Asking questions about MIG/MMA heat input, penetration etc.

[AdrianH]

So I do not clutter up someone else’s thread with my innate questions about why MIG, why Stick etc. I thought best to add this post. Post in question is the one on multipass MIG welding getting towards MMA at the end. vif better on that thread then mods please move and/or edit.
We all try at times to weld metal together a lot thinker then we should be doing and I some times need to understand a bit more of the science to things, just my brain, so here goes.

As per the thread welding two bits of 10mm plate together fillet joint, choice either MIG or MMA.

Am I correct in presuming that with MIG one tries to go for root penetration for strength and in MMA one tries to go for Fillet size?

So why not go for Fillet size with MIG rather then penetration.

As I read it, current density is a factor and for say a constant 125 Amps, the electrode size will effect this. 0.8 MIG wire has a cross section area of 0.502mm2 so will have a current density 249 Amps per square mm.

A 3.2 rod has a csa of 8.8mm2 so would have a current density of 14.2 Amps per square mm

From my reading and trying to understand it all the heat input from a multi-purpose machine would be the same as the formula for ARC Energy is 60VI/1000v

VI is Volts x Amps
v is travel speed in inches or mm (depending on preference)
Heat input is down to efficiency and is 0.8 for MMA/TIG/MIG compared to sub merged arc at 1.
All from TWI web site.
So heat input is the same in both camps.

But trying to understand the figures would suggest that when joining thicker metals together multi-pass MIG could provide more fusion depth and still build up to the same fillet size then MMA with a 3.2mm rod at 125 Amps.

But the general consensus is the other way, why, what have I missed in my thinking?

Probably as clear as mud.

Adrian

 

[TechnicAl]

Heat Input is VAT (volts x amps x time) over ROL (run out length)
It is a measure of the potential for penetration but its not the full story.
Cutting a long story short MIG and MMA can basically do the same but MIG is quicker because its higher current (mostly)

 

[Richard.]

Penetration depth, fusion level and fillet size are all just as important with any of the 3 arc generated processes. We select our process based on its pros and suitability to the application. Eg. 1.5 mm stainless but joint. All 3 of those processes will do it. But Tig is the most suitable because cap height and penetration levels need to be kept under good control. 10mm filet. Tig would be my last choice even with high power it's gonna take ages and lots of localised heat. So that leaves me with a 250-300 amp mig or 120amp 3.2 mma. Mma runs high OCV some times as much as 100 volts which is why you can really burn off that thick rod at lower amperages and gain good fusion levels. Mig OCV is adjustable but usually 15-30 volts. So a lot more current is needed if it's to compete with mma on fusion/penetration depth. Fillet size is relevent to travel speed and number of runs this is the same in mig/mma. Increasing a fillet size by weaving or going sloooow will cause low mechanical toughness/strength.
Increasing the size of a fillet by multi passing adds strength not only because of the extra reinforcement but adding more runs refines the grain in the microstructure. (If done properly and heat in monitored). Both mma/mig are capable of welding 10mm fillets. Difference is only one of them will do it (properly) off a 13amp plug.

 

[AdrianH]

I can appreciate the Open circuit volts is higher on MMA then MIG, but depending on machine the voltage when welding with MMA is more likely to be around 30 ~ 35 Volts The MMA machine being constant current. Higher voltages on start-up helping arc starting etc.

MIG being more of constant voltage so current is more dependent on wire feed.

Yes my questions do come from limited welders, my R-Tech i-180 will do MMA and MIG but can only supply the same energy to either at fill tilt. Will do 180 Amps MIG at around 28 Volts, but 150 Amps MMA at a slightly higher voltage around the 33 Vollts welding.

Both have a higher OCV volts typically 75 Volts for MMA and dropping to around 45 Volts on MIG.

Anyway I need to practice my Stick.

Adrian

 

[Kent]

Ok let's say it like it is and cut the tecnobabble
You can easy put in a bad mig on such a joint that looks basically ok
If you put a 3.2 stick into it and it looks good it generally is good
A good welder with the right size plant could sort it either way with it being 10mm
If I had doubt in the mig plant I should stick weld it over mig
Reason is 12mm is as thick as you can go on single phase on such a joint
The safest recommendation is don't do nothing your not confident in if safety is involved
But why not do some test pieces to settle your mind

 

[AdrianH]

No you don't get what I am asking, and it is not to be given bull with this is how it is done, but for someone to give me reasons I can understand and not comments as above by saying it is easy to but in a bad mig and if it looks good with stick it will be.

I wish to know why one technically picks MMA over MIG, for what actual reasons. Because when I look into it on the technical sites and from my limited understanding of it, the heat inputs for the same size of throat on fillet weld will be virtually the same for both processes.

Lincoln website technical pages have statements like this

"One main welding variable that has virtually no effect on weld penetration is arc voltage. While changes in arc voltage can result in minimal changes in weld penetration, the effect is very minor compared to welding current and the other variables just listed in this article. Arc voltage affects the arc length." and
"Ironically, many welders mistakenly think that arc voltage is the main variable which affects the level of weld penetration. Voltage is often improperly referred to as “heat”, where welders turn up the voltage or "heat" for perceived more penetration and turn down the voltage or “heat” for perceived less penetration."

