surface when you have risen the current. The mentioned pressure is the result of different strong forces (electromagnetic, surface tension, buoyancy and plasma jet) Mainly responsible however is the electromagnetic or LORENTZ force respectively, which is proven to generate an inward flow of the molten metal within the weld pool volume. By the way, when you have a closer look upon Argon_TIG.jpeg, you can slightly recognize that the arc is causing an indentation into the base material. Why am I that strongly dealing with the pressure underneath the arc? Very simple... Since the pressure does arrange that the material is being distributed and furthermore displaced underneath the arc.
O.K. let's see what we do have by now?
A Gas Tungsten Shielded Arc established and maintained in an Argon atmosphere, does have a particular shape and a particular physical behavior.
Hmmm, but on what depends the physical behavior of the special shape of the arc? We have heard that the arc consistency, its temperature profile, its pressure gradient is amongst others - to be neglected herein - as well depending on the current density. What is the current density again depending on? Well, again amongst others it is depending on the tungsten electrode geometry, the electrode diameter, the electrode material's composition and other peripheral parameters (e.g. electrode to work distance, torch positioning angle,...).
How to make it possible to describe the pressure the TIG Arc is performing upon the weld pool? (Excuse me for not coming to the point but I hope that I will find a way to it finally.) Good, this is best possible by using the way of visualizing what's taking place while an arc is burning between the electrode (cathode) and the work (anode). But nonetheless I would like to make a short sidestep towards to describe how tremendously intricate it actually is what we are here talking about. So intricate that the theorists - as well nowadays - are not 100% sure about the complex interactions between the different physical phenomena existing within an arc plasma.
Let the volume of a molten pool (V) be a function of - as we have heard - a large number of different parameters e.g. temperature (T); material density (rho); arc pressure (p); welding speed (v) and let us consider further that these parameters are - in a wide range - temperature depending again what complicates the prediction of the molten pool behavior additionally. I permit myself to decide that the latter should be neglected for a strong simplification to being able to reduce all the intricate combinations to a more simplified form where we can say, the conditions coming from the plasma side (arc) + the conditions coming from the solid state side (base material) yield a very specific molten pool condition which yields a molten pool volume again. Now let us presume that - as already shortly mentioned - the main parameter affecting the height of the arc pressure should be the height of the welding current. Different - very interesting - investigations were conducted in the past with respect to find out more about the coherences of molten pool depression forces induced by the arc. Most of those surveys could confirm that the even stated (height of welding current) is the main parameter to be considered for calculating the arc pressure or its influence on the surface depression of the molten pool, respectively.
Why have I mentioned all this by now?
I hope you may agree when I say that - in a great extent - the pressure of the arc is responsible for the amount of the depth of fusion or - in case of welding autogenously - the depth of penetration. As I have said, please allow to neglect the various interactions - and thus variables - caused by electromagnetic-, surface tension- and other forces, taking place within the molten weld pool and being responsible for the convective flow being induced by these forces. What however is - from my point of view - important, in particular for the case of autogenous GTA-Welding and achieving both root- and surface reinforcement, is the fact of the composition of the base material. But hereunto I would like to come to a bit later on. Alright, let me coming to the visualization of how it might look like when the (Gas Tungsten) arc is autogenously deforming the weld pool. Please see the Surface_Depression_TIG.jpeg, for making it imaginable how a Gas Tungsten Arc is able to depress the weld pool surface and thus, in autogenous welding, making sure the depth of penetration in relation to the workpiece' wall thickness. What's interesting is the detail, that when GTA Welding autogenously the depth of penetration does not increase infinitely by increasing the current. This phenomenon is as well observable when increasing the welding time at constant current when welding with a stationary arc. Proportional to each other do increase both weld width and weld pool area. Many Investigations have been conducted in terms of the influence of surface depression and convection on the arc weld pool geometry. Here one has e.g. found out that there is a kind of "transition current" existing, where relative "shallow penetration" depth is increased to be changed to higher values by so-called "deep surface depression". Very interesting but I won't like to further treat this matter herein since the transition current ranges are far above those values Boon has stated in his post and which laid at 110... 130 Ampere. However, what we can conclude by here is that the surface depression of the weld pool is being influenced mainly by the height of current and other variables.
