The plasma transferred arc weld surfacing process
TWI Bulletin, June 1985
Dick Sharples
Dick Sharples, BA, is a Research Metallurgist in the Materials Department.
The Welding Institute has recently purchased Plasma Transferred Arc (PTA) equipment for weld surfacing. The results of trials to evaluate the process are described in this article together with a brief comparison with other surfacing methods and a summary of applications.
The principle of constricting the plasma arc, so that it becomes more stable and directional, was developed in 1955 [1] for the purpose of metal cutting. With modifications in torch design, which made the plasma arc more suited to surfacing, [2] it was subsequently applied to weld surfacing and equipment became commercially available in 1961. [3]
During the PTA surfacing operation an arc is struck in argon between a non-consumable, thoriated tungsten cathode and the work piece. The arc current creates an argon plasma which is constricted by a nozzle within the torch, to form a high power-density welding heat source relative to conventional welding arcs. A powder consumable is metered into the plasma column through two apertures in the nozzle and carried by a shielding gas (argon or Ar/5%H 2 ) ( Fig.1). The process thus offers independent control of arc energy, penetration and deposition rate. A weld pool forms on the substrate and is protected from the atmosphere by shielding gas flowing concentrically around the plasma arc. Fusion of the deposit with the substrate occurs and dilution levels can be controlled to give consistently low values in 'single pass' deposits.
Start-up is carried out by first striking a nontransferred pilot arc, which is initiated by high frequency current, between the tungsten cathode and the inner nozzle which acts as the anode ( Fig.1). Ionisation of the argon occurs, providing a current path for the main arc to be transferred to the substrate.
Fig.1. The plasma transferred are process: Eutectic + Castolin equipment
Equipment
The PTA equipment at The Welding Institute was supplied by Eutectic/Castolin and consists of a 70 volt open circuit power source rated at 225A continuous, 300A maximum, a console for control of current, voltage and gas flows, a powder feeder capable of delivering powder at rates from 15 to 45g/min, and two welding torches. Deposits from both torches are made in the flat position, one torch is oriented vertically and has a current capacity of up to 200Acontinuous ( Fig.2) while the other is horizontally oriented, being designed for surfacing internal bores (down to 90mm diameter and up to 250mm in length) with a current capacity of 160A continuous ( Fig.3). The work piece is traversed beneath the welding torch, which is mounted on a weave unit allowing adjustment of weave frequency, width, dwell and torch position.
Fig.2. Vertically oriented torch
Fig.3. Horizontally oriented torch, for surfacing of internal bores
Process characteristics
The Welding Institute's equipment was used to carry out a series of trials to evaluate the characteristics of the process. Beads of Stellite 6,* a commonly used cobalt-based hardfacing material, of between ~1-6mm in thickness and ~20mm in width were deposited; the corresponding range of deposition rates was 0.5-3.5 kg/hr and of arc energies (calculated as voltage x current/travel speed) was 1.5-6 kJ/mm. The limit on overlay thickness was imposed by current capacity of the torch at the thick end of the range, and by minimum rates of stable powder feeding at the thin end.
* Stellite is a registered trade name of the Cabot Corporation.
Beads of 4mm and 2mm thickness were deposited with dilutions as low as 1% and 3% respectively, although it was found that beads of ~1 mm thickness could be deposited only at dilutions of greater than 20%. Adequate fusion with the substrate was achieved throughout. Work at The Welding Institute demonstrated that dilution may be controlled to within ±2-3% of the working value, at worst, thus enabling deposits to be made at a working level of 5% dilution. Harris and Smith, [4] in a systematic study of the process, concluded that a working level of 3% dilution could be adhered to without any significant risk of lack of fusion. Control of dilution is largely through adjustment of welding current and plasma-gas flow rate; for 4mm thick beads of Stellite 6 there is a 2-3% dilution increase per 10A increase in current, and a similar increase in dilution per 1 litre/min increase in plasma-gas flow rate. For normal running the plasma-gas flow is set at 3-4 litre/min, and a shielding gas flow of 8-10; litre/min is usually adequate.
