THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 45 E. 47th St., New York, N.Y The Society shall not be responsible for statements or opinions advanced In papers or discussion at rnootings of tho Society or of its Divisions or Sections, or printed In Its publications. Discussion is printed 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 99 NEW MACHINING METHOD FOR NICKEL BASED THERMAL SPRAYS Joel H. Cohen Advanced Repair Processes Development Engineering GE Aircraft Engines Cincinnati, Ohio James H. Dailey GE Superabrasives Worthington, Ohio ABSTRACT The routine overhaul and repair of gas turbine components requires dimensional restoration of many components. This process is usually accomplished by using various thermal sprays to build up the worn part surface and then machining or grinding the thermal sprays to correct part tolerances and dimensions. These surfaces usually contain numerous holes that generate heavily interrupted surfaces to be machined. The combination of interrupted cuts and a very wear resistant thermal spray causes rapid tool wear on conventional carbide cutting tools. This rapid tool wear also produces poor surface finish, part taper, chipping around the hole edges, and increased tool pressure that could result in lifting or peeling of the sprayed material from the parent metal. This paper summarizes the results of machining nickel based thermal sprays with polycrystalline cubic boron nitride (PCBN) cutting tools. The tests have shown a minimum 2-5x improvement in surface finish, dimensional control, part taper and up to 10x increase in productivity. The PCBN tools also generated lower cutting forces resulting in reduced stress and higher bond strengths in the part. This paper presents the data collected and the recommended machining parameters developed under controlled conditions for machining air plasma sprayed Metco 44, Metco 450, Inconel 718, two wire arc applied TAFA 758, TAFA 7MXC, and high velocity oxygen fuel applied Inconel 718. BACKGROUND The operation of gas turbines can be described as installation, operation, repair and reinstallation. This is generally true for aero as well as stationary gas turbines. As repair and serviceability are considered prime safety items involving human life, we shall discuss acre applications even though many commercial and stationary turbines applications will be equivalent. In commercial airline service, the key to profitable operation is the control of engine maintenance cost. This type of cost is usually only about 10% of the total operating expense, but it is often the only cost element that is controllable to any great degree. The other 90% of the operating expense is largely fixed in terms of costs related to fuel, salaries and facility maintenance. As a result, control of profit margin is highly dependent on the effective control of engine maintenance expense. However, much of the engine maintenance expense is fixed in terms of costs related to disassembly, reassembly, inspection, engine testing, etc. In fact, the only meaningful cost saving is to replace unserviceable engine hardware with cost effective repaired parts instead of expensive new spare parts. Direct Maintenance Cost (DMC) is the prime measurement for commercial customers for the maintenance portion of the cost of engine ownership in terms of cost per engine flight hour (S/EFH). This cost includes both line and shop maintenance. The cost elements involved are as follows: Labor Costs Those costs associated with disassembly, inspection, in-house repair, assembly, test and any other activity directly associated with engine maintenance. Material Costs The cost of replacement parts used in place of unserviceable parts or of other material used in engine maintenance. Repair Costs The cost of repairing parts in outside shops to serviceable condition for use in place of unserviceable parts. As far as repair costs are concerned, it goes without saying that the lower the cost the better. The average repair cost is about 20-40% of the new part price. Since this represents a 60-80% reduction in cost on most all the repaired parts used, it is easy to see what a dramatic effect this has on DMC. Most all hot structures require repair and subsequent dimensional restoration. Occasionally dimensional restoration requires only light machining; however, upon second repair or where welding and heat treatment are involved, thermal sprays are required to present adequate material for the light machining. The thermal sprays generally used are nickel based materials Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands June 1-16, 1994

