The Use of Lasers in Gas Turbine Manufacturing

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the 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 fifteen months after the meeting. Printed in USA. Copyright 1985 by ASME The Use of Lasers in Gas Turbine Manufacturing by D. S. DUVALL Manager, Manufacturing Process Development Pratt & Whitney United Technologies Corporation East Hartford, Connecticut USA ABSTRACT The laser is being increasingly used as a manufacturing tool in the fabrication of aircraft gas turbine engine components. Both solid-state pulsed and gas continuous-wave lasers are employed for a variety of manufacturing tasks including welding, cutting, hole drilling, surface alloying, and marking of parts for identification. In these applications, the laser provides increased productivity and reduced cost compared to conventional methods. Examples of specific uses of the laser in gas turbine fabrication are described along with the benefits achieved. Metallurgical effects associated with various laser processes are also discussed. Presented at the 1985 Beijing International Gas Turbine Symposium and Exposition Beijing, People's Republic of China September 1-7, 1985

2 INTRODUCTION The potential for using the laser as a manufacturing tool was recognized early in the development of this energy source. However, it was not until the mid 1970's that widespread production applications started to appear. During the 1980's, use of industrial lasers has increased substantially. Today, it is estimated that over 6000 lasers are employed in factories in the United States for manufacturing operations, and as many as 30,000 lasers may be so employed by the year 2000 (Reference 1). This growth in industrial laser usage has been brought about by demonstrated advantages of the laser over conventional methods in preciseness, flexibility, and processing economy coupled with the improved reliability of recent laser equipment. Today there are two basic types of lasers used in manufacturing--solid state systems and gas systems. Solid state systems employ either ruby, neodymium-doped yttrium-aluminum garnet (YAG), or neodymium-doped glass as the lasing medium. Carbon dioxide is used as the lasing medium in industrial gas lasers. Both basic types of lasers can deliver energy in either pulses or in continuous wave (CW). Delivered power can range from a few watts to several kilowatts. Gas lasers generate the highest power outputs, while solid-state lasers are typically operated in the few hundred watt range. The advantage of the laser from a manufacturing viewpoint is its capability to deliver a concentrated density of high energy to a small, focused spot coupled with the ability of the energy spot to be rapidly moved along any three dimensional path. While an electron beam offers similar benefits, the laser's light beam is free from magnetic influences, electrical circuitry limitations, and the need to operate in a vacuum environment. The latter characteristic gives the laser an advantage over electron-beam processing in initial equipment costs and productivity (since vacuum pumpdown times are avoided). The laser is being used for some operations, like welding, where electron-beam methods are also widely used. But it is also being incorporated for processes where the electron beam is technically suitable but has never generally been employed because of cost. Examples include hole drilling, localized heat treating, cutting, surface alloying, and identification marking. The gas-turbine industry, which became one of the major users of electron-beam welding, is rapidly adopting laser processing. Recent production applications for aircraft gas turbines involve hole drilling, welding, cutting, surface alloying and parts marking. Examples of these applications are described below. HOLE DRILLING Precision drilling of small (e.g mm) holes was one of the first major applications for the laser in gas turbine construction. Blades and vanes in the high pressure turbine section of modern aircraft engines require as many as several hundred cooling holes on each airfoil to provide film air cooling (Figunel). Holes must be precisely located on the airfoil with entry angles designed from 90 degrees to as shallow as seven degrees. Hole diameters must be maintained within a few hundredths of a millimeter to ensure that the optimum amount of cooling air will be delivered to the external airfoil surfaces. Too little airflow (e.g. too small a hole diameter) causes part overheating and poor durability; too large a hole diameter wastes cooling air and penalizes engine efficiency. Prior to the laser, these holes were commonly produced by techniques such as electrical discharge machining (EDM). While technically satisfactory, EDM became unacceptably expensive as the numbers of cooling holes per airfoil were increased with more modern blade and vane designs. In the mid 1970's, Pratt & Whitney began using laser drilling as a lower cost replacement to EDM. Initially, 8-joule glass lasers were employed which delivered one energy pulse every four seconds. These were subsequently replaced with 30-joule ruby lasers capable of one pulse per second. As an indicator of the economic benefits which were 2