The deposition rate for the PTA process is equal to the product of the powder feed rate and the powder utilisation efficiency (some powder is lost because of 'overspray'); the latter depends on the size of the weld pool and was measured to be 95% for 4mm thick beads but only 80% for 2mm thick beads.
The appearance and cross section of a multi-run two layer deposit (consisting of three overlapping 2mm thick beads, with one bead on top) is shown in Fig.4; the low level of dilution is evident. The deposits have a regular shape and show only a small degree of surface ripple, allowing a machined finish to be achieved after only a small amount of metal removal. There is no slag to remove, and thus further passes may be made with only simple cleaning of previous deposits. Adequate tie-in between adjacent beads is obtained by maintaining a 15% overlap and the use of dwell on one side of the weave.
Fig.4. A multi run two layer deposit, made by plasma transferred arc cladding, consisting of three overlapping 2mm thick beads with one on top, as indicated in the schematic cross section
Comparison with other surfacing processes
The potential deposition rate of a surfacing process is of great economic importance in production. Deposition rates generally achieved with the PTA equipment lie between 2-4 kg/hr and thus exceed those for oxy-acetylene and manual TIG surfacing, for which deposition rates attainable are ~1 and 2kg/hr respectively. Deposition rates match those for mechanised TIG, PTA's nearest competitor, but compared to the MMA and MIG surfacing processes, for which deposition rates lie between ~1-8 kg/hr, the PTA process falls behind in this respect. Clearly, the deposition rate of PTA is well below that of large area, high deposition rate processes such as strip electrode submerged arc cladding (for which deposition rates may reach 30 kg/hr) [5] but such methods will not have the precision of control afforded by the PTA process.
In its controllable achievement of dilution levels of 5%, or less, the PTA process offers distinct advantages over all weld surfacing processes except perhaps oxy-acetylene overlaying. Where a minimum iron content for the top of an overlay on steel is specified, to ensure adequate wear resistance in the case of Stellites or corrosion resistance in the case of Monel for example, such a requirement may be met by a smaller number of passes than other processes because of the lower dilution.
The ability of PTA to deposit beads down to 1mm in thickness offers further advantages as a result of reductions in the quantity of expensive, high alloyed overlay material used.
The use of a powder consumable means that PTA may be used to deposit hardfacing materials which are difficult, or impossible, to produce in wire form.
Furthermore, it is often cheaper to buy materials as powder, rather than as wire, although the likely powder utilisation efficiency must always be taken into account when such cost comparisons are made.
The main disadvantage of PTA is the high capital cost of the equipment (~£40 000), in comparison to other weld surfacing processes, and the running cost of the equipment is higher than most processes as a result of the gas consumption.
Industrial application
Applications of the PTA process are almost entirely concerned with the deposition of cobalt and nickel-based materials on medium sized steel components suitable for mechanised processing, such as valves ( Fig.5). Other examples include the surfacing of pipe fittings, shafts and spindles and extruder screw flights. There is much interest in the process for the surfacing of the internal surfaces of small diameter tubes.
Fig.5. Weld surfacing of valve head by PTA ( photograph courtesy of Eutectic + Castolin)
Concluding remarks
The high capital investment initially involved with the PTA process may be justified for the overlaying of discrete areas of medium sized components suitable for mechanised processing. The PTA process offers the possibility of savings as a result of reductions in:
- production time, as a result of mechanisation and the use of fewer, less diluted layers;
- rejection rates, because of the consistent high quality;
- material costs, resulting from the use of thinner and/or fewer layers of expensive overlay materials.
The PTA process may have something unique to offer in its ability to deposit materials only available in powder form, and in its application to the overlaying of the internal surfaces of small diameter tubes.
References
- Gage R M: US Patent No 2,806,124.
- Zuchauski R S and Culbertson R P: 'Plasma arc weld surfacing', Welding Journal, 1962, 41 (6), 548-555.
- Gage R M: 'The plasma arc can now be used for welding and weld surfacing', Welding Design and Fabrication, 1961, 34 (4), 768o.
- Harris P and Smith B: 'Factorial techniques for weld quality prediction', Metal Construction 1983, 15 (11), 661-666.
- Gooch T G: 'Review of overlay welding procedures for light water nuclear reactor pressure vessels,' Weld Res Abroad 1978 24 (6),2-56.