2 sprayed by air plasma, high velocity oxy fuel (HVOF) or some sort of arc wire process. Because of the cost of the process and material sprayed, only enough build-up to machine within serviceable limits is applied. This typically is less than 1.1mm allowing for approximately.25mm machining stock. In most cases, this represents a bolting flange, locating diameter or rotor rub surface. These repair techniques are required for fit and function and/or performance restoration. During the machining operation, important criteria are flatness, surface finish and spray integrity. The flatness is important so that excessive bending loads are not put into the attaching bolt. A very good surface finish is required for the sealing characteristic which is particularly important in bearing sumps or areas of lube oils. Spray integrity or chipping is important so that the clamping force created by the bolting arrangement is uniformly distributed around the hole and in the case of locating characteristics, positive positioning is accomplished. Presented in this paper is the testing of polycrystalline cubic boron nitride (PCBN) cutting tools for the machining (turning) of a variety of commonly used nickel based thermal spray materials as compared with conventional carbide cutting tools. TEST MATERIAL AND SET-UP A survey of the commercial aircraft engine overhaul requirements was made and the most commonly used thermal spray materials and processes were chosen for the machinability testing. Only the more difficult to machine spray materials were tested as the nickel graphite series and aluminum alloys don't present a problem. All of the tested materials are nickel based and are somewhat difficult to machine effectively with good surface finish, minimal taper and minimal machining stresses thus maintaining spray coating integrity. The most common material sprayed is nickel aluminum (95% Ni -5% Al). Most people use the Metco name generically although chemically identical materials are supplied by many manufacturers. This Metco 450 is tough, brittle and difficult to machine efficiently. The assprayed surface is rather abrasive and causes significant tool wear that generally requires a tool change for each machining pass. Chipping is definitely a problem. This test was not designed to simulate actual repairs to gas turbine engine components. The workpieces were designed to be very rigid with excellent support. This would allow the close examination of cutting tool materials and thermal sprays and exclude the influence of part rigidity and fixturing. These machining tests detail the parameters necessary to produce excellent surface finishes and part tolerances that are required when rebuilding a gas turbine engine used in the aircraft industry. The 20cm diameter cylinders used in these tests were held in a three jaw pneumatic chuck and were supported on the non-chuck end by means of a support plate and a live center. This workpiece configuration eliminated chatter due to vibration and also eliminated part deflection. The machine tool used was a Mazak 15HP CNC controlled "Quick Turn" lathe. All evaluations were run without coolant. Most facilities prefer to machine without coolant so that any respray of damaged areas can be completed with minimal cleaning and preparation. Additionally, many coolants contain elements that could contaminate the surface and cause corrosion on hot superalloy components during operation. The cylinders, as stated previously, were air plasma sprayed, HVOF sprayed or wire arc sprayed in order to cover the most common application methods. The Metco 450 and TAFA 75B are nickel aluminum; the Metco 450 was air plasma sprayed to a layer thickness of about 1.27mm while a thicker layer of the TAFA 75B was applied with their arc wire process. Bond strength allowed a thicker layer of the TAFA material. The air plasma sprayed Metco 44 is a nickel, chrome, aluminum material; the TAFA 7MCX had the same chemistry. Inconel 718 is a nickel base structural material comprised of Ni, Cr, Fe, Cb, and Mo. This material was sprayed to 1.27 thick using APS and 2.16mm thick with the HVOF process. The HVOF process allows thicker spray deposits because of the reduced spray stress and higher bond strength, HVOF sprayed materials are generally cleaner and more dense than air plasma sprayed deposits. The deposition rates for APS and HVOF ranged from Kg per hour and the TAFA wire arc system deposition rate was Kg per hour. All cylinders were sprayed with the same powder lot and spray parameters breach material in order