3 gained in switching to laser drilling, the ruby laser can drill a mm diameter hole through 5mm thick nickel or cobalt superalloy material in 15 seconds; an identical hole drilled by EDM takes 120 seconds. To further increase efficiency, the laser drillers are incorporated into fully automated computer-numerically controlled (CNC) systems with five axes of motion for part and beam manipulation at each laser station. Today, a system of 12 lasers drills over eight million holes in approximately 50,000 airfoils a year at depths ranging from 0.2-2mm. Switching to laser drilling for airfoils required solving several technical problems. Initially, laser hole diameter control was insufficient to satisy the precise airflow reproducibly needed from part to part. This was solved by optimizing laser operating parameters; the required control today is monitored and maintained by inexpensive statistical process techniques. A second early problem involved the metallurgical quality of the laser drilled holes. Like EDM drilling, the laser process leaves a thin layer of resolidified material on the hole walls. Early laser drilled holes had excess resolidified material which either contained small microcracks or was prone to initiating cracks during engine service. Again, optimization of laser processing parameters minimized the resolidified layer and produced metallurgically acceptable holes. Figure2 compares the microstructure of holes produced by EDM and optimized laser drilling techniques. Both are satisfactory for longlife engine use. WYLDIINU Fusion welding of gas-turbine components with the laser offers many of the advantages of electron-beam welding.,much higher welding speeds are attainable with both processes than with conventional arc welding. Both introduce low heat input to the materials being joined, thus minimizing distortion, residual welding stresses, and the amount of heataffected parent material. The rapid cooling rates following welding that are inherent with these two processes are compatible with most gas-turbine alloys. Laser welding requires the same precise joint fit-up as electron-beam welding, but, in addition, laser welds must be actively protected from molten weldmetal oxidation by inert gas shielding since welding is usually performed outside of a vacuum-chamber. The productivity advantages of high speed, low heat input, non-vacuum welding offered by the laser have led to its incorporation as a production process for fabricating several types of gas-turbine components. Early applications utilized pulsed lasers; more recently,.cw solid-state and gas-laser devices are being increasingly used. Each variation of the process has advantages and disadvantages. For equivalent average power levels, the pulsed laser can penetrate and weld thicker sections than the CW laser because of the higher power density delivered with each pulse. (For instance, a 400 watt YAG pulsed laser can weld 0.23nm thick nickel superalloys while a 400 watt YAG CW can only penetrate and weld 0.08mm material of the same alloy.) However, the restricted pulse-repetition rate of pulsed lasers requires slower welding speeds than appropriately powered (i.e. higher powered) CW lasers. Nevertheless, pulsed lasers have the advantage of less heat input (higher local energy density) and lower equipment and maintenance costs. Pratt & Whitney is utilizing laser welding to replace conventional arc welding in several applications and is examining its substitution for electronbeam welding in other requirements. One example involves the construction of impingement tubes which direct cooling air flow inside the first-stage turbine blades in the JT9D engine (Figure 3 ). Originally, each tube (made of Inconel 718 nickel superalloy) was fabricated by manual plasma-arc welding 0.8mm thick end caps and electron-beam welding 0.8mm thick root attachments. Following welding, considerable manual grinding operations were needed to contour the parts to the required dimensions. By substituting 300w CW YAG laser welding for both operations, a 93% savings was achieved in labor manhours for welding time and post-weld finishing operations. Welding time to attach the cover was reduced from 66 seconds to 12 seconds; root attachment welding was reduced from 133 seconds per part to 12 seconds; and post-weld finishing (which took 173 seconds per part by the conventional method) was completely eliminated. Other applications now in production for CW solidstate laser welding includes welds on turbine-vane baffles. These sheet-metal assemblies (made of nickel alloys up to 0.6mm thick) are fabricated with longitudinal butt welds and weld attachment of end covers. As with the impingement tubes, no filler metal is added and thus precise joint fit-up is needed. Experience to date indicates that switching to laser welding has reduced rejection rates due to weld imperfections to less than 1% while tripling productivity. Pulsed YAG welding is currently utilized to attach end covers and cooling air metering plates to turbine blades (Figure 4 ). For these applications, the pulsed laser mode was selected to achieve welds with the minimum possible heat input. This reduces the amount of melting of the superalloy airfoil material and eliminates the occurrence of weld-metal or airfoil heat-affected zone cracking. High power (greater than lkw) gas laser welding is also beginning to be adapted for joining thicker materials used in gas turbine construction. Research studies have demonstrated that high quality, high strength fusion weldments can be produced in typical engine materials such as titanium alloys and nickelbase superalloys using high power lasers (References 2,3). Pratt & Whitney is now fabricating certain engine components by joining 2.3nm thick nickel alloys at welding speeds of hundreds of centimeters per minute using a 6kw rated carbon dioxide CW laser. Use of the high power CW laser increases welding speed 12 times over that attainable with current pulsed lasers. Because of the ability to weld outside of a vacuum, welded parts output is 50% greater than achieveable using electron-beam welding. In establishing this procedure, the only unusual requirement was to design the inert-gas shielding system so as to avoid the formation of beam-absorbing plasmas above the weld pool (Reference 4). CUPPING Cutting is the most common application for lasers in several materials working industries and is the single most popular use for high powered CW carbon dioxide lasers (Reference 5). It is attractive because of the high cutting speeds, precise control, narrow cuts, and usefulness on virtually any material that is opaque to laser radiation. For relatively thin materials, pulsed lasers are preferred for cutting since their local pulse energy density is sufficient to expel the material along the cut. When using CW lasers or when cutting thicker materials, it is necessary to employ a gas jet to assist in material 3

