short circuiting, globular, spray and pulsed spray GMAW

Date 01-05-2013 02:21

That's because most people don't know the difference. They think wire is wire. Some call all of it MIG welding, wire welding, etc. They have no idea the difference between voltage settings, wire feed speed, gas shielding, machine capabilities, and other variables that determine which mode of the GMAW process you are using.

Let's start by making sure there is an understanding of the differences between GMAW (Gas Metal Arc Welding) and FCAW (Flux Cored Arc Welding). GMAW is generally a solid wire with an inert shielding gas. FCAW is a flux cored wire with (commonly referred to as dual shield) or without (commonly referred to as innershield) a shielding gas.

When it comes to GMAW we have several choices in how we use and utilize the process.

Short Circuiting Transfer- low voltage (about 16-22 depending upon wire size and usage), low wire feed speed, gas shielding with Ar/CO2 mixes of 75/25, 80/20, or 90/10.
Spray Transfer- high voltage (25-30 maybe even higher), high wire feed speed, and gas shielding with Ar/CO2 of 90/10, 95/5, or Ar/O2 of 98/2. Can be done with a mix of 80/20 but not as well.
Pulsed Spray- controlled pulsing by the machine power source that can be set for it's on/off frequency, time/duration on both on and off settings, but basically a spray transfer function. To explain further, would be similar to using the spray at high heat but it is too hot for what is being done so you use the trigger to arc, off, arc, off, etc, giving the weld pool time to cool so as to not blow through the steel.
Globular- set basically like spray but with the wire speed feed reduced. Has special applications but it not generally used. To many, including myself, it is just a wrongly adjusted machine that is not set properly for either short arc or spray.

Have a Great Day, Brent

By DaveBoyer (*****)

Date 01-05-2013 04:39

To elaborate some on what Brent posted, with short circuit transfer, the end of the wire actually touches into the weld puddle, then burns off creating an arc. This cycle repeats continuously many times per second. Shielding gas can be CO2 or the argon CO2 mixes like Brent mentioned. This works in all positions.

In spray transfer, the wire only touches the work to iniate the arc, then a spray of droplets SMALLER THAN WIRE DIAMETER melt off the end of the wire as it feeds toward the work. The droplets go rather uniformly to the work in a predictable, pattern with little spatter when done right. Shielding gas is usually over 80% argon, but spray can be made to happen with 75% argon & 25% CO2 with enough fussing on a good day. This works in the flat position.

Globular transfer has droplets LARGER THAN WIRE DIAMETER that go all over the place and leave a lot of spatter everywhere. This process was used with CO2 for deep penetration, which it does give, but deposit efficiency is not good and cleanup costs high due to the excessive spatter. This works in the flat position. There is little reason if any to use this process ever.

Pulse allows some of the advantages of spray transfer to be had in an all position process, as well as offering these advantages on materials thin enough to make spray transfer problematic. Proponents claim higher quality than other GMAW processes, and in some cases it might actually be true. Pulse transfer requires a power supply which can deliver the background and peak power levels at the proper time durations. While these have been available for 20+ years really good ones have only been available for about the last 5-10 years from the major American manufacturers.

By welderbrent (*****)

Date 01-05-2013 14:15

I am more than a little grateful for the additional comments by Dave and Obewan. In my efforts to keep it short and simple I missed a couple of key differences, the use of 100% CO2, the differences in droplets, the fact that D1.1 does not allow for Short Circuit Transfer as a Pre-Approved process (you must get it approved by a PQR test and even then the engineer probably will not allow it's use except in special applications).

Hopefully between the three of us you have a pretty good idea of the differences. You may be aware of many of these differences but had never heard the correct terminology applied to the changes in usages. Having been around a good many welders in many parts of the country during the past 40 years I understand that there is much confusion because of all the slang terms associated with GMAW as well as other wire processes.

That's a good reason to be part of AWS, this forum, and purchasing of standards available here. Education is not just limited to 'schools'. One can be self taught if they are truly interested in knowing correct procedures, terminology, and applications. And the more we know about our chosen profession the less confusion there will be over word usage, process application, etc.

Hopefully we have answered your question. I know we have not gone real indepth with our answers and there are acceptable variations in some of the gas shielding, voltages, amperage/wire feed speed numbers that we used. The main point was to simply describe the differences for you so yourself and others can understand the terminology.

Have a Great Day, Brent

By OBEWAN

Date 01-05-2013 13:43 Edited 01-06-2013 07:37

I am glad you are asking these things because there can be a huge difference in penetration and weld quality between the different modes. But, if you have been using published weld procedures for all those years, then those issues were most likely covered and there is no need to state the modes in the procedures as long as the parameters are published.

Many welding codes forbid the use of short circuiting mode for structural work due to light penetration. It is easy to tell the difference between modes. I can even hear the difference by just listening to the arc. Short circuiting has a buzzing sound and spray transfer has a hissing sound.

