How do MIG welding machines work?

Welding process | MIG - 135

MAG 135, Blog post for Youtube channel of Arc-welding

This is a welding process in which the heat source is an electric arc between the workpiece and a fusible electrode (filler metal)
within a protective atmosphere flowing from the torch nozzle.
This atmosphere is said to be active insofar as the gases used are no longer inert gases but binary or ternary mixtures of inert gases with active gases.
The feed rates are higher than in TIG and the polarity is indirect (electrode at the positive pole of the generator) in direct or pulsed current.
The molten metal is deposited in several modes
(short circuit, globular, axial spraying and pulsed) depending on the energy (voltage and current),
the diameter of the filler metal (current density that can pass through the electrode) and the gas (the arc potential depends on the nature of the gas).
This transfer mode is an essential variable in the operating mode.
This process is also referred to as "semi auto", there is a self regulation of the arc.
When the torch moves away from the bead, the intensity increases, the rate of deposition also increases but the arc length (and therefore the voltage) remains constant.

The station will regulate the intensity according to the distance of the torch from the bead.
The gas (usually an Argon based mixture - with the addition of CO2, Oxygen, Nitrogen, Hydrogen) protects the melt,
the filler metal, affects the transfer mode, the shape and the chemistry of the bead. The gases used for this process are generally mixtures.
The gas-wire combination for semi-auto applications is an essential variable.
Depending on the materials to be welded, the gas will have to be adapted:
limited CO2 content for stainless steel (less than 5%) to avoid increasing the % of C (carbon), nitrogen should be avoided for non-alloyed steels
(porosity and ageing) as should hydrogen (cold cracking).

The use of gases such as CO2 will bring oxygen into the atmosphere around the bead.
This is why the filler metal is also enriched with deoxidising elements (Si in particular).
These elements are found in the form of oxides in the weld bead (the silicates, SiO2, which are so hard to detach....)
The good condition of the welding system is important because certain parameters can cause irregularities in the operation of the machine:
wear of the contact tube (change of the contact point), wire drive rollers (slipping or blocking of the wire), condition of the sheath, clogged nozzle, etc.

How steel is processed? TIG 141, Blog post for Youtube channel of Arc-welding

Defects related to the process 135 - MAG

Greyish cord (on stainless steel): poor gas protection, gas flow.
Concave bead: Not enough filler metal.
Channel: chamfer too closed, current too strong, wrong orientation of the torch, especially for fillet welds, feed speed too high.
Wrong geometry for fillet welds: Wrong orientation of the torch, irregular feed speed, stability of the torch, too high a feed speed, arc blowing (presence of a magnetic field).
Filler metal spatter: Unwinding speed too high, gas not suitable for filler metal, contact with wire too long (bending away from torch....), torch tilt.
Uneven penetration: Uneven feed rate, torch stability.
Excessive penetration: Unwinding speed too high in combination with slow feed speed, wrong torch tilt, too much play.
Lack of penetration: Current too low, root clearance (joint preparation, pinching during welding), gaseous atmosphere.
Lack of fusion / bonding: Current too low or speed too low, root clearance and chamfer (access to the bottom of the joint), Energy too low, position and type of joint (heat input / heat output). Beware of short circuit mode.
Oxide inclusion: Si-enriched filler metal, silicates end up in bead skin and must be ground off.
Blowout / Porosities: Poor degreasing of welding edges, poor gas protection (flow rate, clogged nozzle, nozzle or torch leakage, end of cylinder and feed speed too fast), type of gas used (nitrogen, CO2 content), dirty wire (coil lying around the workshop), leakage in the cooling water circuit, volatile elements in the steel or unquenched steel