4 expulsion at the kerf. Although laser cutting is a very productive proces its use in the gas turbine industry has been inhibited by concerns about metallurgical damage of cut surfaces. As in laser drilling, laser cutting leaves a residual layer of resolidified material ajacent to the cut (Figure 5). In high performance components, this layer can penalize mechanical properties and/or be the site of in-situ defects. If this condition cannot be tolerated for a particular application, additional operations are required to "dress" the cut surface, and these may negate the economic advantages of using laser cutting. Nevertheless, laser cutting has successfully been adapted for several gas-turbine construction procedures, and more widespread utilization is likely in the future. One early application involved trepanning holes and creating triangular cutouts in a cobaltalloy combustor as economical replacements for EDM and mechanical punching (Reference 6). Many additional uses are presently being examined including cutting of aluminum, titanium, steel, and nickel and superalloys plus the cutting and trimming of polymer layups for nonmetallic composites. SURFACE ALLOYING Surface alloying is one of several promising surface treatments for the laser. This and other applications including transformation hardening of steels and melting and rapid quenching of thin external layers on various materials have been extensively studied in the laboratory (Reference 7) but have yet to be extensively used in production (Reference 1). Several gas-turbine manufacturers have already adopted the process, however, as an improved way to deposit wear-resistant surfaces on components in place of arc welding. At Pratt & Whitney, lasers are now being routinely employed to apply wear-resistant alloys to the surfaces of turbine-blade shroud notches (Figure 6). Previously, wear-resistant hardface alloys were deposited on these locations by inert gas, tungsten-arc welding. This conventional approach required manual welding by skilled welders. Considerable dilution of the hardface deposit from melted turbine-blade base alloy was unavoidable. The heat input needed to successfully fuse the hardface to the shroud notch often created weld and heat-affected zone cracks with attendant high rework levels. Today, arc welding is being replaced by fully automated hardface deposition using 6kw carbon dioxide CW lasers. Cobalt-chromium-tungsten alloy deposits are fused to the blade shroud notches in thicknesses of mm and subsequently machined to finished dimensions. The process automation allows much more precise and reproducible control of energy input and heating and cooling rates. These factors, coupled with the higher local energy concentration of the laser beam, have reduced base-metal melting and deposit dilution and increased the hardness of the surface treatment. Figure 7 compares the microstructures of these types of hardface deposits produced by arc welding and laser fusion and illustrates the differences in dilution and base-metal heat-affected-zone cracking tendencies. Significant economic benefits have been realized by changing to laser surface treating for these types of applications. Processing times have been reduced by a factor of three. More importantly, the reduction of incidence of deposit cracking has lowered rework requirements from approximately 15% to less than 2%. IDENTIFICATION MARKING The laser offers several advantages as a tool to inscribe permanent identification symbols onto parts. Marking is achieved by laser-induced evaporation of shallow amounts of material as the beam traverses the geometric pattern corresponding to the desired identification. Because the process is quick,versatile, and automatible, it is becoming increasingly employed in the gas turbine industry as it is in other types of manufacture. There are several benefits of laser marking compared to conventional "permanent" methods such as mechanical marking (e.g., roll marking, vibropeening, et al) and electroetching. The laser procedure is normally faster, much easier to automate, and is readily adaptable to complex part surfaces. No force is applied to the part being marked, and there is no wear and replacement problem with the marking "tool." It is easy to program the laser to make any number, type, and size of identification marks in any sequence desired. The greatest disadvantages are the considerably higher equipment costs and the need for skilled maintenance versus traditional marking methods. Gas turbine components are typically marked with very low power (50-100w) CW solid-state lasers that traverse at 10-30cm per second. Depths of engraving vary from 0.01 to 0.1mm depending on the application and the material's sensitivity to localized melting and resolidification. For critical gas-turbine components (e.g., rotating parts), it has been necessary to carefully verify that laser marks do not introduce debits in part strength and performance. Results to date, however, indicate that laser marking is widely acceptable and suggest that its use will continue to increase in the near future. SUMMARY Applications for the laser as a materials-working tool in gas-turbine construction have rapidly increased in recent years. Today, this industry is a leader in the employment of laser processing. Although turbine airfoil hole drilling presently constitutes the greatest use of lasers, welding with the process is increasingly being adopted as a more economic method than conventional arc-welding and electron beam techniques. Laser marking is expanding despite relatively high initial equipment costs because of its versatility to accommodate large numbers of symbol types needed for large numbers of different parts. laser cutting is less widely applied today for aircraft gas turbines than in other industries. Restriction of usage principally relates to concerns about residual metallurgical damage to cut surfaces and the need for subsequent costly machining to remove "damaged" material. Use of the laser to produce unique surface treatments of turbine engine parts has begun. The initial implementation has been for creating wear-resistant faying surfaces. REF=CES 1 J. A. Vaccari, "The Laser's Edge in Metalworking", American Machinist, August, W. A. Baeslack et al, "A Comparative Evaluation of Laser and Gas Tungsten Arc Weldments in High- Temperature Titanium Alloys", Welding Journal, July,