The other factors being: Polarity, arc angle, travel speed, electrode diameter and shielding from my understanding.

So I read and get that 0.8 MIG wire at a set current should provide more fusion depth or penetration then stick due to diameter for any particular current, things like angle and arc speed being similar, both can have the same polarity.

But I could, and probably am missing something fundamental and that is what I am asking.

Yes if I get a chance and a decent press I will try 10mm steel with MMA at X amps and Y Volts and MIG at the same heat input and see if I can split them and what different pressures it takes for the same CSA test pieces.

Adrian

 

[Richard.]

Adrian your question has been answered.
But really when someone selects a process it's simply selected through practicality of the job in hand.
It's a fact mma will penetrate deeper than mig amp for amp.
It's also a fact that mig is twice as fast (roughly) than mma .
Tig can autogenously weld Stanley blades together.
Mma can be taken anywhere with a box of rods.
Mig requires the lowest form of skill.
These are the reasons with many more as to why a process is selected.
I don't see how Lincolns blur on arc voltage has anything to do with your question perhaps I'm missing something there.
If this isn't the answer your looking for then I for one can't help.

 

[AdrianH]

Hi Richard;

I appreciate what you say and I am not saying you or anyone is wrong, I am not disagreeing with any of your statements you have listed there, but I guess I want to see and be able to understand the physics behind each of them rather then just take it as read.

The voltages bit came from your statement

Mma runs high OCV some times as much as 100 volts which is why you can really burn off that thick rod at lower amperages and gain good fusion levels. Mig OCV is adjustable but usually 15-30 volts. So a lot more current is needed if it's to compete with mma on fusion/penetration depth.

So I went off and searched the web looking at Welding penetration and found a Lincoln Page on the Variables that affect weld penetration. The thing I took from that was current. Now again I could be reading or getting points mixed up with reading to much, or about different processes.

http://www.lincolnelectric.com/en-gb/support/process-and-theory/Pages/welding-fundamentals.asp

I will be at college on Thursday and will ask if they have some papers on the theory behind the different welding techniques. Or can recommend something I can read (with pictures).

I know that no amount of theory will help in welding skills or laying good welds. But the reading and understanding keeps my mind active.

Adrian

 

[Richard.]

Hi Richard;

I appreciate what you say and I am not saying you or anyone is wrong, I am not disagreeing with any of your statements you have listed there, but I guess I want to see and be able to understand the physics behind each of them rather then just take it as read.

The voltages bit came from your statement


So I went off and searched the web looking at Welding penetration and found a Lincoln Page on the Variables that affect weld penetration. The thing I took from that was current. Now again I could be reading or getting points mixed up with reading to much, or about different processes.

http://www.lincolnelectric.com/en-gb/support/process-and-theory/Pages/welding-fundamentals.asp

I will be at college on Thursday and will ask if they have some papers on the theory behind the different welding techniques. Or can recommend something I can read (with pictures).

I know that no amount of theory will help in welding skills or laying good welds. But the reading and understanding keeps my mind active.

Adrian

Under stand the physics behind mma/GMAW?????????? It's all here and in tutorials.

Personally I'd say you've got that information between both threads. If your looking for a metallurgical point of view this could be a bit of a As there are so many consumables , fluxes, gases etc you would be better off been specific about a consumable/flux/gas etc and better off asking AL/Hotrodder.

If your going to college to learn welding. Why not just concentrate on the actual process rather than swamp your self with metallurgy .

 

[Richard.]

Wire speed(amps) = pen
Volts= spread
Travel = reinforcement.

Mma no wire speed but the pen still comes from the current.
That current is able to operate deeper pen than mig because the voltage burning the consumable is higher.

 

[Kent]

Adrian,
It might well be how you think and understand things but I promise you find the best 12 welders in Lancashire and ask the question and they will look blankly at you then turn to each other and say " what the heck is this guy saying? " or words to that effect the actual words will be different and less suited for written form on a forum
If you find one who can answer it all to your satisfaction ask him to run a bead and see what you get. Thinking too much about what your doing is realy counter productive it's like working out velocity of a clay target angle speed of shot etc you will miss every time
Just look hard at it. Move mount shoot. Reaction over interaction
Of course I am sure you could look this up and work it all out just like you can the clay pigeon but you will likely miss
Running a critical weld most will spend a bit more time setting up run a test piece and test it
Until fairly recently nobody actually knew more than that switch there the other there and wire about here as readout was not so good and we didn't have computers that did more than play pong and crash out a lot

 

[Wendelspanswick]

Knowing how a weld is formed at a molecular level might not make you a better welder but the quest for knowledge is never a bad thing!

 

[Richard.]

