Case studies

KLM Royal Dutch Airlines has the oldest Maintenance, Repair and Overhaul (MRO) shop for aircraft in the world (more than 80 years old). The company's philosophy in all areas, including thermal spray, has always been to pursue the latest innovations and to maintain well-equipped facilities with state-of-the-art processes, equipment and materials for performing repairs. In the area of thermal spray, the latest procurement is a state-of-the-art unique cabinet with a high enthalpy plasma spray process. This new cabinet and new plasma spray process provides up-to-date technology for applying thermal spray coatings on aircraft engines. In this paper, experiences and the latest results obtained using this system to deposit various types of coatings used on aircraft will be reported.

Introduction

KLM started incorporating thermal spraying in their Engineering & Maintenance Division in 1969 with flame spray equipment (acetylene-oxygen). In the mid seventies the first plasma spray equipment was bought. In 1987 the first fully automated and robotized plasma spray system was acquired and in 1993 the first electric arc spray system entered the engine overhaul shop of the KLM.

In 2002 KLM initiated planning to build a complete new engine shop at Schiphol-Airport for its engine MRO market on General Electric (CF6-50, CF6-80A, CF6-80C2, CF6-80E1) and CFM-International (CFM56/7B) engines. Taking into account all the occupational and environmental laws, it was determined that this would be an excellent opportunity to introduce state-of-the-art processes, techniques and equipment. This was considered particularly important for thermal spraying because for engine repairs the company wanted to establish a centre of excellence in thermal spraying and be at the cutting-edge of this field.

Changes in the Aviation Industry

The last couple of decades have brought many changes in the aviation industry. These changes concern, among others, aviation legislation, occupational health laws, environmental laws, safety laws, European laws (CE directives), standardization (ISO, CEN, NEN, DIN, AMS, etc.), OEM manuals (e.g., new repairs, new consumables). Also, the financial situation of companies in the aviation industry has changed dramatically, caused, for example, by politics, market changes, cost of consumables (e.g., nickel prices), cost of labor and external factors like SARS, BSA, wars and terrorism.

Innovations
A key question for MRO facilities in this changing environment is how to survive in the current market. One of the possible ways of increasing the chances of surviving is to be innovative and introduce new ideas in the MRO market. Innovations can be focused on techniques and labor. In the case of techniques, computerized and automated processes (like: ultra high waterjet stripping, shotpeening, TIG-welding, LASER-cladding and even thermal spraying) can be considered In the case of labor, special air-conditioned working areas, separate cabinets for smokers and improved work conditions can improve the functioning of a thermal spray facility.

In the Netherlands, the latter of these two approaches is especially being promoted. Manual labor involves, for example, bench work, NDT work, high pressure cleaning, welding, chemical cleaning and, again, thermal spraying. Dutch occupational laws are encouraging industry to take the operator out of dangerous processes, where exposure to hazardous materials are possible. Dutch legislation also requires a so-called “minimalization obligation” for laborexposure by means of the latest ‘state-of-the-art’ equipment.

State of the Art
As mentioned in the introduction, in 2002 KLM began planning to build a complete new engine overhaul shop at Schiphol-Airport in the Netherlands. Therefore KLM Engine Services Department started looking at new solutions and state-of-the-art thermal spray equipment. The Process, Equipment and Materials Development Department of the KLM Engineering & Maintenance Division began to prepare specifications for a unique fully automated and robotized thermal spray cabinet with features never seen before in the thermal spray industry.

All the major thermal spray companies were invited to engineer and quote on this unique specification. Two years later in 2004 a new state-of-the-art fully automated and robotized cabinet was built, including 5 different thermal spray systems: 1) a closed-loop electric arc Thermach spray system: AT400, 2) a traditional external Sulzer-Metco APS system: 9MB, 3) a traditional internal Sulzer-Metco APS system: 7MST, 4) a HP-HVOF Praxair-Tafa Inc. system: JP8000 and 5) a High Enthalpy Progressive Surface APS system: 100HE. Included in this cabinet (Fig. 1) is a 4 axes gantry-robot; an indexable turn table; a unique dust suction, filtration and recirculation system; an infra-red part temperature control system; a CO2 cooling system; a cabinet gas and temperature sensor system; and gas flow measurement by ‘critical orifices’.

Features

The new cabinet is equipped with the latest Siemens PLC as slave computer and a Windows XP based PC as master computer. Incorporated in the PC is a Progressive Surface CITS (Computer Integrated Thermal Spraying) software with special features like preventive maintenance and cost-of-ownership calculation. Off-line programming of parts is possible. All the incorporated thermal spray guns are monitored, controlled and recorded through the master-PC.

