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.
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 work piece movement are automated and computer controlled.
PTA hard facing 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; multi pass 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.
In PTA hard facing, 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 work piece (+). The pilot arc is struck by a High Frequency device and the plasma gas flowing around the cathode is ionized at the electrode tip. When the transferred arc is ignited, the work piece becomes part of the electrical circuit and the plasma arc is directed and focused through the torch orifice into the work piece. Powder is metered, under a positive pressure of Argon flow, from the bottom of the torch into a pool of molten metal on the work piece surface.
The torch is then either moved by a side-beam carriage over the work piece, or the work piece is rotated or moved under the torch to produce a weld overlay deposit.
The plasma arc deposit is fully dense and metallurgic ally bonded to the work piece. 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 hard facing by MGAW (MIG) and GTAW (TIG) processes. So it is possible to maintain the noble properties of deposit even in one single pass.
Gas Tungsten Arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used.
GTAW is used to produce a homogenous and low dilution weld overlay of various materials over Mild Steel and Stainless Steel substrates. Some of the materials deposited with GTAW are as follows :
The process can also be automated with PLC controlled manipulator and a voltage feedback (AVC) which senses the voltage and corrects the torch distance. Using Auto-GTAW a uniform overlay can be achieved and process completion time can be reduced significantly.
After desired thickness is achieved the coating is further re-heated and taken to solidus temperature (1020 to 1100 ºC).At this temperature coating fuses and voids and porosity in the coating collapses making the coating more dense and homogeneous. This also forms a fine metallurgical bond with the prepared substrate which makes the coating capable of enduring impacts.
Spray and Fuse is an extension of coatings done by Powder Flame Spray process wherein the sprayed coating is fused with the base material by application of external heat energy using Oxy-Acetylene torch, Induction coil, Furnace, Laser beam or an external heating source. As the process requires the application of heat to fuse the coating to the substrate, consideration shall be given to the possible effects of such heating on the substrate, including distortion, scaling, need to stress relieve and irreversible transformation of mechanical and/or metallurgical properties. Spray and Fuse can be carried out in two ways :
This is a manual process using an oxy/acetylene torch fitted with a hopper. A suitable self-fluxing powder is fed via the hopper into the gas stream, through the flame and on to the component where it is simultaneously fused. The continuous process of pre-heating the work-piece with the flame through which the powder is fed and fused produces a coating with properties dependent on the choice of self-fluxing powder.
It is a manual or mechanized process using a powder flame spray gun to apply the required thickness of coating material to the component. This sprayed coating is subsequently fused. A selected self-fluxing alloy powder is fed via a powder hopper into the carrier gas, through the flame, on to the work-piece until the desired thickness is achieved. Fusing of the deposit is a separate operation performed as soon as possible after spraying.
In both the above methods, the coating is taken to solidus temperature (1020 to 1100°C). At this temperature coating fuses and voids and porosity in the coating collapses making the coating more dense and homogeneous. This also forms a fine metallurgical bond with the prepared substrate which makes the coating capable of enduring impacts. The rate and duration of heating and the temperature range is critical and will vary according to the composition of the coating alloy and the size and complexity of the component. A very good indication of proper fusing is the appearance of the coating. Once properly fused, the coating will have a reflective orange surface or a “glaze”. At this point the reflection of the gun/flame can also be seen on the coating surface.
A variety of materials can be deposited by this process to achieve properties required for different critical applications such as :