Adrian,
It might well be how you think and understand things but I promise you find the best 12 welders in Lancashire and ask the question and they will look blankly at you then turn to each other and say " what the heck is this guy saying? " or words to that effect the actual words will be different and less suited for written form on a forum
If you find one who can answer it all to your satisfaction ask him to run a bead and see what you get. Thinking too much about what your doing is realy counter productive it's like working out velocity of a clay target angle speed of shot etc you will miss every time
Just look hard at it. Move mount shoot. Reaction over interaction
Of course I am sure you could look this up and work it all out just like you can the clay pigeon but you will likely miss
Running a critical weld most will spend a bit more time setting up run a test piece and test it
Until fairly recently nobody actually knew more than that switch there the other there and wire about here as readout was not so good and we didn't have computers that did more than play pong and crash out a lot

Are you suggesting people who can weld don't understand the therory behind it and people who have spent time learning about it can't weld to a standard? Bold statement I must say if I've got that right.

 

[TechnicAl]

In my opinion, understanding how a process or equipment works has to improve your performance and your enjoyment.

I think Kents choice of the 10 best welders in Lancashire would probably differ from mine.

 

[Richard.]

I see nothing wrong or negative in trying to learn more about the process. So long as it's controlled learning. The negative can have effect by trying to take in too much too quick and nothing actually sinks in. especially if it's a complex involved subject like metallurgy. Gaining info off this forum from guys like AL isn't going to hurt. You'll either understand it or you won't. I'm not that clued up on metallurgy and yet I'm as interested in the subject as the op. I certainly wouldn't think for a second clever people in the field are useless with a torch. There are people on here that so prove that theory wrong.

 

[Kent]

I have met plenty who can repeate what they have read or been told years back but when you haven't got a remote and calling out to your mate for an adjustment what do you say. That's the point I am trying to make

Guys above thier mid forties finished thier time before sets had reasonable read out of performance so how exactly do you expect them to think in such terms as first recourse
It's always going to be " give me s tad more wire" or knock the Dail down a midges " etc

What you see feel and hear. If it was any different you could just issue a booklet but as already stated speed of travel. Length of arc angle of dangle are so much more relivent
Perhaps a little exaggeration to make the point clear and you can't learn this from a book or chart

 

[Richard.]

No your right there Kent there is no substitute for hands on experience but all these things play a part and there is a lot more to it. For example knowledgable guys that qualify as "true" welding engineers design complex structures calculated in such a way to eliminate stress points. They also design the wps's which will (if followed) create welds more suitable/stronger than twiddling knobs and going by sound. If a weld fails the welding inspector will say why it's failed the metallurgist can design/improve consumables to suit. I understand your an old school fabricator and you know your stuff as you've done it long enough. You can set your machine up and lash out strong welds. Guess what I can too. I am not a qualified metallurgist or a welding engineer. But I would like to say I could be one or the other One day as I'm on the right tracks, in the right job and have got a few things already under my belt that get me on that ladder. When ever a post comes up about a guy wanting codings/quals you don't seem that keen to agree with it but things have changed since the 80's and to be taken seriously these are the things that we need to approach.

 

[AdrianH]

OK not been around today, visit to Huddersfield then looking at Kitchen tiles etc.

But look chaps, I am not trying to start an argument on here, I know full well that there is one thing to understand the physics/maths/theory behind the welding and another to just know how to do a good weld one comes with study, and one comes with practice and following guidance given by others.

I spent a few years at University in Electronics and a life time in repairing things and designing radio systems, I learnt how a transistor works, not at the atomic level but understanding valence bands, doping to give extra electrons etc. I can and have designed circuits from my general understanding, the spec sheets of devices etc.

So all I am trying to do is understand some of the theory behind welded joints, I have not found that in the tutorials on here but what I have found is the techniques to weld.

All down to the post on someone asking could one use MIG multiple runs and being told to use MMA. When I looked up things on the internet, I got contradicting statements to what has been mentioned, but I would like to understand it all in context.

I started a simple worksheet for myself, say a fillet weld, throat size and length of weld, I will start working out how much wire is needed and how much of a particular MMA rod is needed, I will move on to current densities, heat input and time to lay the weld, (if I can find deposition rate stuff), and hopefully get a bit more of the understanding, and I will be happy, so lets not start any arguments, I will get to satisfy my mind eventually and I will continue to practice the welds as described to me by others when I can.

Adrian

 

[Richard.]

Hi ad. I don't think you would of found contradicting info on the net had the full facts been known.
The question was related around a 150 amp mig or mma for 10mm section.
Anyone suggesting the a hobby mig with as many runs as you like and as much prep work as needed is going to perform that joint as strong as 1 run either side with mma is incorrect. Regarding the two processes (providing the units are up to the job) BOTH will happily weld it nice and strong. As mentioned lots of times now you will need a mig with a minimum of 250 amps behind it to gain the fusion levels required to make it strong. If (which was the case) 10mm joint needed welding strong to cope with load and it needed doing with a 13amp plug then mma is the only suitable process.

 

[AdrianH]

And I am also limited to what I can get off a 13 Amp plug, (I can change it to a 20 Amp plug as I really need 16 Amps).

So I am interested in what I can and can not do with it.

I have no 10 mm plate around at the moment so I can not go in the garage and try anything, but I would ask this question, can you not get any penetration on 10mm plate with a 150 Amp Mig or will the penetration be , of in the order of 0.5mm?? What would be expected?

Adrian