Dust Suction, Filtration and Recirculation System

Probably one of the most advanced parts of the cabinet is the unique dust suction, filtration and recirculation system. The cabinet is designed in such a way that there is an adjustable combined ‘down flow floor air suction’ and ‘horizontal flow plenum air suction’, which ensures almost no dust problems during spraying. In addition, all of the dust-contaminated air is sucked to the newest Donaldson-Torit cartridge filtration system (DFO-56/4). After the first cartridge filtration step, the air is pushed through a second step HEPA #14 filtration unit. After this step the air is monitored by a computer-controlled continuous particle measuring system. Then the air is pushed through a third step HEPA #14 filtration unit. Subsequently, the ultra-filtered air is divided by electrical valves to recirculation piping and exhaust piping outside the building. The recirculation percentage is computer controlled and depends on gas and temperature sensors in the cabinet. Also, the sucked air quantity is adjustable by a frequency-controlled ventilator and is PLC-monitored. Measurements of the filtered air showed that this system had a dust emission lower than 1.5 ng/m3 of air, which is unique in the world for thermal spray systems. This means that the dust content of the recirculated or exhausted air has a value 1000 times lower than that required by the most stringent proposed chromate (VI) exposure level (see Table 1).

100HE® High Enthalpy Plasma Spray gun

One of the unique processes incorporated in the cabinet is the new 100HE enthalpy plasma spray gun of Progressive Surface (Fig. 2). This gun is capable of spraying with a high feedrate (up to 200 g/min) combined with a very high deposition efficiency (DE) (up to 95%) and excellent coating qualities (porosity contents, oxide levels, tensile bond strength and density values reaching those of HVOF coatings).

Experience

KLM sprayed their polyester-aluminum powder as the first powder to check out the capabilities of the new high enthalpy plasma spray system. See Fig. 3 and 4 (gas values in scfh) for the first spray results. Table 2 shows the results of a parameter DOE by Progressive [3] for optimizing coating results of Polyester- Aluminum. The results of this first tested powder were very successful at KLM and KLM started spraying other powders.

All coatings sprayed of figure 6 till 15 showed in general increased densities, lower porosities, higher tensile bond strengths, better uniformities, lower oxide contents, etc. compared to the traditional APS sprayed coatings.

The most important characteristics of the 100HE high enthalpy system are the high feedrates and high DE's compared to a standard APS system. The combination of high feedrates and high DE's resulted generally in a decrease by a factor of almost 2.5 in the spray times (depending on the kind of coating, coating thicknesses and the part geometry). Turnaround- times (TATs) are very important in the aircraft MRO business because reducing the TAT reduces cost. The reduced TATs together with the powder savings derived from high DE’s for the 100HE spray system resulted in a payback time for this new system of 15 months at KLM.

The new system is now being used in production for applying all known thermal spray materials used in the aviation industry for General Electric and CFM-International aircraft engines.

Summary

The KLM introduced a new state of the art fully automated and robotized thermal spray cabinet with 5 thermal spray processes and among all things unique features on dust suction and filtration. Especially the incorporated new 100HE High Enthalpy plasma spray process turned out to be a great success on DE, coating quality, TAT and hardware life. By introducing this unique cabinet KLM engine services is capable of fulfilling the future capacity demanding and has a versatile flexible spray system in house.

Conclusion

KLM has invested in the next generation flexible, automated and robotized thermal spray cabinet including a high enthalpy plasma spray system in order to establish and maintain a center of excellence in thermal spraying. With this system, KLM will stay at the forefront of the aircraft MRO market, which was reported at the AEA (Association of European Airlines) office [1].

Acknowledgement

The authors acknowledge Mr. Lou George, for helpful discussions on automated thermal spray systems, Mr. Larry Pollard, Progressive Surface, for his input on thermal spraying in general and, specifically, on high enthalpy plasma spraying, and Mr. Todd Snyder, Progressive Surface for contributing his expertise in thermal spray development.

References

1. M. van Wonderen, presentation: A state of the Art thermal spray system, EACMT (European Airline Committee on Materials and Technology) General Meeting, Brussels, 22 - 23 June 2004
2. Dutch Health Council: “Chromium and its inorganic compounds” 1998-00664, 24-sept-1998
3. Progressive Surface, Grand Rapids, MI, USA