Development of simulative approaches for the specific design of the properties of plasma sprayed coatings

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Kirsten Bobzin

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+49 241 80 95329

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Thermal Spraying (TS) is a widely used component coating technology that has many applications, such as corrosion protection, wear protection, and thermal insulation in aeronautical engineering and astronautics, in the mold and tool making industry, in the automotive sector, and in the offshore industry. Atmospheric Plasma Spraying (APS) is a process variant of thermal spraying which makes it possible to process any congruently melting powder material through the generation of very high thermal energy.

In the near future, plasma-coated layers shall also be used in casting processes in order to regulate the heat transfer between casting tool and workpiece through adapted layer properties, e.g. through a variable heat transfer coefficient, as well as to address precision problems such as distortion and wastage. The layers have to possess, among other things, thermomechanical properties that are compatible with the substrate, sufficient durability and tensile bond strength, and a defined topography. Both the chemical composition of the coating material and the microstructure of the coating are decisive for the characteristics of the layer.

A precise adjustment of these characteristics is challenging, due to the complex interactions between the numerous process parameters and the impact of disturbances such as electrode and jet wear. The processes taking place during the transition of the layer particle from solid to (partially) molten and from (partially) molten to solid have a significant impact of the accuracy and quality of the resulting layer.

Both the definition of the process parameters, such as current strength and gas flow volume, and disturbances through electrode and jet wear show their influence in the melt-affected processes and have a significant impact on layer microstructure.

The central question is how layer characteristics can be predicted as a function of process parameters and disturbance variables and how these parameters can be adjusted so as to achieve the desired layer characteristics. Seeking to answer this question, we will draw on knowledge gained in the modeling and simulation of the individual APS partial processes.

In the first funding period, the complex physical processes of thermal spraying will be comprehensively described with the help of numerical analyses (subprojects A4, A11, B6). The aim is to be better able to determine the influence of the process parameters on the characteristics of the casting tool coatings (subprojects B1, B8). The coupling between effective layer characteristics and layer microstructure will be realized through homogenization methods (subproject B7). For the experimental detection of the properties of the casting tool coatings as well as for the verification of the developed models, high-resolution diagnostic methods will be used (subproject A5, A6, A8, A11).

 

Continuous modeling of the entire TS process

Continuous modeling of the entire TS process Copyright: SFB 1120
 
 

Sequence of work steps

Step 1:

Stationary magnetohydrodynamic simulation of the plasma torch. The process gas flow and the electrical voltage or current between the electrodes correspond to the process parameters. The velocity and temperature profiles at the nozzle exit serve as input conditions for the free jet simulation.

Step 2:

Illustration of the turbulences by means of large-eddy simulation of the plasma flow at the nozzle outlet of the plasma torch. The three isotherms correspond to the different areas between the plasma core and the mixing zone of the plasma with the environment.

Step 3:

Large-Eddy simulation of a particle-laden free jet. The velocity and temperature profiles at the nozzle outlet of the plasma torch are coupled with the plasma torch simulation. The free jet is particle-loaded and was simulated transiently. The particles in the animation are shown magnified 100 times for better visibility.

Step 4:

Degrees of melting along the particle trajectories in the free jet. The particle-loaded free jet was simulated stationary, each particle trajectory represents the trajectory for several particles of a certain size. The particle melting degrees were determined via a subroutine based on the energy balance between gas and particle phase.

 
*** A10 Video 1 ***
 
A10 Video 1
Injection of the coarse-grained alumina powder into the turbulent plasma jet, recording rate: 87500 frames per second. Cooperation in the field of high-speed videography.