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Thermal Spraying for Power Generation ComponentsK. E. Schneider, V. Belashchenko, M. Dratwinski, S. Siegmann, A. ZagorskiCopyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 3-527-31337-0

8 Outlook, Summary

The targets of the book are to show how to achieve a stable and reliable coating production for power-generation equipment within reasonable time and at optimum cost.

We described the theory, practical approaches, process monitoring possibilities and quality management aspects with the target in mind to show how they help to

Improve quality, process stability, maintenance process.Reduce cost and lead timeSecure process developmentReduce tolerancesProduce the right properties (i.e. microstructure)

We said this book deals with questions that are essential for a good performance of thermal spraying for power generation components. We asked the question: Is this still an art or is it just another manufacturing process that can be controlled by intelligent machinery?

We followed the obvious logic: A stable process comprises the appropriate hardware, rigorous but still realistic specifi cations, material and tools according to specifi cations, optimized process sequence, offl ine programming tools, right quality system, standardization, well-established control and monitoring and, of course, the people.

When we described process stability we changed the focus on the operational window, changing parameters, drifting and monitoring. Ideally we would like to use a non drifting process. Clearly the gun development goes into this direction (see Section 2.4.4.2).

When we looked at the stochastic nature of the process we concluded that we have to live with the phenomenon. The behavior of so many particles can hardly be used to describe the coating formation during the spraying process and to use this description for modeling the quality of a coating on a component. Therefore our approach is to separate the events in the torch and plume from the real painting process on the component. We use the spray pattern as the tool for offl ine programming the coating on the component. Of course the description of the spray pattern is derived by a combination of theoretical explanations of the development of the plume or fl ame and calibration by experiments. This approach is specifi c for equipment and group of components to be coated.

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8 Outlook, Summary242

It is shown that the process capability for coating of a specifi c component is achieved by using the right rules for process and part development. This is not, however, suffi cient. Only trained and motivated people can guarantee the required process capability in this changing, drifting process. Special causes of disturbances of the coating process (see Chapter 3) have to be eliminated and documented. This type of infl uence on the process is a major part of what is considered as an “art”.

Of course, the right equipment, tooling and fi xture maintenance is a condition of every good process.

Probably all efforts will not bring thermal spraying process capability into the order of other manufacturing processes. However sigma values of 4 to 5 seem achievable under the following conditions:

Design for manufacturing must be guaranteed. This means the component must be coatable. In addition, the tolerances given in the specifi cations have to be consistent with coating process possibilities. However, today’s modeling and pre-dictions have limitations. Thickness and porosity can be modeled, microstructure and bonding defects can not. Trial and error is still required in process development of a mature technology. Many published articles and standards are available, e.g. on:

Coating production parameters and their infl uence on microstructure, micro-structure investigations and limitations (e.g. porosity measurements), heat impact in parts and components monitoring.Quality assurance: the 4 Ms rule: materials, machines, measurement methods, man.

All these pieces of information and results have to be documented in such a way that they can be used as a knowledge base for the future. This becomes even more important due to the fact that the basics of thermal spraying still rely on the know-how of experts who have been dealing with it for decades.

For the future we see two aspects that we will describe in more detail in separate sections.

On the equipment side, the development and introduction of new torches. On the process side, the introduction and usage of offl ine programming and monitoring in process development and production.

8.1 Thermal Spray Torches

Further developments of HVOF torches are expected to bring spray guns, which would assure higher deposition effi ciency and production rate. A wider operational window on a temperature–velocity plane is also a requirement emerging from the trend “higher quality at lower cost”.

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2438.1 Thermal Spray Torches

There is a market demand to have advanced plasma spray torches. Customers are expecting the device to provide:

Lower level of plasma fl uctuations, which means reduced scatter of particle parameters and higher level of deposition effi ciency.Less pronounced effects of drifting, which would directly increase the process capability.Longer electrode lifetimes in order to reduce the number of maintenance occurrences and increase the process output.

However, the thermal spray market now is strongly focused on cost issues. This is why to bring the three aforementioned advantages, or even more, is not enough for the new development to win a large market niche. Redevelopment and requalifi cation of the whole coating shop nomenclature of turbine parts with all the setbacks related to the learning process could cost millions €. And the manufacturer simply follows the well-known saying “do not trouble trouble unless trouble troubles you”, i.e. they prefer the conservative evolutionary way of gradual improvements and accumulation of know-how with the existing equipment. In order to break into production the new hardware must be able to reproduce existing coating conditions in terms of particle parameters and, in the ideal case, to reproduce the existing spray patterns in order to avoid reprogramming. Therefore, operational windows of new equipment should include those of existing torches as subsets.

The transition to the new generation of coating equipment may become easier if it is accompanied by the introduction of advanced online monitoring and control technologies. If the existing process can be mapped in terms of particle temperatures and velocities as well as in terms of surface temperatures, it is much easier to fi nd an appropriate operational window for the equipment with signifi cantly or even completely different control parameters like power, mass fl ow rates and gas compositions, which would still fulfi ll the existing coating specifi cations.

It may be also expected that the equipment manufactures will package their hardware with the customer-tailored accessories like nozzle and injectors, which would make the transition to new equipment easier. An expert consulting service in redefi nition of the process operational windows is likely to be another demanded product in a thermal spray market, especially in small and medium-size businesses, which can not afford continuous employment of high-ranked development experts.

Similar requirements may be imposed on other new products for thermal spray, for instance, powders or power sources. Apart from improvement in the process stability coating manufactures also expect from these new products the ability to be integrated into existing process without signifi cant changes in the coating specifi cations.

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8.2 Future Offl ine Programming and Monitoring in Process Development and Production

The fi nal target in production is to:

Achieve process stability at minimum total costIntroduce measuring tools like 6 sigma, TQM, ISO 9000×

This book makes the statement: This can only be achieved by good:

Materials: Quality, shape and size specifi ed according to the demandsMachines: Capability, service and process developmentMeasurement methods: Aspects of coating qualityMan, i.e. trained people

When considering the importance of process development it becomes evident that this step requires personnel with the right education, experience and communication skills. On the other hand production personnel have to take over such a process development, understand the important features and work independent of the process developer. Therefore production has to understand the limitations of offl ine programming. It will not happen that the offl ine program delivers a perfect process. It has to be adapted by production experience.

Coating preparation by blasting or other methods has to make the step from pure testing and experimental to a certain degree of automation which means some modeling. Probably, an identical program structure can be used.

Modeling must be used to describe microstructure and out of that the physical and technological properties in order to correlate them with lifetime of the coatings and the components, respectively.

Therefore the future of modeling and offl ine programming is an integrated model which includes most of the coating development and production steps. This does not mean that all processes have to be in one model. The output of one model must be such that it can be used as input for the next step in coating or part manufacturing.

Monitoring and closed-loop control shall not be independent from the modeling. A correlation has to be established. However, one has to keep in mind the conditions. Process development has to achieve such an operational window, which is not sensitive to small changes, drifts in process and material parameters. Only then should monitoring and – if reasonable – closed-loop control be applied. Nevertheless it is not expected that all parameters are adjustable and controllable. Therefore, also in the future, the spraying operator has the task: He has the prime responsibility for the coating quality.

An important aspect in addition is the cost of the coating process. Because the costs depend on materials cost, maintenance cost and process fl ow with all the people and equipment involved there is a high responsibility on the shop fl oor personnel. It is expected that coating cost can be optimized by optimizing these factors.

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