III III MI1,

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., Now York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society oral its Divisions or Sections, or printed In its publications. Discussion is printed only If the paper is published in an ASME Journal. Papers aro available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94-0T-448 APPLICATIONS OF LASER SYSTEMS TO CUTTING, DRILLING AND WELDING TURBINE ENGINE PARTS Terry L. VanderWert Eden Prairie Operations Lumonics Corporation Eden Prairie, Minnesota III III MI1, ABSTRACT Multi-axis laser materials processing systems are having a significant impact on the way turbine engine parts are being cut, drilled, and welded. The success of laser cutting, drilling and welding is based in the ability to concentrate laser energy into a small area and to produce features having narrow heat affected zones. Reduced tooling expense, fast turnaround, and flexibility for handling design changes and for economical small lot manufacturing are some of the benefits associated with laser processing of turbine engine parts. For example, in replacing hand trimming, laser cutting has increased throughput for trimming a deep drawn gas turbine engine part from 18 pieces per day to 18 pieces in 30 minutes) For another company, laser cutting saved $75,000 in tooling expense for an application involving drilling of 3500, 0.33 mm diameter holes in a 0.36 mm thick turbine engine blade insert' Laser cutting and welding have been key to a major aircraft manufacturer's implementation of Just-In-Time manufacturing practices.' Recent advances in design and control of laser processing systems have increased the number of applications for laser processing turbine engine parts. New processes have increased the range of applications for which laser processing is qualified. In-process gaging has been used to automate processes that were previously manual and, in doing so, increase the quality of the finished part and productivity of the manufacturing operation. This paper reviews the laser processes used to cut, drill, and weld turbine engine parts. Some typical applications are presented to illustrate the benefits of laser systems for processing turbine engine parts. BACKGROUND Laser materials processing involves focusing a high power laser beam onto the surface of a workpiece. A portion of the beam is absorbed; the amount depends on the material type and surface condition. The high intensity (on the order of 10' Watts per cm') produced by absorption of high power (greater than 500 Watts) and focusing the beam to 0.1 to 0.25 mm results in heating, melting and vaporization of the surface. FIGURE 1. LASER DRILLING EFFUSION COOLING HOLES IN A TURBINE ENGINE COMBUSTOR COMPONENT. Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands June 13-16, 1994

2 In cutting and drilling, a high pressure inert or oxidizing gas flows through a nozzle coincident with the laser beam to provide mechanical energy to aid in removing the laser melted metal. In welding, a low pressure, dry inert gas is used to protect the laser melted metal from absorption of or reaction with oxygen and water vapor. LASER CUTTING The success of laser processing is based in the ability to concentrate laser energy into a small area and produce a narrow cut having a small heat affected zone (see Figure 2). The laser cut is characterized by a narrow kerf (or width) and 1-2 degree taper, similar to that produced by fine-blanking. E Kett or eut, width ffl Parent metal E Heat Affected Zone TABLE 1: RECAST LAYER THICKNESS FOR LASER CUT EDGES Recast thickness, mm Average Maximum Type 321 (18 Cr-10.5 Ni-Fe) stainless steel 0.74 mm thick Type 347(18 Cr-11 Ni-Fe) stainless steel 0.51 mm thick mm thick mm thick mm thick mm thick No microcracking was observed for any of the thicknesses Hastelloy X 0.51 mm thick mm thick mm thick mm thick mm thick No microcracking was observed for any of the Thicknesses System: Laserdyne 780 BeamDirectorm/pulsed fast axial flow CO2 Laser Process Conditions: 1000 Watt CW using a 63.5 mm focal length and 1.0 MPa nitrogen through a 1.5 mm inch orifice. FIGURE 2: SCHEMATIC OF LASER CUT EDGE ILLUSTRATING RECAST AND HEAT AFFECTED ZONES. One factor which has limited turbine engine applications of laser systems has been the concern over the effects of the heat affected zone (HAZ) and recast layer on the mechanical properties of the cut edge. Considerable advance both in laser beam quality and in understanding and control of the laser process has led to major reductions in the HAZ and recast layer thicknesses from those found on early laser cut edges. Table 1 shows recast layer thicknesses for several thicknesses of aerospace alloys Such results are easily achieved and leading to significant growth of laser applications. I.no.Pr cutting is a cost effective alternative to conventional trim dies or pinch-off dies especially for complex contours. Since die life for tough aerospace alloys is short, hard tooling can be quite costly to both initially construct and maintain. Representative applications of laser cutting include: laser cutting has replaced conventional machining for trimming a deep drawn gas turbine engine component. Throughput has increased from 18 pieces per day to 18 pieces in 30 minutes.' laser cutting has replaced hand methods for cutting intersecting tubing used in aircraft applications. With the hand method, one assembly was produced every 2-1/2 hours. With laser cutting, the rate has been increased to one assembly every minute. laser cutting flash from forged titanium alloy and stainless steel turbine blades. laser cutting to remove worn or damaged sections of hot section components Laser cutting provides the following benefits: Reduced trimming costs. Time to laser cut and trim formed turbine engine parts is one fifth that for routing, the traditional method for trimming these.' Faster turnaround on production parts. The reduced cycle times mean that the laser can get parts through the trimming stage fast to relieve the bottleneck that occurs with conventional trimming methods.' 2

