ADVANCED COMPOSITE MOLDS- A NEW USE FOR INVAR. Boeing Commercial Airplane Group P.O. Box 3707 Seattle, Washington

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1 ADVANCED COMPOSITE MOLDS- A NEW USE FOR INVAR Steven L. Abberger Boeing Commercial Airplane Group P.O. Box 3707 Seattle, Washington Abstract. Molds constructed from lnvar have been developed for the production of advanced composite aircraft components. Included are early development efforts at the Boeing Company along with later design and fabrication improvements that significantly enhanced this emerging technology. This led to its current application for producing advanced composite primary structures for the Boeing model 777 commercial airplane.

2 Introduction Large commercial aircraft have been using increasing amounts of advanced composites (primarily carbon fibers in an epoxy resin matrix) with each successive model. These composite materials offer excellent high strength to weight,ratios as well as reduce corrosion and fatigue concerns. Common applications have included aircraft control surfaces, body fairings and, more recently, primary structural members. Advanced composite components are produced by consolidating and curing multiple layers of resin impregnated carbon fibers against a rigid mold surface. Processing requires heat (177 C) and pressure (up to 10.5 kg/cm2) to form a dense and accurate aircraft quality part'. lnvar (36% Ni ferrous alloy) has become the material of choice at Boeing Commercial Airplane Group (Seattle, Washington) to construct these critical molds. Why lnvar? Precise aerodynamic surfaces and the exacting fit-up of mating parts drives the requirement for dimensionally accurate advanced composite components. Therefore, it is essential that the thermal expansion mismatch between the composite material and mold be minimized during the 177 C cure cycle. If there is a significant difference in coefficient of thermal expansion (CTE), inaccurate part dimensions and planar warpage in the composite part will occur. Ideally, the CTE of the mold should mirror the part throughout the. 177 C cure cycle. The effective CTE of a carbon fiber/epoxy (CF/Ep) component is typically between 2.9 to 3.6 x 10-6 m/m C. These thermal expansion properties are usually anisotropic, can vary greatly throughout the laminate and, since material orientation of each composite component is unique, predictive mold contour compensation is not often viable. Instead, "matched "thermal growth has been much more successful for producing accurate parts. Prior to lnvar molds, both mild steel and a modified version of the CF/Ep part material were the two primary materials of choice for fabricating molds for this application. The mild steel provides dimensionally stable, durable tooling, however, the CTE of this material is too high (12.1 x 1 o- 6 m/m C). With CF/Ep as the base tooling material, the opposite problem exists. The CTE of this composite tooling material (approximately 3.6 x 1 o- 6 m/m C) is comparable to the aerospace components. yet does not have sufficient damage tolerance for production applications. lnvar, with its compatible thermal expansion of 2.1 x 1 o- 6 m/moc (at 150 C) and mechanical properties near steel, offers an unique combination of low thermal with good durability. (

3 Table I Tooling Material Choices Vs Desired Attributes Low Thermal Durability Expansion Carbon Fiber/Epoxy X Steel X lnvar X X Development Boeing first considered the application of lnvar for composite materials processing in the early 1970's. After briefly struggling with the concept, it was dropped for lack of a reliable welding process, poor material availability and the inability to achieve the required dimensional accuracy of the tooling. The original concept was to stud weld a support structure to thin sheets (2-3 mm thickness) of stretch formed lnvar. The welding process, however, thermally distorted the formed sheets and close dimensional tolerances could not be achieved. In addition, a ready source of large, structural lnvar sheet could not be secured. But the need for a durable, low expansion tooling concept intensified as advanced composite materials were rapidly gaining acceptance. New concepts for mold construction were envisioned that would use heavier lnvar plate sections welded into a robust structure that would then be machined to a very accurate finished contour. Several difficult issues needed to be addressed before the new concept would become production-viable. Material Availability/Cost Procuring enough lnvar to support this development process was never a problem. Numerous mills have been steadily supplying lnvar wire and thin gage strip to electronics and optical equipment industries for years. The problem was that in the early 1980's, thick lnvar plate (6 mm to 50 mm thickness) was not being produced for the simple fact that no market existed for such a product form. lnvar suppliers were initially reluctant to commit the resources to produce plate products to support a then unproven market. The early prototype molds were fabricated from plate that had to be custom rolled from I nvar ingot. However, as lnvar tooling matured and the demand became greater, several lnvar producers realized a potential market opportunity. Not only did these companies begin to routinely produce common sizes of lnvar plate, they also began to stock lnvar plate at local service centers to fill emergent requirements.