5 3 R. F. Duhamel et al, "Laser Welding of Steels and Nickel Alloys", 1983 Conference on Applications of Lasers in Materials Processing, Los Angeles, California, January 24-26, 1983, American Society for Metals 4 M. C. Fowler et al, "Ignition and Maintenance of Subsonic Plasma Waves in Atmospheric Pressure Air by CW Carbon Dioxide Laser Ratiation and Their Effect on Laser Beam Propagation", Journal of Applied Physics, January, C. B. Ritz, "The Laser Marketplace--1984", Lasers & Applications, January, G. F. Benedict, "Production Laser Cutting of Gas Turbine Components", Paper MR80-851, Society of Manufacturing Engineers, C. M. Banns, 'Macro-Materials Processing", Proceedings of IEEE, June, 1982 FIG. 1 JT9D high pressure turbine vane showing airfoil cooling holes produced by laser drilling. LASER - 500X a. b. EDM - 500X FIG. 2 Microstructures of 5mm diameter holes in a nickel alloy showing the amount of resolidified material (arrows) along the hole surfaces. a) drilled by laser b) drilled by electro-discharge machining (EDM) 5

6 FIG. 3 JT9D turbine blade impingement tubes showing locations of laser welds at the root and tip (arrows). FIG. 4 Cover plate attached to a turbine blade root by laser welding. FIG. 5 Residual resolidified layers (arrows)present along a carbon dioxide laser cut in a 0.4mm thick alloy. 6

7 FIG. 6 Deposition of wear resistant cobalt alloy to the shroud notch of a turbine blade by laser melting and fusion. FIG. 7 Microstructures of turbine blade wear-resistant alloy deposits showing differences in base-metal dilution and cracking (arrows) as a function of the process, a) laser b) gas-tungsten arc 7