If you are interested in details on parameter ranges there are many good free booklets that might be available from the welding equipment manufacturers and gas distributors.

In general though, to switch from short circuiting to spray transfer, around 80% or more argon is requried ( some people debate the limits). On electrical, voltage makes a huge difference with around 28-30 volts required to transistion from short circuiting to spray transfer. (Some people debate these limits as well.) Then start increasing the current until there is a sharp point at end of the wire where the arc attaches and you hear a hissing sound. If the wire starts "stubbing out", you are going back out of spray mode. It is not possible to spray with close 100% CO2, but the penetration worries can be overcome by using globular transfer, which is like a mixture of the modes, and it has more splatter.

These are simple explainations, but hopefully they will give you the "big picture".

By pipewelder_1999 (****)

Date 01-06-2013 14:04

Some great explanations above. Wish I had something to add. But here are some videos from another source that show the process variations.

Spray Transfer http://www.youtube.com/watch?v=gTFhh8dAK10&list=UUPat-i5SoYDT0sN16OaIqGw&index=34
Short Circuit http://www.youtube.com/watch?v=hBvEkdSxTuQ&list=UUPat-i5SoYDT0sN16OaIqGw

Globular http://www.youtube.com/watch?v=ngrqcDdVyhM

Here is another that is not high speed or up close but lets you see and hear the welding. Its short circuit and spray only.

http://www.youtube.com/watch?v=PI3pZxoPRvA

Gerald Austin

By welderbrent (*****)

Date 01-06-2013 01:50

Notice too that gas shielding composition is a major contributor.

And "by feel" is out with WPS's that specify the welding perameters. That's what happens when you work to codes and proper welding practices. You don't get to choose. Proven past experiences and practices set the norm, not your gut, feel, and 'cause we have always done it that way'.

Have a Great Day, Brent

By OBEWAN

Date 01-06-2013 03:37

It is not just about penetration. Sometimes is is about lbs. per hour wire deposition rates. There have been many many studies done on the cost options of various different shielding gas options vs. the lbs. per hour wire deposition rates. There is a cost benefit curve that can be plotted. CO2 is cheaper than argon oxygen mixtures, but at some point for very high deposition rate welding there is a point on the curve where the payback for spray arc welding exceeds the higher cost of using more argon. Of course, if the parameters are set incorrectly, the benefit will never be realized.

Spray arc can be very hot! I have run spray arc processes at 450 to 500 amps that were "hi dep" processes. I cooked and badly blistered the back of my hands with one of them once - and I was wearing gloves. They even used to make special gloves with shiny reflective material to deflect the extra heat - like those asbestos fire suits that firemen wear!

By Lawrence (*****)

Date 01-06-2013 06:09

I disagree that most welders work by "feel"

Manufacturers who allow their welders to produce by feel have mostly gone out of business or the work is shipped overseas, where welders who work by feel are paid considerably less.

Quality production is almost always governed by codes or internal standards that clearly provide the welder guidence regarding variables like, shield gas, wirefeed speed/amps and voltage..... Using terms like "more heat" mean almost nothing in the world of GMAW unless you have a very firm grasp of how amperage, voltage and contact tip to work distance relate within the framework of a written procedure. Only then are you likely to have consistantly good results when working without a procedure.

Understanding process is important.. Especially with GMAW, a process that can leave a weld that looks very good on the top but may have serious fusion problems in the sidewalls and root.

It's good that you are asking questions about transfer modes... Understanding the process you use makes you valuable.

The guys have given you a very good "beginning" description of GMAW process... How each mode of transfer is applied is a much larger question.

By hacksaw (*)

Date 01-06-2013 20:28

I've welded on ASME coded pressure vessels, hastoloy, Inconel, monel and chrome piping, much of which was x-rayed to "sever-cycle" and have never been required to follow this. I wouldn't have a problem following it as long as it worked in the real world, and not just on paper. Everytime I've seen radical changes in shop welding processes, because of advice from QC experts, the results are terrible, and cost the business a large sum of money to fix bad welds that don't pass x-ray. Some good examples of this is suddenly changing the brand of wire or the type of gas used to increase production. Much of this stuff will only work if the shop is welding a large amount of the same type of product in the same position on the same alloy, everyday. In this case you can tweak the process by ironing out the wrinkles. I've never really worked at a place like this before. I've always been requires to weld on many different things, in all positions, with many different processes. That's why we're called combination welders. We have to weld everything, at anytime, without doing a practice coupon first.