Since I would like to treat how the weld seam appearance is basically being formed I would like to coming now to the physical coherences in terms of how the weld bead geometry does react under the autogenous Gas Tungsten Arc. When having a look upon a weld pool being generated by a stationary Gas Tungsten Arc and when using relatively low current values (as which I interpret the values of Boon) - which is important since only here one can use the simplification of using a so-called "Gaussian" heat distribution (particular mathematical approximation method, see also:
http://en.wikipedia.org/wiki/Normal_distribution) by considering the heat maximum in the center of the arc - the surface depression is as well relatively low. Let me now assume that - just as we have heard - increasing the welding time of a stationary working Gas Tungsten arc at constant current does not increase the depth of penetration but does increase the area of the weld pool on the workpiece at least as long there has no balance between heat addition and heat dissipation been generated. By the way, in my eyes the depth of surface depression should thus being reduced due to the greater pool volume. However, let us now presume to not use a stationary Gas Tungsten arc but to move our heat source, even to weld e.g. a square groove butt joint, as mentioned by Boon.
What does now happen to the surface depression?
Well, before dealing with this interesting question we have to go back to an increase of welding current and exceeding the limit of where the "transition current" causes a deep surface depression. Here one has to not only to leave the range of relative shallow depth of penetration but has also to leave the pattern of heat distribution ("Gaussian")and thus to achieve a changed current flow. Why is this detail that important? Even since the flow of current stands in a close relationship to the molten metal flow pattern. LIN et al have investigated this and have found out that by an increased surface depression there is generated a displacement of the field force density having similar potential (equipotential lines) and hereby it is assumed a "maldistribution" of current - or in other words an imbalanced heat density at the workpiece (anode) surface. This is by forming a deep crater after the "transition current value" is exceeded (deep surface depression exists) and the main share of both heat and current is being received by the crater side walls, but only a minimum is received by the bottom of the crater itself, for instance to increase the depth of penetration additionally.
Alright, we have now spoken of that there can be assumed a limit in penetration depth in relation to the height of welding current. And furthermore we can assume that when having fixed a specific arc performance to be coupled into the workpiece and presuming the generation of a thermomechanical balance between the heat added and the heat dissipated, we will achieve even a specific volume of (stationary) weld pool. It remains at even this specific size and is neither increased nor decreased in size. This means of course that the depth of penetration may not change as well. Finally concluded: "Specific (Gas Tungsten) arc performance being generated and coupled into a workpiece having even specific dimensions and (thermomechanical) properties yields even a specific depth of penetration."
Resume...
No is my question: "What does happen with the (depressed) weld pool when we switch off the welding current after having held the weld pool itself under stationary conditions (NO movement of the arc)?" And further: "Do we then have a surface reinforcement?"
Oh, I almost forgot to mention but I know you would have already asked. By now we have neglected all the little but often very important details (material composition,...) which can have a strong influence on the weld pool geometry and its behavior. But I hope to be able to treat them in a respective way later on, since actually these details are - at least in basics - not neglectable.
Coming back to the questions of above...
YUSHCHENKO et al have investigated the behavior of "conventional GTA" vs. "A-TIG" (GTAW under activating fluxes) weld pools generated on a base material comparable the AISI 304 grade for finding a way to a theoretical approach of an explanation of the deep penetration effect in A-TIG-Welding. There it could be seen that - at least in these particular experiments - there is a slight surface depression of the solidified weld pool, see as well the jpegs Surface Depression_1 (transversal section) and Surface Depression_2 (longitudinal section). But what as well can be seen is - at least apparently - in the longitudinal macro section (jpeg Surface Depression_2) that the weld bead seems to be reinforced after the weld has been carried out, i.e. the seam itself should be reinforced slightly. Now one could ask: "Is the amount of molten material volume been depressed similar to the reinforcement?" This to reply is - only from the attached picture - surely hard to accomplish. But nonetheless it might be so. A bit better the effect of surface depression caused by the arc can be recognized by having a look upon the longitudinal macro section of an A-TIG welded material of same heat, see also the jpeg Surface Depression_3_A-TIG. Here one can see the "keyhole effect" - as YUSHCHENKO calls it - which is being physically induced by the activating flux. Here one can see, that the welded seam appears to be reinforced behind the crater which contains the "keyhole".