to reduce variability. Teflon plugs were used to mask the holes with no post preparation of the sprayed surface other than the conventional wire brushing. The workpieces were 20.cm diameter, cm long, 6.5mm wall cylinders sprayed with the six different thermal sprays: APS Memo 450, 44, Ince, 718, HVOF 718, TAFA 75B and TAFA 7MXC. The workpiece geometry used for the interrupted tests was identical to the continuous tests except for a hole pattern consisting of six rows of ten 9.5mm diameter longitudinal holes spaced 60 apart on each cylinder, and every other row was displaced by 12.7mm, so that the tool was always in an interrupted cut mode. Conventional C2 grade carbide inserts were used to establish baseline performance data. This tool material is commonly used throughout the industry and the GEAE Standard Practice Manual recommends a cutting speed of 0M/min.,.05mm depth of cut and.05mm per revolution feed rate. Cutting speeds with the C2 carbide inserts were varied up to 90M/min. where severe wear or catastrophic failure of the spray coating occurred. These conditions (C2 carbide at 0M/min., 0.05mm DOC and 0.05mm per revolution), produce a material removal process that is slow and imparts a high shear load on the thermal spray. It does give acceptable surface finishes, taper, and chippage around bolt holes through flange faces and other types of interruptions in the engine components. These conditions sometimes require a second operation utilizing a grinding wheel to obtain required surface finish and/or dimensional tolerances. The baseline machining conditions used with the PCBN inserts were virtually the same as the C2 carbide inserts. The BZN-6000 tools were then tested at speeds varying from 61M/ mm. to 65M/min. with a depth of cut from.05mm to.8mm, and feed rates between 0.05 and 0.25mm per revolution. The tests were conducted using GE Superabrasives BZN*-6000 tool material in a 12.5mm square shape (SNGN ) and a 50 negative rake toolholder other PCBN materials with different chemical or physical properties may not provide the same results. Nose radii used were.08mm radius and 1.2mm radius with little or no difference in performance noted. BZN-6000 in other insert geometries, 80 diamonds (CNMA- *Trademark of General Electric Company, U.S.A.

3 ) and I 2.5mm IC triangles (TNGN ) were evaluated to verify that machining parameters did not change with a weaker cutting edge geometry. The BZN-6000 tipped inserts had a 15 degree by 0.2mm wide chamfer applied to the cutting edge. The C-2 carbide inserts were WON and TPGT geometry. The C-2 carbide inserts were used as a baseline comparison based on recommendations from the GE Standard Practices Manual. The data collected included insert geometry, cutting tool edge preparation, cutting tool material, workpiece speed, feed rate, depth of cut, length of cut, total taper in the area machined, cutting time in minutes, tool flank wear, and surface finish. The material removal rate in cubic inches per minute, taper per inch of machined surface, total material removed (cubic centimeters), and tool efficiency. The term tool efficiency can be defined as follows: Tool efficiency is a calculated value used in these tests to quantify tool performance. It is calculated by dividing the total amount of material removed in cubic centimeters and dividing it by the tool flank wear in millimeters. An example of this would be removing I mm of material and generating.025mm of flank wear on the tool. The flank wear had the decimal point shifted 2 places to the right making 2.5. This number was then divided into the 1 mml to give a tool efficiency of 0.4. TEST RESULTS As discussed previously, two types of cylinders were machined. A straight cylinder (continuous) and a cylinder that had holes simulating an interrupted cut of a bolting flange. The straight cut data is not presented here since the BZN-6000 tipped tools showed minimal wear while the carbide wear was -5X. The data presented is cutting time (minutes) vs. flank wear along with surface finishes for various cutting speeds in the interrupted cut cylinders. Flank wear is important because it directly relates to tool pressure which can cause damage to the thermal sprayed surface including lifting of the thermal spray from the parent metal. Flank wear is also directly related to part taper. The part taper will increase as the tool flank wear increases and the nose radius of the tool is worn away. The tool dulls and significant heat is generated in the part rather than the chip, causing coating damage and distress. Photograph "A" is a typical C2 carbide