4 When using heavy plate to produce lnvar molds, the material cost becomes a significant factor in the overall tool cost. A small batch of custom rolled lnvar plate in 1985 cost $35.20/Kg. In 1996, large quantity orders of common thickness plate are now roughly $13.00/Kg and have been at a stable price for about three years. This price reduction is likely the combined effect of a lower raw m~terial cost of nickel, multiple producers vying for a better market share and the economic benefits of larger mill runs influencing production costs. In perspective, CF/Ep material used for a directly competitive tooling application, has risen steadily to an average cost of $ /kg in These material costs have favored the selection of lnvar molds as a primary tooling -method. Welding Obtaining quality welds on consistent basis proved to be one of the most difficult tasks to achieve. Besides structural integrity, V-groove welds were required to have vacuum integrity at elevated, temperatures/pressures to insure a dense composite laminate could be formed during part processing. Gas tungsten arc (GTA) and gas metal arc (GMA) were the welding methods selected for producing both fillet welds for substructure construction and single/double V groove butt welds needed to attach heavy lnvar plate end to end to form the mold face. But early efforts, using base metal chemistry weld rod and wire, proved unreliable and hot cracking and porosity were often encountered in the welds. Much of the success in solving this problem can be credited to earlier research conducted by C.E. Witherell and others (references 1 thru 3) that supported the development of large liquefied natural gas (LNG) tanks used for cryogenic storage during the 1960s. Three significant changes were incorporated into the welding process to resolve the cracking and porosity problems. First, weld wire chemistry was changed by increasing Ti and. Mn concentrations (to 0. 75o/o and 2.5o/o, respectively) in the lnvar base metal composition. Additions of these deoxidants proved to effectively control hot cracking and reduce porosity. Second, lnvar plate suppliers were required to reduce sulfur and phosphorous in their products to much lower levels than previously produced. By specifying maximum levels to Sand P, the susceptibility of lnvar welds to hot cracking was further decreased. Finally, much closer attention was given to weld preparation in terms of both cleanliness and joint fit-up. The combination of all three changes resulted in a vastly improved joining process where weld anomalies are a rare occurrence.

5 Prototype Molds The first successful prototype mold, built in 1987, demonstrated the feasibility of lnvar tooling for producing a beaded rib component 1. Six sections of lnvar plate (from 12 mm to 30 mm thick) were cut to shape, formed to contour and GTA welded to form a machining blank. Following a full anneal heat treatment, the final contour was achieved by machining and hand finishing. Figure 1 shows the finished lnvar lay-up,mandrel measuring approximately 1.3 by 1.0 meters in size. Figure 1 - Isometric View of lnvar Beaded Rib Prototype Mold A second prototype tool, flat to within +/ mm of nominal, was designed with a rigid substructure as shown in Figure 2. Tools of this type, often with relatively mild compound contours, are common for producing CF/Ep laminates that comprise the aerodynamic surfaces of an airplane. This tool was used to address many of the initial concerns expressed with the introduction of an unfamiliar tooling material. 2 Among the major concerns were long term dimensional stability, thermal conductivity as it effects autoclave cure cycles, surface quality of resultant composite parts and general tool durability. As each 1 For this application, the term "rib" refers to a C-shaped, transversely located structural component on an airplane. The "beads," located on the flange of the rib, provide addition rigidity to the component. 2 Many toolmakers initially envisioned lnvar as an "exotic" alloy and were hesitant to accept this new tooling method. An extraordinary amount of effort was expended to expel rumors about lnvar and prove the validity of this tooling approach.