3 KU X20 1 ni W025. FIGURE 3: SCANNING ELECTRON MICROGRAPH OF LASER CUT EDGE IN 2.0 MM THICK INCONEL 718. THE EDGE WAS PRODUCED WITH A 1500 WATT FAST AXIAL FLOW CO2 LASER, A 63.5 MM FOCAL LENGTH LENS, AND NITROGEN AT 1.0 MPA. THE LASER BEAM FOCAL POINT WAS LOCATED AT THE BACKSURFACE OF THE MATERIAL. THE TOP EDGE OF THE CUT IN THE PHOTOGRAPH IS THE BACKOF THE CUT. Reduced tooling costs. The cost of router tooling has been reduced considerably. Tool life for trimming the tough, difficult-to-machine aerospace materials is short. Laser trimming does not use perishable tooling. Since the laser beam is positioned in five axes, the workpiece remains stationary during cutting and trimming. Consequently, workpiece fixturing is relatively simple since there are no mechanical forces applied to the workpiece during cutting.. Reduced material costs. The narrow kerf of a laser cut has also resulted in significant savings of expensive aerospace alloys. Higher quality because the trim path is more consistent. Trimmed parts can be welded without filler metal. Laser systems for cutting incorporate automatic laser beam focus control to maintain constant location of laser beam focal point relative to the workpiece surface. The focus control automatically compensates for the part-to-part variations common with sheet metal parts. For fast changeover, especially important for small lot size manufacturing, setup of workpiece fixturing is minimized by sensing incorporated into the laser system. A tooling ball located on the fixturing is scanned using sensing built into the laser cutting head. Through the scanning routine, the tooling ball is sensed and its location is determined. From this, fixture offsets are automatically calculated. Since the scanning routine is completed in 5-10 seconds, fixture offsets are recalculated each time the fixture is setup. FIGURE 4: ILLUSTRATION OF A TECHNIQUE BASED ON BUILT-IN SENSING FOR FAST CHANGEOVER BETWEEN DIFFERENT JOBS ON A LASER SYSTEM LASER DRILUNG The ability to control heat input in laser processing is providing design and manufacturing engineers with capability to reduce the size and spacing of features. Turbine engine applications of laser drilling include producing holes for cooling of turbine engine hot section components and for controlling airflow on airfoil surfaces. Thousands of holes are produced by laser processing in turbine engine components such as combustion liners, compressor blades, and nozzle guide vanes. Laser drilling is used to produce small (0.1 mm) diameter holes, holes with high fa 10:1) aspect (depth to diameter) ratios, and holes at shallow (10 ) angles from the surface, all in the toughest aerospace alloys. Precise control of heat input allows features to be cut closely to each other, a feature especially important to designers concerned abut miniaturization. One distinct advantage of laser processing is the capability for low diameter to thickness ratio in percussion drilled or trepanned holes. For thin gauge high temperature alloys, a 1 to 1 diameter to thickness ratio is typically as low as practical for production piercing. A 1.5 to 1 ratio is considered better to maximize yields. A common guideline is that the minimum spacing, or web, is 0.3 times the material thickness, independent of material hardness. Yields and consistency of small laser cut holes is very high; repeatability of ± 0.025mm is easily achieved. There are two types of laser drilling processes: (1) percussion drilling and (2) trepanning (see Figure 5.) Percussion drilling is typically used for production drilling of cooling holes in blades and nozzle guide vanes. As illustrated in Figure 5, the process involves a stationary beam and one of more pulses to penetrate the thickness of the material. 3