insert after one I I.4cm long cut. This shows the wear on the flank with an equivalent wear on the nose of the insert. This wear creates taper and causes frictional heat. Photograph "B" is the same length of cut and in the same material at X higher cutting speed but does not exhibit nose wear. PCBN materials exhibit much higher hot hardness and toughness which translates to greatly reduced wear on the tool. Photograph A These photos are typical of all materials tested. NICKEL/ALUMINUM (APS METCO 450 AND TAFA 75B) Figure 1 is a direct comparison turning Metco 450 with C2 carbide and BZN-6000 at the same cutting speed (M/min.) and feed rate (mm/rev). Note the different wear characteristic. The carbide corner had.86mm wear after 28 minutes of cutting showing about.075mm total taper producing about.6 ism Ra surface finish. The BZN-6000 insert on the other hand had a similar surface finish of.5 ism Ra but the wear was generated in 116 minutes of cut time. Zero taper was observed until about 100 minutes of cutting time when.025mm total taper was noted. This relates to a 50X improvement in tool efficiency. Figure 2 shows the effect of cutting speed(m/min.) on flank wear development and surface finish with the BZN-6000 tool material. When run at M/min. flank wear does increase, however, surface finish is greatly enhanced, well below the 1.5 gm Ra desired. As expected, with increased cutting speed, the tool wear increases, however, tool efficiency is 20-0x better than C2 grade carbide. The TAFA 758, although the same chemistry as Metco 450, is more machineable possibly due to the finer, more evenly distributed oxide microstructure. C2 carbide worked well up to 91M/min. showing.41mm flank wear with no nose wear nor taper (see Figure ). Surface finishes with C2 carbide ranged from 4.14 Lm Ra at 0M/min. down to 1.55 pm Ra at 90M/min. The BZN-6000 insert did much better having less than.5 gm Ra at anything between M/min. and virtually no taper. The calculated tool efficiency was between 40 and 250X that of carbide. NICKEL CHROME ALUMINUM MATERIAL (APS 44, TAFA 7 MXC) The Metco 44 is a tougher material than the Metco 450 because of the addition of chromium. Although tough, surface finishes are very good when applied properly. Machining with carbide or PCBN yielded below.75 Ra finishes, equivalent flank wear and good dimensional stability. The major benefit to the PCBN tool is the cutting speed. Carbide tools could run between 0-91M/min. where BZN-6000 was run 91-18M/min (see Figure 4). This is a productivity measurement not an engineering type need. The TAFA 7MXC material again showed better machinability characteristics as compared to the APS Metco 44. Carbide did reasonably well up to 91M/min. with.076mm taper and more than.5imm flank wear. Surface finishes were around 1.25 pm Ra. By using the BZN-6000, tool speeds could go to 150M/ min. with no flank wear or taper. Finishes typically were better

4 than.5 gm Ra (see Figures 5 and 6). Testing of faster speeds produced unacceptable flank wear and taper. The accelerated wear caused spray coating failure. Cutting efficiency using PCBN tools are still 10-50X that of conventional carbide. INCONEL 718 HVOF AND APS TESTING The tests on HVOF and APS applied Inconel 718 indicate that there was very little difference in the machining characteristics of the two application methods when machining continuous cuts at workpiece speeds between 0-65M/min. using the BZN-6000 tool material. The tests on the interrupted cut cylinders indicate that surface finishes less than.5 gm Ra, and little if any tool wear are produced when machining speeds are between 0-76M/min. When the surface speed was increased to 90M/min. or higher, rapid tool wear was observed with both BZN-6000 and C2 carbide. The tests conducted at 0M/min.,.05mm DOC, and.05mm/ rev on the interrupted cut HVOF applied Inconel 718 indicate that both the BZN-6000 material and the C2 carbide had minimal wear and produced consistently good surface finishes. Increasing the workpiece speed to 60M/min. caused accelerated wear on the C2 carbide insert at all feed rates and depths of cut that were tested. The BZN-6000 tipped tool produced excellent results up to of.1mm DOC and.1mm/rev. The BZN-6000 tool material was then tested at 76M/min.,.1mm DOC, and.1min/rev. This test produced only.0mm of tool wear and the surface finish on the HVOF applied Inconel 718 was.28 gm Ra. The workpiece speed was then increased to 90M/min. and the depth of cut and the feed rate were maintained at.1mm. The tool wear was.22mm and the surface finish averaged.6 gm Ra. The