Plasma Transferred Arc Process ( PTA Process ) is used to fuse a metallic coating to a substrate in order to improve its resistance against wear
and/or corrosion.
This technique is called hardfacing, wear surfacing, or more commonly wearfacing.
During the process, metal powder is fed into a molten weld puddle (fusion bath) generated by the plasma arc at high temperature (up to 20,000 °C).
All welding parameters, including powder feed, power
input, plasma gas and shielding gas, as well as torch and workpiece movement are automatized and computer controlled in PLASMA TEAM equipment.
PTA hardfacing is a true welding process, with a metallic bond between the substrate and deposit.
Deposit thickness can range from 0.6 to 6.0 mm, width from 3 to 10 mm when using a single pass; multipass welding reaches deposit thickness up 20 mm and width over 30 mm.
The core of PTA process is PLASMA. The plasma (a gas sufficiently ionized to be electrically
conductive) can be viewed as the natural state of matter (the so called fourth state of matter), with the other states existing only as variants to the normal.
Plasma state constitutes more than 99.9% of all matter in the universe. Thermal plasma describes a gas which is at least 1% ionized, with a temperature greater than 13,000 °C, and is a good electrical
conductor.
In PTA hardfacing, two DC power supplies are used to first establish a non-transferred arc (pilot arc) between the tungsten
electrode (-) and the anodic nozzle (+) and then a transferred arc between the tungsten electrode (-) and the workpiece (+).
The pilot arc is struck by an High Frequency device and the plasma
gas flowing around the cathode is ionized at the electrode tip.When the transferred arc is ignited, the workpiece becomes part of
the electrical circuit and the plasma arc is directed and focused through the torch orifice into the workpiece. 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 to produce a weld overlay deposit.
The plasma arc deposit is fully dense and metallurgically bonded to the workpiece. The deposit microstructure is dense, with formation of dendrites during solidification.
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 hardfacing by MGAW (MIG) and GTAW (TIG) processes. So it is possible to maintain the noble properties of deposit even in one single pass. |
