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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is piinted only If the paper is published in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94-GT 450 kr: AN ALTERNATIVE THERMAL SPRAY PROCESS TO PLASMA SPRAY IN THE AIRCRAFT AND STATIONARY GAS TURBINE INDUSTRY Menno P. Zwetsloot Hobart Tafa Technologies, Incorporated Amsterdam, The Netherlands R 1E Elliott R. Sampson Hobart Tafa Technologies, Incorporated Concord, New Hampshire ABSTRACT Technological advances in arc spray have produced an arc spray system that competes very favorably with other thermal spray processes. In the past arc spray was thought of as a process for very large parts that need thick buildups. However, an attachment device has been developed which focuses the pattern and accelerates the particles and is known as the Arc Jet system. The advantages of this device were discussed by 2wetloot, Sampson and Thorpe (1993), Reference 4. This attachment device coupled with the introduction of metal cored wires that provide the same chemistries as plasma sprayed powders, provides application engineers with a viable economic alternative to existing spray methods. A comparative evaluation of a standard production plasma spray system was conducted with the arc spray process utilizing the device mentioned above. This evaluation was conducted by an airline company on four major parts coated with nickel aluminum and results show that, for those applications, the arc spray process demonstrated significant benefit. It is expected that other applications may benefit from the use of the arc spray process as well. DESCRIPTION OF THE PROCESS A schematic of the Arc Jet process is in Figure la followed by the description of the process. Figure lb shows the plasma process which can have the powder injected into the stream either externally or internally. Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands June 13-16, 1994

2 deer I Er"Atz--k Electric arc meiting wire Ord!1==.,Jr olgif Wee electrodes, Powder Figure lc Schematic View of The Plasma Process Figure la Schematic View of The Arc Jet Process Two consumable wire electrodes are fed into the gun, where they meet and form an arc in an atomonizing air stream. The process is energy-efficient; only enough input energy to melt the wire. is used. Spray rate per kw is about Slb/h. Substrate temperatures can be low because the energy input per pound of metal is only about one-eighth that of other spray methods. Figure lb Arc Jet Attachment on A Standard Arc Gun The Arc Jet has an attachment on the front of a standard production arc gun and it introduces secondary air axially. This accelerates the particles being stripped from the arc "ball" and produces a more concentrated pattern. Plasma provides controllable temperatures well above the melting point of any known substance (30,000 F). To generate the plasma, an inert gas is passed through a DC arc, superheating the gas (plasma). Powder feedstock is introduced and is carried to the workplace by the plasma stream. Provisions for cooling or regulation of the spray rate maybe required to maintain substrate temperatures in the 200 to 400 F range. Typical spray rate per KW is 0.2 lb/h; power size of systems ranges from 14 to 80 kw. The Arc Jet process provides high quality.microstructures and bond strengths that exceed plasma for thick coatings up to.080" at any angle from 30 to 90. This was shown in Unger and Grossklaus 1993, Reference 3. All bond tests were conducted to the ABTM 633 specification. The gun used in each case was Arc Jet held at 80 to bond slugs held in a rotating fixture. Results are shown in Figures 2a and 2b. - Bond Strength vs Thickness Nickel - 5% Coatings (758) 410 Stainless Substrate 0.0i $ Thiartes soncho4 Figure 2a Bondstrength Thickness 2 Vs.

3 Bond Strength vs Spray Angle Nickel - 5% Coatings (758) Coating quality Spray times Survey of operators Machinists survey Conclusions For this survey, the following main engine parts were used: to Spray Angl 000f**3 Figure 2b Bondstrength Vs. Spray Angle Compressor case, Spool Frame Case Figures 4a and 4b show the actual parts. - Figure 3 a. Standard Arc-Spray Pattern b. Arc Jet Spray Pattern Figure 4a Compressor Case The comparisons showed that the chipping at the edges of the machined coating was eliminated, the finish was as good or better for the arc coatings and the machining time appeared to be improved for the arc sprayed coatings over the parts that they do for plasma. This was enough to make the decision to request a direct evaluation of the two processes on four actual parts. AIRLINE EVALUATION At a European Airline (using the plasma process) an extensive survey was conducted to determine if and to what extent the Arc Jet system was better than standard plasma spray. In this survey, the following were evaluated: Figure 4b Compressor Spool COATING QUALITY All materials to be sprayed (Table 1), are approved by the OEM and covered by proceedures in the standard practices manual. Bond 3

