100HE® Plasma Torch Stability
Due to globalization, industry is under intense pressure to lower costs and improve quality. The thermal spray industry is not immune to these demands. In order to achieve these goals, better process control and plasma guns offering improved performance are required. In addition to closed-loop control of the process, monitoring systems that measure particle temperature and velocity are also beginning to be used. Although the algorithms have not yet been fully developed to close the loop on the plasma process using this data, it has proven itself as an excellent tool for parameter development, process monitoring and gun hardware design.
In 2001, Progressive Surface introduced the 100HE High Enthalpy Air Plasma System. The performance attributes of this gun include consistently high quality coatings, superior deposition efficiencies and high spray rates. The design features for this gun include: enhanced plasmatron, single anode and cathode, operates with ternary gas mixture, and three different powder feeding (axial, radial and external).
A major influence on coating quality is the stability of the plasma. The unique design of the 100HE achieves this with an anode-cathode design that produces an elongated and stabilized arc. The high voltage, low amperage arc reduces anode-cathode wear and improves the thermal efficiency of the torch. The anode consists of three tungsten rings separated by annular grooves whose depth and width are designed to cause the arc to attach to the anode bore and prevent migration past the rings, thus stabilizing the arc length. This design provides an uniquely stable plasma plume thus significantly improving coating quality and deposit efficiency.
The robust design of the anode and cathode combined with the use of a high voltage, low amperage arc and the utilization of a ternary gas mixture consisting of two diatomic gases (N2 and H2) plus Argon achieves maximum enthalpy resulting in high deposition efficiencies and consistently high quality coatings at high spray rates. The 100HE can also utilize Helium instead of Hydrogen as the third gas to increase particle velocity. An example is spraying Tungsten Carbide / Cobalt and achieving an average particle velocity of 527 m/sec (see Figure 1).
Performance Analysis
In January 2003 the 100HE was subjected to a sixteen hour durability run and was stopped every hour and then restarted. Stainless Steel powder (316SS, 22μ to 53μ) was sprayed at 100 grams/minute, every hour for a period of one minute. Particle temperature and velocity were measured and plume deviation was also recorded utilizing the SprayWatch™ System from Osier Ltd. (Figure 2).
100HE gun parameters were data-logged every second on Progressive Surface's CITS closed-loop controller (Figure 3).
Table 1 shows set points, as well as measured minimum, maximum and average values for the gun parameters. The extremely small standard deviation from setpoint for all parameters over the 16 hour run demonstrates the robust nature of this 100HE system. The gun consumables for this test, i.e., anode, cathode and radial nozzle already had 20 hours of service life.
Plots of gun current and voltage (Figure 4) as well as particle temperature and velocity (Figure 5) highlight the consistency possible with advanced plasma gun hardware and controls.
Summary
The stability of the plasma process has the biggest impact on the quality of the coating. Process instability also increases the cost of spraying. It leads to many problems, e.g., rework, short gun component life and inefficiencies that increase powder and gas consumption and labor. This test of the 100HE demonstrates process stability despite using aggressive plasma parameters; 80KW power, N2 flow of 100 SCFH and H2 flow of 60 SCFH. Examination of the gun hardware after conclusion of the test showed no deterioration.
Process stability combined with high deposition efficiencies has enabled the 100HE to improve the economics of plasma spraying.
Progressive Surface provides surface treatment equipment for thermal spray, waterjet, grit blasting and shot peening processes. For more information contact Progressive Surface at +1-800-968-0871 / +1-616-957-0871
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Figure 1 100HE plasma with high velocity nozzle using Helium as the third gas
Figure 2 Temperature, particle velocity, relative flux and plume deviation measurements as shown on the SprayWatch™ system.
Figure 3 CITS Closed-loop process control screen.
Table 1 Setpoints and data-logged averages over 16 hour spray event using Progressive Surface CITS closed-loop control.
Figure 4 Graph of 100HE plasma gun average voltage and current for each hour of the 16 hour test.
