"Net Energy" Control of Plasma Spraying Process

Many complex thermal spray coatings require more precise control of the plasma process. Due to the dynamic characteristics of plasma spraying, and the extremely strict requirements for process control verification and status diagnosis, the control system must carry out powerful system design and extremely precise computer closed-loop control. Praxair Surface Technologies' Model 7700 and Model GTS plasma spray control systems feature "net energy" control that provides unique plasma stability through its advanced technology.

What is "net energy"

"Net Energy" is the amount of energy in the plasma plume that transfers thermal energy to the spray material injected into the plume. The DC arc that heats the plasma gas oscillates and oscillates at a very high frequency (15000cps) on the anode, thus generating extremely high noise. Due to the burning, roughening, continuous oxidation of the material surface and the change of their metallic composition, the state of the arc termination point (the arc root point, see the right figure) and the arc root point surface at the cathode and anode is changing. These changes affect voltage, current and heat loss (on the gun) from those parts. This condition in turn causes a change in the arc. Under constant power input conditions (ie, system power or gun input power), changes in the arc cause the energy in the plasma plume to change over time. In short, the more stable and constant the plasma energy (form and temperature of the plasma arc flame) is, the better and more reproducible the heating of the particles is, and the same is true for plasma coating performance.

"Controlling the net energy of the plasma reduces the variability in coating properties while spraying. Net energy control allows us to quickly and precisely control the previously uncontrollable fluctuations in the plasma arc"

What is Net Energy Control?

"Net energy" control is the detection of changes in the physical state of the plasma process hardware and maintains the generated plasma energy constant (i.e. net energy or gun output) by adjusting the input of direct current (DC) power (system power or gun input power) Power), so that the heating conditions of the powder remain unchanged. That is to say, the closed-loop control of the system power (spray gun input power) cannot guarantee the stability of the plasma flame flow. The desired constant energy output is maintained by controlling the actual energy (net energy) in the plasma plume and adjusting it "in real time". That is to say, the closed-loop control of the net energy (spray gun output power) can ensure the stability of the plasma flame flow. The 7700 and GTS systems with net energy control can produce high quality plasma coatings with high consistency and stability.

The Importance of Net Energy Control

By controlling the energy required to heat the powder, Net Energy Control maintains process reproducibility from run to run, even if parts of the standard spray gun are changed or burned out or cables age. The control of "net energy" enables the system to monitor and display the efficiency of the plasma spray gun in real time. This useful parameter allows the control system to react to the state of the plasma process hardware, regulate and maintain the target plasma, and alert the operator when operating parameters exceed preset limits, or when parts fail or require maintenance .

Having the correct energy value is beneficial to both coating quality and coating cost. Some of the benefits of Net Energy Control are listed below:
Optimization of Energy Usage - Maximization of deposition efficiency is the result of optimized energy usage.
Reduced material usage and waste generation - If the plume power is changed, the average deposition efficiency will be lower than expected, and more material will be used, resulting in more waste that must be disposed of.
Reduced use of consumable parts - Monitoring of gun efficiency will optimize the use of gun parts and allow longer parts life.
Better product quality - changes in coating quality that can only be detected with metallographic laboratory work can still occur if individual parameters are simply monitored (see image below). As can be seen from the figure, there are only very small changes in the process parameters, while the changes in the coating can be large. These differences cannot be found in parameters, but can be seen in differences in "net energy" and gun efficiency.
Reduce rework/reduce costs/increase yield – better, more consistent coating quality reproducibility reduces the amount of rework. The combination of low rework rates and high deposition efficiency increases workpiece turnover, which reduces costs and frees up the capacity of the coating cell to increase throughput if needed.
"Net Energy" Controlled Plasma Spraying

The picture on the right is a metallographic photo of the plasma sprayed metal coating during the spraying process test. Only the voltage, current and cooling water flow are changed parameters, and the rest remain unchanged. The power on the gun for the coating on the right is 12.22 kW; the power on the gun for the coating on the left is 11.63 kW. Although the spray gun power has only decreased by 4.8%, the coating quality has changed greatly, which confuses many coaters, because it is difficult to explain clearly from the analysis of independent process parameters.

If there is a "net energy" control function, it is easier to identify and explain. Because the net energy of the spray gun when spraying the coating on the right is 4.4KW, the efficiency of the spray gun is 40.7%; while the net energy of the coating on the left is 3.2KW, the efficiency of the spray gun is 30.9%. Compared with the two, the net energy is reduced by 27.3%, and the spray gun efficiency is reduced by 24.1%. Such a large difference will of course cause variations in the quality of the coating.

Because what plasma torch adopts is arc plasma heat source. When the spray gun is working, a non-transferred arc will be generated between the cathode in the gun and the anode of the nozzle. When the working gas flows through this arc, it will be heated or ionized to form a plasma flame flow, thereby also obtaining the heat of heating the powder and the acceleration of the particles. speed. Due to the burning, evaporation and oxidation of the material surface on the cathode and anode arc root points, the chemical composition of the metal surface is also constantly changing. The change of the surface state of the cathode and anode arc root points not only draws energy from the arc, but also affects the voltage, current and heat loss of the sprayed parts. At the same time, the hydrodynamic characteristics of the working gas flowing through the arc and the interaction between the two will also cause changes in the arc. Under constant power input conditions, changes in the arc cause the energy in the plasma plume to vary with time.

Therefore, net energy control can play a key role in ensuring the consistency and reproducibility of coating quality when spraying large quantities of workpieces. Similarly, if a large workpiece is sprayed and it takes a long time, the use of net energy control can ensure consistent coating quality without worrying about the impact of small changes in the state of the spray gun on the coating.

What is the value of a net energy control system?
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The value of a net energy control system depends on the work available, the application and the amount of coating work being done. To prove its true value, compare the cost today with that of a system with net energy control. Making such a real-world comparison can be an expensive proposition, so a business model is drawn up to compare two given scenarios. The variation of some values is listed in the following two tables.