4 strength tests for the approvals were extensive and are well documented in Unger, Robert H. and Grossklaus, Warren D. (1992) reference 3. Coating quality was evaluated by comparing the arc sprayed microstructures to repair manual specifications. Figure 5 shows the comparison between plasma and arc sprayed Nickel microstructures. This illustrates that the overall structure of the arc coating is very similar to the plasma coating. The airline quality department reported that the arc coating was as good or better than the plasma and met all overhaul manual requirements. unit. For the NiAl and the NiCrAl the 200 Amp. unit is sufficient for the spray parameters used were in the range of Amp. For, the parameter used was 200 Amps. The plasma system used was a standard production external feed plasma unit. Plasma coatings were sprayed a maximum of 12 lbs./hour. For production, a Asp. will be used. This delivers 18 lbs/hour for aluminum. Also, at higher spray rate the deposit efficiency will increase. (Figure 6) Production spray time will be reduced even further when the 300 Amp. parameter is used. A preliminary one part spraying and machining evaluation for the amount of chipping and machining time was then conducted. The results showed no chipping at all and a thirty percent reduction in machining time. These results led to the decision to further evaluate this process. Figure 6 Deposit Efficiency Vs. Amperage of In general there are four main reasons for the time reduction when plasma spraying and arc spraying are compared, they are as follows: Figure 5 Photomicrographs (200x) a. Plasma sprayed NiAl b. Arc-Sprayed NiAl SPRAY TIME In the process of choosing the process, the spray time per part is of significant importance. The following times were found. For comparison the plasma spray times are utilized and verified during the evaluation. The spray times mentioned in Table 2 needs some explanation. All tests were performed with a 200 Amp. Arc Jet Arc has rates. for the are: NiAl NiCrAl Al higher spray The spray rate used materials lbs/hour lbs/hour lbs/hour Low heat input. Because of the low heat input even thicker coatings could be applied without having to stop for cooling. increased deposit efficiency. A graph of deposit efficiency vs spray rate is shown in figure 6. Less "Do Over" of parts chipping during machining. 4

5 Part Description Thickness Bondcoat Thickness Topcoat Compressor Case Spool Frame Case Nickel-5 Nickel-5 Nickel Chrome Nickel-5 AlUminum Table 1 General Electric Approved Materials Sprayed For Survey Part Description Spray Times Arc Plasma Compressor Case min. Spool min. Frame Case min min. Table 2 Spray Times for Arc Vs Plasma at European Overhaul Facility SPRAY COSTS The thermal spray costs consists of a number of factors. There are the direct costs like consumed powders, gases, spray times, etc. Nickel aluminum wire in some cases has been found to cost more than powder. This wire has had price decrease over the past two years due to the increased volume used. More difficult to measure and transfer into costs are the re-spraying and set-up times. From the extensive testing period, the airline expected to save $25,000 U.S. dollars per year. These estimated numbers were based on some very conservative numbers. OPERATORS SURVEY Six operators on two shifts were surveyed at one European overhaul shop. Their comments are summarized in Table 3. Based on our experience over the past four years and as shown in Sampson (1994) reference 2, we would expect savings up to 2 to 3 times this estimate in the first year of operation. 5

6 Advantages Disadvantages Unit is mobile, small & compact Very simple to use-easy set-up No powder feeder to clean Spray material doesn't have to be kept in an oven Only two parameters to control Low noise level compared to plasma Low heat input Pew spare parts a low cost compared to plasma Gun is heavier More dust than plasma for aluminum Table 3 Results Of The Operator Survey MACHINISTS SURVEY Pour machinists on two shifts were surveyed at the same overhaul facility. Their comments about machining the parts sprayed for this evaluation are listed in Table 4. Coating Advantages Disadvantages NiCrAl Structure looks finer None Less chipping at the edges Tool cuts better (You can hear it) Overall quality improved After machining surface looks very smooth (easy to produce required surface finish) NiAl All diameters (0D,ID) machine better None Material machines very easy Overhaul quality improved Minimum chipping at the edges Al After machining coating looks good Coating as sprayed looks coarse Table 4 Results Of The Machinists Survey 6

7 CONCLUSIONS Results of the operator and machinists survey demonstrated the benefits Of the concentrated spray Stream generated by the Arc Jet system. There were eight advantages to only two disadvantages from the operators and the results from the machinists listed ten advantages and only one disadvantage. Reduction of spray times up to 50 percent for the Arc Jet combined with a reduction of machining times with no do overs of chipped coating were also determined during the evaluation. Aircraft Power Plant Overhaul", paper presented at Aachen, pp J.80. ACKNOWLEDGEMENTS The authors wish to thank the coating engineer, spray shop managers, operators and machinists who supplied the information that this paper is based on. Based on this comparative evaluation the Arc Jet process demonstrated benefits over the production spray process for the parts repaired. It is anticipated that there are other repair applications in the aircraft engine industry and the industrial gas turbine industry that might be served as well by application of the Arc Jet process. References 1. Cohen & Dailey, (1994) "New Machining Method for Nickel Based Thermal Sprays" presented at International Gas Turbine and Aero Engine Conference, 11229, Netherlands 2. Sampson, Elliott R., (1993) "Economies of Arc Spray Vs. Plasma for Aircraft Components", Proceedings, 5th National Thermal Spray Conference, Anaheim, CA, pp Unger, Robert B., Grossklaus, Warren D. (1992) "A Comparison of the Technical Properties of Arc Sprayed Versus Plasma Sprayed Nickel-5 ", Proceedings, 28th Annual Aerospace/Airline Plating & Metal Finishing Forum & Exposition, Orlando, FL. 4. Zwetsloot, Menno P., Sampson, Elliott R and Thorpe, Merle (1993) "An Alternative Thermal Spray Method for 7