This transition phenomena of Globular to spray mode as voltage , and especially current is raised, is very commonly known to Manual Metal Arc Welding (MMAW) and Gas-Metal Arc Welding (GMAW) practitioners, but physics behind it is invoked rarely. See for example
- (https://www.aedmotorsport.com/news/mig-welding-transfer-methods#:~:text=In%20MIG%20welding%2C%20there%20are,Spray%20Arc%20and%20Pulsed%20MIG.) Short circuit is the coldest form of MIG welding and uses low voltage. In the Short Circuit transfer method, the consumable electrode wire arcs and touches the base material and shorts. This creates a small, quickly solidifying, weld metal puddle that drips into the weld joint fusing the materials together sometimes referred to as “fast freezing.” Short Circuit method is great for thinner materials but you risk “cold lapping” on thicker materials. This method also creates an increased amount of spatter. ...
Globular transfer method is very similar to the short circuit transfer method, which the consumable electrode wire arcs and touches the base material and shorts. The difference comes in how long the consumable electrode melts. In Globular method, the wire is heated longer and creates a large volume of weld metal that drips into the weld joint. It uses a high heat input and also risks less fusion because of large amounts of spatter disrupting the weld puddle. You are limited to flat and horizontal fillet welds with this method.
- (https://www.esabna.com/us/en/news/newsletters/july-2013/selecting-the-mode-of-transfer.cfm)
Globular Transfer In the globular transfer mode, the weld metal transfers across the arc in a gravity feed. The droplets across the arc are usually larger than the diameter of the electrode. Globular transfer does not produce a very smooth weld bead appearance and some spatter can occur. The use of a globular transfer is usually limited to heavier plate thicknesses and limited to the flat and horizontal positions. Globular transfers are typically found in solid MIG wires, gas shielded metal cored wires and gas shielded flux cored wires when 100% CO2 shielding gas is applied.
Spray Transfer The spray transfer is named for the spray of tiny molten droplets across the arc, not unlike the spray coming out of a garden hose when the opening is restricted. A spray transfer is usually smaller than the diameter of the wire and uses relatively high voltage and wire feed speeds or amperage. Unlike the short circuit transfer, once the arc is established, the arc is "on" at all times. There is very little spatter with the spray transfer mode and it is usually used on thicker metals in the flat and horizontal positions. The spray transfer is normally found in solid MIG wires and metal cored wires with a high ratio of Argon in the shielding gas, usually above 90%. A partial or semi spray transfer is found in gas shielded flux cored wires when an Argon CO2 shielding gas is used.
- (https://www.jasic.co.uk/post/mig-welding-modes-of-transfer)
Globular Transfer Mode
The globular transfer method is in effect an uncontrolled short circuit which occurs when the voltage and wire are above the dip range but too low for spray. Large irregular globules of metal are transferred between the torch and work piece under the force of gravity.
The disadvantages of this method of transfer are that it produces a large amount of spatter as well as high heat input. In addition, globular transfer is limited to flat and horizontal fillet welds above 3mm. Lack of fusion is often common because the spatter disrupts the weld puddle. Also, because globular transfer uses more wire it is generally considered less efficient.
The advantages of globular transfer are that it runs at high wire feed speeds and amperages for good penetration on thick metals. Also, when weld appearance is not critical it can be used with inexpensive, CO2 shielding gas.
Spray Arc Mode
The Spray arc mode is used with high voltage and current. Metal is projected in the form of a fine spray of molten droplets of the electrode, propelled across the arc to the work piece by an electromagnetic force without the wire touching the weld pool. Its advantages include high deposition rates, good penetration, strong fusion, excellent weld appearance with little spatter as no short circuits are occurring. The disadvantages of the spray arc mode are mainly due to the high heat input which can cause problems on thinner material and the limited range of welding positions where the mode can be used. Generally, the minimum thickness to be welded will be around 6mm.
Some research papers on this very common welding phenomena include
............................................................................................................
Now my questions are
How high voltage and current Applied to GMAW makes the liquid metal droplets generated at electrode tip to be detached faster and in form of small droplets? Why large metallic melt droplets are not preferred at high V or I?
One might point out to the elevated melting rate at high V,I that causes the spray transfer, but how would higher liquid flow rate be related to disintegration into droplets? Is the scenario similar to liquid atomization at high flow rate (High Weber number scenario, lower pressure head due to high velocity head aspires surrounding gas and atomize fluid), even when phase transformation is involved here?
Liquid metal contains both +ve charged ionic core and -ve charged delocalized electrons in charge-balancing amount, so would there be any role of electrostatic/electrodynamic attraction on the molten droplet? I do not think so.
Smaller droplets' easily forming means lowering surface energy. Through which mechanisms some inert/active gases can affect surface tension of liquid metal? (excessive surface oxidation to molten liquid excluded?)
Sumit,
It is not by chance that you split your question into 7 private sub-questions. This question is quite capacious.
But you are right to pay attention to surface tension. I could try to answer your individual questions if you pointed to specific published experiments confirming the effects you indicated. I am most interested in the first two questions.
I would be grateful for more detailed information.
Thanks for the articles to help me understand the problem. It seems to me that the change in the diameter of the drops from voltage and current can occur as follows. With an increase in the arc power (product of current and voltage), the metal temperature rises. Surface tension decreases with increasing temperature as a result of the thermocapillary effect (dependence of surface tension on temperature). You can write down the droplet separation condition, process the experiments that you indicated, and get a real change in surface tension with increasing current. Perhaps it is possible to get the drop-off frequency. In principle, this could be the material for a new and possibly joint article. I will be glad to be of service.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Fluid Mechanics
- Fluid Dynamics