Understanding Common Welding Terms

What are the three main arc welding processes and how do they differ?

Stick is the casual term for SMAW (shielded metal arc welding)
Stick welding is a process that melts and joins metals together by heating them with an arc between a covered metal electrode and the work piece. Shielding gas is obtained from the electrode outer coating, often called flux. Filler metal is usually obtained from the electrode core.

MIG is the casual term for GMAW (gas metal arc welding), sometimes referred to as solid wire welding.
MIG welding is also an arc welding process that joins metals by heating them with an arc. This arc is between a continuously fed filler metal electrode and the workpiece. Shielding is provided by externally supplied gas or gas mixtures.

TIG is the causal term for GTAW (gas tungsten arc welding)
TIG welding is a welding process that joins metals by heating them with a tungsten electrode, which does not become part of the completed weld. Argon inert gas or inert gas mixtures are used for shielding, and filler metal is sometimes used.

What is a shielding gas and what are the common types used?
Shielding gas is a protective gas used to prevent contamination from the air from affecting the weld pool. The welding process you're using and the base material you are welding will dictate what shielding gas you need for the job. For instance, 100% argon is a common shielding gas for TIG welding, while 75% argon/ 25% carbon dioxide mixture is common for MIG welding mild steel.
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What is filler metal?
​Filler metal is the the metal or alloy added in making a welded joint. Basically,  what fills the weld joint or the gap between the parts. These filler metals come in a variety of types and sizes.
For MIG you'll find spools of solid wire, a continuous length of wire that is wound onto a spool. While electrodes, metal rods with a flux coating, are used in stick welding to protect the weld from contamination. And solid metal filler rods are used in TIG welding.

What is amperage?
Amperage is the measurement of the flow of electrons moving in a circuit and is sometimes referred to as current. The number of amps produced by the power source determines the amount of heat available to melt the filler metal and the workpiece. So the more amps your machine is able to provide, the more heat you're able to use. If thicker material is being welded, you will need more amps.
The different types of current used in welding is also important. Alternating current (AC) reverses its direction at regular intervals and is commonly used for aluminum welding. While direct current (DC) flows in just one direction and does not reverse flow. It generally provides a smoother welding output, with less spatter and a more stable arc making it the preferred form of welding for most applications.
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What is voltage in welding?
Voltage measures the pressure of the electrons flowing through the electrical current. So it doesn't flow, but it causes amperage to flow. Your voltage controls your arc length and as your voltage increases, the weld bead will flatten out.
Regardless of the amperage output, a constant voltage (CV) welding power source has output that provides relatively stable and consistent voltage. However, a constant current (CC) welding power source has limited maximum short-circuit current. Welding power sources are CV, CC or both. Processes that require a stable current to maintain a consistent arc length to prevent the electrode from sticking, like Stick or Tig, use CC. While MIG and flux-cored welding require a stable voltage to maintain a consistent wire feed speed to prevent the welding arc from becoming unstable so they use  use CV.

What is slag?
The term slag is used to describe the hardened layer left on the top of a weld made using MIG, flux-cored or stick welding. This layer protects the weld from oxidation and atmospheric contamination. It also helps keep the molten weld pool in the joint as it cools, something that is especially important for out-of-position welding. Slag can be removed after welding or in between passes by chipping or grinding.
What is porosity?
Porosity is a defect in a finished weld that is caused by gas entrapment while the weld is solidifiying. This issue is most commonly seen in MIG welding.
What is spatter?
Spatter is the droplets of molten material blown away from your welding arc. These particles are not part of the completed weld, but they may stick to the metal workpiece. This requires chipping or grinding off after welding.