MIG Welding

101 flashcards by MentisLibrary

GMAW from the machine outward: the four transfer modes, shielding gas choice, wire selection, voltage and wire feed speed, gun and drive roll setup, technique by position, and the defect each wrong setting produces.

What does GMAW stand for, and what is it called in the shop?
Gas Metal Arc Welding. Welders call it MIG welding, from the older term metal inert gas.
Why does MIG run on a constant voltage power source?
Constant voltage holds arc voltage nearly steady and lets current rise or fall on its own, so arc length stays stable while the wire feeds at a fixed speed.
What is arc self regulation in a wire feed process?
If the gun moves closer, the arc shortens, current rises, and the wire burns off faster until arc length returns to where the voltage setting puts it.
Name the four metal transfer modes used in GMAW.
Short circuit, globular, spray and pulsed spray.
Describe short circuit transfer.
The wire touches the puddle, shorts, pinches off and re-establishes the arc many times a second. Metal crosses only while the wire is shorted, never across the open arc.
Why is short circuit MIG a classic source of lack of fusion?
The arc is on only part of the time and heat input is low, so wire can be laid onto a joint face that never reached melting temperature.
Describe globular transfer.
Drops larger than the wire diameter fall across the arc under gravity. It shows up with CO2 shielding, or at voltages between short circuit and spray.
Describe spray transfer and what it requires.
A fine stream of droplets smaller than the wire crosses the arc without ever shorting. It needs an argon rich gas and a current above the transition point.
Why is conventional spray transfer limited to flat and horizontal work?
The puddle is large and very fluid, so gravity pulls it out of the joint in vertical and overhead positions.
What does pulsed spray transfer do that conventional spray cannot?
It alternates a high peak current that detaches one droplet with a low background current that keeps the arc alive, so average heat input stays low enough for thin material and out of position work.
What is duty cycle, and how is it stated?
The share of a ten minute period a machine can weld at a given output without overheating. A 250 A machine at 40 percent can run 4 minutes at 250 A, then must cool for 6.
MIG or stick: which would you choose for windy outdoor structural repair?
Stick, or self shielded flux cored. MIG loses its gas shield in wind and turns porous.
What is the main production advantage of MIG over stick welding?
Continuous wire and no slag. There is no stub loss, no electrode changes, and no chipping between passes, so arc-on time per hour is far higher.
What is metal cored wire, and how does it behave?
A tubular wire filled with metal powders and deoxidizers rather than slag forming flux. It runs on gas shielding, DCEP, and gives a spray-like transfer with very high deposition and almost no slag.
Which polarity does solid wire MIG use, and why?
DCEP, direct current electrode positive, also called reverse polarity. It gives a stable arc, good bead shape and reliable transfer with solid wire and gas.
Which polarity does self shielded flux cored wire normally use?
DCEN, electrode negative. Gas shielded flux cored and metal cored run DCEP, like solid wire.
What do the drive rolls do, and how do the three common groove types differ?
They grip and push the wire. Smooth V-groove is for solid steel wire, knurled V-groove for tubular flux cored wire, and U-groove for soft aluminum wire.
How do you set drive roll tension correctly?
Hold the gun so the wire feeds into a gloved hand or a block of wood at a few inches distance and back the tension off until the rolls slip, then tighten just past that point.
What is birdnesting, and what causes it?
The wire tangles into a nest between the drive rolls and the gun. It is caused by anything that stops the wire at the gun while the rolls keep pushing: a clogged or kinked liner, a plugged contact tip, or excessive roll tension.
What does the gun liner do, and how is it matched to the job?
It guides the wire from the feeder to the contact tip. Steel wire uses a coiled steel liner. Aluminum uses a nylon or PTFE liner, because a steel liner shaves the soft wire.
What is the contact tip's job, and how do you know it is worn?
It transfers welding current to the wire in the last inch before the arc. A worn tip has an oval or keyholed bore, feeds erratically, and causes an arc that wanders and burns back.
Where should the contact tip sit relative to the nozzle?
For short circuit work, flush with the nozzle or extending up to about 1/8 in. For spray transfer, recessed roughly 1/8 to 1/4 in inside the nozzle.
Why does spatter build up in the nozzle matter?
Spatter narrows the nozzle bore and disturbs gas flow, which starves the puddle of shielding and shows up as porosity.
Where should the work clamp go, and why does it matter?
On clean bare metal as close to the weld as practical, clamped tight. Paint, rust and mill scale at the clamp add resistance and heat.
What is a spool gun, and when do you need one?
A gun with a small spool of wire mounted at the handle, so the wire travels only a few inches before entering the liner. It is the standard answer for feeding soft aluminum wire.
What is a push-pull gun?
A gun with a motor at the handle that pulls wire while the feeder pushes, keeping the wire under light tension over long cable runs.
What is the difference between a regulator and a flowmeter?
A regulator reduces cylinder pressure to a usable working pressure. A flowmeter measures and sets the volume of gas per unit time, usually in cubic feet per hour.
What is C25 shielding gas, and what is it used for?
A mixture of 75 percent argon and 25 percent carbon dioxide. It is the standard short circuit gas for carbon steel.
What are the trade-offs of 100 percent CO2 shielding on carbon steel?
Deeper, wider penetration and lower gas cost, at the price of a harsher arc, much more spatter, and no spray transfer.
Which shielding gas supports spray transfer on carbon steel?
An argon rich mix, commonly 90 percent argon with 10 percent CO2, or 98 percent argon with 2 percent CO2.