6 issues was successfully addressed, acceptance of lnvar as not only an acceptable tooling concept, but often the preferred method, rapidly progressed. Figure 2 - Second Prototype lnvar Mold Demonstrating Rigid Substructure Design (Tool Size: 3.5 m by 2.4 m) lnvar Tooling Today Today's design and fabrication methods for lnvar molds, while still primarily based on the early prototype tooling, have evolved though continuous process improvements. Nearly every aspect of lnvar mold construction has been modified and enhanced to continually reduce tooling costs while improving overall design and function. Literally hundreds of lnvar molds have been produced to support every current Boeing commercial airplane model including the new model 777 aircraft. Figures 3 and 4 show an lnvar mold, over 12 meters long and weighing in excess of 9,000 Kg, used to produce an advanced composite component for the 777 rear stabilizer (tail section).

7 Figure 3 - Substructure of 777 lnvar Mold as Seen During Construction (Note: Tool Positioned on Its Side) While there is a wide variety of lnvar mold designs, many tools have similar fabrication techniques. The typical lnvar mold begins with a detailed computer modeled design based on lnvar's unique mechanical properties and available structural shapes (6, 10 and 19 mm thick hot rolled plate are most readily available). Construction of the mold is initiated by high definition plasma cutting of the lighter gage plates into substructure components. The individual components are designed to provide maximum mold rigidity while minimizing weight. The open structure is desired to improve airflow around the mold which enhances h~at-up and cool down during the autoclave cure cycle. These cut pieces are then assembled and welded into an interlocking eggcrate configuration substructure that will provide stability and support to the mold. The mold surface is developed by chip forming the 19 mm thick plate into a rough mold configuration and attaching this formed plate to the substructure. For radical contours that are difficult to form, invar castings are substituted for formed plate. GTA and GMA are still the primary weld processes but are now augmented by the use of flux core arc (FCA) welding for out-of-position fillet welds and submerged ard welding for very high quality vacuum tight joints. The entire welded structure is then fully annealed, cleaned and painted with a corrosion resistant urethane primer. The final mold surface contour is achieved by 5-axis CNC machining and abrasive hand finishing. To meet the demanding

8 dimensional accuracy requirements, all points on the finished mold surface must be within +/ mm of nominal. Figure 4- Completed 777 lnvar Mold The lnvar tooling process has been expanded to now include a 42% nickel-iron composition for higher temperature applications. The 36o/o Ni lnvar is still used exclusively for 177 C processing of thermoset epoxy systems. For thermoplastic advanced composites, however, the 42o/o Ni lnvar is preferred for processing temperatures of 371 oc to 427 C. At this higher temperature, the 42% Ni lnvar thermal expansion characteristics more closely match the thermoplastic material's CTE. In addition, an age-hardenable version of lnvar 3 is currently under evaluation for select applications where maximum mold damage tolerance is desired. Conclusion Since lnvar molds were successfully developed at Boeing, the technology has rapidly matured and spread to other aerospace companies throughout the world. A conservative estimate is that over 220 metric tons of lnvar products are 3 NILO Alloy 365, I NCO Alloys International (Huntington, West Virginia, USA) 0 )

9 c \ ~ ( ( ; consumed for this application annually, producing a variety of advanced composite molds ranging in size 0.2 m to 12 meter in length and weighing up to 10 metric tons. As long as interest in advanced composite materials continues, this new use for the unique family of lnvar alloys will no doubt continue to expand. References 1. C.E. Witherell, "Welding Nickel-iron Alloys of the lnvar Type," Welding Research Suppliment, (4) (1964), M.D. Bellware, "How to Weld lnvar," Welding Engineer, (11) (1964 ), K. Nohara and A. Ejima, "Fe-36% Ni lnvar Type Alloy KLN 36 for LNG Facilities" (Kawasaki Steel Technical Report No. 13, 1985).