By aevald

Date 01-06-2013 22:24

Hello hacksaw, although I do believe that you have indeed been successful on all of the items that you have mentioned regarding welding by "feel", we no longer live in the type of world that allows many of us to simply operate that way. Responsibility, liability, and any other term that you care to allow a lawyer to include in their summation has long since changed the "old ways" of doing things. As to a WPS, they are likely developed by a welder such as yourself or other similarly talented individuals along with QC departments/welding inspection folks and incorporated into the production and veryfication systems that support and can be referenced by shops and faciities who need those sorts/types of things.
Not experiencing failures with your welding applications and experiences likely shows that you have essentially been compliant with a WPS if there was one applied to your work. Not having a WPS to back-up what you have done is a recipe for disaster should there be an issue that someone wants to press in a court of law. "Most" WPS's include ranges that allow for a considerable amount of variance to allow for differences in welder's comfort zones(we all know that there are few welders who weld "comfortably" or in the same exact manner), yet there are still limitations to these ranges and they generally are backed up with destructive testing and other forms of testing to insure successful welding practices. Sorry to keep harping about the legal mumbo-jumbo, yet this is the world that we live in and sometimes it is necessary and other times it is likely taking it a bit to the other end of the scale. Well there's my $.02 worth. Have a great day and best regards, Allan

By Lawrence (*****)

Date 01-06-2013 22:41

Obviously you have a golden arm Hacksaw. Thank you for the lengthy pedigree. Now there is no mistaking how good you really are.

If you have welded on coded pressure vessels and had your work radiographically tested, but have never seen a welding procedure specification, this is an obvious sign that you are a poorly managed master crafaftsman who is working for bosses who should be fired for not doing their jobs.

Glad we got that all cleared up.

But again it's good that you are beginning to ask some technical questions (some day that golden arm may begin to shake) and learning the basic transfer modes of GMAW is a good place to begin to iron out those technical wrinkles..

Cheers

By pipewelder_1999 (****)

Date 01-07-2013 00:36

Hello Hacksaw,

I too have welded a few different things. One of the things that I have encountered often is a variation in individual companies sharing of information with welders. Sometimes they allow us just to "weld along" and other times they are micro managing different details. And sometimes they provide information to us with an understanding that we actually read some of the stuff that was in our 11th grade welding textbooks.

As a welder with some experience in the QC side of things, I think that the procedures, specifications, and quality control are all part of a companies desire to either
(1) Comply with a jusridictional, customer supplied, or engineered requirement or
(2) Do the previous AND constantly improve the quality and reliability of what is welded.

I have seen a many field combo welders swear up and down that FCAW will never work on ductwork and seen 1st year apprentices with a background in a fab shop smoke em out of a duct welding 4 to 1 the length of weld. The technology for welding is improving every day. Those that embrace the technology and experiment with it to see what works and what doesn't work will lead their industry.

The transfer modes are a basic part of the knowledge of a process. Not only are the modes different, they also affect the ranges of qualification for a welder performance qualification and welding procedure qualification for many codes and standards. Here is an article from the NBIC (The organization responsible for the code that you have worked under many times if doing any boiler or pressure vessel repairs). http://www.nationalboard.org/Index.aspx?pageID=179

Many of the process variables that are contained within a welding procedure fall within a useable range. It is often the job of the welder to apply the "feel" to get everything going right for that specific joint. However that does not mean the details in the procedure are of no value. The ranges are there for a reason. I do know that many times, you and I never see the WPS until the day the 3rd party or custiomer inspector comes.

Almost all of the variables on a welding procedure can affect the weld in some way. Transfer mode, which is related to Voltage, Wire Feed, Electrical Stickout, shielding gas etc, is a very important part of GMAW and has been for years. Its not something new. Or at least not to me. However I have had very few employers bring it up to me in the shop or field. They did often give me a WPS which did refer to it.

When I am on the QC side of things working within the requirements of a code, you can be sure I am checking the variables as related to GMAW. I am pretty sure there is a power company engineer up north that was not happy with me for bringing up the fact that he approved a GMAW Short Circuit Transfer WPS written for 3/8" thick material in 1 1/8" wall pipe. And the welders had only tested on 3/8" plate.

Anyway. Hope some of the info helps. Understand that many of the experience people have can vary widely from yours. Even within different companies working within the same industries, I have see wide ranges of how welding is controlled. In my current job as a maint welder on piping machinery and tanks at a paper mill, my only requirement is to "fix it". I use some of that stuff I learned in the 11th grade when it gets outside the scope of 70SX, 6010, and 7018. But I also read TAPPI, NBIC, and ASME articles whenever possible.

Have a nice day

Gerald Austin
weldingclassroom.org
course.weldingclassroom.org

By eekpod

Date 01-07-2013 11:37

That was some of the best explanations I have seen in a long time, thanks for that reminder.

Hacksaw
Most of the newer machines have pre-set programs in them that will automatically do spray, pulsed, whatever. You have to look up the thickness of the material you are welding, the sire diam. and shielding gas, then it will tell you what "program" number to set the machine on and make adjustements from there. Our Newer Lincoln machines were like that but I don't recall the model.