4 Percussion drilong Contour cutting of holes (trepanning) FIGURE 5: ILLUSTRATION OF THE TWO PRIMARY LASER DRILLING PROCESSES. Trepanning, on the other hand, involves producing a hole or feature by contour cutting the feature shape. Trepanning is selected over percussion drilling for producing: 1. holes that are larger than those which can be produced by percussion drilling 2. shaped features (see Figure 6) 3. holes having greater consistency than is possible with percussion drilling. Both processes can be tailored to produce a wide range of feature cross-sections and shapes, some complex shapes made possible only as a result of the greater capability (fast servo update times) of modern controls. Top View Crossection along A-A FIGURE 6: SCHEMATIC OF TOP VIEW AND CROSSECTION OF ONE SHAPED HOLE PRODUCED BY LASER DRILLING The benefit of this trepanning over mechanical piercing was illustrated in an application involving trepanning 3, mm diameter holes in a formed 0.36 mm thick Inconel 625 aircraft engine turbine blade cooling insert. FIGURE 7: TREPANNING 0.36 MM DIAMETER HOLES IN 0.38 MM THICK INCONEL 625. The 0.93:1 aspect (diameter:thickness) ratio was difficult to reliably produce with piercing. The manufacturing problem was complicated by the fact that multiple setups were required because of the multiple radii on the part. With laser drilling, tooling costs were reduced by $300,000 for a family of six parts. The real cost savings was greater since laser dilling also eliminated the need for hard pierce tooling. Laser processing is also ideal for drilling the many holes in effusion cooled combustion liners (see Figure 1). Thousands of small holes (0.5 mm diameter) are drilled at to the surface of a roll-formed and welded 1.5 mm thick Hastelloy X cylinder. A key requirement is that holes be drilled along a "waterline" within 0.25 mm. Run out of the rollformed and welded blank made automatic focus control essential for automated production. However, the traditional approach to focus control, that is to offset the laser beam focal point position along the axis of the laser beam (see Figure 8), could not be applied; the "Abbe' error" that resulted from positioning along the axis of the laser beam when drilling holes at an angle other than perpendicular to the surface exceeded the specified tolerance on hole location. To meet the specifications, a new design of automatic focus control was required. As illustrated in Figure 8, the new focus control design provide the user ability to program the multiaxis laser system to offset in the direction of the surface deviation, rather than along the axis of the beam. LASER WELDING Laser welding is growing in application because of its small heat affected zone, ability to weld in an ambient environment (does not require a vacuum), and welding speeds for sheetmetal components of 250 to 750 mm per minute. 4

5 With the Laserdyne fully integrated automate focus control (AFC), the direction of local point motion is VS programmable (caned Selectable Seek). With the lens servo focus control, the lens and focal point move Position error Is avoided by offset in a direction perpendicular to the ideal surface rather than in the direction of the beam. Position error (Abbe error) with traditional focus control. Design Actual position of position of the surface the surface Design position of the surface Actual position of the surface FIGURE 9: LASER WELDING SHEETMETAL COMPONENT FOR AIRCRAFT DUCTING FIGURE 8: ILLUSTRATION OF THE ABBE' ERROR PRODUCED WHEN USING TRADITIONAL AUTOMATIC FOCUS CONTROL FOR DRILLING AT AN ANGLE TO THE SURFACE. These make laser welding a high quality and economical alternative to vacuum brazing, electron beam, and hand welding. Until recently, however, applications have been limited by the difficulty in automating the welding process, especially for sheetmetal parts. As with laser processing in general, a key to producing repeatable laser welds is maintaining the laser beam focal point at a fixed position relative to the surface. Focus controls based on the capacitance method have not been applicable to welding. The reason for this is that a conductive plasma that forms above the weld metal interferes with the ability to sense the worlcpiece surface. Consequently, the conventional focus control follows the plasma rather than the metal surface. An automatic focus control specifically designed to address this problem and provide focus control for welding has been developed. The availability of automatic focus control for laser welding is certain to expand the number of laser welding applications. CONCLUSIONS Capabilities inherent in laser processing combined with advances in laser system design and capability are increasing the importance of laser processing in aerospace part manufacturing today by addressing needs for: Greater accuracy and consistency. Greater process control. Greater flexibility for economical low volume and small lot size manufacturing. Miniaturization. Higher tlwoughput by increased machine utilization as well as faster processing speeds. Faster turnaround on development parts as a means of reducing time to bring a new product to market. REFERENCES Magers, W., personal communication, Rathinavelu, M., "CO2 Laser Cutting and Drilling Applications in Aerospace Fabricating", Paper MF91-403, Fabtech International '91 Conference Papers, SME/FMA, pp "Multi-Axis Laser Boosts 3-D Production", Modern Application News, February 1992, p "Laser Tube Cutting", Production Technology Bulletin 85-16, General Dynamics Corporation, October Vacarri, J., "Precision Laser Cuts Improve Quality", American Machinist, April 1991, pp "Selectable Seek controls hole diameter and location", Industrial Laser Review, July 1993, p Moore, W., Foundation of Mechanical Accuracy, Moore Special Tool Company, Bridgeport, Cr, p.47. 5