But, "Where does this surface reinforcement - as far as it is even there - come from?" and before replying this question one could ask as well: "Does the surface reinforcement occur as well when the weld pool is generated and being held stationary?" Can there even be a reinforcement of the seam - or better spot - after stationary weld pools have solidified? Hmmm, actually this is hard to imagine since there is actually no force which could cause the reinforcement of the melt. The only aspect which I could imagine to be responsible for a reinforcement of stationary weld pools is the natural thermomechanical increase in the weld pool's volume by increasing the temperature. What does this mean again? Let the "original" volume of the material (under room temperature conditions "T 0") be "V 0". When increasing the temperature it can be assumed that the materials volume does increase as well. Let now be the arbitrary increased volume of the specific element of molten material at an arbitrary increased temperature "T 1" be "V 1" and the volume of a specific element of the molten material at melting temperature "T Melt" be "V Melt" then we can write for the volume fraction: V 1 > V 0 << V Melt. But how to evaluate the volume of an element of molten metal? I hope that I am doing right now... The volume is the ratio of mass over density. When we now assume that the volume of a constant mass does increase as the temperature increases (what it evidentially does) then the density must decrease as the mass remains constant. I have tried to find some thermomechanical values of stainless steel comparable to AISI 304. First off, the specific weight of AISI 304 is - to my best knowledge - approx. 7.9 g/cm³ (1 g/cm³ = 0.036127292 lb/in³ and thus the density of AISI 304 equals 2,8540561 lb/in³). Let us now presume to have a volume of 5.0 cm³ of AISI 304. Then we obtain a mass of 39.5 g (~ 0,087 lb.). Now it's becoming a bit difficult since it was hard - at least for me - to find the values for the density of an AISI 304 melt. Since the density of a metal is, as already described previously, a function of the temperature and varies of course due to the lattice transformations. However, to not complicating the entire matter, let us presume that the molten metal has a density of ~ 7.3 g/cm³ (forgive me that I remain metric) at melting point ~ 1680 [K] (absolute temperature) what was an approximate value I have found. This means that we have a reduction in density of ~ 7.6%. By knowing now that the volume is supposed to increase since the density has decreased, let's see what should happen. Volume (V) = Mass (m) / density (rho). Thus 39.5 [g] / 7.3 [g/cm³] ~ 5.41 [cm³]. Thus we have an approximately increase in volume of ~ 0.41 cm³, which is an increase in volume of ~ 8.2 %. Although these are surely pure theoretical and inaccurate values we have calculated with but this however means in my eyes: When we could make sure that the area the melt is existing might be "separated" against the surrounding area of the non molten base material (e.g. by extremely cooling the adjacent areas of the seam) and thus generating a steep temperature gradient, I could imagine that the volume of the weld pool would be "depressed" by the adjacent areas and could - under as well presuming a rapid enough cooling of the melt - be - at least theoretically - reinforced in relation to the non molten base material. As I wrote you sometime that in the coherence with an IIW Commission XI Intermediate Meeting I've had the opportunity to visit the British TWI (The Welding Institute) in Cambridge. As you know perhaps the TWI is one of the worldwide leading research institutes in the field of Electron Beam (EB) Welding. When we were guided through the institutes