What causes porosity during welding?

In any welding process, porosity can be caused by the presence of contaminants or moisture in the welding zone, which includes the base metal, filler metal, shielding gas, and the surrounding atmosphere. Contaminants can include oil, dirt, grease, or cutting fluids. Concurrently, moisture can collect in the flux, shielding gas, or on the base metal, or come from the atmosphere.

Porosity occurring in a welding process that utilizes an external shielding gas can occur from using too much or too little gas flow, poor gas quality, or a defective welding torch, gun, or hose.

Operator technique can also cause porosity. Electrode, torch, or gun angle can lead to porosity, as can excessive arc length, electrode extension, or travel speeds.

Plasma transfer arc (PTA) welding is a process, in which the joining of materials is produced by the heat of a constricted arc between an electrode and a base metal.  In PTA welding, a shielded arc is struck between a non-consumable electrode (Tungsten) and the torch body, and this arc transforms an inert gas (Argon) into plasma by heating it to a high temperature. The PTA welding process uses this plasma to transfer an electric arc to a work piece. Metal powder is metered, under a positive pressure of Argon flow, from the bottom of the torch into a pool of molten metal on the workpiece surface.

The torch is then either moved by a side-beam carriage over the workpiece, or the workpiece is rotated or moved under the torch, or a combination of both to produce a weld overlay deposit. The plasma arc deposit is fully dense and metallurgically bonded to the workpiece.  One of the most important features of the PTA process is the control of dilution. PTA produces dilution as low as 5%, compared to 20-25% typically obtained when hard-facing by MIG and (TIG) processes.  So it is possible to maintain the noble properties of deposit even in one single pass.