tests on the APS applied Inconel 718 interrupted cut material were conducted prior to the tests detailed above. The C2 carbide tools were tested at 0-50M/min.. The C2 carbide tools had minimal wear and produced surface finishes of p.m Ra. The BZN-6000 material was tested at 90M/min.,.25mm DOC and.05mm/rev. The tool wore rapidly causing a large contact point between the tool and workpiece. This caused excessive force and tool pressure which resulted in a bond line failure between the thermal spray and the base metal. Additional tests should be conducted on the APS Inconel 718 to determine if the same machining conditions that worked well for the BZN-6000 material in the HVOF tests will produce equal results in this material. range. The initial cut on the "as sprayed" surface is typically.2-.25mm with subsequent finishing cuts between mm. The feed rates for the cutting tool should be between mm/rev depending on the surface finish required. It may be possible to offset the cost of new part fixturing to improve the rigidity and eliminate part vibration through the productivity gains realized by using the BZN-6000 tool material. These gains would include reduced labor and machine time with the ultimate gain of reduced turnaround time to reinstall the engine on the aircraft. The machining of the Inconel 718 APS in interrupted cut has not been optimized. Additional tests are planned to produce recommended machining parameters to optimize the performance of the BZN-6000 tool material. The tests on the remaining Inconel 718 shows that use of BZN can be justified if dimensional stability is the prime consideration. Conventional C2 carbide will provide adequate performance if material removal is the goal of the machining operation. The flank wear on any tool material is a critical factor in machining thermal sprays. As the flank wear increases, the total contact area between the tool and workpiece increases. This increase in contact area increases the cutting forces and can cause damage or separation of the thermal spray from the base material. It is recommended that the cutting tool be replaced when the flank wear reaches mm. This procedure would eliminate tool pressure causing damage the thermal spray. REFERENCES GE Aircraft Engines Commercial Engines Standard Practices Manual (*GER 9250, Section Machining Thermal Sprays, Section Thermal Spraying Parameters. GE Aircraft Engines CFM Manual Turbofan Engine Standard Practices Manual CFMI-TP.SP.2, Definitive Tests Section , , , for the CFM 56 Engine. ACKNOWLEDGEMENTS The authors would like to thank the following individuals for their contributions to this paper: GE Aircraft Engines Advanced Repair Process Development Engineering: Warren Grossklaus, Brian Dorrel. GE Superabrasives Americas Application Development Center: David Ratliff, Don Johnson, Dwight Dyer. CONCLUSIONS AND RECOMMENDATIONS The test results show substantial performance and economic advantages will be obtained by using the BZN-6000 tool material in five of the six materials machined. The BZN-6000 PCBN tool material produced excellent results at workpiece speeds up to 65M/min. This workpiece speed should not be considered an absolute limit. It is recognized that the majority of parts machined in the overhaul and repair of aircraft engines cannot be safely fixtured or machined at these speeds. Speeds in the M/min.. range are considered practical. The rigid and precise machine tool, test part, and tool fixturing allowed these higher speeds to be evaluated safely. The recommended machining parameters for the repair of aircraft engine components should be in the M/min. speed 4

5 Os EC C-2 Cartide C-2 Certide 8 BilNECOO g I C-2 CaMide 0M/nr BZN 6C /m U Carbide 90Wmin 9 Ei2N Wm 0-2 Carbide 90Winin 0-2 Carbide 0Wmin gzi ?I Cutting lime Minutes Rgure rco 450 Air Plasma Spray Into muted CU SZN 6000 vs. 0 2 Carbide 01.4/min,.05MM/Rev,.05mm DOC Tod Wear and Surface Finish Cutting Time- Minutes Figure 4. METCO 44 Interrupted Cut vs. 0-2 Carbide 0.10Wmin..05MWRev..05mro DOG C-2 90M/m C-2 90M/m EB Flank Wear (mm) BZN M/m Cutting Time Minutes Figure 2. MEMO 450 Air Plasma Spray Interrupted Cut BZN Almin..05MWRev..05mm DOC BZN M/m Cuffing Time Minutes Figure 5. TAFA-7 MXC intenupted Cut BZN-5000 vs. 0-2 Carbide 90150Wmin,.05MM/Rev,.05mm DOC Tod Wear and Surface Finish 0 C-2 90Wmin 0.2 CM/min Wmin EH t C.2 90Wmin Wmin 8 EiZN-6000 BZN Cutting Time - Minutes Figure. TAFA-75 Interrupted Cut BZN-6000 vs. C-2 Carbide /mIn..05MWRev,.05mm DOG Cutting Time - Minutes Figure 8.TAFA 7 MXC Air Plasma Spray Interrupted Cut BIN M/min..05MM/Rev..05mm DOC 5