laboratories we have visited as well the EB-labs. There I had the opportunity to talk to Mr. Chris Punshon who is with the "EB Applications Team". It was an enjoyment, by the way. Amongst others he presented us an EB-welded cylindrical steel component (square groove butt joint) having had a thickness of 40 mm. As you know, no kind of filler is (normally) used in EB-welding. However, what I have observed - I guess you know what comes now - was the fact, that although no kind of filler has been used to weld the 40 mm square groove, there was a huge reinforcement both root and surface side recognizable. This effect can be detected as well for larger wall thickness joints. For a better understanding I attach a picture coming from an IIW Commission IV paper written by Allan Sanderson who was the former head of the EB Welding department in TWI, see also the attached Electron_Beam_Weld.jpeg. Here one can see a 150(!) mm thick C-Mn steel square groove butt joint welded at 1 mbar (1 mbar = 2,0885434 lbf./in²) at 200 kV plate voltage, 300 mA current and a welding speed of 100(!) mm/min. Well, as I have now spoken with Chris Punshon (always kept the "Autogenous Welding" topic in mind :-)) I have asked his opinion about where the effect of reinforcement on both sides of the presented EB-Weld might come from. I am honest. He answered: "Huuh, that's a good question!" Then he explained, that the part does transversal shrink by 0.5 mm after whilst being welded. And this effect is supposed to be the reason of the detectable reinforcements. Very interesting as I find, isn't it? Although I had no more chance to ask him if the width reduction is 0.5 mm per each side of the part and thus would yield a total reduction of 1.0 mm (there was no more time to continue the discussion - unfortunately) it would probably pay off - and would be interesting as well - to calculate if the volume of solidified molten metal (even either recognizable reinforcements) would correlate the volume element calculable by: Thickness (t) x Width Reduction (delta W = W - W zero).
Yes I know, Boon does not use stationary process conditions and he does not use the Electron Beam. He is using a continuously moving heat source - even the Gas Tungsten Arc - for executing the Autogenous Welding of his 304 material. Yes, of course. But what I have liked to show previously is that it's worth to make a short sidestep and to have a short view if the discussed effects might be recognized in general in welding, and I would have liked to show the slight difference in the molten pool behavior between a "stationary" and a "moving" weld pool, although I would like to come back to the volume of a melt in another coherence a bit later on.
However, the stationary weld pool of a Gas Tungsten Arc on a stainless steel base material can be assumed to be more or less flat after having solidified - even since there is no complete temperature "separation" between the melt itself and the adjacent base material but there is a heat transfer caused by the well-known physical properties (conduction,...) from the highest point in temperature (melt) to its vicinity (room temperature base material). But what is then the reason for the basically surface reinforcement of a seam been performed by using a moving heat source - as to be seen in jpeg Surface Depression_2. I guess this is an important question to be replied, since there must exist particular forces making sure that even this reinforcement does occur. I honestly hope that I am on the right path by writing all this down, since you could surely ask me: "What the heck do you guy mean by telling all of this?" Hmmm, surely you are right, I guess it is likely not that easy to follow me :-).
O.K. let me try to explain what's driving me...
As I have mentioned already in the above regions of this try of a response I have had a look into our company's scrap box, where I have found some interesting pieces of stainless steel (304 grade) samples been welded by our apprentices and client's welders. Among others I found a piece which had been welded by using the GTAW process. As I could see, it was a piece where the welder - whoever it was - seemed to had carried out Autogenous Gas Tungsten Arc Welding on a square groove butt joint. Although it was in a thickness range (t = 4 mm) laying above that range Boon has used, I could detect an interesting - at least in my eyes - phenomenon. For a better imagination, please see also the attached jpeg Stainless_Surface_Depression_Imbalance. What can be seen there is both concavity and convexity. When I have seen this, I have asked myself what might have been the reason for this imbalance between the areas of surface concavity and convexity (reinforcement). Well, in my interpretation it appears explainable. It might be assumed that the reinforcement is caused by a direct change between the stationary weld pool's influencing forces (arc pressure upon the melt and forces being mentioned already above which induce a particular melt behavior again [surface tension driven = Marangoni]) and the displacement of these forces by inducing an imbalance between the forces being more or less "symmetric" arranged around the arc core and hereby making sure that the symmetry of the (stationary) weld pool is being generated. In other words: "The whole melt starts to be moved, thus showing a divergence in symmetry, thus shows a divergence in temperature profile, thus showing a divergence in solidification and thus showing a different seam profile. This behavior is, amongst others, being caused by the relatively clear to imagine fact. The welding arc as an electrical conductor is arranged always under strong influences of the respective conditions to be found upon the material welded. This means that by changing the material's conditions the electrical behavior (conductivity) of the base material is changed as well. By changing the materials behavior in terms of conducting the current - in other words changing the current density over the weld pool area - one changes the pressure conditions on the one hand and - and this is much more important - changes the distribution of forces being responsible for the weld pool motion. This means, when having a high current density one has a high temperature and hereby one has a low surface tension. On the other hand when having a lower current density one has a lower temperature and thus one has a higher surface tension. When now changing the condition in terms of the heat source from "stationary" (and thus quasi symmetric arranged forces) to "moving" and thus towards a displacement in force arrangement one has a temperature gradient from "high" (in front of the arc) to "low" (behind the arc). This again yields a weld pool motion from the front to the back of the weld pool area, since the surface tension is higher behind the arc (lower electrical conductivity [of the molten metal] leads to lower temperatures and this leads to higher surface tension forces). Hereby, so at least my assumption, the "compression" of the melt - and thus the final "endeavor" for reinforcement - might be explained.
What does it mean all in all by now? Hmmm, I am fairly tempted to say that we stringently might need the heat source motion for even achieving a reinforcement when welding Gas Shielded Tungsten Arc autogenously. But what explains then the behavior to be seen upon the jpeg Stainless_Surface_Depression_Imbalance? Here we have both over the course of one seam. Concavity and Convexity. This is only an assumption and I do not know if I am right but... Let us assume that the "normal" behavior of a stationary GTA welded - in this case - stainless steel leads to a more or less "even" surface (i.e. neither concave nor convex). And let us now assume that - for achieving even a reinforcement - we need to move the arc (as described above). Then I assume that - as well as there is a threshold value in current for achieving a higher penetration grade - there is a kind of a threshold value for moving the weld pool in a way that the melt does solidify with a reinforcement. When now having a look upon the above mentioned picture, in my eyes it looks like as that the welder has exceeded even this specific threshold value in even welding this specific base material with even this specific thickness. By the way. I have not taken a picture from the rear side of the sample. What the welder has tried was to get a full penetration through the entire wall thickness. He has achieved that. There, where we can see a concavity on the front side, we could see a reinforcement at the rear side of the sample. On the other hand there, where we can see a reinforcement on the front side, we would see no reinforcement at the rear side. And even this is easily explainable. What - so my personal interpretation - what has the welder tried? When we are having a look upon the magnitude of the surface depression at the jpeg, we can see - or at least suppose - that the height of the surface depression is not that great. At least not great enough for achieving a full penetration over the entire sheet metal's thickness. Thus the welder had to reduce the welding speed in an appropriate amount for even achieving this full penetration. However, when he has "interrupted" the continuous (low) welding speed (eventually jiggling?) he has interrupted as well the entire process of weld pool motion, caused an imbalance in both current- and temperature distribution and this again led to a solidification (reinforcement) of an increased amount of molten metal behind the arc. So far even my humble explanation. By the way, there have been conducted investigations in terms of finding out even the GTAW threshold value for yielding a significant increase in the depth of penetration. What there could be found out is that the depth of penetration (for a specific height of surface tension [1300 dyne/cm]) is not that large. Up to a current of approx. 300 Ampere and using a 90° electrode tip angle one could see that the maximum depth of penetration was 1 mm(!) Not that high, isn't it? And thus, as well, when we do have a look upon what Boon has stated what the current values are in his application, one might assume that there must fit everything very accurately for achieving both a reinforcement at the front a n d at the rear side of the sample.
So far so good... Really..? ------> Please see # 2 as the continuation of my response... (hopefully you'll read it) ;-)
For the moment all the best,
Stephan